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In the Earth's general circulation, polar jets act as baroclinic pumps of angular momentum and heat. Reanalysis datasets indicate that shear changes near jets induce upward displacements of the jet cores, suggesting a weakening thermodynamic pumping over the last 50 years. From secondary flow theory, a well‐established principle in fluid dynamics, an increased frequency of heatwaves and persistent winter storms is expected. The ageostrophic wind shear between 700 and 50 hPa indicates the strength of this secondary circulation. The weakening tendency during the reanalysis period also exists in multimodel simulations under the RCP8.5 emissions scenario for the 21st century. The reduction between the periods 2005–2025 and 2081–2100 reaches 18%, 5.3% and 19%, respectively, for the North America, Mid‐Europe and East Asia sectors of the Northern Hemisphere polar jet. Within this background, cold‐surge events are the result of synergic co‐working of several factors. The occurrence trend for transitional season winter extreme events also is examined. The winter extremes seemingly have larger temperature drops. However, in a warming climate, they emerge more rapidly from extreme cold states. The storm tracks, especially over North America, have equatorward extensions, indicating that winter storms can reach lower latitudes. Due to the temperature‐dependence of air viscosity, secondary flows decrease more slowly than the main zonal flow. This imposes an important adjustment to the traditional polar amplification effects on midlatitude winter extremes. During a warmer winter (the primary manifestation of a warmer climate), spatially uniform positive trends in cold extreme events are not expected. There are, however, regions experiencing more winter extremes. These regions show consistent patterns in both the reanalysis period and the remainder of the 21st century. [ABSTRACT FROM AUTHOR]

The mid-Piacenzian Warm Period (MPWP, ~ 3.264–3.025 Ma) is the most recent example of a persistently warmer climate in equilibrium with atmospheric CO2 concentrations similar to today. Towards studying patterns and dynamics of a warming climate the MPWP is often compared to today. Following the Pliocene Model Intercomparison Project, Phase 2 (PlioMIP2) protocol we prepare a water isotope-enabled Community Earth System Model (iCESM1.2) simulation that is warmer and wetter than the PlioMIP2 multi-model ensemble (MME). While our simulation resembles PlioMIP2 MME in many aspects we find added insights. (1) Considerable warmth at high latitudes exceeds previous simulations. Polar amplification (PA) is comparable to proxies, enabled by iCESM1.2's high climate sensitivity and a distinct method of ocean initialization. (2) Major driver of warmth is the downward component of clear-sky surface long-wave radiation ( Δ T rlds _ clearsky ). (3) In iCESM1.2 modulated dominance of dynamic (δDY) processes causes different low-latitude (~ 30 S°–10°N) precipitation response than the PlioMIP2 MME, where thermodynamic processes (δTH) dominate. (4) Modulated local condensation leads to lower δ18Op across tropical Indian Ocean and surrounding Asian-African-Australian monsoon regions. (5) We find contrasting changes in tropical atmospheric circulations (Hadley and Walker cells). Anomalous regional meridional (zonal) circulation, forced by changes in tropical-subtropical (tropical) diabatic processes, presents a more comprehensive perspective than explaining weakened and expanded Hadley circulation (strengthened and westward-shifted Walker circulation) via static stability. (6) Enhanced Atlantic meridional overturning circulation owes to a closed Bering Strait. [ABSTRACT FROM AUTHOR]

Polar amplification appears in response to greenhouse gas forcing, which has become a focus of climate change research. However, polar amplification has not been systematically investigated over the Earth's three poles (the Arctic, Antarctica, and the Third Pole). An index of polar amplification is employed, and the annual and seasonal variations of land surface temperature over the Earth's three poles are examined using MODIS (Moderate Resolution Imaging Spectroradiometer) observations for the period 2001–2018. As expected, the warming of the Arctic is most conspicuous, followed by the Third Pole, and is weakest in Antarctica. Compared to the temperature changes for the global land region, positive polar amplification appears in the Arctic and the Third Pole on an annual scale, whereas Antarctic amplification disappears, with a negative amplification index of −0.72. The polar amplification for the Earth's three poles shows seasonal differences. Strong Arctic amplification appears in boreal spring and winter, with a surface warming rate of more than 3.40 times the global mean for land regions. In contrast, the amplification of the Third Pole is most conspicuous in boreal summer. The two poles located in the Northern Hemisphere have the weakest amplification in boreal autumn. Differently from the positive amplification for the Arctic and the Third Pole in all seasons, the faster variations in Antarctic temperature compared to the globe only appear in austral autumn and winter, and the amplification signal is negative in these seasons, with an amplification index of −1.68 and −2.73, respectively. In the austral winter, the strong negative amplification concentrates on West Antarctica and the coast of East Antarctica, with an absolute value of amplification index higher than 5 in general. Generally, the polar amplification is strongest in the Arctic except from June to August, and Antarctic amplification is the weakest among the Earth's three poles. The Earth's three poles are experiencing drastic changes, and the potential influence of climate change should receive attention. [ABSTRACT FROM AUTHOR]

We estimate snow albedo feedback effects of anthropogenic increases in global radiative forcing, which includes carbon dioxide, methane, nitrous oxide, CFC11, CFC12, black carbon, anthropogenic sulfur emissions, total solar irradiance, and local sulfur emissions by compiling annual observations (1972–2008) for radiative forcing, temperature, snow cover, sulfur emissions, and various teleconnections for 255 5 ∘ × 5 ∘ grid cells in the Northern Hemisphere. Panel DOLS estimates of the long-run relations indicate that the effect of radiative forcing on temperature increases with latitude (consistent with polar amplification), eliminating snow cover increases local temperature by about 2.8 °C, and a 1 °C temperature increase reduces snow cover by about 1%. These values create a snow albedo feedback (SAF) that amplifies the temperature increase of higher forcing by about 3.4% relative to its direct effect while an increase in sulfur emissions increases the temperature reduction by about 0.4% relative to its direct effect. The 3.4% SAF is smaller than values generated by process-based climate models and may be associated with the empirical estimates for snowmelt sensitivity Δ S c / Δ T s . To narrow estimates for the SAF from climate models, we conclude with suggestions for a new experimental design that controls for the simultaneous relation between temperature and snow cover. [ABSTRACT FROM AUTHOR]

The spatial pattern of intermodel spread of surface temperature (TS‐SPREAD) is similar to the absolute error of modeled surface temperature relative to observations, implying the possibility of using intermodel spread to understand the model biases. The regions with maximum TS‐SPREAD are located in the northern polar (NP) and southern polar (SP) regions, and regions with the large orographic features, such as the Tibetan Plateau (TP). We find: (a) Winter amplification exists in the TS‐SPREAD over both the NP and SP regions, but the maximum TS‐SPREAD over the TP happens during spring and summer with weak seasonality there. (b) Surface energy balance analyses show salient land‐sea contrast in the interlinkage between the physical processes. (c) Over the Arctic Ocean and the Southern Ocean, the maximum spreads of sea‐ice‐induced surface albedo appear during summer when the TS‐SPREADs are the weakest because the effect of surface albedo is offset by the heat storage in the oceans. (d) Through the interseasonal linkage of the heat storage term, i.e., the summer‐storing‐winter‐releasing of oceanic heat content, the summer surface albedo may indirectly contribute to the winter amplification of the TS‐SPREAD over the polar oceans. Such interseasonal linkage of the processes does not exist over the land areas. A method of distinguishing the roles of the local and nonlocal dynamical processes in the contribution of the clear‐sky downward longwave radiation to the TS‐SPREAD is also provided. Plain Language Summary: Understanding the origin of climate model biases is essential to the projection of future climate change, and it is very hard task for the climate modeling community. Here, we show that there exists similarity in the spatial patterns of the intermodel spread, mean absolute errors, and absolute mean errors of surface temperature simulations in climate ensembles. Therefore, we may use the intermodel spread to help understand the mean model biases. We find the regions with maximum surface temperature spread (TS‐SPREAD) are mainly located in the northern polar (NP) and southern polar (SP) regions, and regions with large orographic feature, such as the Tibetan Plateau, indicating the polar and topographic amplifications of TS‐SPREAD in climate models. In the paper, we reveal the process‐chains that lead to the spatial pattern and seasonality of amplified TS‐SPREAD over these regions. We suggest that sea‐ice contributes to the TS‐SPREAD over the NP and SP oceans, not in a direct way, but by its coupling with ocean storage, interseasonal heat release through turbulent fluxes, and also the horizontal heat transport, both across the border of and inside the polar regions. The findings may help climate modelers to improve their models. Key Points: The pattern‐similarity among intermodel spread, mean absolute bias, and absolute mean bias enable us to use the former to understand the latter twoThe interseasonal linkage between surface albedo, heat storage, turbulent fluxes, and dynamical transport causes winter amplification over polar oceansA method is provided to determine the local versus nonlocal contributions to surface downward longwave radiation [ABSTRACT FROM AUTHOR]

Based on inferences from proxy records the Miocene (23.03–5.33 Ma) was a time of amplified polar warmth compared to today. However, it remains a challenge to simulate a warm Miocene climate and pronounced polar warmth at reconstructed Miocene CO2 concentrations. Using a state‐of‐the‐art Earth‐System‐Model, we implement a high‐resolution paleobathymetry and simulate Miocene climate at different atmospheric CO2 concentrations. We estimate global mean surface warming of +3.1°C relative to the preindustrial at a CO2 level of 450 ppm. An increase of atmospheric CO2 from 280 to 450 ppm provides an individual warming of ∼1.4°C, which is as strong as all other Miocene forcing contributions combined. Substantial changes in surface albedo are vital to explain Miocene surface warming. Simulated surface temperatures fit well with proxy reconstructions at low‐ to mid‐latitudes. The high latitude cooling bias becomes less pronounced for higher atmospheric CO2 concentrations. At such CO2 levels simulated Miocene climate shows a reduced polar amplification, linked to a breakdown of seasonality in the Arctic Ocean. A pronounced warming in boreal fall is detected for a CO2 increase from 280 to 450 ppm, in comparison to weaker warming for CO2 changes from 450 to 720 ppm. Moreover, a pronounced warming in winter is detected for a CO2 increase from 450 to 720 ppm, in contrast to a moderate summer temperature increase, which is accompanied by a strong sea‐ice concentration decline that promotes cloud formation in summer via enhanced moisture availability. As a consequence planetary albedo increases and dampens the temperature response to CO2 forcing at a warmer Miocene background climate. Key Points: At a CO2 level of 450 ppm, a Miocene simulation shows a global mean surface warming of +3.1°C relative to the preindustrial stateAtmospheric CO2 increase from 280 to 450 ppm causes a warming of ∼1.4°C, which is as strong as all other forcing factors combinedAt higher atmospheric CO2 levels, the Miocene climate shows a reduced polar amplification linked to a breakdown of seasonality in the Arctic [ABSTRACT FROM AUTHOR]

Polar amplification appears in response to greenhouse gas forcing, which has become a focus of climate change research. However, polar amplification has not been systematically investigated over the Earth’s three poles (the Arctic, Antarctica, and the Third Pole). An index of polar amplification is employed, and the annual and seasonal variations of land surface temperature over the Earth’s three poles are examined using MODIS (Moderate Resolution Imaging Spectroradiometer) observations for the period 2001–2018. As expected, the warming of the Arctic is most conspicuous, followed by the Third Pole, and is weakest in Antarctica. Compared to the temperature changes for the global land region, positive polar amplification appears in the Arctic and the Third Pole on an annual scale, whereas Antarctic amplification disappears, with a negative amplification index of −0.72. The polar amplification for the Earth’s three poles shows seasonal differences. Strong Arctic amplification appears in boreal spring and winter, with a surface warming rate of more than 3.40 times the global mean for land regions. In contrast, the amplification of the Third Pole is most conspicuous in boreal summer. The two poles located in the Northern Hemisphere have the weakest amplification in boreal autumn. Differently from the positive amplification for the Arctic and the Third Pole in all seasons, the faster variations in Antarctic temperature compared to the globe only appear in austral autumn and winter, and the amplification signal is negative in these seasons, with an amplification index of −1.68 and −2.73, respectively. In the austral winter, the strong negative amplification concentrates on West Antarctica and the coast of East Antarctica, with an absolute value of amplification index higher than 5 in general. Generally, the polar amplification is strongest in the Arctic except from June to August, and Antarctic amplification is the weakest among the Earth’s three poles. The Earth’s three poles are experiencing drastic changes, and the potential influence of climate change should receive attention.

The future response of the atmospheric circulation to increased anthropogenic forcing is uncertain, in particular due to competing influences of the large projected warming at the surface in the Arctic, and at upper-levels in the tropics. In the present study two ensembles of fully-coupled 21st century climate simulations are used to analyze changes in the wintertime eddy-driven jet in the North Atlantic and the relation to the well-defined thermal signatures of climate change. The models project a robust reinforcement of the eddy-driven jet and a decrease in waviness and blockings, that we attribute to a narrowing of the westerly flow in mid-latitudes. Composite analyses suggest that this signal is driven by the opposite influence of Arctic and tropical warming on each flank of the jet. We find that a significant portion of the multi-model spread in the jet metrics can be explained by the ratio between these two signals. The tug-of-war between the two effects influences by how much wintertime cold extremes diminish at the end of the 21st century. Models with dominant tropical warming (i. e. narrower and stronger eddy-driven jet) exhibit less decrease in cold extremes with climate change, due to the maintenance of cooler conditions in the subpolar North Atlantic and subarctic seas compared to models with a predominance of Arctic warming.

Understanding polar amplification (PA) and its underlying processes is key to accurately predicting the climate system's response to increasing anthropogenic forcings. We examine the amplified warming in the Arctic and Antarctic in 17 global climate models from the Coupled Model Intercomparison Project 6 (CMIP6) against satellite data. Large hemispheric differences in PA strength was found in the CMIP6 models. Changes in surface temperature and strength of PA is closely coupled to changes in albedo. The planetary albedo of Earth (αp) is partitioned into a component associated with surface albedo (defined as surface contribution to planetary albedo, αsurfp ${\alpha }_{\mathit{surf}}^{p}$), and a component associated with atmospheric albedo (atmospheric contribution to planetary albedo, αatmp ${\alpha }_{\mathit{atm}}^{p}$). To assess the hemispheric differences in PA strengths, the relative importance of αsurfp ${\alpha }_{\mathit{surf}}^{p}$ and αatmp ${\alpha }_{\mathit{atm}}^{p}$ were investigated. The surface reflection looks different as seen at the surface (defined as surface albedo, αsurf) compared to αsurfp ${\alpha }_{\mathit{surf}}^{p}$ (as seen at the top of the atmosphere). We find a stronger correlation between surface temperature and αsurf in the Arctic than in the Antarctic, with correlation coefficients of −0.94 and −0.88, respectively. Interestingly, the correlation for surface temperature and αsurfp ${\alpha }_{\mathit{surf}}^{p}$ is stronger in the Antarctic than in the Arctic with correlation coefficients of −0.93 and −0.90, respectively. In the southern high latitudes, albedo changes at the surface are more important than changes in the atmosphere, while the opposite applies in the northern high latitudes. Surface temperature changes in the low‐ and mid‐latitudes are strongly associated with changes in αatmp ${\alpha }_{\mathit{atm}}^{p}$, dominated by changes in cloud properties. Plain Language Summary: The temperature is increasing faster in high latitudes compared to other regions of Earth. This is known as polar amplification (PA). The amount of light reflected from a surface is called albedo. Using 17 climate models and satellite data, this study compares the effects of albedo from surface and atmosphere on PA. We find a large spread in PA among models, with stronger PA in the northern hemisphere than in the southern hemisphere. Surface albedo looks different when looking from the surface compared to when looking from the top of the atmosphere (TOA), because of the effects from the atmosphere when looking from the TOA. We find greater correlation between surface temperature and albedo at the surface in the Arctic than the Antarctic. Interestingly, from the TOA we see greater correlation between surface temperature and surface albedo in the Antarctic than in the Arctic. We also find that in Antarctica, changes in surface albedo as seen from the TOA are more sensitive to changes in surface temperature than changes in atmospheric albedo are, while the opposite applies in the Arctic. These differences may provide part of the answer to why there is greater PA in the northern than in the southern hemisphere. Key Points: A strong correlation between CO2‐induced temperature change and planetary albedo is found in all regionsThere is a large spread in polar amplification between modelsCorrelation between CO2‐induced temperature change and contribution from surface albedo to total planetary albedo is clear in the Antarctic, but less clear in the Arctic [ABSTRACT FROM AUTHOR]

Statistical aspects of the quantitative estimation of the climate change polar amplification (PA) are considered. A theoretical study is provided of the distribution density for the PA sample coefficient defined as the ratio of the linear trend coefficients of spatially averaged surface air temperature. It is shown that heavy tails of the distribution lead to instability of the PA sample estimates. A statistically valid quantification of the PA can be obtained from the results of the ensemble modeling of climate change based on the Fieller's method. The applicability of the method in the case of a non-stationary climate system response to external forcing is considered. [ABSTRACT FROM AUTHOR]

A common approach to assessing how polar amplification affects lower latitude climate is to perform coupled ocean-atmosphere experiments in which sea ice is perturbed to a future state. Recent work with a simple 1-dimensional energy balance model (EBM) shows that sea ice perturbation experiments impose an artificial heat flux on the climate system. We explore this effect in a broader range of models and suggest a technique to correct for the artificial heat post-hoc. Our technique successfully corrects for artificial heat in the EBM, and a possible generalization of this approach is developed to correct for artificial heat in an albedo modification experiment in a comprehensive earth system model. Generalizing this technique to sea ice perturbation methodologies that employ a ‘ghost flux’ seen only by the sea ice model would require a better understanding of the effect of ghost fluxes on the surface energy budget. Applying the correction to the comprehensive albedo modification experiment, we find stronger artificial warming than in the EBM. Failing to account for the artificial heat also leads to overestimation of the climate response to sea ice loss (SIL), and can suggest false or artificially strong ‘tugs-of-war’ between low latitude warming and SIL over some fields, for example Arctic surface temperature and zonal wind.

As a step towards understanding the fundamental drivers of polar climate change, we evaluate contributions to polar warming and its seasonal and hemispheric asymmetries in Coupled Model Intercomparison Project phase 6 (CMIP6) as compared with CMIP5. CMIP6 models broadly capture the observed pattern of surface- and winter-dominated Arctic warming that has outpaced both tropical and Antarctic warming in recent decades. For both CMIP5 and CMIP6, CO2 quadrupling experiments reveal that the lapse-rate and surface albedo feedbacks contribute most to stronger warming in the Arctic than the tropics or Antarctic. The relative strength of the polar surface albedo feedback in comparison to the lapse-rate feedback is sensitive to the choice of radiative kernel, and the albedo feedback contributes most to intermodel spread in polar warming at both poles. By separately calculating moist and dry atmospheric heat transport, we show that increased poleward moisture transport is another important driver of Arctic amplification and the largest contributor to projected Antarctic warming. Seasonal ocean heat storage and winter-amplified temperature feedbacks contribute most to the winter peak in warming in the Arctic and a weaker winter peak in the Antarctic. In comparison with CMIP5, stronger polar warming in CMIP6 results from a larger surface albedo feedback at both poles, combined with less-negative cloud feedbacks in the Arctic and increased poleward moisture transport in the Antarctic. However, normalizing by the global-mean surface warming yields a similar degree of Arctic amplification and only slightly increased Antarctic amplification in CMIP6 compared to CMIP5.

The drivers of polar amplification are investigated by isolating the role of sea‐ice processes, moist energy transport, and the seasonal cycle of insolation in two models, an energy balance model and an idealized general circulation model. Compared to a simple ice‐albedo feedback (temperature‐dependent surface albedo), the addition of thermodynamic‐ice processes and the seasonal cycle of insolation profoundly affects seasonal polar warming. Climatologically limited‐extent ice in the warm season permits only small increases in absorbed solar radiation, producing weak warming, while thick, cold ice in the cold season enables a large radiatively forced response. Despite this enhanced winter warming, the annual‐mean polar amplification is modestly reduced by thermodynamic‐ice processes. When latent heat transport is disabled, polar amplification is further reduced by a factor of 1.8 across the range of ice representations, suggestive of a nearly additive warming by ice and moist‐transport processes. Plain Language Summary: Since the 1970s, simulations of climate change have predicted warming that is greatest in polar regions. This polar‐amplified warming is ubiquitous, though uncertain in magnitude, in climate models subjected to increases in greenhouse gases and has been variously attributed to the ice‐albedo feedback, associated with the retreat of reflective sea ice; the lapse rate feedback, associated with the uneven warming of the Arctic atmosphere; and changes in heat transport by atmospheric circulations. In this study, we turn to highly simplified climate models to isolate the role of sea ice and moisture transport on polar amplification and to explore their interactions. We find that, in order to reproduce the seasonality of polar warming in a manner consistent with both observations and state‐of‐the‐art climate models, it is necessary to model the changing thickness of sea ice and not just its retreat. This effect works in concert with moisture transport by the atmosphere to further enhance polar amplification. Together, the findings imply that sea‐ice loss leads to winter warming that would promote a positive lapse rate feedback and that the increase in moist energy transport would also lead, in more complex models, to additional warming by increasing the water‐vapor greenhouse effect. Key Points: Ice thermodynamics promote a realistic seasonality of polar warming that is greatest in winterMoist energy transport and the ice‐albedo feedback make comparable and nearly additive contributions to polar‐amplified warmingThe seasonal cycle of insolation has no impact on annual‐mean polar amplification with a simple ice‐albedo feedback [ABSTRACT FROM AUTHOR]

The Earth's North and South Poles and the Tibetan Plateau — collectively referred to as the 'three poles' — are regions that are very sensitive to climate changes and are known to amplify shifts in global temperature. In the case of the polar regions this effect is well known as polar amplification. The climate patterns during the Middle Miocene Climatic Optimum (MMCO) are considered a past analog to future climate, which could develop patterns and characteristics bearing similarity to MMCO by the end of the century. Therefore, analyzing MMCO environmental conditions in these regions in comparison to a pre-Industrial reference state provides valuable insights into possible future surface temperature changes at the Earth's three poles. We conducted simulations of the MMCO, focusing on surface temperature anomalies at the Earth's three poles. As a modelling tool we employed two versions of the Community Earth System Model (CESM), one of them equipped with model dynamics to simulate stable water isotopes. During the MMCO, we show 1) temperature anomalies at the three poles exceeded global averages, with Antarctica warming most, followed by the Tibetan Plateau and the Arctic, highlighting the role of the Tibetan Plateau as a key factor in global climate change often overlooked; 2) temperature anomalies at the Tibetan Plateau are higher in boreal winter (DJF), while in the Arctic and Antarctic temperature anomalies are larger in boreal summer (JJA), highlighting a seasonal shift between the maximum of climate change signals at the third pole; 3) Changes in clear-sky surface downwelling longwave radiation fluxes (∆ T rlds _ clearsky) and changes in surface albedo forcing (∆ T SAF) constitute the primary and secondary mechanisms driving the annual surface temperature anomaly at the Earth's three poles. In contrast, the leading physical processes governing the seasonal cycle are subject to regional variation. • Earth's 'three poles' surface temperature anomaly exceeds global average, with Tibetan Plateau amplification surpassing the Arctic. • Tibetan Plateau temperature anomalies are higher in DJF, unlike the Arctic and Antarctic,which are more pronounced in JJA. • surface downwelling longwave radiation (∆ T rlds _ clearsky) contributes most to annual surface warming at the three poles. [ABSTRACT FROM AUTHOR]

Presently, there are no global measurement constraints on the surface emissivity at wavelengths longer than 15 μm, even though this surface property in this far-IR region has a direct impact on the outgoing longwave radiation (OLR) and infrared cooling rates where the column precipitable water vapor (PWV) is less than 1 mm. Such dry conditions are common for high-altitude and high-latitude locations, with the potential for modeled climate to be impacted by uncertain surface characteristics. This paper explores the sensitivity of instantaneous OLR and cooling rates to changes in far-IR surface emissivity and how this unconstrained property impacts climate model projections. At high latitudes and altitudes, a 0.05 change in emissivity due to mineralogy and snow grain size can cause a 1.8-2.0 W m(-2) difference in the instantaneous clear-sky OLR. A variety of radiative transfer techniques have been used to model the far-IR spectral emissivities of surface types defined by the International Geosphere-Biosphere Program. Incorporating these far-IR surface emissivities into the Representative Concentration Pathway (RCP) 8.5 scenario of the Community Earth System Model leads to discernible changes in the spatial patterns of surface temperature, OLR, and frozen surface extent. The model results differ at high latitudes by as much as 2°K, 10 W m(-2), and 15%, respectively, after only 25 y of integration. Additionally, the calculated difference in far-IR emissivity between ocean and sea ice of between 0.1 and 0.2, suggests the potential for a far-IR positive feedback for polar climate change.

Large-ensemble simulations of the atmosphere-only time-slice experiments for the Polar Amplification Model Intercomparison Project (PAMIP) were carried out by the model group of the Chinese Academy of Sciences (CAS) Flexible Global Ocean-Atmosphere-Land System (FGOALS-f3-L). Eight groups of experiments forced by different combinations of the sea surface temperature (SST) and sea ice concentration (SIC) for pre-industrial, present-day, and future conditions were performed and published. The time-lag method was used to generate the 100 ensemble members, with each member integrating from 1 April 2000 to 30 June 2001 and the first two months as the spin-up period. The basic model responses of the surface air temperature (SAT) and precipitation were documented. The results indicate that Arctic amplification is mainly caused by Arctic SIC forcing changes. The SAT responses to the Arctic SIC decrease alone show an obvious increase over high latitudes, which is similar to the results from the combined forcing of SST and SIC. However, the change in global precipitation is dominated by the changes in the global SST rather than SIC, partly because tropical precipitation is mainly driven by local SST changes. The uncertainty of the model responses was also investigated through the analysis of the large-ensemble members. The relative roles of SST and SIC, together with their combined influence on Arctic amplification, are also discussed. All of these model datasets will contribute to PAMIP multi-model analysis and improve the understanding of polar amplification. [ABSTRACT FROM AUTHOR]

The Miocene epoch, spanning 23.03–5.33 Ma, was a dynamic climate of sustained, polar amplified warmth. Miocene atmospheric CO2 concentrations are typically reconstructed between 300 and 600 ppm and were potentially higher during the Miocene Climatic Optimum (16.75–14.5 Ma). With surface temperature reconstructions pointing to substantial midlatitude and polar warmth, it is unclear what processes maintained the much weaker‐than‐modern equator‐to‐pole temperature difference. Here, we synthesize several Miocene climate modeling efforts together with available terrestrial and ocean surface temperature reconstructions. We evaluate the range of model‐data agreement, highlight robust mechanisms operating across Miocene modeling efforts and regions where differences across experiments result in a large spread in warming responses. Prescribed CO2 is the primary factor controlling global warming across the ensemble. On average, elements other than CO2, such as Miocene paleogeography and ice sheets, raise global mean temperature by ∼2°C, with the spread in warming under a given CO2 concentration (due to a combination of the spread in imposed boundary conditions and climate feedback strengths) equivalent to ∼1.2 times a CO2 doubling. This study uses an ensemble of opportunity: models, boundary conditions, and reference data sets represent the state‐of‐art for the Miocene, but are inhomogeneous and not ideal for a formal intermodel comparison effort. Acknowledging this caveat, this study is nevertheless the first Miocene multi‐model, multi‐proxy comparison attempted so far. This study serves to take stock of the current progress toward simulating Miocene warmth while isolating remaining challenges that may be well served by community‐led efforts to coordinate modeling and data activities within a common analytical framework. Plain Language Summary: As human activity continues to increase atmospheric carbon dioxide concentrations, scientists turn to warm intervals in Earth's history to develop insight into the behavior of the climate system under elevated carbon dioxide and temperature. One such interval is the Miocene epoch which has become increasingly relevant as reconstructions of Miocene atmospheric CO2 concentrations point to values ranging between current concentrations of ∼400 ppm and those projected for the end of this century under Shared Socioeconomic Pathways 3 and 4. In this study, we evaluate the surface warming patterns simulated by a range of different climate models configured with Miocene paleogeography and CO2 concentrations spanning 200–850 ppm. We also synthesize available Miocene surface temperature reconstructions. The primary factor controlling the amount of global warming seen across the Miocene simulations analyzed is the CO2 concentration that was prescribed within a given simulation. On average, Miocene elements other than CO2, such as Miocene paleogeography and ice sheets, raise global mean temperature by ∼2°C. While some Miocene simulations with high CO2 forcing overlap with the reconstructed global mean surface temperature estimates for their target Miocene interval, they still generally fail to capture the reconstructed pattern of warming. Key Points: A synthesis of Miocene modeling efforts, and surface temperature reconstructions, is presented within a single analysis frameworkMiocene global mean surface temperature estimates span ∼5.3°C–11.5°C higher than preindustrial, only ∼2°C is explained by non–CO2 boundary conditions in climate modelsSome simulations overlap with reconstructed global mean surface temperature estimates but fail to capture the weak temperature gradient [ABSTRACT FROM AUTHOR]

Arctic amplification (AA) of surface warming is a prominent feature of anthropogenic climate change with important implications for human and natural systems. Despite its importance, the underlying causes of AA are not fully understood. Here, analyzing coupled climate model simulations, we show that AA develops rapidly (within the first few months) following an instantaneous quadrupling of atmospheric CO2. This rapid AA response—which occurs before any significant loss of Arctic sea ice—is produced by a positive lapse rate feedback over the Arctic. Sea ice loss is therefore not needed to produce polar‐amplified warming, although it contributes significantly to this warming after the first few months. Our results provide new and compelling evidence that AA owes its existence, fundamentally, to fast atmospheric processes. Plain Language Summary: Climate warming is greater in the Arctic than at lower latitudes, a phenomenon known as Arctic amplification. Despite its importance for humans and ecosystems, the causes of Arctic amplification are not fully understood. Sea ice loss has long been thought to be a primary cause. However, we show here that Arctic amplification happens faster than sea ice loss in climate models when atmospheric CO2 is increased. This indicates that atmospheric processes alone are capable of causing Arctic amplification. Key Points: Arctic amplification develops rapidly (within a few months) in climate models forced with abrupt CO2 quadruplingThis rapid response is produced by a positive lapse rate feedback over the ArcticThe response occurs before Arctic sea ice loss becomes significant [ABSTRACT FROM AUTHOR]

The lapse rate feedback is the dominant driver of stronger warming in the Arctic than the Antarctic in simulations with increased CO2. While Antarctic surface elevation has been implicated in promoting a weaker Antarctic lapse rate feedback, the mechanisms in which elevation impacts the lapse rate feedback are still unclear. Here we suggest that weaker Antarctic warming under CO2 forcing stems from shallower, less intense climatological inversions due to limited atmospheric heat transport above the ice sheet elevation and elevation‐induced katabatic winds. In slab ocean model experiments with flattened Antarctic topography, stronger climatological inversions support a stronger lapse rate feedback and annual mean Antarctic warming comparable to the Arctic under CO2 doubling. Unlike the Arctic, seasonality in warming over flat Antarctica is mainly driven by a negative shortwave cloud feedback, which exclusively dampens summer warming, with a smaller contribution from the winter‐enhanced lapse rate feedback. Plain Language Summary: Models project stronger surface warming in the Arctic than the Antarctic under climate change. A climate feedback in which more warming occurs near the surface than at higher altitudes in the atmosphere promotes this stronger Arctic warming. Antarctica's surface elevation is thought to weaken this feedback in comparison to the Arctic, but how this occurs is unclear. Here we show that Antarctic elevation weakens surface warming by changing the base state vertical temperature structure. When Antarctic topography is flattened in model experiments, Antarctica experiences more warming under climate change, resembling Arctic warming. Similarly to the Arctic, flat Antarctica warms most during the winter, but this seasonality is driven by different climate feedbacks in the Arctic versus Antarctic. These results indicate the importance of base state temperatures for warming under climate change and suggest that strong polar amplification is possible without local sea ice loss. Key Points: Antarctic elevation causes asymmetry in climatological inversions, lapse rate feedbacks, and warming between the polesIn model experiments with flattened Antarctica, strengthened inversions lead to stronger lapse rate feedback and Antarctic amplificationShortwave cloud feedback promotes seasonality in flat Antarctic warming; lapse rate feedback more strongly promotes seasonality in Arctic [ABSTRACT FROM AUTHOR]

In the Tropical Rain belts with an Annual cycle and Continent Model Intercomparison Project (TRACMIP) ensemble of aquaplanet climate model experiments, CO2‐induced warming is amplified in the poles in 10 out of 12 models, despite the lack of sea ice. We attribute causes of this amplification by perturbing individual radiative forcing and feedback components in a moist energy balance model. We find a strikingly linear pattern of tropical versus polar warming contributions across models and processes, implying that polar amplification is an inherent consequence of diffusion of moist static energy by the atmosphere. The largest contributor to polar amplification is the instantaneous CO2 forcing, followed by the water vapor feedback and, for some models, cloud feedbacks. Extratropical feedbacks affect polar amplification more strongly, but even feedbacks confined to the tropics can cause polar amplification. Our results contradict studies inferring warming contributions directly from the meridional gradient of radiative perturbations, highlighting the importance of interactions between feedbacks and moisture transport for polar amplification. Plain Language Summary: In both observations and computer model simulations, the polar regions (especially the Arctic) warm more than the rest of the world in response to increased greenhouse gas concentrations. Scientists disagree on the reasons for this polar amplification of warming. The melting of ice floating in the ocean, which lets more sunlight be absorbed, is often given as an explanation, but climate models with no sea ice also display polar amplification. We ran hundreds of experiments with a simple climate model in order to understand the reasons for polar amplification in more complex models that lack sea ice. We found that the main reason is that the atmosphere transports energy from the tropics to the poles, so much so that even processes that initially add energy mostly to the tropics cause polar amplification. Our methods produce different explanations from past studies because they did not fully account for this movement of energy. Key Points: Polar amplification occurs robustly in the TRACMIP aquaplanet simulationsMoisture transport mediates the contributions of different forcing and feedback components to polar amplificationThe instantaneous CO2 forcing and water vapor feedback are the largest contributors to polar amplification [ABSTRACT FROM AUTHOR]

The future response of the Antarctic ice sheet to rising temperatures remains highly uncertain. A useful period for assessing the sensitivity of Antarctica to warming is the Last Interglacial (LIG) (129 to 116 ky), which experienced warmer polar temperatures and higher global mean sea level (GMSL) (+6 to 9 m) relative to present day. LIG sea level cannot be fully explained by Greenland Ice Sheet melt (~2 m), ocean thermal expansion, and melting mountain glaciers (~1 m), suggesting substantial Antarctic mass loss was initiated by warming of Southern Ocean waters, resulting from a weakening Atlantic meridional overturning circulation in response to North Atlantic surface freshening. Here, we report a blue-ice record of ice sheet and environmental change from the Weddell Sea Embayment at the periphery of the marine-based West Antarctic Ice Sheet (WAIS), which is underlain by major methane hydrate reserves. Constrained by a widespread volcanic horizon and supported by ancient microbial DNA analyses, we provide evidence for substantial mass loss across the Weddell Sea Embayment during the LIG, most likely driven by ocean warming and associated with destabilization of subglacial hydrates. Ice sheet modeling supports this interpretation and suggests that millennial-scale warming of the Southern Ocean could have triggered a multimeter rise in global sea levels. Our data indicate that Antarctica is highly vulnerable to projected increases in ocean temperatures and may drive ice-climate feedbacks that further amplify warming. [ABSTRACT FROM AUTHOR]

One of the most visible signs of global warming is the fast change in the polar regions. The increase in Arctic temperatures, for instance, is almost twice as large as the global average in recent decades. This phenomenon is known as the Arctic Amplification and reflects several mutually supporting processes. An equivalent albeit less studied phenomenon occurs in Antarctica. Here, we used numerical climate simulations obtained from CMIP5 and CMIP6 to investigate the effects of +1.5, 2 and 3 °C warming thresholds for sea ice changes and polar amplification. Our results show robust patterns of near-surface air-temperature response to global warming at high latitudes. The year in which the average air temperatures brought from CMIP5 and CMIP6 models rises by 1.5 °C is 2024. An average rise of 2 °C (3 °C) global warming occurs in 2042 (2063). The equivalent warming at northern (southern) high latitudes under scenarios of 1.5 °C global warming is about 3 °C (1.8 °C). In scenarios of 3 °C global warming, the equivalent warming in the Arctic (Antarctica) is close to 7 °C (3.5 °C). Ice-free conditions are found in all warming thresholds for both the Arctic and Antarctica, especially from the year 2030 onwards.

In this study, we use the National Center for Atmospheric Research Community Earth System Model to investigate the contribution of sea ice and land snow to the climate sensitivity in response to increased atmospheric carbon dioxide content. We focus on the overall effect arising from the presence or absence of sea ice and/or land snow. We analyze our results in terms of the radiative forcing and climate feedback parameter. We find that the presence of sea ice and land snow decreases the climate feedback parameter (and thus increases climate sensitivity). Adjusted radiative forcing from added carbon dioxide is comparatively less sensitive to the presence of sea ice or land snow. The effect of sea ice on the climate feedback parameter decreases as sea ice cover diminishes at higher CO2 concentration. However, the influence of both sea ice and land snow on the climate feedback parameter remains substantial under the CO2 concentration range considered here (to eight times preindustrial CO2 content). Approximately, one quarter of the effect of sea ice and land snow on the climate feedback parameter is a consequence of the effect of these components on longwave feedback that is mainly associated with cloud change. Polar warming in response to added CO2 is approximately doubled by the presence of sea ice and land snow. Relative to the case in which sea ice and land snow are absent in the model, in response to increased CO2 concentrations, the presence of sea ice and land snow results in an increase in global mean warming by over 40%. Plain Language Summary: Sea ice and land snow are two crucial components that affect the climate response to external forcings. Feedbacks between ice/snow and climate change cause amplified surface warming in high latitudes. In this study, we use a climate model to estimate the contribution of sea ice and land snow to climate change in response to increased CO2 concentrations. We compare the climate response to increased CO2 between the simulations with sea ice and/or land snow and the simulations without them. We show that the existence of sea ice and land snow substantially amplifies the global temperature response to increased CO2 with sea ice having a stronger effect than land snow. Under higher CO2 levels, the effect of sea ice diminishes more rapidly than does the effect of land snow. About one quarter of the total climate feedback from sea ice and land snow is associated with the change in longwave radiation. Also we show that the effect of sea ice and land snow on the sensitivity of top‐of‐atmosphere net energy flux to the global mean temperature change is approximately additive. Key Points: We use NCAR CESM to investigate the overall contribution of sea ice and land snow to climate feedback to increased carbon dioxide contentSea ice and land snow lead to substantial increases in polar warming to increased CO2 content with sea ice playing a more important roleThe presence of sea ice and land snow decreases the climate feedback parameter with about one quarter of the effect from longwave feedback [ABSTRACT FROM AUTHOR]

Arctic amplification (AA)—referring to the enhancement of near-surface air temperature change over the Arctic relative to lower latitudes—is a prominent feature of climate change with important impacts on human and natural systems. In this review, we synthesize current understanding of the underlying physical mechanisms that can give rise to AA. These mechanisms include both local feedbacks and changes in poleward energy transport. Temperature and sea ice-related feedbacks are especially important for AA, since they are significantly more positive over the Arctic than at lower latitudes. Changes in energy transport by the atmosphere and ocean can also contribute to AA. These energy transport changes are tightly coupled with local feedbacks, and thus their respective contributions to AA should not be considered in isolation. It is here emphasized that the feedbacks and energy transport changes that give rise to AA are sensitively dependent on the state of the climate system itself. This implies that changes in the climate state will lead to changes in the strength of AA, with implications for past and future climate change.

Over the last century Earth's surface temperatures have warmed by order 1 K as a global average, but with significant variation in latitude: there has been most surface warming at high Northern latitudes, around 3 times more than in low latitude regions (termed Arctic Amplification), while there has been least warming over the Southern Ocean. Many contributing processes have been suggested to explain this asymmetrical latitudinal warming pattern, but quantification of the contributing factors responsible remains elusive. Complex general circulation climate models can reproduce similar asymmetrical patterns of warming, but it can be difficult to interpret the contributing processes. Meanwhile, idealised conceptual energy balance climate models have been able to reproduce a general polar amplification of warming whose origins can be interpreted, but this warming is often symmetrical across both hemispheres and may not be responsible for the real-world pattern. Here, we use a conceptual Energy Balance Model, with imposed closures for initial horizontal diffusivity and cloudiness drawing upon observational constraints and including temperature-dependent diffusivity and a sub-surface ocean heat reservoir, to show that the magnitude of present-day Arctic Amplification may arise through relatively simple thermodynamic (Clausius–Clapeyron) and radiative (climate feedback) processes. The current asymmetry between hemispheric warming may arise due to the transient heat transport up through the base of the surface ocean mixed layer from the slow-responding deep ocean to the fast-responding surface ocean being dominated by upwelling in the Southern Ocean. It should be noted that the processes identified here are not a unique in offering a potential solution, and so significant, or dominant, roles for dynamical processes remain plausible explanations for Arctic Amplification. • Data constrained Energy Balance Model for Earth's surface temperature presented. • EBM has explicit cloud representation for shortwave and longwave radiation. • Arctic amplification arises through both thermodynamic and radiative processes. • Hemispheric asymmetry in warming arises through upwelling in Southern Ocean. [ABSTRACT FROM AUTHOR]

Orbital forcing affects climate system by regulating the amount of solar radiation received at the Earth's surface, which is vital for global hydrological cycles. However, a systematic understanding of the global water cycle with synergistic changes in ice sheets and sea levels under orbital forcing during the late Paleozoic ice age (LPIA) remains elusive. Here, we perform cyclostratigraphic analysis and sedimentary noise modeling for coastal sedimentary records (Well Greer #2 in the Permian Basin) from the early Permian and two sets of high-resolution paleoclimate simulation experiments using the Community Earth System Model. We provide the first high-resolution absolute astronomical time scale for the Permian Basin, which suggests that the timing of the early Permian Lower Leonard and Upper Wolfcamp Formations range from 286.29 ± 0.55 Ma to 289.05 ± 0.55 Ma, and the Lower Wolfcamp Formation ranges from 292.97 ± 0.16 Ma to 298.9 ± 0.15 Ma, respectively. In addition, we find a coupled phase relationship between high sea levels and eccentricity maxima, suggesting that eccentricity may have played a critical role in the early Permian sea-level oscillations. Paleoclimate simulations show that eccentricity and obliquity maxima lead to an overall increase in global mean temperature. Significant changes in surface temperature at middle to high latitudes are manifested as the polar amplification effect during the LPIA. Compared with obliquity, eccentricity leads to higher annual and summer variability in global surface temperature, and enhanced polar amplification results in ice retreat and sea-level rise. Evidence from Earth system modeling elucidates the phase relationship between orbital cycles and sea levels. Therefore, eccentricity, as the leading driving force in low latitudes, may dominate climate changes in the Earth's climate system and regulates global hydrological cycles during the LPIA. • The first high-resolution ATS (∼8.7 Myr) for the Permian Basin is established. • A coupled phase relationship between high sea levels and eccentricity maxima. • The polar amplification effect during the LPIA is significant. • Eccentricity dominates the climate change in the Earth system during the LPIA. [ABSTRACT FROM AUTHOR]

Stable water isotopologues in paleoclimate archives ( δ 18 O ) have been widely used as an indicator to derive past climate variations. The modern observed spatial δ 18 O -temperature relation in the middle and high latitudes has been used to infer the paleotemperatures changes from ice core data. However, various studies have shown that the spatial slope is larger than the temporal slope at the drill site by a factor of 2. Physically, the different spatial and temporal slope has been suggested to result from the amplified local surface air temperature cooling in the polar region at Last Glacial Maximum (LGM), according to the slope ratio equation derived in our previous study. To explicitly confirm the “polar amplification” effect in understanding the differences between temporal and spatial isotope–temperature relations, here we use the same isotope-enabled atmospheric general circulation model with a moisture-tracing module embedded to quantitatively estimate the contributions of different sources to the precipitated heavy oxygen isotopes in the middle and high latitudes. Our results show that the major sources of δ 18 O in precipitation over middle and high latitudes are from oceans where the sea surface temperature cooling at Last Glacial Maximum (LGM) is less than −2 ° C , while the local moisture sources with a higher cooling can be also relevant for polar regions, such as north Greenland. Additionally, the neglect of the strengthened local inversion layer strength at LGM could be the main cause for the overestimated source temperature cooling by the slope ratio equation, especially for the polar regions in the Northern Hemisphere.

We examine the effects of non-linear interactions between surface albedo, water vapor and cloud cover (referred to as climate variables) on amplified warming of the polar regions, using a new energy balance model. Our simulations show that the sum of the contributions to surface temperature changes due to any variable considered in isolation is smaller than the temperature changes from coupled feedback simulations. This non-linearity is strongest when all three climate variables are allowed to interact. Surface albedo appears to be the strongest driver of this non-linear behavior, followed by water vapor and clouds. This is because increases in longwave radiation absorbed by the surface, related to increases in water vapor and clouds, and increases in surface absorbed shortwave radiation caused by a decrease in surface albedo, amplify each other. Furthermore, our results corroborate previous findings that while increases in cloud cover and water vapor, along with the greenhouse effect itself, warm the polar regions, water vapor also significantly warms equatorial regions, which reduces polar amplification. Changes in surface albedo drive large changes in absorption of incoming shortwave radiation, thereby enhancing surface warming. Unlike high latitudes, surface albedo change at low latitudes are more constrained. Interactions between surface albedo, water vapor and clouds drive larger increases in temperatures in the polar regions compared to low latitudes. This is in spite of the fact that, due to a forcing, cloud cover increases at high latitudes and decreases in low latitudes, and that water vapor significantly enhances warming at low latitudes. [ABSTRACT FROM AUTHOR]

Human carbon use during the next century will lead to atmospheric carbon dioxide concentrations ( pCO2) that have been unprecedented for the past 50-100+ million years according to fossil plant-based CO2 estimates. The paleobotanical record of plants offers key insights into vegetation responses to past global change, including suitable analogs for Earth's climatic future. Past global warming events have resulted in transient poleward migration at rates that are equivalent to the lowest climate velocities required for current taxa to keep pace with climate change. Paleobiome reconstructions suggest that the current tundra biome is the biome most threatened by global warming. The common occurrence of paleoforests at high polar latitudes when pCO2 was above 500 ppm suggests that the advance of woody shrub and tree taxa into tundra environments may be inevitable. Fossil pollen studies demonstrate the resilience of wet tropical forests to global change up to 700 ppm CO2, contrary to modeled predictions of the future. The paleobotanical record also demonstrates a high capacity for functional trait evolution as an additional strategy to migration and maintenance of a species' climate envelope in response to global change. [ABSTRACT FROM AUTHOR]

Abstract: Amplified warming of the high latitudes in response to human‐induced emissions of greenhouse gases has already been observed in the historical record and is a robust feature evident across a hierarchy of model systems, including the models of the Coupled Model Intercomparison Project Phase 5 (CMIP5). The main aims of this analysis are to quantify intermodel differences in the Arctic amplification (AA) of the global warming signal in CMIP5 RCP8.5 (Representative Concentration Pathway 8.5) simulations and to diagnose these differences in the context of the energy and water cycles of the region. This diagnosis reveals an emergent behavior between the energetic and hydrometeorological responses of the Arctic to warming: in particular, enhanced AA and its associated reduction in dry static energy convergence is balanced to first order by latent heating via enhanced precipitation. This balance necessitates increasing Arctic precipitation with increasing AA while at the same time constraining the magnitude of that precipitation increase. The sensitivity of the increase, ~1.25 (W/m2)/K (~240 (km3/yr)/K), is evident across a broad range of historical and projected AA values. Accounting for the energetic constraint on Arctic precipitation, as a function of AA, in turn informs understanding of both the sign and magnitude of hydrologic cycle changes that the Arctic may experience. [ABSTRACT FROM AUTHOR]

Past greenhouse periods with elevated atmospheric CO2 were characterized by globally warmer sea-surface temperatures (SST). However, the extent to which the high latitudes warmed to a greater degree than the tropics (polar amplification) remains poorly constrained, in particular because there are only a few temperature reconstructions from the tropics. Consequently, the relationship between increased CO2, the degree of tropical warming, and the resulting latitudinal SST gradient is not well known. Here, we present coupled clumped isotope (▵ 47)-Mg/Ca measurements of foraminifera from a set of globally distributed sites in the tropics and midlatitudes. ▵ 47 is insensitive to seawater chemistry and therefore provides a robust constraint on tropical SST. Crucially, coupling these data with Mg/Ca measurements allows the precise reconstruction of Mg/Casw throughout the Eocene, enabling the reinterpretation of all planktonic foraminifera Mg/Ca data. The combined dataset constrains the range in Eocene tropical SST to 30-36 °C (from sites in all basins). We compare these accurate tropical SST to deep-ocean temperatures, serving as a minimum constraint on high-latitude SST. This results in a robust conservative reconstruction of the early Eocene latitudinal gradient, which was reduced by at least 32 ± 10% compared with present day, demonstrating greater polar amplification than captured by most climate models. [ABSTRACT FROM AUTHOR]

Dramatic climate changes have occurred in recent decades over the Arctic region, and very noticeably in near-surface warming and reductions in sea ice extent. In a climatological sense, Arctic cyclone behaviour is linked to the distributions of lower troposphere temperature and sea ice, and hence the monitoring of storms can be seen as an important component of the analysis of Arctic climate. The analysis of cyclone behaviour, however, is not without ambiguity, and different cyclone identification algorithms can lead to divergent conclusions. Here we analyse a subset of Arctic cyclones with 10 state-of-the-art cyclone identification schemes applied to the ERA-Interim reanalysis. The subset is comprised of the five most intense (defined in terms of central pressure) Arctic cyclones for each of the 12 calendar months over the 30-yr period from 1 January 1979 to 31 March 2009. There is a considerable difference between the central pressures diagnosed by the algorithms of typically 5–10 hPa. By contrast, there is substantial agreement as to the location of the centre of these extreme storms. The cyclone tracking algorithms also display some differences in the evolution and life cycle of these storms, while overall finding them to be quite long-lived. For all but six of the 60 storms an intense tropopause polar vortex is identified within 555 km of the surface system. The results presented here highlight some significant differences between the outputs of the algorithms, and hence point to the value using multiple identification schemes in the study of cyclone behaviour. Overall, however, the algorithms reached a very robust consensus on most aspects of the behaviour of these very extreme cyclones in the Arctic basin.