Solar variation accounts for less than half of global warming in 20th Century, UA geoscientist finds

Press Release


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University of Arizona News Services
From: Melanie Lenart (for News Services)

Contact(s): Paul Damon, phone: 520-621-4659, fax: 520-621-2672, pedamon@geo.arizona.edu, office: Gould-Simpson Room 311 (4th St. East of Park Ave.)

October 2, 1997

It's a fact: Global temperatures have heated up by about a degree Fahrenheit in just over a century. But scientists are still trying to figure out whodunit -- the sun, with its cyclical variations, or humans, by blanketing the planet with more greenhouse gases?

Armchair detective work by University of Arizona geosciences Professor Emeritus Paul Damon and a colleague finds solar variation alone can explain only 40 percent of the warming measured in the 20th century through 1975. It explains even less of the heat spell of the last couple of decades.

''That rapid warming that occurred in the '80s and '90s, there's no way to fit it,'' Damon said. By default, then, he puts most of the culpability on human input of greenhouse gases via cars, factories and deforestation.

Damon's analysis, undertaken with assistance from Russian physicist Alexei Peristykh, attempts to pinpoint the cause behind a clear signal of global warming since about 1850. The observed temperature increase roughly coincides with both the invention of modern industry and a rise in solar activity. However, the increase goes beyond what would be expected from the sun's high-energy cycle, which Damon projects will peak in 2030.

Damon and Peristykh compared a variety of data to reach their conclusions. They used a well-known link between global radiocarbon production and solar activity to model sun cycles. Then they compared this model to temperatures measured in the last century or so as well as longer records of climate, such as those estimated from variations in tree-ring growth and changes in oxygen isotopes in cores of ancient ice.

A 250,000-year ice core record compiled by scientists working with Willy Dansgaard in Iceland indicates longterm temperatures fluctuated widely from glacial to interglacial periods, sometimes 9 degrees Fahrenheit at a time. But a few thousand years after the Younger Dryas glaciers retreated about 12,000 years ago, temperatures settled down, never straying more than 1-degree Fahrenheit from the global average.

''The important thing to remember is that global temperatures have been stable for the past 8,000 years, so it doesn't take much solar forcing to explain the record,'' Damon said.

Even when the Little Ice Age reached its coldest point in the span from 1570 to 1730, average global temperature appears to have cooled by no more than 2 degrees Fahrenheit, he noted. However, these relatively small temperature changes can loom large in human perception, in part because they fluctuate in certain regions much more than the world as a whole.

This long reign of stable climate balanced near the point of glaciation could end if predictions on the overall warming effect of polluting the atmosphere come to pass. International scientists agree that global temperatures can be expected to rise by about 2 to 8 degrees Fahrenheit by the end of the next century, mainly due to the continuing release of greenhouse gases.

The debate over which is most responsible for warming the planet -- something beyond human control, like solar variation, or something within our ability to change, such as fossil fuel use -- is heating up as international diplomats prepare to debate a policy response to global warming during a December conference to be held in Kyoto, Japan.

The current media attention on how much the sun is to blame for warming carries echoes of earlier debates. While a senator in 1992, Vice President Al Gore held a Senate hearing on the role of solar variability in global warming. Damon testified at the hearing, giving a similar verdict on the limitations of solar variability as an explanation for global warming.

The professor emeritus has been puzzling over this cosmic question almost since he first joined the University of Arizona in 1957. Although there is no direct record of solar variability, he was involved in a successful effort to create a reasonable proxy for it by plotting radiocarbon production. The greater the warmth of the sun, the fewer radiocarbon atoms produced.

Scientists can gauge its past production because this radioactive type of carbon, which has eight neutrons in each atom instead of the more common six, has a predictable life span. It exists for thousands of years before converting into a form of nitrogen when one of its neutrons transforms into a proton by emitting its electron.

Damon's longtime work in isotopes such as radiocarbon inspired international geologists attending the Goldschmidt Conference held in Tucson in June to honor him with a symposium dedicated to isotope geochemistry.


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