Temperatures Are Likely to Go From Warm to Warmer
By ANDREW C. REVKIN
Published: December 31, 2002
Climate experts say global temperatures in 2003 could match or beat the modern record set in 1998, when temperatures were raised sharply by El Niño, a periodic disturbance of Pacific Ocean currents that warms the atmosphere.
El Niño that year was the strongest ever measured. A new one is brewing in the Pacific but is expected to remain relatively weak, experts say. Still, they say, a persistent underlying warming trend could be enough to push temperatures to record highs.
Some of the warming could be the result of natural climate variation, but the experts say it is almost impossible to explain without including the heat-trapping properties of rising levels of carbon dioxide and other greenhouse gases emitted by smokestacks and tailpipes.
The mounting evidence of human contributions to climate warming has raised pressure on American policy makers to reconsider their reliance on voluntary measures for reducing heat-trapping emissions.
At a meeting of climate scientists organized by the Bush administration this month, White House officials said President Bush was no longer locked into the stance he announced last year -- calling for nothing beyond voluntary measures to slow the growth in emissions until 2012.
And Senator John McCain, Republican of Arizona, and Senator Joseph I. Lieberman, Democrat of Connecticut, plan to introduce legislation early in 2003 that would gradually establish mandatory greenhouse gas restrictions and a system in which companies could trade credits they would earn by making emissions cuts.
The European Union, Japan and most other industrial powers have ratified the Kyoto Protocol, a treaty that, once in effect, will require them to make reductions.
The growing shift toward action in the American debate over greenhouse emissions comes after a decade of mounting evidence that the recent warming is caused mainly by rising concentrations of such substances.
The main means of tracking climate change has been to synthesize hundreds of measurements of surface temperatures around the world into a global average.
This average reading is meaningless for any particular spot, but it is a valuable way to measure long-term trends, and it puts the planet in its warmest period in a millennium, with the trajectory still headed upward.
According to the Commerce Department, the global average surface temperature increased at a rate of about one degree per 100 years over the 20th century, but since 1976 the earth has been warming at the rate of about three degrees per century.
The Hadley Center for Climate Prediction and Research in Britain put the odds at 50-50 for 2003 to match or exceed the temperature record set in 1998. Dr. James E. Hansen, the director of the National Aeronautics and Space Administration's Goddard Institute for Space Studies, put the odds higher than that, barring a big new sun-blocking volcano or the like.
A decade-long paucity of big volcanic eruptions and a peak in solar intensity can account for only part of the overall warming, he said, adding, ''Clearly it's primarily due to human forcing.''
The global average temperature reached 58.0 degrees in 1998, while the average from 1880 to 2001 was 56.9 degrees.
Preliminary estimates put the global temperature in 2002 at 57.9 degrees.
Areas like Alaska have experienced sharper warming, in patterns that largely match projections produced by computer simulations of the climatic effect of rising greenhouse gases.
The National Oceanic and Atmospheric Administration reported this month that satellite tracking of surface conditions on Greenland's vast ice sheet saw more melting last summer than at any time in the 24-year satellite record.
Arctic sea ice also retreated more than it had done before in that span, the agency said.
These time spans are short when it comes to climate, and polar experts say it remains exceedingly difficult to ascribe regional changes to human actions rather than natural cycles in ocean and weather circulation around the Northern Hemisphere.
The continuing global climb in temperatures, however, is getting harder to link to natural climate fluctuations, many scientists say.
Even if 2003 does not set a record, many experts say, it almost undoubtedly will follow a generation-long rise in temperatures that has put the planet on course for substantial shifts in drought and storm patterns, continuing and significant retreats of terrestrial ice, and a resulting rise in sea levels in coming decades.
This month, American and British climate teams and the World Meteorological Organization reported that 2002 would nudge out 2001 as the second-warmest year since the late 1800's.
Some scientists still doubt that the human influence will alter the climate beyond the range of natural variability, which they say has produced significant shifts in past eras and will inevitably do so again.
''We don't really know enough about the climate to say with any confidence how much of this warming is natural and how much is caused by human activities,'' said Dr. John R. Christy, the director of the Earth System Science Center at the University of Alabama in Huntsville.
But his view is held by an ever-shrinking minority of climate experts, partly because new analyses are questioning some of their conclusions.
In 1990, Dr. Christy and a team that included NASA satellite experts pioneered a method for measuring the average temperature of the atmosphere above the surface, using instruments on weather satellites.
In a series of papers examining three decades of satellite data, they reported cooling or only slight warming, and the findings were highlighted by skeptics of the greenhouse theory among climatologists and policy makers.
A new analysis of the same data by an independent team of scientists suggests that much more warming is under way in the upper atmosphere -- more than three times as much as Dr. Christy estimated. These analyses are more in line with surface trends and estimates produced by computer models.
The new results were described in a news release this month by the Commerce Department but have not yet been published in a peer-reviewed journal.
The scientific teams with differing views of the satellite temperature data argued at several scientific meetings this year, including the meeting convened this month by the administration to set priorities for climate research.
At the meeting, Dr. Christy and the head of the other group, Frank J. Wentz, the founder of Remote Sensing Systems, a company that analyzes satellite data for the government, agreed to share more data and information on the way they arrived at their results.
While the debate about the amount of atmospheric warming plays out, there is little disagreement about the extent of warming at the surface.
The shifts around the Arctic -- whether natural or human-induced -- are profound, said Dr. Waleed Abdalati, NASA's director of polar programs.
''The Greenland melting or the changes in ocean circulation or sea ice, any one of those is kind of a 'Wow, that's interesting,' '' he said.
''But when you see them collectively and kind of working in concert with one another,'' Dr. Abdalati added, ''that's very significant.''
The result could be some significant surprises, he said.
This year, Dr. Abdalati was a co-author of a study showing that the surface melting in Greenland, for example, was unexpectedly accelerating the seaward crawl of the ice sheet as the melt water percolated down through more than a half mile of ice and lubricated the interface between the grinding sheet and the rock below.
Should the Greenland ice continue to accelerate, that could require scientists to change their projections of how much a little warming could raise sea levels.
Photo: The surface of the vast ice sheet in Greenland melted more last summer than at any time in the 24 years that conditions had been tracked. (Roger J. Braithwaite/University of Manchester) Chart: ''Warming on Earth, and Above'' Although the earth's surface has warmed, analysis of satellite data by NASA and university researchers in Huntsville, Ala., failed to show much warming in the troposphere. But a new analysis finds more warming. Chart shows the a new analysis is finding that the earth's surface is warmer and the troposhere. (Sources: National Oceanic and Atmospheric Administration, NASA)