Long-Term Variations in Solar Activity and Their Apparent Effect on the Earth's Climate

The varying length of the 11-year cycle has been found to be strongly correlated with longterm variations of the northern hemisphere land surface air temperature since the beginning of systematic temperature variations from a global network, i.e. during the past 130 years. Although direct temperature observations before this interval are scarce, it has been possible to extend the correlation back to the 16th century due to the existence of a series of proxy temperature data published by Groveman and Landsberg in 1979. Reliable sunspot data do not exist before 1750, but we have been able to derive epochs of minimum sunspot activity from auroral observations back to 1500 and combine them with the direct observations to a homogeneous series. Comparison of the extended solar activity record with the temperature series confirms the high correlation between solar activity and northern hemisphere land surface air temperature and shows that the relationship has existed through the whole 500-year interval for which reliable data exist. A corresponding influence of solar activity has been demonstrated in other climatic parameters. Thus, both the date of arrival of spring in the Yangtze River Valley as deduced from phenological data and the extent of the sea-ice in the Atlantic sector of the Arctic Sea have been shown to be correlated with the length of the sunspot cycle during the last 450 years.