The role of solar variability on the Earth's climate has remained very controversial. Satellite measurements have shown that the total irradiance varies both on time scales of days and in concert with the solar cycle. The amplitude of the variation is at the 0.1% level, which should be of little direct climatic consequence in the troposphere but may be more important in the Earth's upper atmosphere.
However, many stars having properties similar to the Sun's often show brightness variability on time scales of a few years with amplitudes of up to a few percent. A solar variability smaller than that could account for the entirety of the climate warming recorded in the past century.
The last decade has seen numerous attempts to establish a relationship between the Earth's environment and variations occurring on the Sun. For example, in one recent report a remarkable correlation (coefficient of 0.95) was presented between the length of the solar activity cycle and the long-term variations of the air temperature above the Northern Hemisphere land mass during the past 130 years. All of these studies are based on statistical analyses and contain no physical link between the quantities, and barely any explanation of what may be causing the suggested relations.
Many researchers have proposed that direct evidence for a Sun-climate connection is given by the almost- coincident timing of the Little Ice Age in Europe and the Maunder minimum of the Sun (recall that the brightness of the Sun is minimum for minimum solar activity). Inferences such as these, and those over longer time scales, are all based on proxies of the detailed behavior which are still far from robust, rather than on precise solar observation. One thing is certain: the Sun is the single largest energetic input to the Earth's climate system and has been assumed constant until recently.
A pressing need in the area of global change studies is to establish the climate-system response to a variety of possible scenarios of greenhouse gas increases expected to occur in the next few decades. This task is complicated because the Earth's climate system has substantial internal variability. One can study this problem by examining four major external forcing processes for the climate system, whose characteristics are presently not well understood: greenhouse gases, aerosols, variations of the solar energy input, and surface reflectivity. The changes affecting them are either human-produced or reconstructible (anthropogenic forcings are in principle reconstructible, although the job is not easy, and the volcanic record is also quite complete, although at times the amounts and composition of the ejected material must be estimated by indirect means).
This leaves the solar variations as a major source of uncertainty .
Last revised: Wed May 17 11:28:21 MDT 2000