6.4.3 Sea Level Variations Over the Last Glacial-Interglacial Cycle
126.96.36.199 What Is the Influence of Past Ice Volume Change on Modern Sea Level Change?
Palaeorecords of sea level history provide a crucial basis for understanding the background variations upon which the sea level rise related to modern processes is superimposed. Even if no anthropogenic effect were currently operating in the climate system, measurable and significant changes in relative sea level (RSL) would still be occurring. The primary cause of this natural variability in sea level has to do with the planet’s memory of the last deglaciation event. Through the so-called glacial isostatic adjustment (GIA) process, gravitational equilibrium is restored following deglaciation, not only by crustal ‘rebound’, but also through the horizontal redistribution of water in the ocean basins required to maintain the ocean surface at gravitational equipotential.
Models of the global GIA process have enabled isolation of a contribution to the modern rate of global sea level rise being measured by the TOPography EXperiment (TOPEX)/Poseidon (T/P) satellite of –0.28 mm yr–1 for the ICE-4G(VM2) model of Peltier (1996) and –0.36 mm yr–1 for the ICE-5G(VM2) model of Peltier (2004). These analyses (Peltier, 2001) imply that the impact of modern climate change on the global rate of sea level rise is larger than implied by the uncorrected T/P measurements (see also Chapter 5).
By employing the same theory to predict the impact upon Earth’s rotational state due to both the Late Pleistocene glacial cycle and the influence of present-day melting of the great polar ice sheets on Greenland and Antarctica, it has also proven possible to estimate the extent to which these ice sheets may have been losing mass over the past century. In Peltier (1998), such analysis led to an upper-bound estimate of approximately 0.5 mm yr–1 for the rate of global sea level rise equivalent to the mass loss. This suggests the plausibility of the notion that polar ice sheet and glacier melting may provide the required closure of the global sea level rise budget (see Chapters 4 and 5).