1000 years of climate change

The magnitude of solar forcing varies with the amount of solar activity. In addition to the 10-12 year variations, solar activity varies over centennial and millenial periods. During the past 1000 years, of particular interest were the Medieval Warming Period (high solar activity), the Maunder Minimum, (minimal solar activity) and the sharp multi-decadal increase in solar maxima during the first half of the 20th Century. Satellite observations of solar irradiance for the past two decades have allowed calibration of variations in solar activity proxies. A number of attempts at reconstructing past climate variations via these proxies has shown that direct (irradiance) solar forcing can explain observed northern hemisphere temperatures from 1000 to 1900, but fails to account for most of the observed warming in the past 100 years, especially after 1975. Because solar direct forcing is able to account for climate changes prior to 1900, indirect solar forcing effects seem to be small, but merit continued study especially until both paleo-temperatures and magnitude of forcings are known more precisely. (C) 2004 COSPAR. Published by Elsevier Ltd. All rights reserved.