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The soils of the Northern Hemisphere permafrost region are estimated to contain 1100 to 1500 Pg of carbon (Pg C). A substantial fraction of this carbon has been frozen and therefore protected from microbial decay for millennia. As anthropogenic climate warming progresses much of this permafrost is expected to thaw. Here we conduct perturbed physics experiments on a climate model of intermediate complexity, with an improved permafrost carbon module, to estimate with formal uncertainty bounds the release of carbon from permafrost soils by year 2100 and 2300. We estimate that by 2100 the permafrost region may release between 56 (13 to 118) Pg C under Representative Concentration Pathway (RCP) 2.6 and 102 (27 to 199) Pg C under RCP 8.5, with substantially more to be released under each scenario by year 2300. A subset of 25 model variants were projected 8000 years into the future under continued RCP 4.5 and 8.5 forcing. Under the high forcing scenario the permafrost carbon pool decays away over several thousand years. Under the moderate scenario forcing a remnant near-surface permafrost region persists in the high Arctic which develops a large permafrost carbon pool, leading to global recovery of the pool beginning in mid third millennium of the common era (CE). Overall our simulations suggest that the permafrost carbon cycle feedback to climate change will make a significant but not cataclysmic contribution to climate change over the next centuries and millennia. [ABSTRACT FROM AUTHOR]

Browning of inland waters has been noted over large parts of the Northern Hemisphere and is a phenomenon with both ecological and societal consequences. The increase in water color is generally ascribed to increasing amounts of dissolved organic matter of terrestrial origin. However, oftentimes the increase in water color is larger than that of organic matter, implying that changes in the amount of organic matter alone cannot explain the enhanced water color. Water color is known to be affected also by the quality of organic matter and the prevalence of iron. Here we investigated trends in water color, organic matter and iron between 1972 and 2010 in 30 rivers draining into the Swedish cost, and performed a laboratory iron addition experiment to natural waters, to evaluate the role of iron and organic matter in determining water color. By comparing the effect of iron additions on water color in the experiment, to variation in water color and iron concentration in the monitoring data, we show that iron can explain a significant share of the variation in water color (on average 25 %), especially in the rivers in the north of Sweden (up to 74 %). Further more, positive trends for iron are seen in 27 of 30 rivers (21-468 %) and the increase in iron is larger than that of organic matter, indicating that iron and organic matter concentrations are controlled by similar but not identical processes. We speculate that increasing iron concentrations can be caused by changes in redox conditions, that mean that more anoxic water with high concentrations of soluble FeII are feeding into the surface waters. More studies are needed about why iron is increasing so strongly, since both causes and consequences are partly different from those of increasing organic matter content. [ABSTRACT FROM AUTHOR]

As a result of climate change, rainfall regimes became more extreme over the course of the 20th century, characterised by fewer and larger rainfall events. Such changes are expected to continue throughout the current century. The effect of changes in the temporal distribution of rainfall on ecosystem carbon fluxes is poorly understood, with most available information coming from experimental studies of grassland ecosystems. Here, continuous measurements of ecosystem carbon fluxes and precipitation from the worldwide FLUXNET network of eddy-covariance sites are exploited to investigate the effects of differences in rainfall distribution on the carbon balance of seasonally water-limited shrubland and forest sites. Once the strong dependence of ecosystem fluxes on total annual rainfall amount is accounted for, results show that sites with more extreme rainfall distributions have significantly lower gross productivity, slightly lower ecosystem respiration and consequently a smaller net ecosystem productivity. [ABSTRACT FROM AUTHOR]

The boreal forest is the world’s largest terrestrial biome, covering all continents in the northern hemisphere. Much research has focused on the effects of forest management and climate change on biodiversity and ecosystem level processes of the boreal forest. However, even though climate change and the increasing rate of resource exploitation are likely to intensify the arrival and establishment of exotic species with the potential to become invasive, the boreal forest continues to be viewed as inhospitable to incoming species and we have little understanding of its invasive species status. We reviewed the literature and compiled information on the current status of invasive species across all taxa present in the North American boreal forest. We found that an increasing number of exotic plants, insects, earthworms, slugs and pathogens are establishing in the boreal forest. Research is scarce and their ecological effects are poorly understood. However, given that some of the reported species represent a major driver of change in many ecosystems globally, we expect that this review will provide direction for invasive species research as well as preventative measures aimed at better understanding and conserving Earth’s largest terrestrial biome. [ABSTRACT FROM PUBLISHER]

In a comment on a 2017 paper by Cheung et al., Kravtsov states that the results of Cheung et al. are invalidated by errors in the method used to estimate internal variability in historical surface temperatures, which involves using the ensemble mean of simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5) to estimate the forced signal. Kravtsov claims that differences between the forced signals in the individual models and as defined by the multimodel ensemble mean lead to errors in the assessment of internal variability in both model simulations and the instrumental record. Kravtsov proposes a different method, which instead uses CMIP5 models with at least four realizations to define the forced component. Here, it is shown that the conclusions of Cheung et al. are valid regardless of whether the method of Cheung et al. or that of Kravtsov is applied. Furthermore, many of the points raised by Kravtsov are discussed in Cheung et al., and the disagreements of Kravtsov appear to be mainly due to a misunderstanding of the aims of Cheung et al. [ABSTRACT FROM AUTHOR]

Zonal and meridional surface temperature gradients are considered to be determinants of large-scale atmospheric circulation patterns. However, there has been limited investigation of these gradients as diagnostic aids. Here, the twentieth-century variability in the Northern Hemisphere equator-to-pole temperature gradient (EPG) and the ocean-land temperature contrast (OLC) is explored. A secular trend in decreasing EPG and OLC is noted. Decadal and interannual (ENSO-related) variations in the joint distribution of EPG and OLC are identified, hinting at multistable climate states that may be indigenous to the climate or due to changing boundary forcings. The NH circulation patterns for cases in the tails of the joint distribution of EPG and OLC are also seen to be different. Given this context, this paper extends past efforts to develop insights into jet stream dynamics using the Lorenz-1984 model, which is forced directly and only by EPG and OLC. The joint probability distribution of jet stream and eddy energy, conditional on EPG and OLC scenarios, is investigated. The scenarios correspond to (i) warmer versus colder climate conditions and (ii) polarized ENSO phases. The latter scenario involves the use of a heuristic ENSO model to drive the Lorenz-1984 model via a modulation of the EPG or the OLC. As with GCMs, the low-order model reveals that the response to El Niño forcing is not similar to an anthropogenic warming signature. The potential uses of EPG and OLC as macro-level indicators of climate change and variability and for comparing results across GCMs and observations are indicated. [ABSTRACT FROM AUTHOR]

Several numerical simulations were performed, using a global climate model that includes a realistic land surface model, to investigate the impact of Asian tropical vegetation changes on the climate. The control simulation, under conditions of the actual vegetation, and three vegetation-change impact experiments were performed. The horizontal resolution of the model used in these simulations was finer than those of the models used in previous vegetation-change impact studies. In, which is a companion of this paper, the results of the Northern Hemisphere summer June–July–August (JJA) case were described. In the present paper, the results of the analysis concern the Northern Hemisphere winter; that is, the December–January–February (DJF) case are discussed as Part II. It was clarified, from the results of the bare soil and C4 grass experiments, that the decrease in the roughness length, and from the results of the green-less experiment, that the decrease in the latent heat flux exert strong influences on horizontal and convective atmospheric circulations and the distribution of precipitation. Other energy and water balances at the land surface are also significantly influenced by the vegetation changes. The vegetation changes were implemented only in the Asian tropical region. There were, however, possible influences of the vegetation change on the midlatitude atmospheric circulation. It was considered that the vegetation changes from the forest type to grassland or bare soil induced modifications in the Hadley and Walker circulations. In particular, the divergence/convergence anomaly pattern that appeared at the upper-atmospheric level in the C4 grass experiment was very similar to that of an ENSO event. The height anomalies at the 500-hPa level were also similar to those found in an ENSO event. The possibility exists that the deforestation of the Asian tropical region could induce similar teleconnections as those associated with ENSO events. [ABSTRACT FROM AUTHOR]

Kretzschmar et al., in a comment in 2017, use the spread in the output of aerosol-climate models to argue that the models refute the hypothesis (presented in a paper by Stevens in 2015) that for the mid-twentieth-century warming to be consistent with observations, then the present-day aerosol forcing, must be less negative than −1 W m−2. The main point of contention is the nature of the relationship between global SO2 emissions and In contrast to the concave (log-linear) relationship used by Stevens and in earlier studies, whereby becomes progressively less sensitive to SO2 emissions, some models suggest a convex relationship, which would imply a less negative lower bound. The model that best exemplifies this difference, and that is most clearly in conflict with the hypothesis of Stevens, does so because of an implausible aerosol response to the initial rise in anthropogenic aerosol precursor emissions in East and South Asia-already in 1975 this model's clear-sky reflectance from anthropogenic aerosol over the North Pacific exceeds present-day estimates of the clear-sky reflectance by the total aerosol. The authors perform experiments using a new (observationally constrained) climatology of anthropogenic aerosols to further show that the effects of changing patterns of aerosol and aerosol precursor emissions during the late twentieth century have, for the same global emissions, relatively little effect on These findings suggest that the behavior Kretzschmar et al. identify as being in conflict with the lower bound in Stevens arises from an implausible relationship between SO2 emissions and and thus provides little basis for revising this lower bound. [ABSTRACT FROM AUTHOR]

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This paper examines the physical processes controlling how synoptic midlatitude temperature variability near the surface changes with climate. Because synoptic temperature variability is primarily generated by advection, it can be related to mean potential temperature gradients and mixing lengths near the surface. Scaling arguments show that the reduction of meridional potential temperature gradients that accompanies polar amplification of global warming leads to a reduction of the synoptic temperature variance near the surface. This is confirmed in simulations of a wide range of climates with an idealized GCM. In comprehensive climate simulations (CMIP5), Arctic amplification of global warming similarly entails a large-scale reduction of the near-surface temperature variance in Northern Hemisphere midlatitudes, especially in winter. The probability density functions of synoptic near-surface temperature variations in midlatitudes are statistically indistinguishable from Gaussian, both in reanalysis data and in a range of climates simulated with idealized and comprehensive GCMs. This indicates that changes in mean values and variances suffice to account for changes even in extreme synoptic temperature variations. Taken together, the results indicate that Arctic amplification of global warming leads to even less frequent cold outbreaks in Northern Hemisphere winter than a shift toward a warmer mean climate implies by itself. [ABSTRACT FROM AUTHOR]

Stratospheric ozone recovery is expected to figure prominently in twenty-first century climate change. In a recent paper, Hu et al. (2011) argue that one impact of ozone recovery will be to enhance the warming of the surface-troposphere system produced by increases in wellmixed greenhouse gases. Furthermore, this enhanced warming would be strongest in the Northern Hemisphere, which is surprising since previous studies have consistently shown the effects of stratospheric ozone changes to be most pronounced in the Southern Hemisphere. Hu et al. (2011) base their claims largely on differences in the simulated temperature change between two groups of CMIP3 (Coupled Model Intercomparison Project 3) climate models, one group which included stratospheric ozone recovery in its twenty-first century simulations and a second group which did not. Both groups of models were forced with the same increases in well-mixed greenhouse gases according to the A1B emissions scenario. In the current work, we compare the surface temperature responses of the same two groups of models in a different experiment in which atmospheric CO2 was increased by 1% per year until doubling. We find remarkably similar differences in the simulated surface temperature change between the two sets of models as Hu et al. (2011) found for the A1B experiment, suggesting that the enhanced warming which they attribute to stratospheric ozone recovery is actually a reflection of different responses of the two model groups to greenhouse gas forcing. [ABSTRACT FROM AUTHOR]

Variations of land surface parameters over the Tibetan Plateau have great importance on local energy and water cycles, the Asian monsoon, and climate change studies. In this paper, the NOAA/NASA Pathfinder Advanced Very High Resolution Radiometer (AVHRR) Land (PAL) dataset is used to retrieve the land surface temperature (LST), the normalized difference vegetation index (NDVI), and albedo, from 1982 to 2000. Simultaneously, meteorological parameters and land surface heat fluxes are acquired from the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) dataset and the Global Land Data Assimilation System (GLDAS), respectively. Results show that from 1982 to 2000 both the LST and the surface air temperature increased on the Tibetan Plateau (TP). The rate of increase of the LST was 0.26±0.16 K decade−1 and that of the surface air temperature was 0.29 ± 0.16 K decade−1, which exceeded the increase in the Northern Hemisphere (0.054 K decade−1). The plateau-wide annual mean precipitation increased at 2.54 mm decade−1, which indicates that the TP is becoming wetter. The 10-m wind speed decreased at about 0.05±0.03 m s−1 decade−1 from 1982 to 2000, which manifests a steady decline of the Asian monsoon wind. Due to the diminishing ground-air temperature gradient and subdued surface wind speed, the sensible heat flux showed a decline of 3.37 ± 2.19 W m−2 decade−1. The seasonal cycle of land surface parameters could clearly be linked to the patterns of the Asian monsoon. The spatial patterns of sensible heat flux, latent heat flux, and their variance could also be recognized. [ABSTRACT FROM AUTHOR]

ABSTRACT The effect of a warmer climate on the properties of extratropical cyclones is investigated using simulations of the ECHAM5 global climate model at resolutions of T213 (60 km) and T319 (40 km). Two periods representative of the end of the 20th and 21st centuries are investigated using the IPCC A1B scenario. The focus of the paper is on precipitation for the NH summer and winter seasons, however results from vorticity and winds are also presented. Similar number of events are identified at both resolutions. There are, however, a greater number of extreme precipitation events in the higher resolution run. The difference between maximum intensity distributions is shown to be statistically significant using a Kolmogorov-Smirnov test. A generalized Pareto distribution is used to analyse changes in extreme precipitation and wind events. In both resolutions, there is an increase in the number of extreme precipitation events in a warmer climate for all seasons, together with a reduction in return period. This is not associated with any increased vertical velocity, or with any increase in wind intensity in the winter and spring. However, there is an increase in wind extremes in the summer and autumn associated with tropical cyclones migrating into the extratropics. [ABSTRACT FROM AUTHOR]

Abstract: The Arctic has undergone substantial changes over the last few decades in various cryospheric and derivative systems and processes. Of these, the Arctic sea ice regime has seen some of the most rapid change and is one of the most visible markers of Arctic change outside the scientific community. This has drawn considerable attention not only from the natural sciences, but increasingly, from the political and commercial sectors as they begin to grapple with the problems and opportunities that are being presented. The possible impacts of past and projected changes in Arctic sea ice, especially as it relates to climatic response, are of particular interest and have been the subject of increasing research activity. A review of the current knowledge of the role of sea ice in the climate system is therefore timely. We present a review that examines both the current state of understanding, as regards the impacts of sea-ice loss observed to date, and climate model projections, to highlight hypothesised future changes and impacts on storm tracks and the North Atlantic Oscillation. Within the broad climate-system perspective, the topics of storminess and large-scale variability will be specifically considered. We then consider larger-scale impacts on the climatic system by reviewing studies that have focused on the interaction between sea-ice extent and the North Atlantic Oscillation. Finally, an overview of the representation of these topics in the literature in the context of IPCC climate projections is presented. While most agree on the direction of Arctic sea-ice change, the rates amongst the various projections vary greatly. Similarly, the response of storm tracks and climate variability are uncertain, exacerbated possibly by the influence of other factors. A variety of scientific papers on the relationship between sea-ice changes and atmospheric variability have brought to light important aspects of this complex topic. Examples are an overall reduction in the number of Arctic winter storms, a northward shift of mid-latitude winter storms in the Pacific and a delayed negative NAO-like response in autumn/winter to a reduced Arctic sea-ice cover (at least in some months). This review paper discusses this research and the disagreements, bringing about a fresh perspective on this issue. [Copyright &y& Elsevier]

The Northern Hemisphere extended winter mean stratospheric vortex alternates between a strong and a weak state, which is manifested in a statistically significant bimodal distribution. In the end of the 1970s a regime change took place, increasing the frequency of the strong phase relative to the weak phase. This paper investigates the connection between the regime behavior of the vortex and the equatorial quasi-biennial oscillation (QBO) in three different datasets. Although there are some differences between the datasets, they agree regarding the general picture. It is found that stratospheric equatorial wind between 70 and 8 hPa shows a bimodal structure in the Northern Hemisphere winter. Such bimodality is nontrivial as it requires only weak variability in the amplitude. Unimodality is found above 8 hPa, where the semiannual oscillation dominates. A strong coincidence is found between strong (weak) vortex winters and winter in the westerly (easterly) QBO regime. Furthermore, the change of the vortex in the late 1970s can be related to a change in the QBO from a period with strong bimodality to a period with weak bimodality. Careful consideration of the statistical significance shows that this change in the QBO can be a random process simply related to the annual sampling of the QBO. Finally, previous findings of phase locking between the QBO and the annual cycle are considered; it is shown that the phase locking is related to the seasonal variations in the bimodality of the QBO. [ABSTRACT FROM AUTHOR]

Based on the availability of hemispheric gridded data sets from observations, analysis and global climate models, objective cyclone identification methods were developed and applied to these data sets. Due to the large amount of investigation methods combined with the variety of different datasets, a multitude of results exist, not only for the recent climate period but also for the next century, assuming anthropogenic changed conditions. Different thresholds, different physical quantities, and considerations of different atmospheric vertical levels add to a picture that is difficult to combine into a common view of cyclones, their variability and trends, in the real world and in GCM studies. Thus, this paper will give a comprehensive review of the actual knowledge on climatologies of mid-latitude cyclones for the Northern and Southern Hemisphere for the present climate and for its possible changes under anthropogenic climate conditions. [ABSTRACT FROM AUTHOR]

The authors examine the effects of latitude and life history on the timing of relationships between satellite measures of normalized difference vegetation index (NDVI) and ground-based measures of tree-ring width in forests at mid- and high latitudes in the Northern Hemisphere. Results indicate a correlation between NDVI and tree rings over the entire growing season for all areas analyzed. For sites south of 40°N, a correlation appears in early spring and late fall while a correlation appears during summer months north of 40°N. For conifers, the correlation appears in summer while deciduous trees show the relationship during early spring and late fall. Of these two correlations, the effect of life history seems to dominate the effect of latitude. The timing of these correlations may help clarify the relationship between climate and tree rings and the effect of canopy duration on carbon uptake by trees. [ABSTRACT FROM AUTHOR]

In this paper, a nonlinear dry model, forced by fixed radiative forcing alone, has been constructed to simulate the Northern Hemisphere winter storm tracks. A procedure has been devised to iterate the radiative equilibrium temperature profile such that at the end of the iterations the model climate closely resembles the desired target climate. This iterative approach is applied to simulate the climatological storm tracks in January. It is found that, when the three-dimensional temperature distribution in the model resembles the observed distribution, the model storm tracks are much too weak. It is hypothesized that this is due to the fact that eddy development is suppressed in a dry atmosphere, owing to the lack of latent heat release in the ascending warm air. To obtain storm tracks with realistic amplitudes, the static stability of the target climate is reduced to simulate the enhancement in baroclinic energy conversion due to latent heat release. With this modification, the storm tracks in the model simulation closely resemble those observed except that the strength of the Atlantic storm track is slightly weaker than observed. The model, when used as a forecast model, also gives high-quality forecasts of the evolution of observed eddies. The iterative approach is applied to force the model to simulate climate anomalies associated with ENSO and the interannual variations of the winter Pacific jet stream/storm tracks. The results show that the model not only succeeds in simulating the climatology of storm tracks, but also produces realistic simulations of storm track anomalies when the model climate is forced to resemble observed climate anomalies. An extended run of the control experiment is conducted to generate monthly mean flow and storm track statistics. These statistics are used to build a linear statistical model relating storm track anomalies to mean flow anomalies. This model performs well when used to hindcast observed storm track anomalies based on observed mean flow anomalies, showing that the storm track/mean flow covariability in the model is realistic and that storm track distribution is not sensitive to the exact form of the applied forcings. [ABSTRACT FROM AUTHOR]

In this paper, the two factors controlling Northern Hemisphere springtime snow albedo feedback in transient climate change are isolated and quantified based on scenario runs of 17 climate models used in the Intergovernmental Panel on Climate Change Fourth Assessment Report. The first factor is the dependence of planetary albedo on surface albedo, representing the atmosphere's attenuation effect on surface albedo anomalies. It is potentially a major source of divergence in simulations of snow albedo feedback because of large differences in simulated cloud fields in Northern Hemisphere land areas. To calculate the dependence, an analytical model governing planetary albedo was developed. Detailed validations of the analytical model for two of the simulations are shown, version 3 of the Community Climate System Model (CCSM3) and the Geophysical Fluid Dynamics Laboratory global coupled Climate Model 2.0 (CM2.0), demonstrating that it facilitates a highly accurate calculation of the dependence of planetary albedo on surface albedo given readily available simulation output. In all simulations it is found that surface albedo anomalies are attenuated by approximately half in Northern Hemisphere land areas as they are transformed into planetary albedo anomalies. The intermodel standard deviation in the dependence of planetary albedo on surface albedo is surprisingly small, less than 10% of the mean. Moreover, when an observational estimate of this factor is calculated by applying the same method to the satellite-based International Satellite Cloud Climatology Project (ISCCP) data, it is found that most simulations agree with ISCCP values to within about 10%, despite further disagreements between observed and simulated cloud fields. This suggests that even large relative errors in simulated cloud fields do not result in significant error in this factor, enhancing confidence in climate models. The second factor, related exclusively to surface processes, is the change in surface albedo associated with an anthropogenically induced temperature change in Northern Hemisphere land areas. It exhibits much more intermodel variability. The standard deviation is about 1/3 of the mean, with the largest value being approximately 3 times larger than the smallest. Therefore this factor is unquestionably the main source of the large divergence in simulations of snow albedo feedback. To reduce the divergence, attention should be focused on differing parameterizations of snow processes, rather than intermodel variations in the attenuation effect of the atmosphere on surface albedo anomalies. [ABSTRACT FROM AUTHOR]

In this paper, the interannual variations in the Northern Hemisphere winter storm tracks have been examined based on 51 winters (December-January-February) of NCEP-NCAR reanalysis data. The leading empirical orthogonal function (EOF) corresponds to the simultaneous strengthening/weakening of both the Pacific and Atlantic storm tracks. Interannual and month-to-month variations in the Pacific and Atlantic storm tracks are found to be significantly correlated. The principal component associated with the leading hemispheric EOF exhibits pronounced interdecadal variability. There appears to be a transition during the early 1970s from a weak storm track state prior to 1972/73 to a strong storm track state subsequently. Decadal mean storm track intensity during the 1990s is about 30% stronger than that during the late 1960s and early 1970s. The relationship between variations in storm track intensity and low-frequency (seasonal mean) flow anomalies has also been examined. It is shown that storm track variations and their associated seasonal mean flow anomalies are highly correlated. Relations to several other modes of interdecadal variability have also been explored. It is shown that part of the storm track variations may be related to the Arctic oscillation and part to the interdecadal ENSO-like variability. However, even when storm track variations that are linearly dependent on these other modes have been removed, substantial interdecadal variations still remain. In order to ascertain that the interdecadal variations have not been introduced into the reanalysis data by changes in the observation network, radiosonde observations along the storm track have been examined. Preliminary analyses suggest that the radiosonde observations are largely consistent with the reanalysis data, except for the possibility that biases in the reanalysis data may have led to an overestimation of the interdecadal variability. Unfortunately, since the peaks of the storm tracks lie over... [ABSTRACT FROM AUTHOR]

Long-term monitoring of surface water quality has shown increasing concentrations of dissolved organic carbon (DOC) across a large part of the Northern Hemisphere. Several drivers have been implicated including climate change, land management change, nitrogen and sulphur deposition and CO 2 enrichment. Analysis of stream water data, supported by evidence from laboratory studies, indicates that an effect of declining sulphur deposition on catchment soil chemistry is likely to be the primary mechanism, but there are relatively few long term soil water chemistry records in the UK with which to investigate this, and other, hypotheses directly. In this paper, we assess temporal relationships between soil solution chemistry and parameters that have been argued to regulate DOC production and, using a unique set of co-located measurements of weather and bulk deposition and soil solution chemistry provided by the UK Environmental Change Network and the Intensive Forest Monitoring Level II Network. We used statistical non-linear trend analysis to investigate these relationships at 5 forested and 4 non-forested sites from 1993 to 2011. Most trends in soil solution DOC concentration were found to be non-linear. Significant increases in DOC occurred mostly prior to 2005. The magnitude and sign of the trends was associated qualitatively with changes in acid deposition, the presence/absence of a forest canopy, soil depth and soil properties. The strongest increases in DOC were seen in acidic forest soils and were most clearly linked to declining anthropogenic acid deposition, while DOC trends at some sites with westerly locations appeared to have been influenced by shorter-term hydrological variation. The results indicate that widespread DOC increases in surface waters observed elsewhere, are most likely dominated by enhanced mobilization of DOC in surficial organic horizons, rather than changes in the soil water chemistry of deeper horizons. While trends in DOC concentrations in surface horizons have flattened out in recent years, further increases may be expected as soil chemistry continues to adjust to declining inputs of acidity. [ABSTRACT FROM AUTHOR]

Summary A paper recently published in “Nature” finds that there is sufficient evidence to identify the effects of human activity on global temperature. The study is based on northern and southern hemispheres’ time series temperature from 1865 to 1994 and the econometric technique applied is Granger causality analysis. In this note, we make some critical remarks on the conclusions of this study which seem to be rather inconsistent from a methodological point of view. It is shown that a more accurate application of Granger causality analysis to this problem may not allow the same strong and unambiguous conclusions. [ABSTRACT FROM AUTHOR]

Accurate quantification of the distribution and variability of atmospheric CO2 is crucial for a better understanding of global carbon cycle characteristics and climate change. Model simulation and observations are only two ways to globally estimate CO2 concentrations and fluxes. However, large uncertainties still exist. Therefore, quantifying the differences between model and observations is rather helpful for reducing their uncertainties and further improving model estimations of global CO2 sources and sinks. In this paper, the GEOS-Chem model was selected to simulate CO2 concentration and then compared with the Greenhouse Gases Observing Satellite (GOSAT) observations, CarbonTracker (CT) and the Total Carbon Column Observing Network (TCCON) measurements during 2009–2011 for quantitatively evaluating the uncertainties of CO2 simulation. The results revealed that the CO2 simulated from GEOS-Chem is in good agreement with other CO2 data sources, but some discrepancies exist including: (1) compared with GOSAT retrievals, modeled XCO2 from GEOS-Chem is somewhat overestimated, with 0.78 ppm on average; (2) compared with CT, the simulated XCO2 from GEOS-Chem is slightly underestimated at most regions, although their time series and correlation show pretty good consistency; (3) compared with the TCCON sites, modeled XCO2 is also underestimated within 1 ppm at most sites, except at Garmisch, Karlsruhe, Sodankylä and Ny-Ålesund. Overall, the results demonstrate that the modeled XCO2 is underestimated on average, however, obviously overestimated XCO2 from GEOS-Chem were found at high latitudes of the Northern Hemisphere in summer. These results are helpful for understanding the model uncertainties as well as to further improve the CO2 estimation. [ABSTRACT FROM AUTHOR]

The temperatures in the Northern Hemisphere from 1854 to 1999 have been analyzed in this article by means of a testing procedure that permits one to consider fractional degrees of integration. The tests are valid under general forms of serial correlation and deterministic trends and do not require estimation of the fractional differencing parameter. The results show that the series follows a fractionally integrated process with the order of integration higher than zero and thus implying long memory behavior. The series was decomposed into four different subsamples, and it was observed that the degree of dependence between the observations substantially increased during the twentieth century. [ABSTRACT FROM AUTHOR]

The largest contributor to the planetary energy imbalance is well‐mixed greenhouse gases (GHGs), which are partially offset by poorly mixed (and thus northern midlatitude dominated) anthropogenic aerosols (AAs). To isolate the effects of GHGs and AAs, we analyze data from the CMIP5 historical (i.e., all natural and anthropogenic forcing) and single forcing (GHG‐only and AA‐only) experiments. Over the duration of the historical experiment (1861–2005) excess heat uptake at the top of the atmosphere and ocean surface occurs almost exclusively in the Southern Hemisphere, with AAs canceling the influence of GHGs in the Northern Hemisphere. This interhemispheric asymmetry in surface heat uptake is eliminated by a northward oceanic transport of excess heat, as there is little hemispheric difference in historical ocean heat storage after accounting for ocean volume. Data from the 1pctCO2 and RCP 8.5 experiments suggests that the future storage of excess heat will be skewed toward the Northern Hemisphere oceans. Plain Language Summary: Climate change is fundamentally an energy balance problem. Due to the influence of greenhouse gas (GHG) emissions, the amount of solar energy absorbed by Earth is currently greater than the amount of energy radiated to space. This energy imbalance is partially offset by particulate matter released into the atmosphere from burning fossil fuels (anthropogenic aerosols, AAs), which is most concentrated in the northern midlatitudes. In this study, model simulations of the historical and future climate are compared to single forcing simulations that apply only GHG or AA emissions. We find that the historical uptake of excess heat is strongly skewed toward the Southern Hemisphere because AAs cancel the influence of GHGs in the Northern Hemisphere. The oceanic storage of that heat shows little difference between the hemispheres due to a strong northward transport of excess heat. In future, the models suggest excess heat storage will skew toward the Northern Hemisphere. Key Points: Single forcing simulations suggest anthropogenic aerosols have caused most historical excess heat uptake to occur in the Southern HemisphereThis interhemispheric asymmetry in heat uptake is eliminated by a northward oceanic transport of excess heatIn the future, the storage of excess heat will be skewed toward the Northern Hemisphere [ABSTRACT FROM AUTHOR]

Data from five sites of the International Long Term Ecological Research (ILTER) network in the North- Eastern Pacific, Western Arctic Ocean, Northern Baltic Sea, South-Eastern North Sea and in the Western Mediterranean Sea were analyzed by dynamic factor analysis (DFA) to trace common multi-year trends in abundance and composition of phytoplankton, benthic fauna and temperate reef fish. Multiannual trends were related to climate and environmental variables to study interactions. Two common trends in biological responses were detected, with temperature and climate indices as explanatory variables in four of the five LTER sites considered. Only one trend was observed at the fifth site, the Northern Baltic Sea, where no explanatory variables were identified. Our findings revealed quasi-synchronous biological shifts in the different marine ecosystems coincident with the 2000 climatic regime shift and provided evidence on a possible further biological shift around 2010. The observed biological modifications were coupled with abrupt or continuous increase in sea water and air temperature confirming the key-role of temperature in structuring marine communities. [ABSTRACT FROM AUTHOR]

The circulation of the Northern Hemisphere extratropical troposphere has changed over recent decades, with marked decreases in extratropical cyclone activity and eddy kinetic energy (EKE) in summer and increases in the fraction of precipitation that is convective in all seasons. Decreasing EKE in summer is partly explained by a weakening meridional temperature gradient, but changes in vertical temperature gradients and increasing moisture also affect the mean available potential energy (MAPE), which is the energetic reservoir from which extratropical cyclones draw. Furthermore, the relation of changes in mean thermal structure and moisture to changes in convection associated with extratropical cyclones is poorly understood. Here we calculate trends in MAPE for the Northern extratropics in summer over the years 1979-2017, and we decompose MAPE into both convective and nonconvective components. Nonconvective MAPE decreased over this period, consistent with decreases in EKE and extratropical cyclone activity, but convective MAPE increased, implying an increase in the energy available to convection. Calculations with idealized atmospheres indicate that nonconvective and convective MAPE both increase with increasing mean surface temperature and decrease with decreasing meridional surface temperature gradient, but convective MAPE is relatively more sensitive to the increase in mean surface temperature. These results connect changes in the atmospheric mean state with changes in both large-scale and convective circulations, and they suggest that extratropical cyclones can weaken even as their associated convection becomes more energetic. [ABSTRACT FROM AUTHOR]

Plant secondary compounds (PSCs), also called secondary metabolites, have high chemical and structural diversity and appear as non-volatile or volatile compounds. These compounds may have evolved to have specific physiological and ecological functions in the adaptation of plants to their growth environment. PSCs are produced by several metabolic pathways and many PSCs are specific for a few plant genera or families. In forest ecosystems, full-grown trees constitute the majority of plant biomass and are thus capable of producing significant amounts of PSCs. We summarize older literature and review recent progress in understanding the effects of abiotic and biotic factors on PSC production of forest trees and PSC behavior in forest ecosystems. The roles of different PSCs under stress and their important role in protecting plants against abiotic and biotic factors are also discussed. There was strong evidence that major climate change factors, CO2 and warming, have contradictory effects on the main PSC groups. CO2 increases phenolic compounds in foliage, but limits terpenoids in foliage and emissions. Warming decreases phenolic compounds in foliage but increases terpenoids in foliage and emissions. Other abiotic stresses have more variable effects. PSCs may help trees to adapt to a changing climate and to pressure from current and invasive pests and pathogens. Indirect adaptation comes via the effects of PSCs on soil chemistry and nutrient cycling, the formation of cloud condensation nuclei from tree volatiles and by CO2 sequestration into PSCs in the wood of living and dead forest trees. [ABSTRACT FROM AUTHOR]

Recent studies have revealed a global, poleward migration trend of tropical cyclones (TCs) in terms of annual‐averaged latitude of lifetime maximum intensity under the global warming. The TCs in the western North Pacific (WNP) make the largest contribution to this trend. One existing hypothesis is that there is a linkage between poleward migration of TCs and anthropogenic forcing. Here we introduce a new measurement, the lifetime‐averaged latitude weighted by TC destructive potential, to detect the meridional migration of the WNP TCs, which is more reliable and meaningful as it considers not only past data uncertainty but also public concerns. Our results show that the trend of WNP TC migration reversed during the recent warming slowing‐down period (after ~1999). Different from the existing hypothesis, it is the change of regional pattern of sea surface temperature that contributes greatly to the meridional migration of WNP TCs. Plain Language Summary: Tropical cyclones (TCs) account for the majority of natural catastrophic losses in the developed world and are the second leading cause (next to floods) of death and injury among natural disasters affecting developing countries. The challenges for climate detection and attribution research with regard to TCs are to determine whether an observed change in TC activity exceeds the natural variability and to attribute significant changes to anthropogenic forcing (e.g., greenhouse gases or aerosols). Recent studies have indicated that the TCs over the western North Pacific (WNP) systematically migrated poleward, which is plausibly linked to the expansion of the tropics associated with anthropogenic forcing. However, utilizing our new measurement, we find that the poleward migration of WNP TCs reversed direction during the recent warming slowing‐down period (after ~1999). We thus propose a new mechanism associated with the regional pattern of sea surface temperature change, which is characterized by a relatively warm or cold sea surface temperature pool in the region (20–40°N, 140–160°E). Compared with the previous theory based on anthropogenic forcing and well‐known dominant modes of variability, the new mechanism provides a more convincing explanation for the migration of WNP TCs since 1980, especially for the recent warming slowing‐down period. Key Points: The trend of tropical cyclone migration over the western North Pacific reversed during the recent warming slowing‐down period (after ~1999)It is the change of regional sea surface temperature that contributed greatly to the meridional migration trend of tropical cyclonesThe effect of regional sea surface temperature change came from its impact on storm potential intensity and large‐scale circulation [ABSTRACT FROM AUTHOR]

Observed long-term variations in summer season timing and length in the Northern Hemisphere (NH) continents and their subregions were analyzed using temperature-based indices. The climatological mean showed coastal–inland contrast; summer starts and ends earlier inland than in coastal areas because of differences in heat capacity. Observations for the past 60 years (1953–2012) show lengthening of the summer season with earlier summer onset and delayed summer withdrawal across the NH. The summer onset advance contributed more to the observed increase in summer season length in many regions than the delay of summer withdrawal. To understand anthropogenic and natural contributions to the observed change, summer season trends from phase 5 of the Coupled Model Intercomparison Project (CMIP5) multimodel simulations forced with the observed external forcings [anthropogenic plus natural forcing (ALL), natural forcing only (NAT), and greenhouse gas forcing only (GHG)] were analyzed. ALL and GHG simulations were found to reproduce the overall observed global and regional lengthening trends, but NAT had negligible trends, which implies that increased greenhouse gases were the main cause of the observed changes. However, ALL runs tend to underestimate the observed trend of summer onset and overestimate that of withdrawal, the causes of which remain to be determined. Possible contributions of multidecadal variabilities, such as Pacific decadal oscillation and Atlantic multidecadal oscillation, to the observed regional trends in summer season length were also assessed. The results suggest that multidecadal variability can explain a moderate portion (about ±10%) of the observed trends in summer season length, mainly over the high latitudes. [ABSTRACT FROM AUTHOR]

Offline forcing methods for ice-sheet models often make use of an index approach in which temperature anomalies relative to the present are calculated by combining a simulated glacial-interglacial climatic anomaly field, interpolated through an index derived from the Greenland ice-core temperature reconstruction, with present-day climatologies. An important drawback of this approach is that it clearly misrepresents climate variability at millennial timescales. The reason for this is that the spatial glacial-interglacial anomaly field used is associated with orbital climatic variations, while it is scaled following the characteristic time evolution of the index, which includes orbital and millennial-scale climate variability. The spatial patterns of orbital and millennial variability are clearly not the same, as indicated by a wealth of models and data. As a result, this method can be expected to lead to a misrepresentation of climate variability and thus of the past evolution of Northern Hemisphere (NH) ice sheets. Here we illustrate the problems derived from this approach and propose a new offline climate forcing method that attempts to better represent the characteristic pattern of millennial-scale climate variability by including an additional spatial anomaly field associated with this timescale. To this end, three different synthetic transient forcing climatologies are developed for the past 120 kyr following a perturbative approach and are applied to an ice-sheet model. The impact of the climatologies on the paleo-evolution of the NH ice sheets is evaluated. The first method follows the usual index approach in which temperature anomalies relative to the present are calculated by combining a simulated glacial-interglacial climatic anomaly field, interpolated through an index derived from ice-core data, with presentday climatologies. In the second approach the representation of millennial-scale climate variability is improved by incorporating a simulated stadial-interstadial anomaly field. The third is a refinement of the second one in which the amplitudes of both orbital and millennial-scale variations are tuned to provide perfect agreement with a recently published absolute temperature reconstruction over Greenland. The comparison of the three climate forcing methods highlights the tendency of the usual index approach to overestimate the temperature variability over North America and Eurasia at millennial timescales. This leads to a relatively high NH icevolume variability on these timescales. Through enhanced ablation, this results in too low an ice volume throughout the last glacial period (LGP), below or at the lower end of the uncertainty range of estimations. Improving the representation of millennial-scale variability alone yields an important increase in ice volume in all NH ice sheets but especially in the Fennoscandian Ice Sheet (FIS). Optimizing the amplitude of the temperature anomalies to match the Greenland reconstruction results in a further increase in the simulated icesheet volume throughout the LGP. Our new method provides a more realistic representation of orbital and millennial-scale climate variability and improves the transient forcing of ice sheets during the LGP. Interestingly, our new approach underestimates ice-volume variations on millennial timescales as indicated by sea-level records. This suggests that either the origin of the latter is not the NH or that processes not represented in our study, notably variations in oceanic conditions, need to be invoked to explain millennial-scale icevolume fluctuations. We finally provide here both our derived climate evolution of the LGP using the three methods as well as the resulting ice-sheet configurations. These could be of interest for future studies dealing with the atmospheric or/and oceanic consequences of transient ice-sheet evolution throughout the LGP and as a source of climate input to other ice-sheet models. [ABSTRACT FROM AUTHOR]

Copyright of Advances in Atmospheric Sciences is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Abstract: Aim: Numerous studies have reported changes in first flowering day (FFD‐changes) in response to changes in climate. However, regarding the direction (advances versus delays) and the intensity (number of days/decade) of FFD‐changes, species show differences even when observed in the same location. Here, we examine the extent to which plant traits can explain observed differences in the response of flowering phenology in trees, shrubs, herbs and grasses. Location: Eighteen sites distributed over the Northern Hemisphere. Methods: We compiled data from the literature on FFD‐changes over recent decades for 562 species (648 observations). We related FFD‐changes to predictor variables associated with (a) changes in climate, (b) local site conditions and (c) traits. Results: Of all FDD‐changes, 80.4% were FFD‐advances, 69.9% not exceeding 5 days/decade, and 10.5% exceeding 5 days/decade, whereas 19.6% reported delays. The intensity of FFD‐advances could be explained by several predictor variables from all three groups (a–c). The importance of these variables differed between the growth forms. Overall, decreasing precipitation was more important than increasing temperature in explaining FFD‐advances. FFD‐advances were strongest in polar tundra and in dry and warm habitats. Traits related to competition and growth rate, like plant height, specific leaf area and leaf dry matter content, had substantial explanatory power in the models. Traits had the highest overall importance in trees and grasses. In herbs they were of equal importance with changes in climate. In shrubs, variables related to site conditions best explained the intensity of FFD‐advances. Main conclusions: Plant traits are important to understand species‐specific and growth form‐specific differences in phenological responses to climatic changes. Hence, in future observations and predictions of plant phenology, traits should be taken into consideration, especially those related to competition and growth rate, as they improve our understanding of adaptations leading to phenological changes. [ABSTRACT FROM AUTHOR]

The dynamic character of an enstrophy-based diagnostic, previously used in the study of atmospheric blocking, is examined here, in near-term future simulations from the Institut Pierre Simon Laplace Climate Model version 4 (IPSL-CM4) and version 5 (IPSL-CM5) climate models of the Northern Hemisphere flow for moderate climate change scenarios. Previous research has shown that integrated regional enstrophy (IE) increases during blocking onset and decay, which is a reflection of planetary-scale instability. In addition, IE has been shown previously to increase during flow regime transitions in general, even those not associated with blocking events. Here, a 31-year IE diagnostic time series is examined for changes in short term (5-40 days) planetary-scale variability that may correspond flow regime changes in an increased carbon dioxide environment. The time-series analysis herein indicates that the IE diagnostic provides evidence for approximately 30-35 atmospheric flow regime transitions per year in a warmer climate, which is similar to that of the control run and the latest 30-year observed climate, as derived from re-analyses. This result has implications regarding the predictability of weather in a warmer world. [ABSTRACT FROM AUTHOR]

Copyright of Advances in Atmospheric Sciences is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Projected changes in temperature extremes, such as regional changes in the intensity and frequency of hot extremes, differ strongly across climate models. This study shows that this disagreement can be partly explained by discrepancies in the representation of the present-day temperature distribution, motivating the evaluation of models with observations. By evaluating climate models on carefully selected metrics, the models that are more likely to be reliable for long-term projections of temperature extremes are identified. The study found that frequencies of hot extremes are likely to increase at a higher rate than the multimodel mean estimate over large parts of the Northern Hemisphere and Australia. This implies that a higher degree of adaptation is required for a given global temperature target. It also found that projected changes in the intensity of hot extremes can be constrained in several regions, including Australia, central North America, and north Asia. In many other regions, large internal variability can often hamper model evaluation. For both aspects-the intensity and the frequency of hot extremes-the total area over which the constraints can be implemented is limited by the quality and completeness of observations. Thereby, this study highlights the importance of long-term, high-quality, and easily accessible observational records for model evaluation, which are vital to ultimately reduce uncertainties in projections of temperature extremes. [ABSTRACT FROM AUTHOR]

The observed late twentieth century warming must be assessed in relation to natural long-term variations of the climatic system. Here, we present a Northern Hemisphere (NH) temperature reconstruction for the mid-to-late Holocene of the past 6000 years, based on a synthesis of existing paleo-temperature proxies that are capable of revealing centennial-scale variability. This includes 56 published temperature records across the NH land areas, with a sampling resolution ranging from 1 to 100 years and a time span of at least 1000 years. The composite plus scale (CPS) method is adopted with spatial weighting to develop the NH temperature reconstruction. Our reconstruction reveals abrupt cold epochs that match well the Bond events during the past 6000 years. The study further reveals two prominent cycles in NH temperature: 1700-2000-year cycle during the mid-to-late Holocene and 1200-1500-year cycle during the past 3500 years. Our reconstruction indicates that the late twentieth century NH temperature and its rate of warming are both unprecedentedly high over the past 5000 years. By comparing our reconstruction with the projected temperature increase scenarios, we find that temperature by the end of the twenty-first century would likely exceed any peaks during the mid-to-late Holocene. [ABSTRACT FROM AUTHOR]

Aims Long-term climate stability is hypothesized to drive the emergence of species assemblages with large species age differences due to the accumulation of relict species and relatively newly arisen species via reduced extinction and increased speciation. Few studies have addressed these predictions and so far no study has done so for plants across the Northern Hemisphere. Here, we linked Quaternary-scale climate variability to phylogenetic age differences between the oldest and youngest group of species in tree assemblages in 100 km × 100 km grid cells across the Northern Hemisphere to test these predictions. Location Northern Hemisphere. Methods Last Glacial Maximum (LGM)-to-present shifts in temperature and precipitation were used as proxies for Quaternary-scale glacial-interglacial climate variability. Simultaneous autoregressive (SAR) models were used to assess the relationships between phylogenetic age differences and Quaternary-scale climate variability. Results We found that phylogenetic age differences overall were largest in China and smallest in Europe, and they declined with increasing temperature instability as predicted, but only in Europe and North America. In China, the relatively mild Quaternary climate changes did not appear to have strongly affected phylogenetic age differences in tree assemblages. Main conclusions Our results show that phylogenetically diverse assemblages with large phylogenetic age differences among species are associated with relatively high long-term climate stability, with intra-regional links between long-term climate variability and phylogenetic composition especially strong in the more unstable regions. These findings point to future climate change as a key risk to the preservation of the phylogenetically diverse assemblages in regions characterized by relatively high paleoclimate stability, with China as a key example. [ABSTRACT FROM AUTHOR]

The last 130 000 years have been marked by pronounced millennial-scale climate variability, which strongly impacted the terrestrial environments of the Northern Hemisphere, especially at middle latitudes. Identifying the trigger of these variations, which are most likely associated with strong couplings between the ocean and the atmosphere, still remains a key question. Here, we show that the analysis of δ18O and dust in the Greenland ice cores, and a critical study of their source variations, reconciles these records with those observed on the Eurasian continent. We demonstrate the link between European and Chinese loess sequences, dust records in Greenland, and variations in the North Atlantic sea ice extent. The sources of the emitted and transported dust material are variable and relate to different environments corresponding to present desert areas, but also hidden regions related to lower sea level stands, dry rivers, or zones close to the frontal moraines of the main Northern Hemisphere ice sheets. We anticipate our study to be at the origin of more sophisticated and elaborated investigations of millennial and sub-millennial continental climate variability in the Northern Hemisphere. [ABSTRACT FROM AUTHOR]

Observations show the tropical belt has widened over the past few decades, a phenomenon associated with poleward migration of subtropical dry zones and large-scale atmospheric circulation. Coupled climate models also simulate tropical belt widening, but less so than observed. Reasons for this discrepancy, and the mechanisms driving the expansion remain uncertain. Here, the role of unforced, natural climate variability-particularly natural sea surface temperature (SST) variability-in recent tropical widening is shown. Compared to coupled ocean-atmosphere models, atmosphere-only simulations driven by observed SSTs consistently lead to larger rates of tropical widening, especially in the Northern Hemisphere (NH), highlighting the importance of recent SST evolution. Assuming the ensemble mean SSTs from historical simulations accurately represent the externally forced response, the observed SSTs can be decomposed into a forced and an unforced component. Targeted simulations with the Community Atmosphere Model, version 5 (CAM5), show that natural SST variability accounts for nearly all of the widening associated with recent SST evolution. This is consistent with the similarity of the unforced SSTs to the observed SSTs, both of which resemble a cold El Niño-Southern Oscillation/Pacific decadal oscillation (ENSO/PDO)-like SST pattern, which is associated with a wider tropical belt. Moreover, CAM5 coupled simulations with observed central to eastern tropical Pacific SSTs yield more than double the rate of widening compared to analogous simulations without prescribed tropical Pacific SSTs and reproduce the magnitude of tropical widening in atmosphere-only simulations. The results suggest that the bulk of recent tropical widening, particularly in the NH, is due to unforced, natural SST variability, primarily related to recent ENSO/PDO variability. [ABSTRACT FROM AUTHOR]

Siberia has experienced a pronounced warming over the past several decades, which has induced an increase in the extent of evergreen conifer forest. However, the potential slowing of the trend of increasing surface air temperature (SAT) has produced intense debate since the late 1990s. During this warming hiatus, the Siberian region experienced a significant cooling during the winter season around 10 times that of the Northern Hemisphere (NH) as a whole. This potentially stresses evergreen conifer forests because cooler winters can cause cold-temperature damage and, hence, increase the mortality of young evergreen conifer forests. In this study, the response of Siberian forest composition during the warming hiatus was investigated using satellite observations coupled with model simulations. Observations indicated that from 2001 to 2012, the apparent area of evergreen conifer forest has increased by 10%, while that of the deciduous conifer forest has decreased by 40%. The transition from deciduous to evergreen conifer forest usually occurs through mixed forest or woody savannas as a buffer. To verify the response of evergreen conifer forest, model experiments were performed using an individual-based forest model. Hysteresis of forest change seen in the model simulations indicates that changes in forest composition dynamics under temperature oscillations induced by internal climate variability may not reverse this composition change. As a result, the evergreen conifer forest expansion under climate warming is expected to be a continuing process despite the occurrence of a warming hiatus, exerting far-reaching implications for climate-change-induced albedo shifts in the Siberian forest. [ABSTRACT FROM AUTHOR]

Seasonal snow-cover modulates water and energy budgets across large areas of the Northern Hemisphere. Previous research, based on satellite imagery interpreted and curated by the Rutgers University Snow Laboratory, has identified significant negative and positive trends in annual snow-covered duration and area at hemispheric and continental scales between 1971 and 2014. This study uses the same dataset to generate more detailed descriptions of spatial variations in these trends, maps intraannual variations in sign, statistical significance and strength, and quantifies associations with latitude and elevation. It also considers the limitations and uncertainties associated with a binary classification of this type, and the implications for trend magnitudes of adopting alternatives to the conventional assumption of 100% (0%) actual fractional snow-covered area in ‘snow-covered’ (‘snow-free’) spatial units at different stages of the snow-season. This prompts adoption of alternative terminology, referring to ‘snow-dominated’ area and duration. In response to questions about the dataset's veracity raised by some prior studies, it discusses climatological factors of potential relevance in explaining spatio-temporal trend patterns, and considers how biases might possibly have been introduced as a result of extraneous influences. [ABSTRACT FROM AUTHOR]

The North Atlantic Oscillation (NAO) plays an important role in the climate variability of the Northern Hemisphere, with significant consequences on long-range pollutant transport. We investigate the evolution of pollutant transport in the 21st century influenced by the NAO under a global climate change scenario. We use a free-running simulation performed by the ECHAM/MESSy Atmospheric Chemistry (EMAC) model coupled with the ocean general circulation model MPIOM, covering the period from 1950 until 2100. Similarly to other works, the model shows a future northeastward shift of the NAO centres of action and a weak positive trend of the NAO index (over 150 years). Moreover, we find that NAO trends (computed over periods shorter than 30 years) will continue to oscillate between positive and negative values in the future. To investigate the NAO effects on transport we consider carbon monoxide tracers with exponential decay and constant interannual emissions. We find that at the end of the century, the south-western Mediterranean and northern Africa will, during positive NAO phases, see higher pollutant concentrations with respect to the past, while a wider part of northern Europe will, during positive NAO phases, see lower pollutant concentrations. Such results are confirmed by the changes observed in the future for tracer concentration and vertically integrated tracer transport, differentiating the cases of "high NAO" and "low NAO" events. [ABSTRACT FROM AUTHOR]

Previous research has shown that subtropical and extratropical circulations are linked seasonally and in response to climate change. In particular, amplification (weakening) of subtropical stationary eddies is linked to a poleward (equatorward) shift of the extratropical circulation in the Northern Hemisphere. Here the mechanisms linking subtropical and extratropical circulation responses to climate change are examined using prescribed sea surface temperature aquaplanet simulations with a subtropical zonal asymmetry that mimics land-ocean contrasts. A poleward circulation shift occurs in response to uniform global warming even in the presence of subtropical stationary eddies. Subtropical stationary eddies exhibit a weak response to global warming; however, regional warming of temperature (or equivalent potential temperature) over land (ocean) increases (decreases) stationary eddy amplitude and shifts the extratropical circulation poleward (equatorward), consistent with comprehensive models. The stationary eddy response to regional warming is connected to regional moist entropy gradient, energy input to the atmosphere, and gross moist stability changes. Stationary eddy amplitude changes directly affect momentum and moist static energy transport following linear wave and mixing length theories. The transport changes do not follow a fixed-diffusivity framework. Extratropical transient eddy transport changes compensate ~70%-90% of the subtropical stationary eddy transport response. This assumes exact subtropical compensation accounts for a large fraction of the meridional shift of the extratropical circulation in response to regional climate change. [ABSTRACT FROM AUTHOR]

Using simulations performed with 24 coupled atmosphere-ocean global climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5), projections of Northern Hemisphere daily snowfall events under the RCP8.5 emissions scenario are analyzed for the periods of 2021-50 and 2071-2100 and compared to the historical period of 1971-2000. The overall frequency of daily snowfall events is simulated to decrease across much of the Northern Hemisphere, except at the highest latitudes such as northern Canada, northern Siberia, and Greenland. Seasonal redistributions of daily snowfall event frequency and average daily snowfall are also projected to occur in some regions. For example, large portions of the Northern Hemisphere, including much of Canada, Tibet, northern Scandinavia, northern Siberia, and Greenland, are projected to experience increases in average daily snowfall and event frequency in midwinter. But in warmer months, the regions with increased snowfall become fewer in number and are limited to northern Canada, northern Siberia, and Greenland. These simulations also show changes in the frequency distribution of daily snowfall event intensity, including an increase in heavier snowfall events even in some regions where the overall snowfall decreases. The projected changes in daily snowfall event frequency exhibit some dependence on the temperature biases of the individual models in certain regions and times of the year, with colder models typically toward the positive end of the distribution of event frequency changes and warmer models toward the negative end, particularly in regions near the transition zone between increasing and decreasing snowfall. [ABSTRACT FROM AUTHOR]

Satellite-derived vegetation phenology has been recognized as a key indicator for detecting changes in the terrestrial biosphere in response to global climate change. However, multi-decadal changes and spatial variation of vegetation phenology over the Northern Hemisphere and their relationship to climate change have not yet been fully investigated. In this article, we investigated the spatial variability and temporal trends of vegetation phenology over the Northern Hemisphere by calibrating and analyzing time series of the satellite-derived normalized difference vegetation index (NDVI) during 1982–2012, and then further examine how vegetation phenology responds to climate change within different ecological zones. We found that during the period from 1982 to 2012 most of the high latitude areas experienced an increase in growing period largely due to an earlier beginning of vegetation growing season (BGS), but there was no significant trend in the vegetation growing peaks. The spatial pattern of phenology within different eco-zones also experienced a large variation over the past three decades. Comparing the periods of 1982–1992, 1992–2002 with 2002–2012, the spatial pattern of change rate of phenology shift (RPS) shows a more significant trend in advancing of BGS, delaying of EGS (end of growing season) and prolonging of LGS (length of growing season) during 2002–2012, overall shows a trend of accelerating change. Temperature is a major determinant of phenological shifts, and the response of vegetation phenology to temperature varied across different eco-zones. [ABSTRACT FROM AUTHOR]

Northern hemisphere elementary circulation mechanisms, defined with the Dzerdzeevski classification and published on a daily basis from 1899–2012, are analysed with statistical methods as continuous categorical time series. Classification consists of 41 elementary circulation mechanisms (ECM), which are assigned to calendar days. Empirical marginal probabilities of each ECM were determined. Seasonality and the periodicity effect were investigated with moving dispersion filters and randomisation procedure on the ECM categories as well as with the time analyses of the ECM mode. The time series were determined as being non-stationary with strong time-dependent trends. During the investigated period, periodicity interchanges with periods when no seasonality is present. In the time series structure, the strongest division is visible at the milestone of 1986, showing that the atmospheric circulation pattern reflected in the ECM has significantly changed. This change is result of the change in the frequency of ECM categories; before 1986, the appearance of ECM was more diverse, and afterwards fewer ECMs appear. The statistical approach applied to the categorical climatic time series opens up new potential insight into climate variability and change studies that have to be performed in the future. [ABSTRACT FROM AUTHOR]

Most small glaciers in the world have significantly decreased their volume during the last century, which has caused water shortage problems. Glacier No. 1, at the headwaters of the Urumqi River, Tianshan, China, has been monitored since 1959 and similarly has experienced significant mass and volume losses over the last few decades. Thus, we examined the trend and potential abrupt changes of the mass balance of Glacier No. 1. Principal component analysis and singular value decomposition were used to find significant relations between the mass balance of Glacier No. 1 and Northern Hemisphere teleconnection patterns using climate indices. It was found that the mass balance of Glacier No. 1 had a significantly decreasing trend corresponding to –14.5 mm/year from 1959 to 2010. A change point was detected in 1997 with 99% confidence level. Two time periods with different mass balances were identified as 1959-1996 and 1997-2010. The mass balance for the first period was –136.4 mm/year and up to –663.9 mm/year for the second period. The mass balance of Glacier No. 1 is positively related to the Scandinavian Pattern (SCA), and negatively related to the East Atlantic Pattern (EA). These relationships are useful in better understanding the interaction between glacier mass balance and climate variability. [ABSTRACT FROM AUTHOR]

The response of the Northern Hemisphere winter stratosphere to the Pacific decadal oscillation (PDO) is examined using the Whole Atmosphere Community Climate Model. A 200-yr preindustrial control simulation that includes fully interactive chemistry, ocean and sea ice, constant solar forcing, and greenhouse gases fixed to 1850 levels is analyzed. Based on principal component analysis, the PDO spatial pattern, frequency, and amplitude agree well with the observed PDO over the period 1900-2014. Consistent with previous studies, the positive phase of the PDO is marked by a strengthened Aleutian low and a wave train of geopotential height anomalies reminiscent of the Pacific-North American pattern in the troposphere. In addition to a tropospheric signal, a zonal-mean warming of about 2 K in the northern polar stratosphere and a zonal-mean zonal wind decrease of about 4 m s−1 in the PDO positive phase are found. When compositing PDO positive or negative winters during neutral El Niño years, the magnitude is reduced and depicts an early winter forcing of the stratosphere compared to a late winter response from El Niño. Contamination between PDO and ENSO signals is also discussed. Stratospheric sudden warmings occur 63% of the time in the PDO positive phase compared to 40% in the negative phase. Although this sudden warming frequency is not statistically significant, it is quantitatively consistent with NCEP-NCAR reanalysis data and recent observational evidence linking the PDO positive phase to weak stratospheric vortex events. [ABSTRACT FROM AUTHOR]

Simulations with climate models show a land-ocean contrast in the response of P − E (precipitation minus evaporation or evapotranspiration) to global warming, with larger changes over ocean than over land. The changes over ocean broadly follow a simple thermodynamic scaling of the atmospheric moisture convergence: the so-called 'wet-get-wetter, dry-get-drier' mechanism. Over land, however, the simple scaling fails to give any regions with decreases in P − E, and it overestimates increases in P − E compared to the simulations. Changes in circulation cause deviations from the simple scaling, but they are not sufficient to explain this systematic moist bias. It is shown here that horizontal gradients of changes in temperature and fractional changes in relative humidity, not accounted for in the simple scaling, are important over land and high-latitude oceans. An extended scaling that incorporates these gradients is shown to better capture the response of P − E over land, including a smaller increase in global-mean runoff and several regions with decreases in P − E. In the zonal mean over land, the gradient terms lead to a robust drying tendency at almost all latitudes. This drying tendency is shown to relate, in part, to the polar amplification of warming in the Northern Hemisphere, and to the amplified warming over continental interiors and on the eastern side of midlatitude continents. [ABSTRACT FROM AUTHOR]