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Author(s): Tim Palmer Author Affiliations: Short-term tests validate long-term estimates of climate change How sensitive is climate to atmospheric carbon dioxide levels? For a doubling of CO[sub.2] concentration from pre-industrial [...], Six-hour weather forecasts have been used to validate estimates of climate change hundreds of years from now. Such tests have great potential -- but only if our weather-forecasting and climate-prediction systems are unified. Climate sensitivity to atmospheric CO2 levels is likely to be high.

Even though Northern Hemisphere atmospheric C[O.sub.2] mixing ratios have recently exceeded 400 parts per million (ppm), few investigations have directly explored the effect of changes in well-mixed greenhouse gases on [...], The climatic impact of C[O.sub.2] and other greenhouse gases is usually quantified in terms of radiative forcing (1), calculated as the difference between estimates of the Earth's radiation field from pre-industrial and present-day concentrations of these gases. Radiative transfer models calculate that the increase in C[O.sub.2] since 1750 corresponds to a global annualmean radiative forcing at the tropopause of 1.82 ± 0.19 W [m.sup.-2] (ref. 2). However, despite widespread scientific discussion and modelling of the climate impacts of well-mixed greenhouse gases, there is little direct observational evidence of the radiative impact of increasing atmospheric C[O.sub.2]. Here we present observationally based evidence of clear-sky C[O.sub.2] surface radiative forcing that is directly attributable to the increase, between 2000 and 2010, of 22 parts per million atmospheric C[O.sub.2]. The time series of this forcing at the two locations--the Southern Great Plains and the North Slope of Alaska--are derived from Atmospheric Emitted Radiance Interferometer spectra (3) together with ancillary measurements and thoroughly corroborated radiative transfer calculations (4). The time series both show statistically significant trends of 0.2 W [m.sup.-2] per decade (with respective uncertainties of ± 0.06 W [m.sup.- 2] per decade and ± 0.07 W [m.sup.-2] per decade) and have seasonal ranges of 0.1-0.2 W [m.sup.-2]. This is approximately ten per cent of the trend in downwelling longwave radiation (5-7). These results confirm theoretical predictions of the atmospheric greenhouse effect due to anthropogenic emissions, and provide empirical evidence of how rising C[O.sub.2] levels, mediated by temporal variations due to photosynthesis and respiration, are affecting the surface energy balance.

Author(s): P. K. Patra [sup.1] [sup.2] , M. C. Krol [sup.3] , S. A. Montzka [sup.4] , T. Arnold [sup.5] , E. L. Atlas [sup.6] , B. R. Lintner [sup.7] [...], Observations of methyl chloroform combined with an atmospheric transport model predict a Northern to Southern Hemisphere hydroxyl ratio of slightly less than 1, whereas commonly used atmospheric chemistry models predict ratios 15-45% higher. The north-south distribution of atmospheric OH The hydroxyl radical is an important atmospheric oxidant, but our knowledge of its global distribution remains imprecise, with estimates for the ratio of Northern Hemisphere to Southern Hemisphere hydroxyl radical concentration varying from 0.85 to 1.4. These authors use a three-dimensional chemistry-transport model that has been well validated for interhemispheric transport using sulphur hexafluoride measurements, to obtain an interhemispheric hydroxyl radical ratio of 0.97[plus or minus]0.12. This information can help improve our understanding of the fate of atmospheric pollutants and greenhouse gases. The hydroxyl radical (OH) is a key oxidant involved in the removal of air pollutants and greenhouse gases from the atmosphere.sup.1,2,3. The ratio of Northern Hemispheric to Southern Hemispheric (NH/SH) OH concentration is important for our understanding of emission estimates of atmospheric species such as nitrogen oxides and methane.sup.4,5,6. It remains poorly constrained, however, with a range of estimates from 0.85 to 1.4 (refs 4, 7,8,9,10). Here we determine the NH/SH ratio of OH with the help of methyl chloroform data (a proxy for OH concentrations) and an atmospheric transport model that accurately describes interhemispheric transport and modelled emissions. We find that for the years 2004-2011 the model predicts an annual mean NH-SH gradient of methyl chloroform that is a tight linear function of the modelled NH/SH ratio in annual mean OH. We estimate a NH/SH OH ratio of 0.97 [plus or minus] 0.12 during this time period by optimizing global total emissions and mean OH abundance to fit methyl chloroform data from two surface-measurement networks and aircraft campaigns.sup.11,12,13. Our findings suggest that top-down emission estimates of reactive species such as nitrogen oxides in key emitting countries in the NH that are based on a NH/SH OH ratio larger than 1 may be overestimated.

Author(s): V. Jomelli [sup.1] , V. Favier [sup.2] , M. Vuille [sup.3] , R. Braucher [sup.4] , L. Martin [sup.5] , P.-H. Blard [sup.5] , C. Colose [sup.3] , D. [...], A moraine chronology determined by surface exposure dating shows that glaciers in the northern tropical Andes expanded to a larger extent during the Antarctic cold reversal (14,500 to 12,900 years ago) than during the Younger Dryas stadial (12,800 to 11,500 years ago), contrary to previous studies; as a result, previous chronologies and climate interpretations from tropical glaciers may need to be revisited. Glaciation in the tropical Andes The Younger Dryas was a cold period immediately prior to the present warm interglacial period. There is clear evidence that it occurred throughout much of the Northern Hemisphere, but its global extent remains debatable. Prior work in the tropical Andes suggested extensive glacial advances during the Younger Dryas. Vincent Favier and colleagues present new cosmogenic dating of glacial deposits -- and a reassessment of existing datasets from other glaciated Andean landscapes -- to show that instead, maximal glacial advances took place in the preceding Antarctic cold reversal. Rather than a major advance, the Younger Dryas now appears to have been associated with modest glacial retreat. The Younger Dryas stadial, a cold event spanning 12,800 to 11,500 years ago, during the last deglaciation, is thought to coincide with the last major glacial re-advance in the tropical Andes.sup.1. This interpretation relies mainly on cosmic-ray exposure dating of glacial deposits. Recent studies, however, have established new production rates.sup.2,3,4 for cosmogenic .sup.10Be and .sup.3He, which make it necessary to update all chronologies in this region.sup.1,5,6,7,8,9,10,11,12,13,14,15 and revise our understanding of cryospheric responses to climate variability. Here we present a new .sup.10Be moraine chronology in Colombia showing that glaciers in the northern tropical Andes expanded to a larger extent during the Antarctic cold reversal (14,500 to 12,900 years ago) than during the Younger Dryas. On the basis of a homogenized chronology of all .sup.10Be and .sup.3He moraine ages across the tropical Andes, we show that this behaviour was common to the northern and southern tropical Andes. Transient simulations with a coupled global climate model suggest that the common glacier behaviour was the result of Atlantic meridional overturning circulation variability superimposed on a deglacial increase in the atmospheric carbon dioxide concentration. During the Antarctic cold reversal, glaciers advanced primarily in response to cold sea surface temperatures over much of the Southern Hemisphere. During the Younger Dryas, however, northern tropical Andes glaciers retreated owing to abrupt regional warming in response to reduced precipitation and land-surface feedbacks triggered by a weakened Atlantic meridional overturning circulation. Conversely, glacier retreat during the Younger Dryas in the southern tropical Andes occurred as a result of progressive warming, probably influenced by an increase in atmospheric carbon dioxide. Considered with evidence from mid-latitude Andean glaciers.sup.16, our results argue for a common glacier response to cold conditions in the Antarctic cold reversal exceeding that of the Younger Dryas.

Author(s): Cawas B. Engineer [sup.1] , Majid Ghassemian [sup.2] , Jeffrey C. Anderson [sup.3] , Scott C. Peck [sup.3] , Honghong Hu [sup.1] [sup.4] , Julian I. Schroeder [sup.1] Author [...], The continuing rise in atmospheric carbon dioxide concentrations suppresses the development of stomatal pores, and thus gas exchange, in plant leaves on a global scale; now, a framework of mechanisms by which carbon dioxide represses development has been identified. Control of plant stomatal development The continuing rise in atmospheric CO.sub.2 levels is suppressing the development of stomatal pores in plant leaves on a global scale. This, combined with the increasing scarcity of water for agriculture, can significantly affect plant carbon assimilation, heat stress and water-use efficiency. Julian Schroeder and colleagues investigate the genes and mechanisms through which CO.sub.2 controls development of the stomatal pores that plants use to regulate gas exchange in leaves. They identify a framework of mechanisms: at high CO.sub.2 levels, extracellular signalling and carbonic anhydrase regulate a novel protease called CRSP and the pro-peptide EPF2; this in turn represses stomatal development. Environmental stimuli, including elevated carbon dioxide levels, regulate stomatal development.sup.1,2,3; however, the key mechanisms mediating the perception and relay of the CO.sub.2 signal to the stomatal development machinery remain elusive. To adapt CO.sub.2 intake to water loss, plants regulate the development of stomatal gas exchange pores in the aerial epidermis. A diverse range of plant species show a decrease in stomatal density in response to the continuing rise in atmospheric CO.sub.2 (ref. 4). To date, one mutant that exhibits deregulation of this CO.sub.2-controlled stomatal development response, hic (which is defective in cell-wall wax biosynthesis, ref. 5), has been identified. Here we show that recently isolated Arabidopsis thaliana [beta]-carbonic anhydrase double mutants (ca1 ca4).sup.6 exhibit an inversion in their response to elevated CO.sub.2, showing increased stomatal development at elevated CO.sub.2 levels. We characterized the mechanisms mediating this response and identified an extracellular signalling pathway involved in the regulation of CO.sub.2-controlled stomatal development by carbonic anhydrases. RNA-seq analyses of transcripts show that the extracellular pro-peptide-encoding gene EPIDERMAL PATTERNING FACTOR 2 (EPF2).sup.7,8, but not EPF1 (ref. 9), is induced in wild-type leaves but not in ca1 ca4 mutant leaves at elevated CO.sub.2 levels. Moreover, EPF2 is essential for CO.sub.2 control of stomatal development. Using cell-wall proteomic analyses and CO.sub.2-dependent transcript analyses, we identified a novel CO.sub.2-induced extracellular protease, CRSP (CO.sub.2 RESPONSE SECRETED PROTEASE), as a mediator of CO.sub.2-controlled stomatal development. Our results identify mechanisms and genes that function in the repression of stomatal development in leaves during atmospheric CO.sub.2 elevation, including the carbonic-anhydrase-encoding genes CA1 and CA4 and the secreted protease CRSP, which cleaves the pro-peptide EPF2, in turn repressing stomatal development. Elucidation of these mechanisms advances the understanding of how plants perceive and relay the elevated CO.sub.2 signal and provides a framework to guide future research into how environmental challenges can modulate gas exchange in plants.

As the primary atmospheric oxidant, the OH radical has a key role in the removal or production of major air pollutants, greenhouse gases and many ozone-depleting substances (1-3). A better [...], The hydroxyl radical (OH) is a key oxidant involved in the removal of air pollutants and greenhouse gases from the atmosphere (1-3). The ratio of Northern Hemispheric to Southern Hemispheric (NH/SH) OH concentration is important for our understanding of emission estimates of atmospheric species such as nitrogen oxides and methane (4-6). It remains poorly constrained, however, with a range of estimates from 0.85 to 1.4 (refs 4,7-10). Here we determine the NH/SH ratio of OH with the help of methyl chloroform data (a proxy for OH concentrations) and an atmospheric transport model that accurately describes interhemispheric transport and modelled emissions. We find that for the years 2004-2011 the model predicts an annual mean NH-SH gradient of methyl chloroform that is a tight linear function of the modelled NH/SH ratio in annual mean OH. We estimate a NH/SH OH ratio of 0.97 ± 0.12 during this time period by optimizing global total emissions and mean OH abundance to fit methyl chloroform data from two surface-measurement networks and aircraft campaigns (11-13). Our findings suggest that top-down emission estimates of reactive species such as nitrogen oxides in key emitting countries in the NH that are based on a NH/SH OH ratio larger than 1 maybe overestimated.

Each year, on average, land and ocean carbon sinks absorb the equivalent of about half of the global fossil fuel emissions, thereby providing a critical service that slows the rise [...], The land and ocean act as a sink for fossil-fuel emissions, thereby slowing the rise of atmospheric carbon dioxide concentrations (1). Although the uptake of carbon by oceanic and terrestrial processes has kept pace with accelerating carbon dioxide emissions until now, atmospheric carbon dioxide concentrations exhibit a large variability on interannual timescales (2), considered to be driven primarily by terrestrial ecosystem processes dominated by tropical rainforests (3). We use a terrestrial biogeochemical model, atmospheric carbon dioxide inversion and global carbon budget accounting methods to investigate the evolution of the terrestrial carbon sink over the past 30 years, with a focus on the underlying mechanisms responsible for the exceptionally large land carbon sink reported in 2011 (ref. 2). Here we show that our three terrestrial carbon sink estimates are in good agreement and support the finding of a 2011 record land carbon sink. Surprisingly, we find that the global carbon sink anomaly was driven by growth of semi-arid vegetation in the Southern Hemisphere, with almost 60 per cent of carbon uptake attributed to Australian ecosystems, where prevalent La Nina conditions caused up to six consecutive seasons of increased precipitation. In addition, since 1981, a six per cent expansion of vegetation cover over Australia was associated with a fourfold increase in the sensitivity of continental net carbon uptake to precipitation. Our findings suggest that the higher turnover rates of carbon pools in semi-arid biomes are an increasingly important driver of global carbon cycle inter-annual variability and that tropical rainforests may become less relevant drivers in the future. More research is needed to identify to what extent the carbon stocks accumulated during wet years are vulnerable to rapid decomposition or loss through fire in subsequent years.

A list of authors and their affiliations appears at the end of the paper. The Pacific, as the world's largest ocean, is intricately involved in the prominent changes in the [...], Atmospheric carbon dioxide concentrations and climate are regulated on geological timescales by the balance between carbon input from volcanic and metamorphic outgassing and its removal by weathering feedbacks; these feedbacks involve the erosion of silicate rocks and organic-carbon-bearing rocks. The integrated effect of these processes is reflected in the calcium carbonate compensation depth, which is the oceanic depth at which calcium carbonate is dissolved. Here we present a carbonate accumulation record that covers the past 53 million years from a depth transect in the equatorial Pacific Ocean. The carbonate compensation depth tracks long-term ocean cooling, deepening from 3.0-3.5 kilometres during the early Cenozoic (approximately 55 million years ago) to 4.6 kilometres at present, consistent with an overall Cenozoic increase in weathering. We find large superimposed fluctuations in carbonate compensation depth during the middle and late Eocene. Using Earth system models, we identify changes in weathering and the mode of organic-carbon delivery as two key processes to explain these large-scale Eocene fluctuations of the carbonate compensation depth.

The South Pacific convergence zone (SPCZ) is the Southern Hemisphere's most expansive and persistent rain band, extending from the equatorial western Pacific Ocean southeastward towards French Polynesia (1,2). Owing to [...]

It is possible that anthropogenic climate change will drive the Earth system into a qualitatively different state (1). Although different types of uncertainty limit our capacity to assess this risk [...]

Studies of both the suspended and the dissolved loads of the world's largest rivers and of hill-slope denudation have shown a strong link between physical erosion and chemical weathering (8,9). [...], Over geologic timescales, C[O.sub.2] is emitted from the Earth's interior and is removed from the atmosphere by silicate rock weathering and organic carbon burial. This balance is thought to have stabilized greenhouse conditions within a range that ensured habitable conditions (1). Changes in this balance have been attributed to changes in topographic relief, where varying rates of continental rock weathering and erosion (1,2) are superimposed on fluctuations in organic carbon burial (3). Geological strata provide an indirect yet imperfectly preserved record of this change through changing rates of sedimentation (1,2,4). Widespread observations of a recent (0-5-Myr) fourfold increase in global sedimentation rates require a global mechanism to explain them (4-6). Accelerated uplift and global cooling have been given as possible causes (2,4,6,7), but because of the links between rates of erosion and the correlated rate of weathering (8,9), an increase in the drawdown of C[O.sub.2] that is predicted to follow may be the cause of global climate change instead (2). However, globally, rates of uplift cannot increase every where in the way that apparent sedimentation rates do (4,10). Moreover, proxy records of past atmospheric C[O.sub.2] provide no evidence for this large reduction in recent C[O.sub.2] concentrations (11,12). Here we question whether this increase in global weathering and erosion actually occurred and whether the apparent increase in the sedimentation rate is due to observational biases in the sedimentary record (13). As evidence, we recast the ocean dissolved [sup.10]Be/[sup.9]Be isotope system as a weathering proxy spanning the past ~12 Myr (ref. 14). This proxy indicates stable weathering fluxes during the late-Cenozoic era. The sum of these observations shows neither clear evidence for increased erosion nor clear evidence for a pulse in weathered material to the ocean. We conclude that processes different from an increase in denudation caused Cenozoic global cooling, and that global cooling had no profound effect on spatially and temporally averaged weathering rates.

The results of a compound-specific carbon-isotope study of Cenomanian/Turonian (C/T) sediments from the southern part of the North Atlantic Ocean, are presented. The abruptness and magnitude of the atmospheric CO(sub2) concentration change is quantified. The data indicates that plants utilizing the C(sub3)-type photosynthetic pathway were succeeded by plants using the C(sub4)-type pathway.

Elevated carbon dioxide boosts carbon partitioning to roots, according to research undertaken on naturally occurring annual grasslands in central coastal California. Higher carbon dioxide was found to substantially boost carbon pools in roots, surface detritus and soil microorganisms in the sandstone grassland. There was a 37% rise in the total amount of carbon in these carbon pools. The shift in partitioning is mainly seen as a rise in root exudation and respiration. It is therefore very hard to detect.

The atmospheric radiocarbon ratio is sensitive to changes in the other global carbon cycle reservoirs and so the high 14C concentration in the atmosphere from the Younger Dryas period indicates a significant change. Sediments from Lake Gosciaz in Poland are used to trace climate changes and atmospheric radiocarbon for the Younger Dryas and early Holocene periods. The evidence suggests that the deep ocean ventilation rate decreased during the Younger Dryas, accounting for the high 14C concentration, most likely due to a reduced formation intensity of North Atlantic Deep Water.

The uplift or rise of the Tibetan plateau probably initiated the world-wide cooling in the Cenozoic era that eventually produced vast ice sheets in both the Northern and Southern Hemispheres. The Tibetan plateau is an elevated part of Asia centering on Tibet and including the Himalayan mountains. The Cenozoic uplift of this region may have had climatic feedbacks that reinforced the trend toward cooling especially by reducing atmospheric CO2 levels through tectonically induced chemical weathering.

Carbon transfer by degassing to the atmosphere and by downward flux of biogenic particles to the deep ocean are mainly responsible for the loss of carbon from surface waters of the equatorial Pacific Ocean. Evaluation of the transformations of carbon and nitrogen compounds in the surface layer showed that downward flux of organic carbon accounted for 53% of total C depletion during boreal autumn 1992. Carbon removed by degassing, on the other hand, accounted for 41% of total C loss from surface waters.

Iron supply to the equatorial Pacific Ocean can regulate the local short-term source of carbon dioxide to the atmosphere, but has minimal effect on atmospheric carbon dioxide partial pressure in the long term. If such a regulation takes place likewise in the Southern Ocean, iron bioavailability at high southern latitudes could impact substantially on atmospheric carbon dioxide partial pressure. This was concluded during a second iron fertilization experiment, where a much larger, biologically triggered uptake of surface-water carbon dioxide occurred.

Net CO2 exchange between terrestrial vegetation and the atmosphere is the sum of two fluxes, uptake by gross primary productivity and release by respiration. Small vertical gradients in 13C and 18O in atmospheric CO2 above vegetation can be used to estimate the two components separately at the local scale. Evapotranspiration rates can be determined by analyzing the 18O content of moisture in air samples.

A study has been conducted to examine the concentration of atmospheric carbon dioxide before 2.2 Gyr ago and identify the factors contributing to the present conditions of the atmosphere. A review of studies on paleosols which existed between 2.75 and 2.2 Gyr ago showed that iron from these profiles existed as iron silicates instead of iron carbonates, suggesting that atmospheric CO2 may have been very much lower in concentration than today's atmospheric CO2.

A study of the influence of a positive feedback between clouds and climate on the climate shift related to a doubling of the atmospheric carbon dioxide concentration revealed the positive nature of the feedback in the Northern Hemisphere and its higher strength at lower latitudes. A two-dimensional radiative-connective model was used. This positive feedback climate magnifies the general global climate responsiveness.

The warming of the Earth soon after it formed probably prevented the onset of an interminable glacial climate due to highly reflective carbon dioxide (CO2) clouds and the Sun's much less intense output of heat at the time. The suggestion that an early greenhouse effect stabilized the Earth's climate does not explain why the oceans failed to freeze before the greenhouse gases could accumulate. Paleoclimatic simulations indicate that Earth may have narrowly escaped being rendered uninhabitable by world-wide glaciation thanks to either internal heat or additional greenhouse gases.

Oceanic uptake of atmospheric carbon dioxide (CO2) is influenced by a thermal skin effect that exists at the ocean surface. This skin, which consists of the uppermost one millimeter of the oceans, develops because it is about 0.3 degrees C cooler than the water below due to the upward heat flux. This skin generates a greater than expected oceanic absorption of atmospheric CO2 which must be taken into account in any attempt to foresee how atmospheric loading of CO2 will affect global climate. The thermal skin effect will also allow for greater agreement between the various calculations of oceanic CO2 uptake.

Records of changes in phytoplankton biomass in the North Pacific ocean since 1900 revealed that these changes were negligible and will therefore not slow the increase in atmospheric carbon dioxide (CO2) due to human industry. The data on marine phytoplankton productivity were obtained using Secchi disks. Hence it is unlikely that increased oceanic productivity will erase the environmental effects of rising anthropogenic CO2 levels.

A study reveals cyclic changes in the rate of burial of biogenic calcium carbonate at the Pacific ocean floor 43 million to 33 million years ago, as Earth exited a [...]

The solar luminosity during the Archean (3.8 to 2.5 billion years ago) was 20-25% lower than at present, and was probably compensated, at least in part, by a stronger greenhouse [...]

Author(s): Michael Hopkin [illus. 1] Rising levels of ozone pollution over the coming century will erode the ability of plants to absorb carbon dioxide from the atmosphere, a new climate-modelling [...]

The iconic 'Keeling curve', a 56-year-old record of rising atmospheric carbon dioxide levels at Mauna Loa in Hawaii, has won a five-year, US$500,000 grant from US philanthropists Wendy and Eric [...]

Previous studies have shown that increased land temperature stimulates a significant rise in atmospheric CO2 concentration at high latitudes. It has been suggested that longer growing seasons account for the increased seasonal amplitude of atmospheric CO2. Experimental warming conducted in the Alaskan arctic tundra supports such an assertion and suggests that increased microbial respiration in the winter season could also account for the fluctuations in atmospheric CO2.

Transplanted African grasses, such as Brachiaria and Andropogon, in South America may reduce atmospheric carbon dioxide through enhancing the soil's carbon storage. However, this effect is likely to reduce over time while it is not significant enough to justify destroying current biomass, such as rainforests, for pasture land. The carbon sink effect is a, possibly temporary, side benefit of grassland that is converted to pasture when African grasses are imported because of better survival rates on poor soil but these grasses are highly flammable and may have a negative impact on the local plants.

The uncertainty in the ecosystem-level responses to atmosphere CO2 concentration (pCO2), ecophysiological features of C3 and C4 plants and delta13C measurements of palaeosols suggest that increase in pCO2 is not the only cause for the C4-to-C3 shift. Interaction of pCO2 with other environmental features such as temperature and biological features of plants can affect the responses of plants to increased levels of pCO2.

Natural tropical wetlands were the main source of a marked rise in global atmospheric methane levels observed during the past few years. After a decade of stability, atmospheric concentrations of [...]

The initial high growth rate and the sudden decline of atmospheric CO2 in 1992 is attributed to El Nino and the volcanic eruption of Mount Pinatubo. El Nino raises the level of CO2 in oceans, and its prolonged presence in 1991-92 considerably increased this level. However, the growth rate started slowing, and it is assumed from observations of the carbon-oxygen ratio that Pinatubo's eruption may have influenced it. Calculations of 13C/12C ratio can help in understanding the changes in atmospheric CO2 growth rate.

Two new techniques for measuring the productivity of marine plankton have been developed. This is important because plankton productivity may indicate levels of atmospheric carbon dioxide (CO2) during ancient glacial periods. R. Francois and colleagues used nitrogen isotope levels to deduce productivity while N. Kumar and colleagues used protactinium-thorium isotope levels. Both techniques show that plankton productivity and CO2 levels fluctuated during glacial epochs.