Thinning of the Arctic sea-ice cover
TL;DR: In this paper, a comparison of sea-ice draft data acquired on submarine cruises between 1993 and 1997 with similar data acquired between 1958 and 1976 indicates that the mean ice draft at the end of the melt season has decreased by about 1.3 m in most of the deep water portion of the Arctic Ocean, from 3.1 m in 1958-1976 to 1.8 m in the 1990s.
Abstract: Comparison of sea-ice draft data acquired on submarine cruises between 1993 and 1997 with similar data acquired between 1958 and 1976 indicates that the mean ice draft at the end of the melt season has decreased by about 1.3 m in most of the deep water portion of the Arctic Ocean, from 3.1 m in 1958–1976 to 1.8 m in the 1990s. The decrease is greater in the central and eastern Arctic than in the Beaufort and Chukchi seas. Preliminary evidence is that the ice cover has continued to become thinner in some regions during the 1990s.
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TL;DR: The Community Climate System Model version 3 (CCSM3) as discussed by the authors is a coupled climate model with components representing the atmosphere, ocean, sea ice, and land surface connected by a flux coupler.
Abstract: The Community Climate System Model version 3 (CCSM3) has recently been developed and released to the climate community. CCSM3 is a coupled climate model with components representing the atmosphere, ocean, sea ice, and land surface connected by a flux coupler. CCSM3 is designed to produce realistic simulations over a wide range of spatial resolutions, enabling inexpensive simulations lasting several millennia or detailed studies of continental-scale dynamics, variability, and climate change. This paper will show results from the configuration used for climate-change simulations with a T85 grid for the atmosphere and land and a grid with approximately 1° resolution for the ocean and sea ice. The new system incorporates several significant improvements in the physical parameterizations. The enhancements in the model physics are designed to reduce or eliminate several systematic biases in the mean climate produced by previous editions of CCSM. These include new treatments of cloud processes, aerosol ...
2,500 citations
Cites background from "Thinning of the Arctic sea-ice cove..."
...The annual-mean ice thickness is between 2 and 2.5 m over the central Arctic basin with thicknesses reaching 3–4 m next to the Canadian Archipelago and in the East Siberian Sea (Fig. 8). CCSM3 agrees well with submarine measurements of sea ice thickness from Bourke and Garrett (1987) and Rothrock et al. (1999) , although the model is too thin by about 1 m within about 400 km of the Canadian Archipelago and too thick by about 2 m in the East ......
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...CCSM3 agrees well with submarine measurements of sea ice thickness from Bourke and Garrett (1987) and Rothrock et al. (1999), although the model is too thin by about 1 m within about 400 km of the Canadian Archipelago and too thick by about 2 m in the East Siberian Sea....
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TL;DR: The authors used a global climate model to compare the effectiveness of many climate forcing agents for producing climate change and found that replacing traditional instantaneous and adjusted forcings with an easily computed alternative, Fs, yields a better predictor of climate change, i.e., its efficacies are closer to unity.
Abstract: [1] We use a global climate model to compare the effectiveness of many climate forcing agents for producing climate change. We find a substantial range in the “efficacy” of different forcings, where the efficacy is the global temperature response per unit forcing relative to the response to CO2 forcing. Anthropogenic CH4 has efficacy ∼110%, which increases to ∼145% when its indirect effects on stratospheric H2O and tropospheric O3 are included, yielding an effective climate forcing of ∼0.8 W/m2 for the period 1750–2000 and making CH4 the largest anthropogenic climate forcing other than CO2. Black carbon (BC) aerosols from biomass burning have a calculated efficacy ∼58%, while fossil fuel BC has an efficacy ∼78%. Accounting for forcing efficacies and for indirect effects via snow albedo and cloud changes, we find that fossil fuel soot, defined as BC + OC (organic carbon), has a net positive forcing while biomass burning BC + OC has a negative forcing. We show that replacement of the traditional instantaneous and adjusted forcings, Fi and Fa, with an easily computed alternative, Fs, yields a better predictor of climate change, i.e., its efficacies are closer to unity. Fs is inferred from flux and temperature changes in a fixed-ocean model run. There is remarkable congruence in the spatial distribution of climate change, normalized to the same forcing Fs, for most climate forcing agents, suggesting that the global forcing has more relevance to regional climate change than may have been anticipated. Increasing greenhouse gases intensify the Hadley circulation in our model, increasing rainfall in the Intertropical Convergence Zone (ITCZ), Eastern United States, and East Asia, while intensifying dry conditions in the subtropics including the Southwest United States, the Mediterranean region, the Middle East, and an expanding Sahel. These features survive in model simulations that use all estimated forcings for the period 1880–2000. Responses to localized forcings, such as land use change and heavy regional concentrations of BC aerosols, include more specific regional characteristics. We suggest that anthropogenic tropospheric O3 and the BC snow albedo effect contribute substantially to rapid warming and sea ice loss in the Arctic. As a complement to a priori forcings, such as Fi, Fa, and Fs, we tabulate the a posteriori effective forcing, Fe, which is the product of the forcing and its efficacy. Fe requires calculation of the climate response and introduces greater model dependence, but once it is calculated for a given amount of a forcing agent it provides a good prediction of the response to other forcing amounts.
1,376 citations
Cites background from "Thinning of the Arctic sea-ice cove..."
...Thus soot may be a partial cause of reduced area and thickness of Arctic sea ice in the past half century [Rothrock et al., 1999]....
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TL;DR: Although the large scatter between individual model simulations leads to much uncertainty as to when a seasonally ice-free Arctic Ocean might be realized, this transition to a new arctic state may be rapid once the ice thins to a more vulnerable state.
Abstract: Linear trends in arctic sea-ice extent over the period 1979 to 2006 are negative in every month. This ice loss is best viewed as a combination of strong natural variability in the coupled ice-ocean-atmosphere system and a growing radiative forcing associated with rising concentrations of atmospheric greenhouse gases, the latter supported by evidence of qualitative consistency between observed trends and those simulated by climate models over the same period. Although the large scatter between individual model simulations leads to much uncertainty as to when a seasonally ice-free Arctic Ocean might be realized, this transition to a new arctic state may be rapid once the ice thins to a more vulnerable state. Loss of the ice cover is expected to affect the Arctic's freshwater system and surface energy budget and could be manifested in middle latitudes as altered patterns of atmospheric circulation and precipitation.
1,278 citations
TL;DR: Reducing soot emissions, thus restoring snow albedos to pristine high values, would have the double benefit of reducing global warming and raising the global temperature level at which dangerous anthropogenic interference occurs.
Abstract: Plausible estimates for the effect of soot on snow and ice albedos (1.5% in the Arctic and 3% in Northern Hemisphere land areas) yield a climate forcing of +0.3 W/m2 in the Northern Hemisphere. The “efficacy” of this forcing is ∼2, i.e., for a given forcing it is twice as effective as CO2 in altering global surface air temperature. This indirect soot forcing may have contributed to global warming of the past century, including the trend toward early springs in the Northern Hemisphere, thinning Arctic sea ice, and melting land ice and permafrost. If, as we suggest, melting ice and sea level rise define the level of dangerous anthropogenic interference with the climate system, then reducing soot emissions, thus restoring snow albedos to pristine high values, would have the double benefit of reducing global warming and raising the global temperature level at which dangerous anthropogenic interference occurs. However, soot contributions to climate change do not alter the conclusion that anthropogenic greenhouse gases have been the main cause of recent global warming and will be the predominant climate forcing in the future.
1,225 citations
TL;DR: In this paper, the authors emphasise that the certainty of conclusions that can be drawn about climate from observations depends critically on the availability of accurate, complete and consistent series of observations.
Abstract: Chapter 2 emphasises change against a background of variability. The certainty of conclusions that can be drawn about climate from observations depends critically on the availability of accurate, complete and consistent series of observations. For many variables important in documenting, detecting, and attributing climate change, data are still not good enough for really firm conclusions to be reached. This especially applies to global trends in variables that have large regional variations, such as pre-
1,220 citations
References
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TL;DR: In this paper, the authors estimated the winter sea ice export through the Fram Strait using ice motion from satellite passive microwave data and ice thickness data from moored upward looking sonars, and found a significant correlation between the area flux and positive phases of the North Atlantic Oscillation (NAO) index over the months of December through March.
Abstract: An important term in the mass balance of the Arctic Ocean sea ice is the ice export. We estimated the winter sea ice export through the Fram Strait using ice motion from satellite passive microwave data and ice thickness data from moored upward looking sonars. The average winter area flux over the 18-year record (1978-1996) is 670,000 square km, approximately 7% of the area of the Arctic Ocean. The winter area flux ranges from a minimum of 450,000 sq. km in 1984 to a maximum of 906,000 sq km in 1995. The daily, monthly and interannual variabilities of the ice area flux are high. There is an upward trend in the ice area flux over the 18-year record. The average winter volume flux over the winters of October 1990 through May 1995 is 1745 cubic km ranging from a low of 1375 cubic km in 1990 to a high of 2791 cubic km in 1994. The sea-level pressure gradient across the Fram Strait explains more than 80% of the variance in the ice flux over the 18-year record. We use the coefficients from the regression of the time-series of area flux versus pressure gradient across the Fram Strait and ice thickness data to estimate the summer area and volume flux. The average 12-month area flux and volume flux are 919,000 sq km and 2366 cubic km. We find a significant correlation (R =0.86) between the area flux and positive phases of the North Atlantic Oscillation (NAO) index over the months of December through March. Correlation between our six years of volume flux estimates and the NAO index gives R =0.56. During the high NAO years, a more intense Icelandic low increases the gradient in the sea-level pressure by almost 1 mbar across the Fram Strait thus increasing the atmospheric forcing on ice transport. Correlation is reduced during the negative NAO years because of decreased dominance of this large-scale atmospheric pattern on the sea-level pressure gradient across the Fram Strait. Additional information is contained in the original.
300 citations
TL;DR: In this article, the authors show that summer ice coverage has been below normal in recent years, with extreme minima in 1990, 1993, and 1995, and linkages are proposed between these ice anomalies and a sharp increase since 1989 in the frequency of low pressure systems over the central Arctic.
Abstract: Sea ice data from November 1978 through September 1995 for the Arctic Ocean and peripheral seas indicate that summer ice coverage has been below normal in recent years, with extreme minima in 1990, 1993, and 1995. The net trend in summer ice cover over the 17-year period is −0.6% per year, with the extent of the perennial ice pack reduced by 9% in 1990–1995 compared with 1979–1989. The reductions are greatest in the Siberian sector of the Arctic Ocean. Linkages are proposed between these ice anomalies and a sharp increase since 1989 in the frequency of low pressure systems over the central Arctic.
268 citations
"Thinning of the Arctic sea-ice cove..." refers background in this paper
...…seasonal duration in (1) the Northern Hemisphere over the period 1978–1995 [Johannessen et al., 1996, ], (2) the eastern Arctic Ocean and Kara and Barents seas over the period 1979–1986 [Parkinson, 1992], and (3) the East Siberian and Laptev seas over the period 1979–1995 [Maslanik et al., 1996]....
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TL;DR: In this article, an ice-ocean model is used to examine the behavior of the Arctic Ocean in response to recent changes in Arctic climate, and the model shows that, starting about 1989, there has been a significant warming and salinification in the Arctic ocean, in agreement with recent observations.
Abstract: An ice-ocean model is used to examine the behavior of the Arctic Ocean in response to recent changes in Arctic climate. The model shows that, starting about 1989, there has been a significant warming and salinification in the Arctic Ocean, in agreement with recent observations. The warming and salinification occur mainly in the upper ocean owing to a sustained increase of Atlantic inflow both at Fram Strait and, most significantly, via the Barents Sea. The increased incoming warm and salty Atlantic Water “flushes” out cold and fresh Arctic Water, thus increasing the temperature and salinity of the upper ocean and resulting in more oceanic heat flux to the mixed layer and ice cover. Concomitantly, the model shows a continuing decrease in ice volume beginning in 1987.
173 citations
TL;DR: In this paper, a British submarine carried out an ice profiling experiment in the Arctic Ocean in which the route closely approximated that of an earlier voyage in October 1976, and there is evidence of a significant decrease in mean ice thickness in 1987 relative to that found in 1976.
Abstract: IN May 1987 a British submarine carried out an ice profiling experiment in the Arctic Ocean in which the route closely approximated that of an earlier voyage in October 1976. Over a zone extending more than 400 km to the north of Greenland there is evidence of a significant decrease in mean ice thickness in 1987 relative to that found in 1976. This thinning amounts to a loss of volume of at least 15% over an area of 300,000 km2. The nature of the ice thickness distributions suggests that the thinning is primarily associated with the presence of a larger fraction of young and first-year ice, a consequence of wind-driven divergence in the ice cover during the months preceding the 1987 observations. Such a large fluctuation in a region previously believed to possess consistently high ice thicknesses illustrates the importance of monitoring Arctic ice thickness more systematically to determine whether such fluctuations fall within the limits of normal inter-annual variability.
127 citations
TL;DR: In this paper, a simplified analytical model of sea ice growth is presented where the atmosphere is in thermal radiative equilibrium with the ice, making the downwelling longwave radiation reaching the ice surface an internal variable rather than a specified forcing.
Abstract: A simplified analytical model of sea ice growth is presented where the atmosphere is in thermal radiative equilibrium with the ice. This makes the downwelling longwave radiation reaching the ice surface an internal variable rather than a specified forcing. Analytical results demonstrate how the ice state depends on properties of the ice and on the externally specified climate.
119 citations