Showing papers on "Ice-albedo feedback published in 2007"
TL;DR: In this paper, a multi-model ensemble mean time series provides a true representation of forced change by greenhouse gas (GHG) loading, 33-38% of the observed September trend from 1953-2006 is externally forced, growing to 47-57% from 1979-2006.
Abstract: [1] From 1953 to 2006, Arctic sea ice extent at the end of the melt season in September has declined sharply. All models participating in the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4) show declining Arctic ice cover over this period. However, depending on the time window for analysis, none or very few individual model simulations show trends comparable to observations. If the multi-model ensemble mean time series provides a true representation of forced change by greenhouse gas (GHG) loading, 33–38% of the observed September trend from 1953–2006 is externally forced, growing to 47–57% from 1979–2006. Given evidence that as a group, the models underestimate the GHG response, the externally forced component may be larger. While both observed and modeled Antarctic winter trends are small, comparisons for summer are confounded by generally poor model performance.
1,536 citations
TL;DR: In this article, an exact method is presented for calculating the changes in sea level that occur when ice and water masses are rearranged on the surface of elastic and viscoelastic non-rotating Earth models.
Abstract: Summary An exact method is presented for calculating the changes in sea level that occur when ice and water masses are rearranged on the surface of elastic and viscoelastic non-rotating Earth models. The method is used to calculate the instantaneous elastic and delayed vi scoelastic sea level changes following the partial melting of late Quaternary ice sheets. We find that there can be large errors in the usual assumption that changes in sea level are uniform over the ocean basins. If a quantity of ice equivalent to a uniform 100-m rise in sea level melts from the Laurentide and Fennoscandian ice sheets, then in the South Pacific the instantaneous rise in sea level can be as large as 120m. In the North Atlantic the instantaneous rise is always less than 100 m. There is a zone in the North Atlantic with almost no sea level change and near Greenland and Norway the sea level falls, rather than rises, by over 100 m. One thousand years after the melting a forebulge migrating towards the ice loads causes water to flow from the South Pacific into the North Pacific suggesting that raised beaches should occur in the South Pacific. The gravitational attraction of an ice mass upon a nearby ocean tends to hold sea level high in the vicinity of the ice. This extra load near the ice may have a significant influence on postglacial isostatic adjustment.
828 citations
TL;DR: In this article, satellite-derived estimates of sea-ice age and thickness are combined to produce a proxy ice thickness record for 1982 to the present, showing that in addition to the well-documented loss of perennial ice cover as a whole, the amount of oldest and thickest ice within the remaining multiyear ice pack has declined significantly.
Abstract: [1] Satellite-derived estimates of sea-ice age and thickness are combined to produce a proxy ice thickness record for 1982 to the present. These data show that in addition to the well-documented loss of perennial ice cover as a whole, the amount of oldest and thickest ice within the remaining multiyear ice pack has declined significantly. The oldest ice types have essentially disappeared, and 58% of the multiyear ice now consists of relatively young 2- and 3-year-old ice compared to 35% in the mid-1980s. Ice coverage in summer 2007 reached a record minimum, with ice extent declining by 42% compared to conditions in the 1980s. The much-reduced extent of the oldest and thickest ice, in combination with other factors such as ice transport that assist the ice-albedo feedback by exposing more open water, help explain this large and abrupt ice loss.
703 citations
TL;DR: The contribution of these smaller glaciers has accelerated over the past decade, in part due to marked thinning and retreat of marine-terminating glaciers associated with a dynamic instability that is generally not considered in mass-balance and climate modeling.
Abstract: Ice loss to the sea currently accounts for virtually all of the sea-level rise that is not attributable to ocean warming, and about 60% of the ice loss is from glaciers and ice caps rather than from the two ice sheets. The contribution of these smaller glaciers has accelerated over the past decade, in part due to marked thinning and retreat of marine-terminating glaciers associated with a dynamic instability that is generally not considered in mass-balance and climate modeling. This acceleration of glacier melt may cause 0.1 to 0.25 meter of additional sea-level rise by 2100.
638 citations
TL;DR: In this article, an earlier start of the positively stratified season significantly increases the period over which the lake warms during the summer months, leading to a stronger trend in mean summer temperatures than would be expected from changes in summer air temperature alone.
Abstract: [1] Lake Superior summer (July–September) surface water temperatures have increased approximately 2.5°C over the interval 1979–2006, equivalent to a rate of (11 ± 6) × 10−2°C yr−1, significantly in excess of regional atmospheric warming. This discrepancy is caused by declining winter ice cover, which is causing the onset of the positively stratified season to occur earlier at a rate of roughly a half day per year. An earlier start of the stratified season significantly increases the period over which the lake warms during the summer months, leading to a stronger trend in mean summer temperatures than would be expected from changes in summer air temperature alone.
473 citations
TL;DR: In this article, the amount of solar energy absorbed in areas of open water in the Arctic Basin has varied spatially and temporally over the past few decades, and the largest increases in total yearly solar heat input, as much as 4% per year, occurred in the Chukchi Sea and adjacent areas.
Abstract: [1] Over the past few decades the Arctic sea ice cover has decreased in areal extent. This has altered the solar radiation forcing on the Arctic atmosphere-ice-ocean system by decreasing the surface albedo and allowing more solar heating of the upper ocean. This study addresses how the amount of solar energy absorbed in areas of open water in the Arctic Basin has varied spatially and temporally over the past few decades. A synthetic approach was taken, combining satellite-derived ice concentrations, incident irradiances determined from reanalysis products, and field observations of ocean albedo over the Arctic Ocean and the adjacent seas. Results indicate an increase in the solar energy deposited in the upper ocean over the past few decades in 89% of the region studied. The largest increases in total yearly solar heat input, as much as 4% per year, occurred in the Chukchi Sea and adjacent areas.
427 citations
TL;DR: In this article, a global multicategory thickness and enthalpy distribution sea ice model coupled with an ocean model is used to estimate the extent of sea ice in the Southern Ocean.
Abstract: Estimates of sea ice extent based on satellite observations show an increasing Antarctic sea ice cover from 1979 to 2004 even though in situ observations show a prevailing warming trend in both the atmosphere and the ocean. This riddle is explored here using a global multicategory thickness and enthalpy distribution sea ice model coupled to an ocean model. Forced by the NCEP–NCAR reanalysis data, the model simulates an increase of 0.20 10 12 m 3 yr 1 (1.0% yr 1 ) in total Antarctic sea ice volume and 0.084 10 12 m 2 yr 1 (0.6% yr 1 ) in sea ice extent from 1979 to 2004 when the satellite observations show an increase of 0.027 10 12 m 2 yr 1 (0.2% yr 1 ) in sea ice extent during the same period. The model shows that an increase in surface air temperature and downward longwave radiation results in an increase in the upper-ocean temperature and a decrease in sea ice growth, leading to a decrease in salt rejection from ice, in the upper-ocean salinity, and in the upper-ocean density. The reduced salt rejection and upper-ocean density and the enhanced thermohaline stratification tend to suppress convective overturning, leading to a decrease in the upward ocean heat transport and the ocean heat flux available to melt sea ice. The ice melting from ocean heat flux decreases faster than the ice growth does in the weakly stratified Southern Ocean, leading to an increase in the net ice production and hence an increase in ice mass. This mechanism is the main reason why the Antarctic sea ice has increased in spite of warming conditions both above and below during the period 1979–2004 and the extended period 1948–2004.
218 citations
TL;DR: In this paper, a mismatch between the AO index and loss of ice can be explained by the frequency of three main sea level pressure (SLP) patterns that yield overall variability in SLP, rather than the presence of a single, coherent physical pattern of SLP reduction associated with the positive mode of AO.
Abstract: [1] The reduction in ice cover observed in the late 1980s and early 1990s has been attributed to the strongly positive Arctic Oscillation (AO) phase during that time. However, despite a change in the AO to more neutral conditions since then, ice extent and the fraction of old ice have continued to decrease. This mismatch between the AO index and loss of ice can be explained by the frequency of three main sea level pressure (SLP) patterns that yield overall variability in SLP, rather than the presence of a single, coherent physical pattern of SLP reduction associated with the positive mode of the AO. These three patterns were in phase during the peak AO period but their frequency has varied differently since then, with two of the patterns continuing to contribute to reduced ice cover in the western Arctic. Hence, regional atmospheric circulation remains a significant factor in recent reductions in ice cover.
203 citations
TL;DR: For example, satellite passive microwave observations from 1979 through 2005 now indicate a significant -8.4±1.5% decade-1 trend (99% confidence level) in September sea ice extent, a larger trend than earlier estimates due to acceleration of the decline over the past 41 years as discussed by the authors.
Abstract: The Arctic sea ice has been pointed to as one of the first and clearest indicators of climate change. Satellite passive microwave observations from 1979 through 2005 now indicate a significant –8.4±1.5% decade–1 trend (99% confidence level) in September sea-ice extent, a larger trend than earlier estimates due to acceleration of the decline over the past 41 years. There are differences in regional trends, with some regions more stable than others; not all regional trends are significant. The largest trends tend to occur in months where melt is at or near its peak for a given region. A longer time series of September extents since 1953 was adjusted to correct biases and extended through 2005. The trend from the longer time series is –7.7±0.6% decade–1 (99%), slightly less than from the satellite-derived data that begin in 1979, which is expected given the recent acceleration in the decline.
198 citations
TL;DR: Sedimentation filling space beneath ice shelves helps to stabilize ice sheets against grounding-line retreat in response to a rise in relative sea level, but smaller sea-level changes are unlikely to have synchronized the behavior of ice sheets in the past.
Abstract: Sedimentation filling space beneath ice shelves helps to stabilize ice sheets against grounding-line retreat in response to a rise in relative sea level of at least several meters. Recent Antarctic changes thus cannot be attributed to sea-level rise, strengthening earlier interpretations that warming has driven ice-sheet mass loss. Large sea-level rise, such as the ≈100-meter rise at the end of the last ice age, may overwhelm the stabilizing feedback from sedimentation, but smaller sea-level changes are unlikely to have synchronized the behavior of ice sheets in the past.
191 citations
TL;DR: In this paper, a high-resolution General Circulation Model (GCM) under different conditions at the Last Glacial Maximum was used to analyze the feedback processes between the ice sheet and the atmosphere.
Abstract: . The ice sheet-climate interaction as well as the climatic response to orbital parameters and atmospheric CO2 concentration are examined in order to drive an ice sheet model throughout an ice age cycle. Feedback processes between ice sheet and atmosphere are analyzed by numerical experiments using a high resolution General Circulation Model (GCM) under different conditions at the Last Glacial Maximum. Among the proposed processes, the ice albedo feedback, the elevation-mass balance feedback and the desertification effect over the ice sheet were found to be the dominant processes for the ice-sheet mass balance. For the elevation-mass balance feedback, the temperature lapse rate over the ice sheet is proposed to be weaker than assumed in previous studies. Within the plausible range of parameters related to these processes, the ice sheet response to the orbital parameters and atmospheric CO2 concentration for the last glacial/interglacial cycle was simulated in terms of both ice volume and geographical distribution, using a three-dimensional ice-sheet model. Careful treatment of climate-ice sheet feedback is essential for a reliable simulation of the ice sheet changes during ice age cycles.
TL;DR: In this article, the authors examined the relationship between atmospheric zonal wave three and Antarctic sea ice variability using an index of Zonal Wave Three and sea ice concentration and found that ZW forces an alternating pattern of equatorward (colder) and poleward (warmer) flow which influences the temperature difference between the atmosphere and the ocean.
Abstract: [1] The study examines the relationship between atmospheric zonal wave three and Antarctic sea ice variability using an index of zonal wave three and sea ice concentration. The net sensible heat flux and the surface air temperature are used to explain the apparent atmosphere-sea ice interaction. The results show that zonal wave three forces an alternating pattern of equatorward (colder) and poleward (warmer) flow which influences the temperature difference between the atmosphere and the ocean. The net sensible heat flux is positive, ocean heat loss is greater, and sea ice growth and expansion is greater in regions of colder air. The reverse is true in regions of warmer air. This influence of zonal wave three on sea ice concentration appears greatest in the southern fall and early winter. It is most clearly seen in the Ross and Weddell Seas outflows and off the Amery iceshelf.
TL;DR: For example, Dyurgerov and Meier as mentioned in this paper estimated that global sea level has risen by about 2 millimeters per year over the past century, about half of which is attributed to thermal expansion of the ocean and melting of temperate-latitude glaciers.
Abstract: Low-elevation land areas and their populations are at risk globally from rising sea level. Global sea level has risen by about 2 millimeters per year over the past century. About half of this rise may be attributed to thermal expansion of the ocean and the melting of temperate-latitude glaciers [Dyurgerov and Meier, 1997]. The remainder of the rise is believed to come from a net loss of mass from the Antarctic and Greenland ice sheets, although the exact contribution is unknown.
TL;DR: In this article, a Mars global circulation model has been used to establish whether these changes contribute to climate change, and the answer is yes: these changes caused elevated air temperatures, increased wind stresses and dust devil production, creating a positive feedback loop between dust erosion and albedo.
Abstract: The surface albedo patterns on Mars, caused by local variation in the ratio of light reflected to light received, are constantly changing. A Mars global circulation model has been used to establish whether these changes contribute to climate change, and the answer is yes. Large swaths of the martian surface have darkened over the past three decades as they were swept free of dust. Climate modelling indicates that these changes caused elevated air temperatures, increased wind stresses and 'dust devil' production, creating a positive feedback loop between dust erosion and albedo. These conditions are consistent with observed polar cap erosion, and may even influence the triggering of large dust storms. For hundreds of years, scientists have tracked the changing appearance of Mars, first by hand drawings and later by photographs1,2. Because of this historical record, many classical albedo patterns have long been known to shift in appearance over time. Decadal variations of the martian surface albedo are generally attributed to removal and deposition of small amounts of relatively bright dust on the surface. Large swaths of the surface (up to 56 million km2) have been observed to darken or brighten by 10 per cent or more3,4,5. It is unknown, however, how these albedo changes affect wind circulation, dust transport and the feedback between these processes and the martian climate. Here we present predictions from a Mars general circulation model, indicating that the observed interannual albedo alterations strongly influence the martian environment. Results indicate enhanced wind stress in recently darkened areas and decreased wind stress in brightened areas, producing a positive feedback system in which the albedo changes strengthen the winds that generate the changes. The simulations also predict a net annual global warming of surface air temperatures by ∼0.65 K, enhancing dust lifting by increasing the likelihood of dust devil generation. The increase in global dust lifting by both wind stress and dust devils may affect the mechanisms that trigger large dust storm initiation, a poorly understood phenomenon, unique to Mars. In addition, predicted increases in summertime air temperatures at high southern latitudes would contribute to the rapid and steady scarp retreat that has been observed in the south polar residual ice for the past four Mars years6,7,8. Our results suggest that documented albedo changes affect recent climate change and large-scale weather patterns on Mars, and thus albedo variations are a necessary component of future atmospheric and climate studies.
TL;DR: In this article, the remarkably low Arctic multi-year (my) sea ice coverage following the summer of 2005 is placed in the context of its variability over the past seven years (2000-2006).
Abstract: [1] The remarkably low Arctic multiyear (MY) sea ice coverage following the summer of 2005 is placed in the context of its variability over the past seven years (2000–2006). Annual cycles of MY ice coverage, from QuikSCAT and satellite passive ice motion, show that the replenishment of MY ice area at the end of this summer is near zero (0.1 × 106 km2) compared to the previous five summers of 1.0, 1.2, 0.4, 0.4, and 0.9 × 106 km2. This is examined in terms of anomalies in ice export and the record of freezing (FDD) and melting degree-days (MDD). The 2005 summer (Jun–Sep) saw the highest Fram Strait ice export (>0.25 × 106 km2) compared to the 7-year mean of 0.14 × 106 km2. This directly explains ∼40% of the decrease in MY coverage of 0.6 × 106 km2 between Jan 2005 and Jan 2006. The cumulative effects of the recent warmer winters and summers, relative to the longer-term record since 1958, explain the balance. For this short record, the combination of spatially averaged FDD and MDD anomalies of the preceding year explain ∼63% of the variance in the replenishment areas.
TL;DR: In this article, a model for the formation and evolution of the melt pond cover is proposed, which is compatible with existing GCM sea ice models, based upon the existing theory of the sea ice thickness distribution.
Abstract: [1] During the Northern Hemisphere summer, absorbed solar radiation melts snow and the upper surface of Arctic sea ice to generate meltwater that accumulates in ponds. The melt ponds reduce the albedo of the sea ice cover during the melting season, with a significant impact on the heat and mass budget of the sea ice and the upper ocean. We have developed a model, designed to be suitable for inclusion into a global circulation model (GCM), which simulates the formation and evolution of the melt pond cover. In order to be compatible with existing GCM sea ice models, our melt pond model builds upon the existing theory of the evolution of the sea ice thickness distribution. Since this theory does not describe the topography of the ice cover, which is crucial to determining the location, extent, and depth of individual ponds, we have needed to introduce some assumptions. We describe our model, present calculations and a sensitivity analysis, and discuss our results.
TL;DR: In this article, a polynomial temperature-dependent scheme was proposed to simulate the annual cycle of the snow and sea ice albedo in the HIRHAM regional atmospheric model.
Abstract: [1] The HIRHAM snow and sea ice albedo scheme and several other existing snow and sea ice albedo parameterizations forced with observed input parameters are compared with observed albedo. For snow on land in non-forested areas, the original linear temperature-dependent snow albedo is suggested to be replaced with a polynomial temperature-dependent scheme. For sea ice albedo none of the evaluated schemes manage to simulate the annual cycle successfully. A suggestion of a new sea ice albedo including the effects of melt ponds, snow on the sea ice and the surface temperature is presented. Simulations with original and new snow and sea ice albedo are performed in the regional atmospheric model HIRHAM and the results are compared. Compared with ERA40 the control simulation with original surface albedo reveals a warm bias during spring in the Arctic. Changing the surface albedo, the biggest differences are found in the same period. Model simulations with old and new surface albedo in HIRHAM clearly reveal that the new albedo scheme is superior to the currently implemented scheme in reproducing the ERA40 temperature climatology. In these experiments the new snow albedo scheme has less impact than the new sea ice albedo. This is probably because areas with changed snow albedo have smaller extent than areas with sea ice in the model setup and are more constraint by the lateral boundaries.
TL;DR: In this paper, the authors proposed a physical mechanism for this phenomenon based on an analysis of additional simulations with the coupled climate models CLIMBER-2 and CLIMber-3α.
Abstract: An increase in atmospheric CO2 concentration and the resulting global warming are typically associated with a weakening of the thermohaline circulation (THC) in model scenarios. For the models participating in the Coupled Model Intercomparison Project (CMIP), this weakening shows a significant (r = 0.62) dependence on the initial THC strength; it is stronger for initially strong overturning. The authors propose a physical mechanism for this phenomenon based on an analysis of additional simulations with the coupled climate models CLIMBER-2 and CLIMBER-3α. The mechanism is based on the fact that sea ice cover greatly reduces heat loss from the ocean. The extent of sea ice is strongly influenced by the near-surface atmospheric temperature (SAT) in the North Atlantic but also by the strength of the THC itself, which transports heat to the convection sites. Consequently, sea ice tends to extend farther south for weaker THC. Initially larger sea ice cover responds more strongly to atmospheric warming; ...
TL;DR: In this article, a series of sensitivity experiments using a coupled regional atmosphere-ocean-ice model of the Arctic has been conducted in order to identify the requirements needed to reproduce observed sea-ice conditions and to address uncertainties in the description of Arctic processes.
Abstract: [1] A series of sensitivity experiments using a coupled regional atmosphere-ocean-ice model of the Arctic has been conducted in order to identify the requirements needed to reproduce observed sea-ice conditions and to address uncertainties in the description of Arctic processes. The ability of the coupled model to reproduce observed summer ice retreat depends largely on a quasi-realistic ice volume at the beginning of the melting period, determined by the relationship between winter growth and summer decay of ice. While summer ice decay is strongly affected by the parameterization of the sea-ice albedo, winter ice growth depends significantly on the parameterization of lateral freezing. Reciprocal model biases due to uncertainties in the atmospheric energy fluxes can be compensated to a certain extent. However, potential underlying weaknesses of the model cannot be eliminated that way. Since lateral freezing also determines the ice concentration during winter, and thus the heat loss of the ocean and the near-surface air temperature, the model tuning possibilities are limited. A large uncertainty in the model relates to the simulation of long-wave radiation most likely as a result of overestimated cloud cover. The results suggest that uncertainties in the descriptions for Arctic clouds, snow, and sea-ice albedo, and lateral freezing and melting of sea ice, including the treatment of snow, are responsible for large deviations in the simulation of Arctic sea ice in coupled models. Improved descriptions of these processes are needed to reduce model biases and to enhance the credibility of future climate change projections.
TL;DR: In this paper, the mass balance of Austfonna, Svalbard's largest ice cap, was investigated by combining surface velocities measured using satellite radar interferometry with ice thicknesses derived from radio-echo sounding.
Abstract: Determining whether increasing temperature or precipitation will dominate the cryo- spheric response to climate change is key to forecasting future sea-level rise. The volume of ice contained in the ice caps and glaciers of the Arctic archipelago of Svalbard is small compared with that of the Greenland or Antarctic ice sheets, but is likely to be affected much more rapidly in the short term by climate change. This study investigates the mass balance of Austfonna, Svalbard's largest ice cap. Equilibrium-line fluxes for the whole ice cap, and for individual drainage basins, were estimated by combining surface velocities measured using satellite radar interferometry with ice thicknesses derived from radio-echo sounding. These fluxes were compared with balance fluxes to reveal that during the 1990s the total mass balance of the accumulation zone was (5.6 � 2.0) � 10 8 m 3 a -1 . Three basins in the quiescent phase of their surge cycles contributed 75% of this accumulation. The remaining volume may be attributable either to as yet unidentified surge-type glaciers, or to increased precipitation. This result emphasizes the importance of considering the surge dynamics of glaciers when attempting to draw any conclusions on climate change based on snapshot observations of the cryosphere.
TL;DR: In this paper, the authors developed a three-step thaw model to assess the impact of predicted warming on an ice-rich polar desert landscape in the Canadian high Arctic, showing mean annual increases of 4.9 and 6.5°C.
Abstract: This paper develops a three-step thaw model to assess the impact of predicted warming on an ice-rich polar desert landscape in the Canadian high Arctic. Air temperatures are established for two climate scenarios, showing mean annual increases of 4.9 and 6.5°C. This leads to a lengthening of the summer thaw season by up to 26 days and increased thaw depths of 12–70 cm, depending on the thermal properties of the soil. Subsidence of the ground surface is the primary landscape response to warming and is shown to be a function of the amount and type of ground ice in various cryostratigraphic units. In areas of pore ice and thin ice lenses with a low density of ice wedges, subsidence may be as much as 32 cm. In areas with a high density of ice wedges, subsidence will be slightly higher at 34 cm. Where massive ice is present, subsidence will be greater than 1 m. Landscape response to new climate conditions can take up to 15 years, and may be as long as 50 years in certain cases.
TL;DR: In this paper, the effects of clouds and sea ice on the solar radiation budget were determined for the Southern Ocean around Antarctica between latitudes 50° and 80°S, where the authors used cloud optical depth and surface albedo to estimate the geographical and seasonal variations of shortwave irradiance and cloud radiative forcing at the surface, both for the present climate and for altered surface and cloud conditions.
Abstract: The effects of clouds and sea ice on the solar radiation budget are determined for the Southern Ocean around Antarctica between latitudes 50° and 80°S. Distributions of cloud optical depth are used, together with distributions of surface albedo, to estimate the geographical and seasonal variations of shortwave irradiance and cloud radiative forcing at the surface, both for the present climate and for altered surface and cloud conditions. Poleward of 68°S in spring, ice causes a greater reduction of solar energy input to the surface than does cloud. However, in summer the clouds are more important than ice at all latitudes in the Southern Ocean. In the present climate the clouds are optically thicker over open water than over sea ice, suggesting a possible negative feedback if the sea ice area shrinks with climatic warming. Compared to the present climate in spring, removing sea ice results in an increase in irradiance reaching the ocean surface, regardless of the type of cloud remaining. However,...
TL;DR: In this paper, the authors analyzed a climate model ensemble with perturbations to parameters in the atmosphere model and found a large range of the sensitivity of Arctic sea ice retreat to global temperature change, from 11 to 18% per °C.
Abstract: [1] The retreat of Arctic sea ice is a very likely consequence of climate change and part of a key feedback process, which can accelerate global warming. The uncertainty in predictions in the rate of sea ice retreat requires quantification and ultimately reduction via observational constraints. Here we analyse a climate model ensemble with perturbations to parameters in the atmosphere model. We find a large range of the sensitivity of Arctic sea-ice retreat to global temperature change, from 11 to 18% per °C. This is placed in the context of the uncertainty obtained by alternative model ensembles. Reasons for the different sensitivities are explored and we find that differences in the amount of ocean and atmospheric heat transported from low to high latitudes dominates over local radiative contributions to the heat budget. Furthermore, we find no significant relationship between the uncertainty in sea ice response to climate change and climate sensitivity.
TL;DR: In this article, the authors found that industrial aerosol loading of the atmosphere should add to the warming effect of greenhouse gases and found a sharp increase in the concentration of industrial pollutants in snow deposited since the early 1950's which makes the trendless nature of various time series surprising.
Abstract: Ice core and mass balance studies from glaciers, ice caps and ice sheets constitute an ideal medium for monitoring and studying present and past environmental change and, as such, make a valuable contribution to the present debate over anthropogenic forcing of climate. Data derived from 32 years of measurements in the Canadian Arctic show no significant trends in glacier mass balance, ice melt, or snow accumulation, although the mass balance continues to be slightly negative. Models suggest that industrial aerosol loading of the atmosphere should add to the warming effect of greenhouse gases. However, we have found a sharp increase in the concentration of industrial pollutants in snow deposited since the early 1950's which makes the trendless nature of our various time series surprising. Spatial differences in the nature of climatic change may account for the lack of trend in the Queen Elizabeth Islands but encourages similar investigations to this study elsewhere in the circumpolar region. A global warming trend over the past 150 years has been demonstrated from instrumental data and is evident in our ice cores. However, the ice core data and glacier geometry changes in the Canadian Arctic suggest the Arctic warming is more pronounced in summer than winter. The same warming trend is not unique when viewed in the context of changes over the past 10,000 or 100,000 years. This suggests the 150-year trend is part of the natural climate variability.
TL;DR: In this article, the effects of albedo on global energy balance and atmospheric circulation are dis-cussed based on the knowledge from observational and modelling studies, the treatment of the ocean surface albedos in AFES (atmospheric general circulation model for the Earth Simulator) has been improved.
Abstract: The albedo of the ocean surface is of primary importance in the radiative energy balance of the Earth. It takes the smallest value of close to 0 over ocean water and the largest value of nearly 1 over snow-covered sea ice. The albedo of ocean water is determined by of the solar zenith angle, slope of the surface and optical properties of the atmosphere and ocean. The albedo of sea ice is significantly influenced by snow cover. During the warm season, ponds of melt water of snow and ice result in large reduction of albedo. Based on the knowledge from foregoing observational and modelling studies, the treatment of the ocean surface albedo in AFES (atmospheric general circulation model for the Earth Simulator) has been improved. Recent modifications to albedo parametrizations incorporated in AFES are described and optimum values for various parame- ters are adjusted to the observation data. The effects of albedo on global energy balance and atmospheric circulation are dis- cussed.
TL;DR: In this paper, an analytical solution to basal ice growth is proposed using theory derived from sea-ice formation, and compared with growth rates derived from numerical modelling, in addition to field observations of ice temperatures.
Abstract: In cold Arctic snowpacks, meltwater retention is a significant factor controlling the timing and magnitude of runoff. Meltwater percolates vertically through the snowpack until it reaches an impermeable horizon, whereupon a saturated zone is established. If the underlying media is below the freezing point, accretive ice formation takes place. This process has previously been crudely parameterized or modelled numerically. Such ice is called either superimposed ice on glaciers or basal ice on bare land.
Using theory derived from sea-ice formation, an analytical solution to basal ice growth is proposed. Results are compared against growth rates derived from numerical modelling. In addition, model results are compared to field observations of ice temperatures. The analytical solution is further extended to account for the temperature gradient inside the underlying media and the variable thermal properties of the underlying media.
In the analysis, observations and references have predominantly relied on knowledge from glaciers. However, the process of accretive ice growth is equally important in seasonal snow packs with a cold snow-ground interface and on Arctic sea ice where the ice-snow interface is well below freezing point. The simplification of this accretive ice growth problem makes the solution attractive for incorporation in large-scale cryospheric models. Copyright © 2007 John Wiley & Sons, Ltd.
01 Jan 2007
TL;DR: In this article, the authors examined the heuristic argument behind this hypothesis using an idealized, but observationally constrained, model of Arctic sea ice with representations of ice and ocean mixed layer thermodynamics, varying open water fraction, an energy balance atmosphere, and scalable CO2.
Abstract: With recent observations of diminishing summer Arctic sea ice extent, the hypothesis of a “tipping point” in summer ice cover has been the focus of a number of studies. This view suggests that as summer Arctic sea ice cover retreats it will reach a critical point after which the ice‐albedo e! ect will cause the summer ice cover to disappear altogether. We have examined the heuristic argument behind this hypothesis using an idealized, but observationally constrained, model of Arctic sea ice with representations of ice and ocean mixed layer thermodynamics, varying open water fraction, an energy balance atmosphere, and scalable CO2. We find that summer ice cover retreats toward an ice-free summer ocean at an accelerating rate in a scenario with exponentially increasing CO2. However, we find no critical CO2 concentration or “tipping point” using observationally based parameter values. We identify in the extended parameter space a bifurcation associated with multiple summer ice cover states and a cusp catastrophe, and we find that it occurs far from the physically realistic parameter regime. Our results suggest that the argument for a “tipping point” in summer Arctic ice cover brought on by ice albedo may not hold up when quantified. The reason is related to the fact that ice cover has only just begun to retreat at the time of maximum sunlight (June), and the minimum ice area occurs in September when there is very little Arctic sunlight.
Dissertation•
01 Dec 2007
TL;DR: In this article, a new sea-ice albedo parameterization scheme has been developed and implemented in ECHAM5 general circulation model, and includes important components like snow aging, ice thickness dependency and an explicit treatment of melt pond albedos.
Abstract: Today we experience an accelerated melting of sea ice in the Arctic which the general circulation models are inadequate to predict. We believe one of the reasons is the shortcomings in the snow and sea-ice albedo schemes for these models. Considering the e ect sea ice has on the Arctic climate due to the ice albedo feedback, accurate, physically-based parameterizations schemes for sea-ice albedo are crucial. A new sea-ice albedo parameterization scheme has been developed and implemented in ECHAM5 general circulation model, and includes important components like albedo decay due to snow aging, ice thickness dependency and an explicit treatment of melt pond albedo. Overall, the new albedo scheme reduces the sea-ice albedo, resulting in an overall reduction in sea-ice thickness, concentration and volume, particularly for northern hemisphere in summer due to the inclusion of melt ponds. We have also investigated procedures for collecting and processing datasets describing the sea-ice environment. The focus has varied from small scales (in-situ measurements), regional scales (airborne measurements) and global scales (remote sensing measurements), providing information of the optical properties of snow and sea ice at di erent resolutions. The small scale measurements were utilized for point-site validation and process studies, while the combinations of airborne measurements provided fractional sea-ice types, sea-ice albedo and sea-ice thickness, thus also estimates for sea-ice area and volume. Such combined datasets are very suitable for validation of climate models. Techniques for appropriate and consistent statistical validation of climate models in general (either to compare climate simulations for di erent parameterizations, or for validating climate model output against observations) have also been investigated. We have developed an adapted signi cance-in-scale-space methodology to detect statistical signi cant di erences between two climatological datasets.
TL;DR: The CLIMBER-GREMLINS model is an atmosphere-ocean-vegetation-northern hemisphere ice-sheet model able to simulate ice sheet growth in response to the transient forcings (insolation and CO2 changes) of the period 126-106kyr BP.
Abstract: The CLIMBER-GREMLINS model is an atmosphere-ocean-vegetation-northern hemisphere ice-sheet model able to simulate ice-sheet growth in response to the transient forcings (insolation and CO2 changes) of the period 126-106kyr BP In the present version of the model, this growth mainly occurs over North America and reaches an equivalent of 17 m in sea-level drop To quantify the role of the vegetation, ocean and icesheet feedbacks in this glaciation of North America, we have conducted sensitivity experiments in which the feedback of each of these components is sequentially switched off These experiments show that, in this model (1) glacial inception does not occur when vegetation is fixed to its interglacial state (experiment testing the response of the atmosphere-ocean-boreal land-ice system), (2) glacial inception occurs faster than in the standard experiment when the ocean surface characteristics (surface temperature and sea ice extent) are prescribed to their interglacial seasonal cycle (experiment testing the sensitivity of the atmosphere-vegetation-boreal land-ice system), (3) the ice-sheet albedo and altitude feedbacks are not crucial for starting the glaciation, but the albedo feedback doubles the ice volume growth rate, (4) the potential effect of a reduction in the thermohaline circulation is tested via an additional experiment in which we have forced it to its ‘off’ mode The results show that this mode favours the fastest land ice growth of all our experiments
Journal Article•
TL;DR: In this paper, the authors present a web-based activity that allows students to learn about how scientists are able to piece together a record of Earth's climate thanks to a unique ice core drilling program in Greenland.
Abstract: The hydrologic cycle is a very basic scientific principle. Water evaporates, rises, cools, condenses, and then precipitates back to Earth. But one role the hydrologic cycle plays is anything but basic--it helps scientists understand ancient climates, also referred to as paleoclimates. [ILLUSTRATION OMITTED] In this article, background information is presented on how the hydrologic cycle provides scientists with clues to understanding the history of Earth's climate. Also detailed is a web-based activity that allows students to learn about how scientists are able to piece together a record of Earth's climate thanks to a unique ice core drilling program in Greenland. Background Sea level is constantly changing. It rises and falls in relationship to the relative positions of land and sea. Two types of sea level change exist--isostatic and eustatic. Isostatic sea level change occurs when the land is depressed very slowly under the weight of glaciers. If the volume of water in the oceans remains constant and the land is pressed downward, it seems that the sea level has risen. As the glaciers melt and the weight of ice is removed, the land rebounds back and the relative position of the land to sea level makes it look like the sea level has dropped. Eustatic sea level change occurs when the land remains in one position; the sea level rises or falls relative to the land in response to changes such as added or reduced amounts of water in the seas. A number of Earth's mechanisms can affect sea level change such as seafloor spreading rates, but generally these are not on the same time scale as glaciation, the formation of the huge, slow moving "rivers of ice" that are the largest reservoirs of fresh water on Earth. Many land features record sea level changes over time. The shorelines and coral reefs are among the most obvious. Where the wave action erodes the shoreline, "benches" are carved. Reef-forming coral only grow at certain depths where the temperature and amount of light is conducive. As sea level changes, coral reefs and shoreline wave benches record these changes. Glaciers are also an important indicator of sea level changes and, in addition, are directly involved in the hydrologic cycle, sea level change, and global temperatures. As global temperatures decrease, the water that comes in the hydrologic cycle is not returned to the oceans as rainwater but stored in the form of snow and ice in glaciers on land. Though glaciers accumulate on land depressing the land surface in those areas and causing an isostatic sea level rise, most of the relative change is the eustatic lowering of global sea level due to the volume of water removed from the ocean. As global temperatures fall, glaciers accumulate and global sea level falls. Ice cores taken from glaciers are a very important tool for extracting the history of Earth's climate. Scientists obtain ice cores by boring into ice sheets using hollow drill bits, which produces a series of short ice cylinders with a total length that can be more than 3,000 m. Ice is particularly important in climate studies because ice traps the atmospheric components present when the ice was formed, and ice itself can reveal temperatures present at its formation. [FIGURE 1 OMITTED] Scientists refer to this principle as "reading the pages of Earth's history." Though the "pages" are made of rock, ice, or biologic composition, and the "book" is not easy to read, it is the way that science obtains incredibly accurate proxy records. Proxy refers to the fact that parameters such as temperature are not read as marks on a thermometer but as ratios of oxygen isotopes, the direction of curl of foraminifera shells, or other natural phenomena that serve as accurate indicators of temperature change. In the same way that ice thaws from ponds about the same time each year due to spring temperatures, the ice, rock, and biological records contain indicators that the temperature has reached a certain level. …