A surprising thing happened to global warming on the way to the databank. Scientists expected the growth rate of the climate effect of heat-trapping greenhouse gases to accelerate. Instead, it has declined about 25 percent since 1980.
Global warming hasn't stopped. But the latest analyses suggest humanity may have more time to cope with what many scientists think is inevitable manmade climate change.
No one knows why this has happened. There has been no decline in carbon dioxide (CO2) emissions. Yet the trend includes a leveling off of the growth rate of atmospheric CO2 as well as a drop in the growth of methane. If the trend continues, atmospheric CO2 won't double by 2050 as many global warming forecasts anticipate. It will be early in the 22nd century before that occurs, says climatologist James Hansen, who heads NASA's Goddard Institute for Space Studies in New York.
At the 160-nation climate summit held in Kyoto, Japan, in December 1997, industrialized countries agreed to cut emissions of CO2, methane, and nitrous oxide - the main climate forcers - below 1990 levels within 10 to 14 years. They are still arguing over how to make the cuts. Now, currently unknown climate processes have slowed the greenhouse gas buildup for them.
For Dr. Hansen and other climate scientists, this intriguing mystery highlights the fact that decades of progress in their science have left them with major knowledge gaps. It shows that "our understanding of ... greenhouse gases is not all that good," Hansen says. He explains that "we really have to understand the cycles of these greenhouse gases if we're going to reliably forecast what's going to happen in the next century."
Even the well-documented global warming trend has its puzzles. There's little dispute that Earth's annual average surface temperature has warmed about a degree Fahrenheit in this century. A slow cooling over the past few thousand years, which may have been headed toward a new ice age, has suddenly reversed. Many scientists suspect that manmade greenhouse gas pollution is at least partly responsible for that reversal.
Why then has most of the US - a major CO2 polluter - "been cooling while the rest of the world as a whole is in its period of most rapid warming?" Hansen asks. He also wonders, "Is it likely that there will be a sudden dramatic shift to warmer conditions in the United States if global warming continues?"
Climate scientists would like to keep Earth under constant detailed surveillance. Their wish is about to be realized. NASA and its international partners plan to orbit a fleet of satellites, the Earth Observing System. Between them, they will monitor many aspects of Earth as an interlinked geophysical and biological system and the sun whose energy drives it. Two units are already in orbit - Landsat 7, a mapping satellite, and Quicksat, launched last month to monitor ocean winds and clouds.
The flagship will be the Terra satellite, to be launched later this year. NASA supplied the spacecraft and three of its five instruments. Canada and Japan contributed the other two. Until now, the Landsat series of mapping satellites has provided the only continuous record of largely geographical changes over the globe for 26 years. Orbiting over the poles like Landsat, Terra should return global data of unprecedented accuracy for many more factors important for climate change.
V. Ramanathan from the Scripps Institution of Oceanography in La Jolla, Calif., describes Terra's mission as a systematic attempt to pin down things that are "key to forecasting" climate change. He explains that the flow of such high quality measurements from the entire Earth surveillance effort will catalyze climate science over the next decade. Then, he said, "some giants will take the voluminous data ... and solve some of the vexing environmental problems."
Climate researchers also look forward to a unique satellite first proposed by Vice President Al Gore. Now called Triana, it will sent by NASA about a million miles out from Earth to a point called L1. There gravitational forces hold a satellite in a stable location. Triana will view the entire sunlit side of Earth as it rotates through 24 hours. It will transmit daily images of cloud cover and cloud height, water vapor, ozone, dust, and vegetation. Objects as small as 8 kilometers across can be imaged at every point on Earth. Triana will also monitor the sun.
Climate scientists now look forward to answering their big questions: What does the past reveal about changes? What are the patterns and modes of human-forced climate change? What are the multiple stresses on Earth's global systems and how to they interact?
The fact that scientists haven't been able to provide good answers is "seriously blocking" their ability to advise decision-makers on climate-related issues, Scripps director Charles Kennel said.
Meanwhile, those decisionmakers still have to decide what to do now, if anything, to mitigate long-term climate change. The fact that greenhouse gases are not building up as fast as expected does not let them off the hook, says Jerry Mahlman, director of the Geophysical Fluid Dynamics Laboratory in Princeton, N.J., a leading center for computer-based climate simulations.
"How much change will we bargain for?" he asks. While little can be done in the long run to avoid doubling CO2, how much more of an increase do we want beyond that in coming centuries? "This is the real emerging greenhouse controversy," Dr. Mahlman says.
(c) Copyright 1999. The Christian Science Publishing Society