GLOBAL HOLOCENE GLACIER FLUCTUATIONS – WHO'S IN THE DRIVER'S SEAT?

Most glaciers around the world fluctuated synchronously during the twentieth century on decadal timescales, suggesting that the dominant control is global temperature change. Local and regional differences in glacier activity can be explained by changes in precipitation and ocean-atmosphere interactions, which are superimposed on the dominant temperature control. Similarly, geologic, dendrochronologic, and documentary evidence demonstrates that glaciers throughout the world advanced and retreated synchronously on decadal timescales over the past millennium and centennial timescales throughout the Holocene, implying a global forcing mechanism. We propose two mechanisms that can account for the global synchronism of glaciers. First, periods when glaciers were more extensive in both the Northern and Southern hemispheres correlate with times of low sunspot activity. Second, changes in insolation explain the observed differences in the extents of glacier advances in the two hemispheres. Early Holocene glacier advances in the Northern Hemisphere were less extensive than advances of the late Holocene because insolation in summer in the Northern Hemisphere decreased through time. In contrast, early Holocene glacier advances in the Southern Hemisphere were more extensive than those of the late Holocene because insolation in summer in that hemisphere increased through time. However, these two forcings alone cannot explain the fact that almost all glaciers in both hemispheres are currently retreating. A third forcing – anthropogenic greenhouse gas emissions – must be controlling glacier activity today.