For many years scientists have speculated that changes in the amount of energy given off by the Sun can influence the Earth's climate. There is no doubt that variations do occur in various characteristics of the Sun on a range of time scales. The 11-year cycle in the number of sunspots on the face of the Sun is well known. But other parameters, including the size of the Sun, vary too, and over different time scales from tens of years to thousands of years.
What is less clear is whether or not these changes produce significant variations in the total energy output of the Sun. The total solar energy received by the Earth, or solar constant, has only been measured accurately since the advent of the satellite era, 40 years ago. In addition, changes which have been detected over the past 20 years are very small in magnitude, and probably too small to account for all of the observed global warming that has occurred during this period. While changes in total solar energy may be greater on longer time scales, this is only a speculative possibility. Nevertheless, scientists have proposed that longer-term changes to the nature of the sunspot cycle may have been the cause of the Little Ice Age that occurred between the 16th and 19th centuries.