A star in the greenhouse; can the sun dampen the predicted global warming? - use of solar observation in research

Science News, Oct 24, 1992 by Richard Monastersky

To test this idea, the researchers used calcium emissiosn as a poor man's index of the sun's total brightness. Because past studies have examined variations in calcium emissions from different parts of the sun, the researchers could calculate how the emission would dim if they hypothetically removed all the magnetic activity including the network. In this case, calcium emissions dropped, but not enough to make the sun look like one of the noncycling stars.

If Baliunas and Jastrow are correct and the sun really did enter a noncycling mode during the Maunder minimum, then something else must have happened in the sun aside from losing the network. To get the calcium emissions down into the range of emissions from the noncycling stars, Lean and her co-workers determined that the sun's total face would have to dim substantially--enough to match the darkest one-ninth of the current solar disk. If this were to happen, total solar output would drop about 0.24 percent below its current levels, they report in the Aug. 3 GEOPHYSICAL RESEARCH LETTERS.

That may be enough to account for the cooling during the Litttle Ice Age. Calculations suggest that a dimming of 0.25 percent would chill Earth's climate by about 0.2 [degrees] C to 0.6 [degrees] C--roughly the amount that scientists think the temperature dropped relative to conditions before the Little Ice Age.

But today's temperatures rest about a full degree higher than those of the Little Ice Age. So despite the correlation found by the danish workers, the work by Lean and her colleagues suggests the sun could not, on its own, explain all the warming from Little Ice Age to the present. This finding supports the theory that rising concentrations of greenhouse gases caused much of the climate warmingd since the late 19th century.

Lean's results concerning potential solar behavior also have implications for how the sun were to dim by 0.25 percent, it would certainly exert a cooling effect on the climate, but that effect would be a modest one. "Given our contemporary understanding, the sun is not going to ever vary enough to counter the greenhouse warming," says Lean. Energy calculations as well as sensitivity experiments with general circulation models suggest the sun would have to cool by roughly 2 percent to make up for a doubling in carbon dioxide, she says.

Lean and her colleagues readily admit that their work is speculative. "Statistically, it's very difficult because there's not much information. We put it out as a speculation, as a technique that we could apply to make some estimate of what's going on," says one of Lean's coauthors, Oran White of the National Center for Atmospheric Research in Boulder, Colo.

But despite the uncertainties in their approach, Lean thinks it offers a more informed picture than less regorous efforts. As an example of these, she mentions the 1989 Marshall Institute report, which suggested that the sun's output could change by 0.5 percent. The report also claimed that solar claimed that solar activity should decrease in the 21st century and exert a major cooling force on the climate. Jastrow, who worked with Baliunas on the star comparison study, sits on the board of the Marshall Institute.