Has Solar Forcing been an Important Influence on Climate in the Late Holocene?

Raymond S. Bradley [rbradley@geo.umass.edu], University of Massachusetts, Amherst.

            A number of empirical studies have attempted to link changes in total solar irradiance during recent decades to changes in patterns of global cloudiness, or to circulation changes associated with variations in the ultra-violet part of the solar irradiance spectrum. Several general circulation and global energy balance models have been used to assess these possible effects on global and regional climate. These models suggest that climate effects are possible, even with relatively small changes in total irradiance. Furthermore, general circulation models indicate that there are distinct changes in circulation which lead to regional patterns of climate change, rather than simply overall warming or cooling. Here, I examine paleoclimatic evidence that has been linked to changes in solar activity, including the connection that has been made between solar activity, regional drought and the Classic Mayan collapse. There is a fairly coherent pattern of climate change associated with solar activity changes during the Holocene. Periods of low solar activity are generally associated with lower temperatures at mid to high latitude sites, and weaker monsoon activity in the Tropics. However, most studies do not recognize any link with solar forcing, so there is a danger of the “reinforcement syndrome”, whereby only a very small number of well-publicized studies establish a paradigm that may not be supported by most records. More research is needed to fully determine whether solar forcing has played an important role in past climate changes. A well-designed strategy, using a network of high resolution proxies, is needed to do this.