| Lecture 21 Notes November 18, 2004 |
Ice
Age Climates
 |
During the great
Pleistocene ice ages, ice sheets covered a great deal
of North America and a comparatively much smaller areas of Eurasia and
sea ice was far more extensive. Note that the land distribution is a
little strange at left because it reflects lower sea level by about
130m. The reason why Alaska and eastern Siberia remained ice free is
still debated. Ice sheet growth is favored when summertime temperatures
are low, which implies that summertime temperatures must not have been
low enough in these regions during the ice age. Instead, some scientist
argue that ice sheet growth was limited by the supply of moisture, so
the ice free regions must have been dry. While this argument appears in
the scientific literature today, I believe it to be far less important
for ice sheet growth than summertime temperature (recall that
Milankovitch said roughly the same thing). |
The shape of ice sheets also affects its sensitivity to summertime temperature. The figure below shows that the ice sheet is steepest at the edge. Because it flattens out at higher elevations, a small change in the freezing elevation causes the ablation zone to expand a great deal.
The accumulation zone on a healthy ice sheets is much larger than the ablation zone. Recall that Antarctica has very little area that ever experiences melt. Atmospheric moisture content decreases with height, so ice sheets tend to grow themselves into an envioronment where they eventually receive very low levels of accumulation. This creates a strong negative feedback on their vertical growth.
The record of ice volume is well represented by oxygen isotopes in ocean sediments and the history of temperature is determined by oxygen isotopes in ice cores. The text explaines how oxygen isotopes provide this information on p. 272-273. Ice cores also capture air bubbles that preserve the history of atmospheric gases back in time.
According to Milankovitch, the history of ice volume should match estimates of insolation in high northern latitudes with summertime insolation variability driven by orbital cycles. There are some characteristics in common, like both time series have similar time scales of variability, but it is far from a "good" match (see figure 14-8). Furthermore the best match is achieved only after shifting the insolation curve so it leads the ice volume curve by 6,000 years in time! There is no defensible reason for such a shift, although plenty of scientific papers have attempted to give one.
Local ice sheet expert Dr. Gerard suggests an alternative idea. He says one should expect a strong relationship between summertime insolation and ice growth and melt, so we should compare insolation with the slope of the ice volume curve! His figure below shows a nice match between curves with no shift in time. This simple idea will no doubt be viewed as a break through in the paleoclimate literature, but it is yet unpublished.
