Tripati, Aradhna K. and Roberts, Christopher D. and Eagle, Robert A. (2009) Coupling of CO_2 and Ice Sheet Stability Over Major Climate Transitions of the Last 20 Million Years. Science, 326 (5958). pp. 1394-1397. ISSN 0036-8075 http://resolver.caltech.edu/CaltechAUTHORS:20091216-121815722
Examining fossil foraminifera the authors were able to determine what the carbon dioxide levels were over the last 20 million years. They also determined the acidity of the oceans (which should become more acid as carbon dioxide levels rise). Both were increased during periods of high temperature and reduced during periods of low temperature.
This conclusions tells it all:
These results show that changes in pCO2 and climate have been coupled during major glacial transitions of the past 20 My, just as they have been over the last 0.8 My, supporting the hypothesis that greenhouse gas forcing was an important modulator of climate over this interval via direct and indirect effects. Variations in pCO2 affect the radiative budget and energy balance of the planet. Such changes will inevitably have consequences for temperature, the hydrologic cycle, heat transport, and the accumulation and ablation of sea ice and glacial ice. The data presented here do not preclude alternative mechanisms for driving climate change over the past 20 Ma. However, they do indicate that changes in pCO2 were closely tied to the evolution of climate during the Middle and Late Miocene and the Late Pliocene glacial intensification, and therefore, it is logical to deduce that pCO2 played an important role in driving these transitions. High-resolution records of pCO2 and other climate parameters should help to resolve whether pCO2 was a trigger and/or feedback (or both).
Carbon dioxide levels are closely tied to global temperatures. IN addition, the authors have this to say about glacial ice depending on carbon dioxide levels:
By comparing our reconstruction to the published data sets described above, we are able to estimate past thresholds for the buildup of ice in different regions. When pCO2 levels were last similar to modern values (that is, greater than 350 to 400 ppmv), there was little glacial ice on land or sea ice in the Arctic, and a marine-based ice mass on Antarctica was not viable. A sea ice cap on the Arctic Ocean and a large permanent ice sheet were maintained on East Antarctica when pCO2 values fell below this threshold. Lower levels were necessary for the growth of large ice masses on West Antarctica (~250 to 300 ppmv) and Greenland (~220 to 260 ppmv).
Carbon dioxide levels have gone up over the last century to levels that have been closely correlated with loss of glacial and sea ice in the Arctic. And that is just what we are seeing. We are also above the concentration associated with the increase of Greenland glaciers. And we continue to see loss of these also.
So, historic data using another approach indicates that carbon dioxide levels are tightly correlated with temperature and pH changes, as have so many other approaches. The physics indicates that this should be true and it is nice to see that fossil data confirms this.
Glaciers represent a tremendous resource used by mankind to store the fresh water it needs. What will happen when these are gone?