September 30, 1997
Solar radiation will not displace the dominant role of atmospheric carbon dioxide in global warming, but could be a significant contributing factor, according to the new report, by Richard C. Willson, senior research scientist at Columbia's Center for Climate Systems Research, in the Sept. 26 issue of the magazine Science.
Greenhouse warming, in which gases created by human activity trap more solar heat in the atmosphere, is expected to increase temperatures on Earth by about 3.6 degrees Fahrenheit over the next 50 to 100 years. By contrast, according to Dr. Willson, solar forcing -- the sun's effect on long-term climate -- might account for between 0.7 and 1.4 degrees of warming over the next 100 years, if sustained at the pace his observations suggest. The globe has already warmed by about one degree since 1880, scientists say.
"Solar forcing would provide only about one-fourth as much warming, if the solar trend persists over the same period," Dr. Willson said. "Solar forcing could be significant, but not dominant."
Solar luminosity is closely linked to sunspot activity, which waxes and wanes over a cycle that lasts about 10 to 11 years. The new research shows dramatically that total solar irradiance, or the total radiant power received by the Earth from the sun, was about 0.036 percent higher in 1996 than in 1986, a finding that may mean that the luminosity of each new solar cycle is stronger than the last.
"The change that Willson measures is small," said James E. Hansen, adjunct professor of earth and environmental science at Columbia and director of the NASA Goddard Institute for Space Studies. "By itself, it would not be a significant source of climate change, but the question is whether there are changes on the century time scale. There, the changes may be significant. His measurements are the first indication that there is long-term change."
Dr. Willson, who began the research at NASA's Jet Propulsion Laboratory in Pasadena, Calif., and now pursues it under NASA auspices from his home in Altadena, Calif., spliced together data from three probes in Earth orbit that together have monitored the sun since 1978: ACRIM I (Active Cavity Radiometer Irradiance Monitor) and ACRIM II, and the less precise Earth Radiation Budget Satellite (ERBS). The two ACRIM satellites were able to calibrate the degradation of their own sensors, a phenomenon that results from exposure to the sun's high-energy, short-wavelength radiation; the ERBS was not. Comparisons in space are essential in monitoring total solar irradiance precisely, Dr. Willson said.
ACRIM II was launched in September 1991, nearly two years behind schedule and after ACRIM I had already ceased gathering data, preventing comparisons between data from the two more sophisticated probes. To relate the measurements from the two ACRIM experiments precisely, Dr. Willson had to rely on ERBS data gathered during periods of overlap with the ACRIM sensors. Because the Earth's atmosphere absorbs or reflects much solar radiation, direct measurements of the sun's energy have only been possible since the late 1970s, when solar probes were lofted into orbit above the atmosphere.
The satellite data show that total solar irradiance reached a low point in 1986, climbed to a peak in about 1991, then declined to another low in 1996. "The significant finding is that the minimum total solar irradiance in 1996 is clearly higher than the one ten years earlier," Dr. Willson said.
What determines how much energy the sun's fusion mechanism pumps out is not well understood. Sunspots, the most visible evidence of solar dynamics, are cooler, darker regions that emit only 30 to 80 percent as much light as the normal sun, with temperatures ranging from 8,000 to 10,000 degrees F. Normal surface temperatures on the sun are about 11,000 degrees F. One of the discoveries of Dr. Willson's ACRIM I experiments was that periods of high sunspot activity, counter to intuition, actually result in increased total solar irradiance because sunspots are accompanied by regions of increased brightness that overpower the dimming from sunspots.
The greatest correlation between sunspot activity and Earth's climate occurred between 1640 and 1720, when solar activity in the form of sunspots and other phenomena declined and temperatures in northern Europe fell by two degrees Fahrenheit; the period is sometimes called the "Little Ice Age."
Dr. Willson holds a doctorate in atmospheric physics from the University of California, Los Angeles, and was a scientist at the California Institute of Technology until 1995, when he joined Columbia's Center for Climate Systems Research. He has developed versions of the active cavity radiometer and used them to measure total solar irradiance in balloon, rocket, space shuttle and satellite experiments, and expects to launch his ACRIM III probe in 1999 as part of the Earth Observation Program of NASA's Mission to Planet Earth. His work is supported by the National Aeronautics and Space Administration.
The Center for Climate Systems Research, the University's partnership with NASA Goddard Institute to understand and predict climate change, is a part of the Columbia Earth Institute, launched in January of this year to promote wise stewardship of our planet.