Showing papers on "Ice-albedo feedback published in 2010"

The central role of diminishing sea ice in recent Arctic temperature amplification.

Abstract: The rise in Arctic near-surface air temperatures has been almost twice as large as the global average in recent decades-a feature known as 'Arctic amplification'. Increased concentrations of atmospheric greenhouse gases have driven Arctic and global average warming; however, the underlying causes of Arctic amplification remain uncertain. The roles of reductions in snow and sea ice cover and changes in atmospheric and oceanic circulation, cloud cover and water vapour are still matters of debate. A better understanding of the processes responsible for the recent amplified warming is essential for assessing the likelihood, and impacts, of future rapid Arctic warming and sea ice loss. Here we show that the Arctic warming is strongest at the surface during most of the year and is primarily consistent with reductions in sea ice cover. Changes in cloud cover, in contrast, have not contributed strongly to recent warming. Increases in atmospheric water vapour content, partly in response to reduced sea ice cover, may have enhanced warming in the lower part of the atmosphere during summer and early autumn. We conclude that diminishing sea ice has had a leading role in recent Arctic temperature amplification. The findings reinforce suggestions that strong positive ice-temperature feedbacks have emerged in the Arctic, increasing the chances of further rapid warming and sea ice loss, and will probably affect polar ecosystems, ice-sheet mass balance and human activities in the Arctic.

Predicting global atmospheric ice nuclei distributions and their impacts on climate

Abstract: Knowledge of cloud and precipitation formation processes remains incomplete, yet global precipitation is predominantly produced by clouds containing the ice phase. Ice first forms in clouds warmer than -36 °C on particles termed ice nuclei. We combine observations from field studies over a 14-year period, from a variety of locations around the globe, to show that the concentrations of ice nuclei active in mixed-phase cloud conditions can be related to temperature and the number concentrations of particles larger than 0.5 μm in diameter. This new relationship reduces unexplained variability in ice nuclei concentrations at a given temperature from ∼103 to less than a factor of 10, with the remaining variability apparently due to variations in aerosol chemical composition or other factors. When implemented in a global climate model, the new parameterization strongly alters cloud liquid and ice water distributions compared to the simple, temperature-only parameterizations currently widely used. The revised treatment indicates a global net cloud radiative forcing increase of ∼1 W m-2 for each order of magnitude increase in ice nuclei concentrations, demonstrating the strong sensitivity of climate simulations to assumptions regarding the initiation of cloud glaciation.

A link between reduced Barents-Kara sea ice and cold winter extremes over northern continents

Abstract: The recent overall Northern Hemisphere warming was accompanied by several severe northern continental winters, as for example, extremely cold winter 2005/2006 in Europe and northern Asia. Here we show that anomalous decrease of wintertime sea ice concentration in the Barents-Kara (B-K) Seas could bring about extreme cold events like winter 2005/2006. Our simulations with the ECHAM5 general circulation model demonstrate that lower-troposphere heating over the B-K Seas in the Eastern Arctic caused by the sea ice reduction may result in strong anti-cyclonic anomaly over the Polar Ocean and anomalous easterly advection over northern continents. This causes a continental-scale winter cooling reaching -1.5°C, with more than three times increased probability of cold winter extremes over large areas including Europe. Our results imply that several recent severe winters do not conflict the global warming picture but rather supplement it, being in qualitative agreement with the simulated large-scale atmospheric circulation realignment. Furthermore, our results suggest that high-latitude atmospheric circulation response to the B-K sea ice decrease is highly nonlinear and characterized by transition from anomalous cyclonic circulation to anticyclonic one and then again back to cyclonic type of circulation as the B-K sea ice concentration gradually reduces from 100% to ice free conditions. We present a conceptual model which may explain the nonlinear local atmospheric response in the B-K Seas region by counter play between convection over the surface heat source and baroclinic effect due to modified temperature gradients in the vicinity of the heating area.

Large-scale atmospheric circulation changes are associated with the recent loss of Arctic sea ice

Abstract: Recent loss of summer sea ice in the Arctic is directly connected to shifts in northern wind patterns in the following autumn, which has the potential of altering the heat budget at the cold end of the global heat engine.With continuing loss of summer sea ice to less than 20% of its climatological mean over the next decades,we anticipate increased modification of atmospheric circulation patterns. While a shift to a more meridional atmospheric climate pattern, the Arctic Dipole (AD), over the last decade contributed to recent reductions in summer Arctic sea ice extent, the increase in late summer open water area is, in turn, directly contributing to a modification of large scale atmospheric circulation patterns through the additional heat stored in the Arctic Ocean and released to the atmosphere during the autumn season. Extensive regions in the Arctic during late autumn beginning in 2002 have surface air temperature anomalies of greater than 3 °C and temperature anomalies above 850 hPa of 1 °C. These temperatures contribute to an increase in the 1000–500 hPa thickness field in every recent year with reduced sea ice cover. While gradients in this thickness field can be considered a baroclinic contribution to the flow field from loss of sea ice, atmospheric circulation also has a more variable barotropic contribution. Thus, reduction in sea ice has a direct connection to increased thickness fields in every year, but not necessarily to the sea level pressure (SLP) fields. Compositing wind fields for late autumn 2002–2008 helps to highlight the baroclinic contribution; for the years with diminished sea ice cover there were composite anomalous tropospheric easterly winds of∼1.4 m s –1 , relative to climatological easterly winds near the surface and upper troposphericwesterlies of∼3 m s –1 . Loss of summer sea ice is supported by decadal shifts in atmospheric climate patterns. A persistent positive Arctic Oscillation pattern in late autumn (OND) during 1988–1994 and in winter (JFM) during 1989–1997 shifted to more interannual variability in the following years. An anomalous meridional wind pattern with high SLP on the North American side of the Arctic—the AD pattern, shifted from primarily small interannual variability to a persistent phase during spring (AMJ) beginning in 1997 (except for 2006) and extending to summer (JAS) beginning in 2005.

Contemporary sea level rise.

Abstract: Measuring sea level change and understanding its causes has considerably improved in the recent years, essentially because new in situ and remote sensing observations have become available. Here we report on most recent results on contemporary sea level rise. We first present sea level observations from tide gauges over the twentieth century and from satellite altimetry since the early 1990s. We next discuss the most recent progress made in quantifying the processes causing sea level change on timescales ranging from years to decades, i.e., thermal expansion of the oceans, land ice mass loss, and land water–storage change. We show that for the 1993–2007 time span, the sum of climate-related contributions (2.85 ± 0.35 mm year −1 )i s only slightly less than altimetry-based sea level rise (3.3 ± 0.4 mm year −1 ): ∼30% of the observed rate of rise is due to ocean thermal expansion and ∼55% results from land ice melt. Recent acceleration in glacier melting and ice mass loss from the ice sheets increases the latter contribution up to 80% for the past five years. We also review the main causes of regional variability in sea level trends: The dominant contribution results from nonuniform changes in ocean thermal expansion.

Increasing fall-winter energy loss from the Arctic Ocean and its role in Arctic temperature amplification

Abstract: [1] Arctic surface temperatures have risen faster than the global average in recent decades, in part due to positive feedbacks associated with the rapidly diminishing sea ice cover. Counter-intuitively, the Arctic warming has been strongest in late fall and early winter whilst sea ice reductions and the direct ice-albedo feedback have been greatest in summer and early fall. To reconcile this, previous studies have hypothesized that fall/winter Arctic warming has been enhanced by increased oceanic heat loss but have not presented quantitative evidence. Here we show increases in heat transfer from the Arctic Ocean to the overlying atmosphere during October-January, 1989-2009. The trends in surface air temperature, sea ice concentration and the surface heat fluxes display remarkable spatial correspondence. The increased oceanic heat loss is likely a combination of the direct response to fall/winter sea ice loss, and the indirect response to summer sea ice loss and increased summer ocean heating.

The sea ice mass budget of the Arctic and its future change as simulated by coupled climate models

Abstract: Arctic sea ice mass budgets for the twentieth century and projected changes through the twenty-first century are assessed from 14 coupled global climate models. Large inter-model scatter in contemporary mass budgets is strongly related to variations in absorbed solar radiation, due in large part to differences in the surface albedo simulation. Over the twenty-first century, all models simulate a decrease in ice volume resulting from increased annual net melt (melt minus growth), partially compensated by reduced transport to lower latitudes. Despite this general agreement, the models vary considerably regarding the magnitude of ice volume loss and the relative roles of changing melt and growth in driving it. Projected changes in sea ice mass budgets depend in part on the initial (mid twentieth century) ice conditions; models with thicker initial ice generally exhibit larger volume losses. Pointing to the importance of evolving surface albedo and cloud properties, inter-model scatter in changing net ice melt is significantly related to changes in downwelling longwave and absorbed shortwave radiation. These factors, along with the simulated mean and spatial distribution of ice thickness, contribute to a large inter-model scatter in the projected onset of seasonally ice-free conditions.

Has the ozone hole contributed to increased Antarctic sea ice extent

Abstract: [1] Since the 1970s sea ice extent has decreased dramatically in the Northern Hemisphere and increased slightly in the Southern Hemisphere, a difference that is potentially explained by ozone depletion in the Southern Hemisphere stratosphere. In this study we consider the impact of stratospheric ozone depletion on Antarctic sea ice extent using a climate model forced with observed stratospheric ozone depletion from 1979 to 2005. Contrary to expectations, our model simulates a year-round decrease in Antarctic sea ice due to stratospheric ozone depletion. The largest percentage sea ice decrease in our model occurs in the austral summer near the coast of Antarctica, due to a mechanism involving offshore Ekman sea ice transport. The largest absolute decrease is simulated in the austral winter away from the coast of Antarctica, in response to an ocean warming that is consistent with a poleward shift of thc large-scale pattern of sea surface temperature. Our model results strongly suggest that processes not linked to stratospheric ozone depletion must be invoked to explain the observed increase in Antarctic sea ice extent.

Cryo‐hydrologic warming: A potential mechanism for rapid thermal response of ice sheets

Abstract: [1] Cryo-Hydrologic (CH) warming is proposed as a potential mechanism for rapid thermal response of glaciers and ice sheets to climate warming. We present a simple parameterization to incorporate CH warming in thermal models of ice sheets using a dual-continuum concept, which treats ice and the cryo-hydrologic system (CHS) as overlapping continua with heat exchange between them. The presence of liquid water in the CHS due to surface melt leads to warming of the ice. The magnitude and time-scale of CH warming is controlled by the average spacing between elements of the CHS, which is often of the order of just 10's of meters. The corresponding time-scale of thermal response is of the order of years-decades, in contrast to conventional estimates of thermal response time-scales based on vertical conduction through ice (∼102–3 m thick), which are of the order of centuries to millennia. We show that CH warming is already occurring along the west coast of Greenland. Increased temperatures resulting from CH warming will reduce ice viscosity and thus contribute to faster ice flow.

Modeling the impact of declining sea ice on the Arctic marine planktonic ecosystem

Abstract: [1] We have developed a coupled 3-D pan-Arctic biology/sea ice/ocean model to investigate the impact of declining Arctic sea ice on the marine planktonic ecosystem over 1988–2007 The biophysical model results agree with satellite observations of a generally downward trend in summer sea ice extent during 1988–2007, resulting in an increase in the simulated photosynthetically active radiation (PAR) at the ocean surface and marine primary productivity (PP) in the upper 100 m over open water areas of the Arctic Ocean The simulated Arctic sea ice thickness has decreased steadily during 1988–2007, leading to an increase in PAR and PP in sea ice-covered areas The simulated total PAR in all areas of the Arctic Ocean has increased by 43%, from 146 TW in 1988 to 209 TW in 2007; the corresponding total PP has increased by 50%, from 456 Tg C yr−1 in 1988 to 682 Tg C yr−1 in 2007 The simulated PAR and PP increases mainly occur in the seasonally and permanently ice-covered Arctic Ocean In addition to increasing PAR, the decline in sea ice tends to increase the nutrient availability in the euphotic zone by enhancing air-sea momentum transfer, leading to strengthened upwelling and mixing in the water column and therefore increased nutrient input into the upper ocean layers from below The increasing nutrient availability also contributes to the increase in the simulated PP, even though significant surface nutrient drawdown in summer is simulated In conjunction with increasing surface absorption of solar radiation and rising surface air temperature, the increasing surface water temperature in the Arctic Ocean peripheral seas further contributes to the increase in PP As PP has increased, so has the simulated biomass of phytoplankton and zooplankton

Influence of Arctic sea ice extent on polar cloud fraction and vertical structure and implications for regional climate

Abstract: Recent satellite lidar measurements of cloud properties spanning a period of five years are used to examine a possible connection between Arctic sea ice amount and polar cloud fraction and vertical distribution. We find an anti-correlation between sea ice extent and cloud fraction with maximum cloudiness occurring over areas with little or no sea ice. We also find that over ice free regions, there is greater low cloud frequency and average optical depth. Most of the optical depth increase is due to the presence of geometrically thicker clouds over water. In addition, our analysis indicates that over the last 5 years, October and March average polar cloud fraction has increased by about 7 and 10 percent, respectively, as year average sea ice extent has decreased by 5 to 7 percent. The observed cloud changes are likely due to a number of effects including, but not limited to, the observed decrease in sea ice extent and thickness. Increasing cloud amount and changes in vertical distribution and optical properties have the potential to affect the radiative balance of the Arctic region by decreasing both the upwelling terrestrial longwave radiation and the downward shortwave solar radiation. Since longwave radiation dominates in the long polar winter, the overall effect of increasing low cloud cover is likely a warming of the Arctic and thus a positive climate feedback, possibly accelerating the melting of Arctic sea ice.

A new projection of sea level change in response to collapse of marine sectors of the Antarctic Ice Sheet

Abstract: We present gravitationally self-consistent predictions of sea level change that would follow the disappearance of either the West Antarctic Ice Sheet (WAIS) or marine sectors of the East Antarctic Ice Sheet (EAIS). Our predictions are based on a state-of-the-art pseudo-spectral sea level algorithm that incorporates deformational, gravitational and rotational effects on sea level, as well as the migration of shorelines due to both local sea-level variations and changes in the extent of marine-based ice cover. If we define the effective eustatic value (EEV) as the geographically uniform rise in sea level once all marine-based sectors have been filled with water, then we find that some locations can experience a sea level rise that is ∼40 per cent higher than the EEV. This enhancement is due to the migration of water away from the zone of melting in response to the loss of gravitational attraction towards the ice sheet (load self-attraction), the expulsion of water from marine areas as these regions rebound due to the unloading, and the feedback into sea level of a contemporaneous perturbation in Earth rotation. In the WAIS case, this peak enhancement is twice the value predicted in a previous projection that did not include expulsion of water from exposed marine-sectors of the West Antarctic or rotational feedback. The peak enhancements occur over the coasts of the United States and in the Indian Ocean in the WAIS melt scenario, and over the south Atlantic and northwest Pacific in the EAIS scenario. We conclude that accurate projections of the sea level hazard associated with ongoing global warming should be based on a theory that includes the complete suite of physical processes described above.

Temperature differences between the hemispheres and ice age climate variability

Abstract: [1] The Earth became warmer and cooler during the ice ages along with changes in the Earth’s orbit, but the orbital changes themselves are not nearly large enough to explain the magnitude of the warming and cooling. Atmospheric CO2 also rose and fell, but again, the CO2 changes are rather small in relation to the warming and cooling. So, how did the Earth manage to warm and cool by so much? Here we argue that, for the big transitions at least, the Earth did not warm and cool as a single entity. Rather, the south warmed instead at the expense of a cooler north through massive redistributions of heat that were set off by the orbital forcing. Oceanic CO2 was vented up to the atmosphere by the same redistributions. The north then warmed later in response to higher CO2 and a reduced albedo from smaller ice sheets. This form of north‐ south displacement is actually very familiar, as it is readily observed during the Younger Dryas interval 13,000 years ago and in the various millennial‐scale events over the last 90,000 years.

Sea ice induced changes in ocean circulation during the Eemian

Abstract: We argue that Arctic sea ice played an important role during early stages of the last glacial inception. Two simulations of the Institut Pierre Simon Laplace coupled model 4 are analyzed, one for the time of maximum high latitude summer insolation during the last interglacial, the Eemian, and a second one for the subsequent summer insolation minimum, at the last glacial inception. During the inception, increased Arctic freshwater export by sea ice shuts down Labrador Sea convection and weakens overturning circulation and oceanic heat transport by 27 and 15%, respectively. A positive feedback of the Atlantic subpolar gyre enhances the initial freshening by sea ice. The reorganization of the subpolar surface circulation, however, makes the Atlantic inflow more saline and thereby maintains deep convection in the Nordic Seas. These results highlight the importance of an accurate representation of dynamic sea ice for the study of past and future climate changes.

Sensitivity of arctic summer sea ice coverage to global warming forcing: towards reducing uncertainty in arctic climate change projections

Abstract: Substantial uncertainties have emerged in Arctic climate change projections by the fourth Intergovernmental Panel on Climate Change assessment report climate models. In particular, the models as a group considerably underestimate the recent accelerating sea ice reduction. To better understand the uncertainties, we evaluated sensitivities of summer sea ice coverage to global warming forcing in models and observations. The result suggests that the uncertainties result from the large range of sensitivities involved in the computation of sea ice mass balance by the climate models, specifically with the changes in sea ice area (SIA) ranging from 0.09 × 10 6 to −1.23 × 10 6 km 2 in response to 1.0 K increase of air temperature. The sensitivities also vary largely across ensemble members in the same model, indicating impacts of initial condition on evolution of feedback strength with model integrations. Through observationally constraining, the selected model runs by the sensitivity analysis well captured the observed changes in SIA and surface air temperatures and greatly reduced their future projection uncertainties to a certain range from the currently announced one. The projected ice-free summer Arctic Ocean may occur as early as in the late 2030s using a criterion of 80% SIA loss and the Arctic regional mean surface air temperature will be likely increased by 8.5 ± 2.5 °C in winter and 3.7 ± 0.9 °C in summer by the end of this century.

Anthropogenic land cover changes in a GCM with surface albedo changes based on MODIS data

Abstract: This study uses a global climate model (GCM) to investigate the climate response at the surface and in the atmosphere caused by land use change. The climate simulations are performed with the National Center for Atmospheric Research Community Land Model 3.5 (CLM3.5) coupled to the Community Atmosphere Model 3 (CAM3) and a slab ocean model. We use the Moderate Resolution Imaging Spectroradiometer (MODIS) surface albedo product to represent surface albedo in the CLM3.5 for both present day and to reconstruct the surface albedo for natural pre-agriculture conditions. We compare simulations including vegetation changes and surface albedo changes to simulations including only surface albedo changes. We find that the surface albedo change is most dominant in temperate regions while the change in evapotranspiration drives the climate response in the tropics. Our results show that land cover changes contribute to an annual global warming of 0.04 K, but there are large regional differences. In North America and Europe, the surface temperatures decrease by − 0.11 and − 0.09 K, respectively, while in India the surface temperatures increase by 0.09 K. When we fix the vegetation cover in the simulations and let the climate changes be driven only by the differences in surface albedo, the annual global mean surface warming is reduced, and all three regions are now associated with surface cooling. We also show that the surface albedo value for cropland is of major importance in climate simulations of land cover change. The surface albedo effect is the main driving mechanism when the change in surface albedo between agricultural and natural vegetation is substantial. Finally, we argue that differences in the surface albedo value of cropland implemented in earlier land use change studies explain the diversity in the sign and magnitude of the climate response. Copyright © 2009 Royal Meteorological Society

New Unified Sea Ice Thickness Climate Data Record

Abstract: With the recent dramatic record-low ice extent of 2007 and with the third-lowest extent having been recorded in 2010, the changing Arctic climate, and particularly the rapidly changing sea ice cover, is often in the news. The climate models of the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report forecast that rising Arctic temperatures and the reduction of sea ice will be the earliest and strongest indications of global warming. However, these models generally underestimate the observed rate of change in summer ice cover over the past 3 decades [Stroeve et al., 2007].

Solar radiation and ice melting in Lake Vendyurskoe, Russian Karelia

Abstract: Field experiments were conducted during two melting periods, April 2006 and April 2007, in Lake Vendyurskoe. The observation programme included weather, ice and snow thickness and structure, water temperature and solar radiation transfer through the ice. Albedo showed a systematic decrease from 0.5–0.8 for wintertime dry ice and snow to 0.1 for wet bare ice in spring, with spatial standard deviation of about 10%. The e-folding depth of light level was 60–80 cm for congelation ice and 15 cm for snow-ice. Light transmissivity of the ice cover increased from melting but decreased from ice deterioration; it varied between 0.25–0.35 in 2006, while in 2007 there was a systematic trend from 0.1 to 0.5 in six days. The heat budget was governed by net solar radiation with daily peaks up to 400–500 W m−2 on clear days. The average daily melt was 1.2 cm at the surface, 0.5 cm at the bottom and 1–2 cm (thickness equivalent) in the interior.

Effects of sea ice on atmospheric pCO2: A revised view and implications for glacial and future climates

Abstract: [1] Sea ice is a key component in the global carbon cycle and climate system. In the traditional view, the sole effect of expanded sea ice coverage is to reduce the atmospheric pCO2 by inhibiting air-sea gas exchange. However, this view neglects the effect that sea ice capping has on the biological production. By limiting light for photosynthesis, larger sea ice coverage would reduce the strength of the biological pump and therefore increase atmospheric pCO2. Recently, Kurahashi-Nakamura et al. (2007) suggested that the opposing impact of biology on atmospheric pCO2 will more than offset the gas exchange effect, such that atmospheric pCO2 will actually increase with larger sea ice coverage. In an effort to resolve this controversy, we use an intermediate-complexity, global model of biogeochemistry and climate to determine the sensitivity of atmospheric CO2 concentration to changes in the sea ice coverage, driven by prescribed changes in sea ice albedo. When sea ice in our model is increased by 34% globally relative to the control run, gas solubility, ice capping effect and stratification increase, while biological production decreases; overall atmospheric pCO2 is reduced by 9.4 ppmv. Our results broadly support the notion that the biological response of sea ice capping is as important as its physical response. Furthermore, we show that the overall change in atmospheric pCO2 is indeed inversely related to sea ice coverage, but it is not because sea ice caps off gas exchange but because gas solubility is increased by lower temperatures that accompany sea ice expansion in our model simulations.

The Importance of Ice Vertical Resolution for Snowball Climate and Deglaciation

Abstract: Sea ice schemes with a few vertical levels are typically used to simulate the thermodynamic evolution of sea ice in global climate models. Here it is shown that these schemes overestimate the magnitude of the diurnal surface temperature cycle by a factor of 2–3 when they are used to simulate tropical ice in a Snowball earth event. This could strongly influence our understanding of Snowball termination, which occurs in global climate models when the midday surface temperature in the tropics reaches the melting point. A hierarchy of models is used to show that accurate simulation of surface temperature variation on a given time scale requires that a sea ice model resolve the e-folding depth to which a periodic signal on that time scale penetrates. This is used to suggest modifications to the sea ice schemes used in global climate models that would allow more accurate simulation of Snowball deglaciation.

Notes on a Catastrophe: A Feedback Analysis of Snowball Earth

Abstract: The language of feedbacks is ubiquitous in contemporary earth sciences, and the framework of feedback analysis is a powerful tool for diagnosing the relative strengths of the myriad mutual interactions that occur in complex dynamical systems. The ice albedo feedback is widely taught as the classic example of a climate feedback. Moreover, its potential to initiate a collapse to a completely glaciated snowball earth is widely taught as the classic example of a climate “tipping point.” A feedback analysis of the snowball earth phenomenon in simple, zonal mean energy balance models clearly reveals the physics of the snowball instability and its dependence on climate parameters. The analysis can also be used to illustrate some fundamental properties of climate feedbacks: how feedback strength changes as a function of mean climate state; how small changes in individual feedbacks can cause large changes in the system sensitivity; and last, how the strength and even the sign of the feedback is dependent ...

Cryospheric Contributions to Sea‐Level Rise and Variability

Abstract: Global mean sea level rose by ~1.8 mm/yr over the last 50 years, increasing to ~3.1 mm/yr during the 1990s (Church et al, 2004, Holgate and Woodworth, 2004, Cazenave and Nerem, 2004). Thermal expansion of ocean water is estimated to account for 0.4 mm/yr of sea level rise (SLR) for the past 4-5 decades rising to 1.5 mm/yr during the last decade (Levitus et al., 2005, Ishii et al., 2006, Willis et al., 2004, Lombard et al., 2006), Contributions from water on land are probably very small, with sequestration by dams more or less balanced by release of groundwater, but uncertainties are large (Cazenave et al., this Workshop). The most important source of the remainder is likely to be land ice which, if it were all to melt, would cause >60 meters SLR. Small glaciers and ice caps, including those around Greenland and Antarctica, represent ~ 1% of this, with 11% in Greenland, and 88% in Antarctica. Glaciers in most mountain regions are known to be retreating, and a recent assessment, using the spatially limited in-situ measurements and a statistical method of global area weighting of known ice masses, showed the contribution from ice caps and glaciers at 0.3-0.45 mm/yr SLR over the last 100 years rising to 0.8 mm/yr over the last decade (Dyurgerov and Meier, 2005).

Sources of spread in simulations of Arctic sea ice loss over the twenty-first century.

Abstract: We show that intermodel variations in the anthropogenically-forced evolution of September sea ice extent (SSIE) in the Arctic stem mainly from two factors: the baseline climatological sea ice thickness (SIT) distribution, and the local climate feedback parameter. The roles of these two factors evolve over the course of the twenty-first century. The SIT distribution is the most important factor in current trends and those of coming decades, accounting for roughly half the intermodel variations in SSIE trends. Then, its role progressively decreases, so that around the middle of the twenty-first century the local climate feedback parameter becomes the dominant factor. Through this analysis, we identify the investments in improved simulation of Arctic climate necessary to reduce uncertainties both in projections of sea ice loss over the coming decades and in the ultimate fate of the ice pack.

Modeled winter sea ice variability and the North Atlantic Oscillation: a multi-century perspective

Abstract: The relationship between winter sea ice vari- ability and the North Atlantic Oscillation (NAO) is examined for the time period 1860-2300. This study uses model output to extend recently reported observational results to multi- century time scales. Nine ensemble members are used in two Global Climate Models with forcing evolving from pre- industrial conditions through the so-called A1B scenario in which carbon dioxide stabilizes at 720 ppm by 2100. Throughout, the NAO generates an east-west dipole pattern of sea ice concentration (SIC) anomalies with oppositely signed centers of action over the Labrador and Barents Seas. During the positive polarity of the NAO, SIC increases over the Labrador Sea due to wind-driven equatorward advection of ice, and SIC decreases over the Barents Sea due to wind- driven poleward transport of heat within the mixed layer of the ocean. Although this NAO-driven SIC variability pattern can always be detected, it accounts for a markedly varying fraction of the total sea ice variability depending on the strength of the forced sea ice extent trend. For the first half of the 20th century or 1990 control conditions, the NAO-driven SIC pattern accounts for almost a third of the total SIC var- iance. In the context of the long term winter sea ice retreat from 1860 to 2300, the NAO-driven SIC pattern is robustly observable, but accounts for only 2% of the total SIC vari- ance. The NAO-driven SIC dipole retreats poleward with the retreating marginal ice zone, and its Barents Sea center of action weakens. Results presented here underscore the idea that the NAO's influence on Arctic climate is robustly observable, but time dependent in its form and statistical importance.

Arctic sea ice response to atmospheric forcings with varying levels of anthropogenic warming and climate variability

Abstract: [1] Numerical experiments are conducted to project arctic sea ice responses to varying levels of future anthropogenic warming and climate variability over 2010–2050. A summer ice-free Arctic Ocean is likely by the mid-2040s if arctic surface air temperature (SAT) increases 4°C by 2050 and climate variability is similar to the past relatively warm two decades. If such a SAT increase is reduced by one-half or if a future Arctic experiences a range of SAT fluctuation similar to the past five decades, a summer ice-free Arctic Ocean would be unlikely before 2050. If SAT increases 4°C by 2050, summer ice volume decreases to very low levels (10–37% of the 1978–2009 summer mean) as early as 2025 and remains low in the following years, while summer ice extent continues to fluctuate annually. Summer ice volume may be more sensitive to warming while summer ice extent more sensitive to climate variability. The rate of annual mean ice volume decrease relaxes approaching 2050. This is because, while increasing SAT increases summer ice melt, a thinner ice cover increases winter ice growth. A thinner ice cover also results in a reduced ice export, which helps to further slow ice volume loss. Because of enhanced winter ice growth, arctic winter ice extent remains nearly stable and therefore appears to be a less sensitive climate indicator.

On the mystery of the perennial carbon dioxide cap at the south pole of Mars

Abstract: A perennial ice cap has long been observed near the south pole of Mars. The surface of this cap is predominantly composed of carbon dioxide ice. The retention of a CO_2 ice cap results from the surface energy balance of the latent heat, solar radiation, surface emission, subsurface conduction, and atmospheric sensible heat. While models conventionally treat surface CO_2 ice using constant ice albedos and emissivities, such an approach fails to predict the existence of a perennial cap. Here we explore the role of the insolation-dependent ice albedo, which agrees well with Viking, Mars Global Surveyor, and Mars Express albedo observations. Using a simple parameterization within a general circulation model, in which the albedo of CO_2 ice responds linearly to the incident solar insolation, we are able to predict the existence of a perennial CO_2 cap at the observed latitude and only in the southern hemisphere. Further experiments with different total CO_2 inventories, planetary obliquities, and surface boundary conditions suggest that the location of the residual cap may exchange hemispheres favoring the pole with the highest peak insolation.

Sea ice sensitivity to the parameterisation of open water area

Abstract: Based on version 2 of the Louvain-la-Neuve sea Ice Model (LIM2), sensitivity experiments reveal simple relations between ice conditions and the characteristic thickness parameter ho that appears in...