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Little Global-Scale Warming Seen During Past 22 Years

Freshly updated global temperature measurements combined with evidence from new research continues to show little global-scale warming of the atmosphere during the past 22 years, a scientist from The University of Alabama in Huntsville (UAH) reported on Monday.

"In looking at all of the pieces of the puzzle, we see a picture of the past 22 years that we hadn't anticipated -- that the bulk of the atmosphere has shown very little warming," said Dr. John Christy, a professor of atmospheric science and director of UAH's Earth System Science Center. "We see warming over the northern third of the globe both at the surface and in the five-mile-deep layer of air above.

"For the bulk of the atmosphere, however, we see a general cooling trend over the remaining two-thirds of the globe, from 20 degrees north latitude to the South Pole."

Christy's discussion of the data, part of an update related to the United Nations Intergovernmental Panel on Climate Change 2001 Assessment, came on Tuesday in Albuquerque, NM, at the annual meeting of the American Meteorological Society. Christy is one of eight "lead authors" of the 2001 report. He serves as a lead author for the chapter on observed climate variability and change.

"For people living north of 20 degrees north, which includes most of the world's population, the perception of warming over the past 22 years is real, both at the surface and in the air above," Christy said. "But that isnıt seen on a global scale for the bulk of the atmosphere."

Recent research provides new pieces to a global climate puzzle that has gone unsolved since eleven years ago, when Christy and Dr. Roy Spencer, a NASA space scientist at the National Space Science and Technology Center, developed a technique to use NOAA satellites to monitor temperatures in the atmosphere.

Satellite measurements of temperatures in the troposphere, the lowest five miles of the atmosphere, as well as independent measurements from balloons, show no evidence of significant global-scale warming.

That's at odds with surface temperature records, which show warming during the same period of time. The apparent disagreement between the two datasets has been the source of scientific investigation and controversy.

The recent research includes regional comparisons of temperature trends from both surface readings and satellite data. That analysis found that the surface and satellite long-term climate trends match for North America, Europe and Australia -- regions with extensive and reliable networks for gathering air temperatures.

"However, where a significant fraction of the data is sea surface temperatures rather than air temperatures, the surface measurements tend to show significantly more warming than the tropospheric temperatures," Christy said.

Other research included an analysis of temperature data gathered by buoys scattered around the tropical Pacific Ocean. Each buoy has thermometers measuring temperatures of both the sea surface at one meter depth and the air three meters above the sea surface.

For each buoy, the long-term climate trend reported by each instrument was different. For multi-buoy averages, the difference between air and sea climate trends was as much as 0.15 degrees Celsius per decade over a span of only four meters.

"A detailed examination of temperatures just above the sea surface and at the sea surface for the tropical third of the globe shows that the air right next to the sea surface isn't warming as fast as the water," Christy said. "The weight of this new research result adds evidence that the satellite and balloon-based climate records are reliable. They also raise questions about the viability of sea surface temperatures as a proxy for tracking air temperature variations over long time periods."

Traditionally, sea water temperatures have been used for tracking climate changes over the oceans because problems related to sea temperature measurement by ships were believed to be less troublesome than problems related to measuring air temperatures on deck.

Air temperature data gathered at sea can experience exposure problems, including heat contamination from a shipıs superstructure or the varying altitude of thermometers from ship to ship. Water temperature data are subject to vagaries including varying depths at which the water is collected and different types of measuring systems, from buckets to engine intakes.

Earlier, Christy and others used data gathered using weather balloons to confirm the accuracy of satellite readings of temperatures in the troposphere. If the new data shows that the satellite measurements are accurate over regions where there is reliable surface temperature data, that implies the satellite sensors are also accurate and reliable over the rest of the globe, although atmospheric and surface trends may vary for specific regions.

This may improve the scientific community's confidence in the satellite data, which cover more than 95 percent of the globe, Christy said. The satellite data coverage includes remote ocean, desert and wilderness regions for which climate data are either scarce or not available at all.

During the past 22 years, the satellite dataset shows a warming trend of about 0.22 degrees C (0.4 degrees Fahrenheit) per decade for the northern third of the globe, Christy said. That covers the area from the North Pole to 20 degrees north latitude, including all of North America, Cuba and most of Mexico, all of Europe, the northern half of Saharan Africa and most of Asia.

The satellite data show that the atmosphere over the southern two-thirds of the globe has actually cooled by about 0.04 degrees C per decade over the past 22 years.

Looked at as a composite, the satellites show a "global" warming trend of about 0.04 degrees C per decade -- with the bulk of that warming concentrated in the Northern Hemisphere.






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