Initiation Of Ice Sheets Research Articles

Climate and the Initiation of Maritime Ice Sheets

The lower latitude extension of polar water during Quaternary glaciations has two opposing effects on the mass balances of adjacent maritime ice sheets. Cooler air temperatures reduce ablation and increase the fraction of precipitation falling as snow, but also lead to reduced atmospheric moisture content and reduced precipitation. The effects of these contrasting processes on ice-sheet initiation are investigated using a coupled ice-sheet-atmospheric moisture model of the Loch Lomond ice sheet in Scotland (10 000 a B.P.). There is a delicate balance between the degree of cooling required to initiate ice accumulation and that which restricts the flow of moisture over the area. At the regional scale of Scotland, the combined effects of orographic enhancement and the inherent feedback between the rising ice-surface elevation and increasing mass balance are dominant, and ice-sheet growth accelerates. Comparison of the results from models using different wind directions suggests that south-westerly winds were prevalent during the Loch Lomond glaciation in contrast to the dominant westerly winds of the present day. The modelling experiments demonstrate the sensitivity and complexity of the links between ocean surface cooling and ice-sheet growth, particularly at the early stages of a glacial cycle.

Climate and the Initiation of Maritime Ice Sheets

The lower latitude extension of polar water during Quaternary glaciations has two opposing effects on the mass balances of adjacent maritime ice sheets. Cooler air temperatures reduce ablation and increase the fraction of precipitation falling as snow, but also lead to reduced atmospheric moisture content and reduced precipitation. The effects of these contrasting processes on ice-sheet initiation are investigated using a coupled ice-sheet-atmospheric moisture model of the Loch Lomond ice sheet in Scotland (10 000 a B.P.). There is a delicate balance between the degree of cooling required to initiate ice accumulation and that which restricts the flow of moisture over the area. At the regional scale of Scotland, the combined effects of orographic enhancement and the inherent feedback between the rising ice-surface elevation and increasing mass balance are dominant, and ice-sheet growth accelerates. Comparison of the results from models using different wind directions suggests that south-westerly winds were prevalent during the Loch Lomond glaciation in contrast to the dominant westerly winds of the present day. The modelling experiments demonstrate the sensitivity and complexity of the links between ocean surface cooling and ice-sheet growth, particularly at the early stages of a glacial cycle.

Can Milankovitch Variations Initiate The Growth Of Ice Sheets In A Global Circulation Model?

The GISS climate model is used to investigate whether the growth of ice sheets could have been initiated by solar insolation variations. Three different orbital configurations are used, corresponding to 116 000 yr B.P., 106 000 yr B.P., and a modified insolation field with greater reductions in summer insolation at high northern latitudes. The time periods chosen are those in which geophysical evidence implies that ice sheets were growing rapidly.The results show that the model fails to maintain snow cover through the summer at locations of suspected initiation of the major ice sheets, despite the reduced summer insolation. When 10 m-thick ice was inserted in all locations where continental ice sheets existed during the last glacial maximum, the model failed to maintain it as well: the ice would melt away in less than five years. Only cooling the ocean to its full ice-age value allowed the model to keep any of the additional ice, and then only in a very restricted region in northern Baffin Island.The experiments indicate there is a wide discrepancy between the model’s response to Milankovitch variations and the geophysical evidence of ice sheet initiation. As the model failed to grow or sustain low-elevation ice during the time of high latitude maximum solar radiation reduction (120 000–100 000 yr B.P.), it is unlikely it could have done so at any time within the last several hundred thousand years. Either the model is not nearly sensitive enough, or we do not understand the connection between Milankovitch variations and the growth of ice sheets.

Can Milankovitch Variations Initiate The Growth Of Ice Sheets In A Global Circulation Model?

The GISS climate model is used to investigate whether the growth of ice sheets could have been initiated by solar insolation variations. Three different orbital configurations are used, corresponding to 116 000 yr B.P., 106 000 yr B.P., and a modified insolation field with greater reductions in summer insolation at high northern latitudes. The time periods chosen are those in which geophysical evidence implies that ice sheets were growing rapidly. The results show that the model fails to maintain snow cover through the summer at locations of suspected initiation of the major ice sheets, despite the reduced summer insolation. When 10 m-thick ice was inserted in all locations where continental ice sheets existed during the last glacial maximum, the model failed to maintain it as well: the ice would melt away in less than five years. Only cooling the ocean to its full ice-age value allowed the model to keep any of the additional ice, and then only in a very restricted region in northern Baffin Island. The experiments indicate there is a wide discrepancy between the model’s response to Milankovitch variations and the geophysical evidence of ice sheet initiation. As the model failed to grow or sustain low-elevation ice during the time of high latitude maximum solar radiation reduction (120 000–100 000 yr B.P.), it is unlikely it could have done so at any time within the last several hundred thousand years. Either the model is not nearly sensitive enough, or we do not understand the connection between Milankovitch variations and the growth of ice sheets.

Can Milankovitch orbital variations initiate the growth of ice sheets in a general circulation model?

The Goddard Institute for Space Studies climate model is used to investigate whether the growth of ice sheets could have been initiated by solar insolation variations. Three different orbital configurations are used, corresponding to 116 and 106 kyr B.P., and a modified insolation field with greater reductions in summer insolation at high northern latitudes. The time periods chosen are those in which geophysical evidence suggests ice sheets may have been growing rapidly. The reduced summer insolation field characteristic of all the experiments is thought to be a necessary condition for allowing snow to last through the summer, and ice sheets to build. The results show that the model fails to maintain snow cover through the summer at locations of suspected initiation of the major ice sheets, despite the reduced summer and fall insolation. When 10‐m‐thick ice was inserted in all locations where continental ice sheets existed during the Last Glacial Maximum, the model failed to maintain it as well, producing energy and mass imbalances which would remove the ice within 5 years. Only when the ocean surface temperatures were adjusted to their peak ice age values was the model able to keep any of the additional ice, and then only in a very restricted region of northern Baffin Island. The experiments indicate there is a wide discrepancy between the model's response to Milankovitch perturbations and the geophysical evidence of ice sheet initiation. As the model failed to grow or sustain low‐altitude ice during the time of high‐latitude maximum solar radiation reduction (120–110 kyr B.P.), it is unlikely it could have done so at any other time within the last several hundred thousand years. If the model results are correct, it indicates that the growth of ice occurred in an extremely ablative environment, and thus demanded some complicated strategy, or else some other climate forcing occurred in addition to the orbital variation influence (and CO2 reduction), which would imply we do not really understand the cause of the ice ages and the Milankovitch connection. If the model is not nearly sensitive enough to climate forcing, it could have implications for projections of future climate change.

Ice-sheet initiation and climatic influences of expanded snow cover in Arctic Canada

It has been suggested that the Laurentide Ice Sheet originated with extensive perennial snow cover, and that the snow cover affected climate so as to aid ice-sheet development. In this study, a large increase in extent of October 1st snow cover in the Canadian Arctic from 1967–1970 to 1971–1975 is compared to changes in October means of other climate variables. Over the area of snow-cover expansion, mean surface air temperature decreased by up to 3°C, mean 500-mbar height was lowered by over 60 m, and precipitation was increased by up to a factor of two. These effects, if applied to the entire summer, together with the temperature change computed by Shaw and Donn for a Northern Hemisphere summer insolation minimum (the Milankovich effect), can account for glacierization of the Central Canadian Arctic.