The Sun's brightness has been increasing steadily for the past 3 years. But there is no reason for alarm! The change is expected and astronomers predict that the Sun's radiation will start reducing again after peaking this year.

In an invited talk this week at the International Astronomical Union's General Assembly in Manchester (UK), Dr. Judith Lean of the Naval Research Laboratory in Washington DC explained how high precision instruments on board spacecraft have been monitoring the Sun since 1984 for the slightest changes. Building up a detailed picture of how the Sun's output varies over time will help determine how much (if any) of the global warming presently affecting the Earth is caused by long-term changes to the Sun.

The sun from 'Solarmax' movie. Photo: ESA

Astronomers at ground-based observatories have studied the Sun's activity for the past 400 years by counting the number of dark sunspots on its disk. Their records reveal a repeating cycle in which the numbers of sunspots hits a maximum approximately every 11 years. The two most recent cycles completed (identified as cycles nos. 21 and 22, corresponding roughly to 1974-85 and 1985-96), turned out to have two of the three highest peaks of activity ever recorded.

Sunspots are dark, and so reduce the total radiation from the Sun. But another manifestation of solar activity, called faculae, are bright and enhance the Sun's output. Typically, the increase due to faculae more than outweighs the decrease due to sunspots. Drawing on their understanding of the Sun's behaviour, astronomers can calculate the net effect of these two competing influences, quite separately from the observations.

Since 1997 we have been in solar cycle number 23. The Sun's activity and brightness has increased steadily. Now, near the predicted time of maximum, levels of radiation are 0.06 percent higher than in 1996. However, in 1990 - the same stage of the last cycle - the rise had been 0.09 percent. Dr. Lean said, "Levels of solar radiation appear not to have reached those seen in the two previous cycles. The consensus drawn from long-term observations in space, and independent calculations is that the increase will likely be about 30% less during the present activity cycle than during the peaks of two prior cycles."

However, she also offers a note of caution, highlighting the difficulty of the sensitive observations, made with instruments known as radiometers. "Not all radiometers agree about the level of solar radiation at the present time. Those on the SOHO spacecraft, for example, suggest larger increases in cycle 23 than 0.06 percent. Differences among radiometers due to slight changes in sensitivity remain a significant source of uncertainty in our knowledge of long-term solar radiation levels. For example, what was suspected to be a long-term increase in solar radiation between 1986 and 1996 disappeared when effects of this kind in the radiometers were identified and properly taken into account."

There appears to have been little or no long-term trend underlying the 11-year cycle of change in the Sun's radiation over the last 30 years. This is consistent with the findings from other ways of tracking solar activity. During this time, however, Earth's average surface temperature has increased by a few tenths of a degree. This suggests that factors other than the Sun are the primary cause of recent climate change. Even so, it is suspected that long-term changes in solar radiation contributed a few tenths of a degree of warming during the first part of the 20th century, and could contribute to future climate change. "To understand what that future change might be, we need to continue with high precision monitoring of the Sun's radiation" adds Dr. Lean.