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Term 'global change' is used to describe the changes of the large scale weather systems on Earth, and especially the surface temperature increase, by man-made increase of atmospheric CO2. There is no denying the fact that mankind has been able to change the atmosphere during a relatively short time period, and that at least in one case, i.e., the ozone hole, the effects have been serious. However, there is still quite a bit controversy about the effects of excess CO2, the true amount of temperature increase, and the claim that no other mechanism than man-made CO2 could be operating if such an increase exists. It is the last point that is of importance for space physics, since it is possible that also the changes in Sun's activity could be causing some changes in terrestrial climate. Here we will concentrate on this subject. In addition, we discuss the CO2 matter from the perspective of upper atmosphere, also an appropriate subject in space research.
The 11-year solar cycle is the best known variability in the Sun. However, all solar cycles are not equally intense, and it is possible that longer period modulations are operating. For example, long periods of almost no sunspot activity at all have been recorded (see the table). Especially the Maunder's minimum (e.g., Eddy, 1976) is well known; it is perhaps noteworthy that it coincided with a "small ice age". In addition, there are some indications that the Sun's activity has had a peak in the 12th century.
There is one important, indirect way the atmosphere is coupled to the Sun's activity. The galactic cosmic rays increase the amount of C-14 in the atmospheric CO2 and, consequently, also in vegetation. During the increased solar activity close to solar cycle maximum years, Earth is better shielded from the cosmic rays than during the minimum years, and the amount of C-14 decreases. Thus the C-14 content of, for example, annual rings of old trees may reveal something about the Sun's performance during the last few millenia. Indeed, studies have indicated that there is a connection between long term climate change and Sun's activity (Friis-Christensen and Lassen, 1991; Lassen and Friis-Christensen, 1995).
One possible mechanism operating is that during high activity levels the decreased amount of galactic cosmic rays could lead to reduced cloud formation in the atmosphere, and hence to increased temperatures.
It has been shown that increase of CO2 and CH4 will cool the thermosphere (e.g., Roble and Dickinson, 1989), and some measurement indicate that this could be happening (e.g., Ulich and Turunen, 1997).