Like thermometers in space satellites are taking the temperature of the Earth's surface or skin. According to scientists, the satellite data confirms the Earth has had an increasing "fever" for decades.

For the first time, satellites have been used to develop an 18- year record (1981-1998) of global land surface temperatures. The record provides additional proof Earth's snow-free land surfaces have, on average, warmed during this time period, according to a NASA study appearing in the March issue of the Bulletin of the American Meteorological Society. The satellite record is more detailed and comprehensive than previously available ground measurements. The satellite data will be necessary to improve climate analyses and computer modeling.

Menglin Jin, the lead author, is a visiting scientist at NASA's Goddard Space Flight Center, Greenbelt, Md., and a researcher with the University of Maryland, College Park, Md. Jin commented until now global land surface temperatures used in climate change studies were derived from thousands of on-the- ground World Meteorological Organization (WMO) stations located around the world, a relatively sparse set of readings given Earth's size. These stations actually measure surface air temperature at two to three meters above land, instead of skin temperatures. The satellite skin temperature dataset is a good complement to the traditional ways of measuring temperatures.

A long-term skin temperature data set will be essential to illustrate global as well as regional climate variations. Together with other satellite measurements, such as land cover, cloud, precipitation, and sea surface temperature measurements, researchers can further study the mechanisms responsible for land surface warming.

Furthermore, satellite skin temperatures have global coverage at high resolutions, and are not limited by political boundaries. The study uses Advanced Very High Resolution Radiometer Land Pathfinder data, jointly created by NASA and the National Oceanic and Atmospheric Administration (NOAA) through NASA's Earth Observing System Program Office. It also uses recently available NASA Moderate Resolution Imaging Spectroradiometer skin temperature measurements, as well as NOAA TIROS Operational Vertical Sounder (TOVS) data for validation purposes. All these data are archived at NASA's Distributed Active Archive Center.

Inter-annually, the 18-year Pathfinder data in this study showed global average temperature increases of 0.43 Celsius (C) (0.77 Fahrenheit (F)) per decade. By comparison, ground station data (2 meter surface air temperatures) showed a rise of 0.34 C (0.61 F) per decade, and a National Center for Environmental Prediction reanalysis of land surface skin temperature showed a similar trend of increasing temperatures, in this case 0.28 C (0.5 F) per decade. Skin temperatures from TOVS also prove an increasing trend in global land surfaces. Regional trends show more variations.

"Although an increasing trend has been observed from the global average, the regional changes can be very different," Jin said. "While many regions were warming, central continental regions in North America and Asia were actually cooling."

One issue with the dataset is that it cannot detect surface temperatures over snow. In winter, most of the land areas in the mid to upper latitudes of the Northern Hemisphere are covered by snow. Of Earth's land area, 90 percent of it is snow free in July, compared to only 65 percent in January. For this reason, the study only focused on snow free areas. Still, in mountainous areas that are hard to monitor, like Tibet, satellites can detect the extent of snow coverage and its variations.

The satellite dataset allows researchers to also look at daily trends on global and regional scales. The largest daily variation was above 35.0 C (63 F) at tropical and sub-tropical desert areas for a July 1988 sample, with decreasing daily ranges towards the poles, in general. Daily changes were also closely related to vegetation cover. The daily skin temperature range showed a decreasing global mean trend over the 18-year period, resulting from greater temperature increases at night compared to daytime.

Things like clouds, volcanic eruptions, and other factors gave false readings of land temperatures, but scientists factored those out to make the skin temperature data more accurate. Scientists are considering extending this 18-year satellite- derived skin temperature record up to 2003. The mission of NASA's Earth Science Enterprise is to develop a scientific understanding of the Earth system and its response to natural or human-induced changes to enable improved prediction capability for climate, weather, and natural hazards. NASA funded the study.