Glacial terminations

Marine, polar ice core, and terrestrial records all highlight the sudden and dramatic nature of glacial terminations, the shifts in global climate that occurred as the world passed from dominantly glacial to interglacial conditions. The dramatic nature of these terminations is immediately evident in O Figure 12.4.

One metric for the rapidity of these glacial terminations is provided by very well-documented rates of global sea level rise. During glacial terminations, sea level rose roughly 100 m in less than 10,000 years, a mean rate of roughly 1 m per century. This is at least an order of magnitude faster than anything that has been suggested for future climate change scenarios associated with global warming (Houghton 2004). In addition during the last interglacial, sea level was about 20 m higher than today. Thus, both the magnitudes and rates of sea level rise have been significantly larger than either those that have been documented in recent years or predicted for future climate change scenarios.

Because the melting of large, northern hemisphere, continental ice sheets which no longer exist is thought to be the reason that past rates of sea level rise were so large, one might be tempted to think that understanding the controls of sea level on these timescales is not relevant to the future. There are two strong arguments against this line of reasoning. First, and most importantly, large and growing coastal populations in the modern day are highly vulnerable to sea level rise, whereas during the last interglacial no such settlements existed. Thus, even a much more gradual, smaller amplitude sea level rise occurring today will have much greater human impact than those of the past. Furthermore, extant ice sheets on Greenland and Antarctica represent fresh water storage equivalent to many meters and many tens of meters of sea level, respectively. The possibility of rapid destabilization of even a small part of these ice sheets in the future, although less likely than many other climate change predictions, cannot be excluded from consideration. Thus, an understanding of the dynamics of large ice sheets... gained through investigation of their behavior in the past, may be important for our future.

Given the growing, and increasingly urbanized, coastal population and the great potential for nonlinear "threshold crossing'' response of coastal zones and the ecosystems they support to sea level rise, there is an urgent need to understand the processes driving and responding to sea level rise on timescales from decades to millennia. Many of these processes, such as postglacial rebound of continental plates, continue to effect sea level change today. The past record of sea level change is thus highly relevant to its likely future course.

Although sea level is a broadly global property, two features of glacial terminations tend to qualify the view that they are a single, rapid, globally coherent, monotonic process. When the inferred temperature change in Antarctica and Greenland are carefully synchronized on the basis of their records of changing atmospheric methane concentrations, it is clear that during the course of the warming trend from Glacial to Holocene, the record from Antarctica leads that from Greenland; moreover during the period of warming, the sharp fluctuations in temperature that mark the Greenland record in particular, are largely in antiphase with the changes in much of Antarctica. Recent high resolution Uranium Thorium dating reinforces the idea of an Antarctic lead, since it suggests that the penultimate glacial termination occurred 135 ± 2.5 ka (Henderson and Slowey 2000). This date precedes the Northern Hemisphere June insolation peak by nearly 10,000 years. Thus, although consistent with primary deglaciation forcing by insolation levels in the southern hemisphere, or the tropics, this evidence runs counter to the view that insolation forcing at high northern latitudes was the initial trigger for deglaciation. Any concept of a rapid shift associated with glacial terminations thus has to be qualified by the realization that the rate of change is not uniform over the whole globe and that the oscillations that are superimposed on the trend are not globally parallel in sign.

0 0

Post a comment