20 year global temperature record shows warming and cooling trends

provided by The University of Alabama in Huntsville


arth's atmosphere has gotten about one-tenth of a degree Celsius warmer in the past 20 years, with most of the warming concentrated in the Northern Hemisphere, according to data gathered by NOAA satellites and released by The University of Alabama in Huntsville.

All of the global warming trend found in the 20-year temperature dataset, however, can be attributed to the major El Niño, Pacific Ocean heating event of 1997-1998, says Dr. John Christy, an associate professor of atmospheric science in UAH's Earth System Science Laboratory.

"When we look at the first 19 years of satellite data, from 1979 through 1997, there is no global climate trend at all -- either up or down," Christy said "Due to the powerful El Niño, 1998 was a singular year.

"Obviously, El Niños are part of the natural weather cycle and shouldn't be discounted. When we look at long-term trends, however, we shouldn't assign excess importance to individual unusual or extreme short-term events, such as this El Niño or the cooling that followed the eruption of the Pinatubo volcano in 1991.

"You can't draw any conclusions about long-term global climate trends based on a heat wave here or a cold snap there.

"For instance, 1998 was easily the hottest year in the 20-year global temperature record and the 1998 average temperature in the contiguous 48 states appears to be the warmest since 1896. But the United States didn't have a single state record its statewide all-time high temperature in 1998. That didn't make 1998 any cooler from a global perspective.

"Some individual stations in the United States had record highs, but that happens frequently. There are more than 7,000 individual stations, so that's to be expected."

Globally, the temperature trend from January 1979 through December 1998 was warming at the rate of about 0.06 Celsius per decade. That equals a warming trend of just over one-half degree Celsius (about one degree Fahrenheit) per century.

Scientifically, any trend that small over a period of time as short as 20 years could be considered statistical "noise," according to Christy.

While large portions of the Northern Hemisphere showed a warming trend over the 20-year study period, most of the Southern oceans showed a long-term cooling. That split saw the Northern Hemisphere warm by about 0.2 C in 20 years, while the Southern Hemisphere got neither warmer nor cooler.

Almost all of North America, the North Atlantic and Europe saw temperatures warm slowly from 1979 through 1998. The region showing the "fastest" warming trend was along the coast of Canada's Northwest Territories, where temperatures warmed as much as 1.6 C (almost 2.9 Fahrenheit).

The next fastest warming region included most of Siberia, Mongolia, Northern China, Korea and Japan, with temperatures rising as much as 1.4 C (more than 2.5 Fahrenheit) in 20 years.

At the same time, 20-year cooling trends were found over Labrador, Antarctica, most of the Southern oceans, and a region stretching eastward from Central Africa across the Middle East, the Indian subcontinent and Southeast Asia.

While most of the noteworthy warming in the past 20 years was over the more industrial and more heavily populated Northern Hemisphere, Christy said it is unlikely that this regional warming can be attributed to man-made air pollution or an enhanced "greenhouse effect."

Burning coal, petroleum products, natural gas and wood returns carbon dioxide to the atmosphere. Carbon dioxide is a greenhouse gas which traps heat rising from Earth's surface. In recent years, climate scientists have theorized that the additional carbon dioxide might create an enhanced greenhouse effect, which would cause global warming.

While carbon dioxide levels in the atmosphere have risen in recent years due to burning fossil fuels, the extra CO2 gas has generally spread almost evenly around the globe instead of concentrating in the Northern Hemisphere.

What does stay in the atmosphere over the Northern Hemisphere is sulfate pollution caused by burning fossil fuels, Christy said. Sulfate particles, which generally stay in the atmosphere less than two weeks, reflect sunlight back into space. This should cause a cooling effect that would be strongest near and down wind from the site of the pollution.

As part of an ongoing NASA/UAH joint research project, Christy and Dr. Roy Spencer, a space scientist in the Earth Science Laboratory of the Global Hydrology and Climate Center in NASA's Marshall Space Flight Center, use data gathered by microwave sounding units on NOAA's TIROS-N satellites to get accurate temperature readings for almost all regions of the Earth. This includes remote ocean, desert and jungle regions for which reliable temperature data are not otherwise available.

The satellite-based instruments measure the temperature of the atmosphere from the surface to an altitude of about eight kilometers above sea level. Once the monthly temperature data is collected and processed, it is placed in a "public" computer file for immediate access by atmospheric scientists in the United States and abroad.

Neither Spencer nor Christy receives any research support or funding from oil, coal or industrial companies or organizations, or from any private or special interest groups.

  Copyright 1999 The University of Alabama in Huntsville Dr. John Christy, (256) 922-5763; Phillip Gentry, (256) 890-6414.