Professor John Christy's work on global warming has turned heads, but it's difficult to say what it means.
Are humans responsible for global warming? Is the planet heating up?
Ask John Christy and he will answer: No, and no.
A professor of atmospheric science at the University of Alabama in Huntsville, Christy is a member of the Intergovernmental Panel on Climate Change established by the United Nations 12 years ago. As such, he is one of the world's pre-eminent experts on atmospheric conditions. Yet Christy is something of a maverick. Years ago he cast doubt on the idea that global warming is caused by humans _ or that the phenomenon exists at all.
"The usual predictions show escalating atmospheric temperatures, and we're just not seeing that rise," he says. "This indicates that the cause of recent surface warming may be due to factors other than human activities."
Christy backs up his hypotheses with rigorously vetted data from satellites and weather stations around the globe.
To most people, numbers are hard and immutable. But Christy knows that they're subject to interpretation _ and misinterpretation. For years, he listened as politicians and scientists spoke of rising global temperatures. But he questioned the data behind their predictions. He knew that buildings had been erected around thermometers and that nearby forests had been cut down, driving up recorded temperatures independently of any global climate change.
He knew that scientists didn't have a way to get accurate readings of temperatures above the Earth's surface, so they didn't know what was happening to the lower troposphere, the first 5 miles of air hovering above Earth.
To fill in that vast gap, Christy began to work with Roy Spencer, a satellite meteorologist at NASA's Marshall Space Flight Center in Huntsville, extracting data from polar-orbiting satellites. The satellites carried instruments that measure the intensity of microwave radiation emitted by oxygen.
Weather forecasters had used this data in a limited way to record temperatures at 20 different levels of the atmosphere but never to get a global average for the troposphere. Figuring out how to infer tropospheric temperature from the data was Christy and Spencer's genius.
In theory, if the atmosphere heats up like a giant greenhouse, then the troposphere ought to be warming as rapidly as the Earth's surface, if not faster. According to Spencer and Christy's satellite data, however, the lower troposphere was surprisingly cool. Since 1979, it had warmed only 0.2 degree Fahrenheit, whereas the surface had warmed between 0.48 and 0.7 degrees Fahrenheit. The disparity suggested to Christy that prevailing climate models were wrong.
When he published his figures in 1990, Christy was attacked in scientific journals, in the media, and as the years passed, on the Web. But he and Spencer have fought back. Their weapon: math. In the end, Christy says, the errors fell to less than a tenth of a degree for the 20-year period.
Eventually, scientific opinion turned in Spencer and Christy's favor. In 1996, the American Meteorological Society presented them with an award for "fundamentally advancing our ability to monitor climate." In 1997, the Hadley Centre for Climate Prediction and Research in England independently verified their data.
While his data have won praise, his conclusions have not. In a report released on Jan. 12, 2000, the National Research Council declared that the disparity between surface and troposphere temperatures is probably real, but that it's difficult to say why it exists or what it means.
More galling for Christy is a report by the U.N. climate panel, a document that Christy himself co-wrote, that essentially contradicts his interpretation of the surface data. The burning of fossil fuels has "contributed substantially to the observed warming over the last 50 years," the report concludes. Moreover, it warns that temperatures could rise even higher than previously predicted.
Christy and panel members like James E. Hansen, director of NASA's Goddard Institute for Space Studies, agree that temperatures on the surface and in the troposphere don't necessarily move in lockstep. But Hansen believes that the troposphere will continue to warm. The discrepancy that Christy found, he says, will disappear as climate models and measurements improve.
Christy thinks it equally likely that the Earth's surface will cool. The surface warming that alarms so many atmospheric scientists is, to Christy, well within the realm of natural variation, or measurement error. "Most of this warming occurred in the early part of the 20th century, before humans had boosted concentrations of greenhouse gases," he says. Sunspots, volcanic eruptions, El Ninos, variations in aerosols, water vapor and other unknown factors may all tweak the planet's temperature up and down, Christy says.