Experts hope to clear up warming controversy

At Wednesday's seminar, two scientists hope to clarify that, during the past few decades, solar radiation has played a relatively minor role in climate change in comparison to greenhouse gases.

November 24, 1999
Web posted at: 2:14 p.m. EST (1914 GMT)

Two scientists hope to clarify that greenhouse gases really are the driving force behind recent climate change and that, especially during the past few decades, solar radiation has played a relatively minor role in this global problem.

Dr. Judith Lean and Dr. Jerry Mahlman will be the lead speakers Wednesday at a seminar devoted to this topic hosted by the U.S Global Climate Change Research Program.

The extent to which climate change is a problem caused by human activity or by natural fluctuations in solar radiation, or other mechanisms such as volcanic eruptions, is very controversial. According to USGCRP's Dr. Anthony Socci who is helping facilitate the seminar, reputable media and non-scientific sources have made some disturbing claims that the sun is in fact completely to blame for the entire 20th century surface warming. "We hope to clear up some of this confusion," said Socci.

Expert members of the scientific community, Lean and Mahlman intend to explain that when the climate warming energy represented by changes in the concentration of greenhouse gases is compared to that which is exerted by changes in the sun's radiation, the sun's effect is quite small. This is true at least during recent decades when direct measurements of solar radiation exist.

Lean, a research physicist in the Space Science Division of the Naval Research Laboratory in Washington, D.C., is an expert on solar radiation. She has done extensive research on the mechanisms and measurements of variations in the sun's radiative output, and the effects of this variability on the Earth's global climate and space weather. She has also worked on projects geared toward better understanding of how the sun affects Earth climate in relation to the influences of greenhouse gases and ozone depletion. Lean has testified before Congress on the role of solar radiation and fluctuations of solar energy in climate change, and chaired a National Academy of Sciences working group on the topic.

Space-based measurements have shown that the sun's output fluctuates on an 11-year cycle. In her presentation, Lean will discuss how she and her colleagues have looked carefully at the historical record of the sun's varying activity levels, including direct observations of solar radiation over the last 20 years and indirect evidence of solar activity implied through the study of ice cores and tree rings. Lean, and collaborator Dr. David Rind of NASA's Goddard Institute for Space Studies in New York, made simulations with computer models of climate change in response to changes in solar radiation during the past 400 years. They then used the model results to compare with both pre-industrial and current climate change trends to determine the role of the sun in the heating on the Earth.

The general conclusion of their study is that the sun may have played a dominant role in pre-industrial climate change (from 1600 to 1800, for example) but it has not played a significant part in long-term climate change during the past few decades. It is furthermore unlikely that the sun accounted for more than half, at most, of climate change from 1900 to 1970. "A larger role for the sun in explaining the observed climate warming over the 20th century is inconsistent with direct measurements of solar output and with proxy evidence of solar variability during the pre-industrial era," said Lean in an abstract prepared for seminar attendees.

The extent to which climate change is a problem caused by human activity or by natural fluctuations in solar radiation, or other mechanisms such as volcanic eruptions, is very controversial.

"Because the sun's radiation varies continuously, determining its role in future climate change will require continuous, direct observations of solar radiation from space," said Lean.

Mahlman is the director of the National Oceanic and Atmospheric Administration Geophysical Fluid Dynamics Laboratory, one of the country's largest and most prestigious climate modeling labs. His discussion will center on model calculations that were used to examine the varying effects of natural variability, carbon dioxide and other greenhouse gases, sulfate particles and changing solar output on the climate of the 20th century.

"In general, these calculations make it clear that it is scientifically very difficult to construct an explanation for the 20th century warming that does not include a major role for the added greenhouse gases resulting from human activities," said Mahlman in a document prepared for seminar attendees.

Based on his study of the model calculations and climate change observations in the 20th century, Mahlman will offer the following conclusions during his presentation:

• Global climate of the 20th century has warmed by 0.7-0.8 degrees C.
• Natural (unforced) climate variability cannot explain the magnitude of the observed warming over the 20th century.
• Solar irradiance variations are large enough to shape, but not dominate, the observed warming.
• The extended warming period between 1910-1940 can be explained by natural variability plus added greenhouse gases. It can also be explained by added greenhouse gases plus increased solar irradiance.
• Added greenhouse gases provide, by far, the most plausible hypothesis for explaining the warming of the 20th century.

Wednesday's seminar at the Dirksen Senate Office Building in Washington, D.C., is one in a series hosted by the USGCRP. The seminar series is a vehicle for disseminating information collected by members of the scientific community who are actually studying climate change and have been for the last 20 years.

The USGCRP was created as a result of the 1990 Global Change Research Act. Made up of a coalition of 10 to 12 federal agencies, USGCRP's mission is to investigate the environmental stresses on the Earth, including climate change, ozone depletion, land cover and land use issues, and biodiversity. USGCRP presents the results of its research activities to Congress on a yearly basis.

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