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Satellite Thermometers Show Earth Has a Fever
Turns out moms and dads aren't the only ones whom rely on thermometers to test for a "fever" ...scientists do too! Thanks to satellite technology, climate researchers now have more evidence that our planet has experienced abnormal warmth for at least two decades.

Global land surface temperature, July 2003
Image Above: Click image above to see larger picture. Global land surface temperature, July 2003. This image shows land surface temperature for the entire month of July 2003, one of the warmest months on record throughout much of Europe. This image was derived using data from NASA's Moderate Resolution Imaging Spectroradiometer (MODIS) sensor. Credit: NASA

Recognizing previous climate studies relied on temperature data from the relatively few observing stations around the globe, Menglin Jin -- visiting scientist at NASA's Goddard Space Flight Center -- set out to use satellite-based data for a more complete and detailed representation of the Earth's temperatures.

Satellite Data Fills in the Gaps

Satellites were used to develop an 18-year record (1981-1998) of global land surface temperatures. The data showed that the Earth's snow-free land surfaces, on average, warmed during the period. The findings are in a NASA study published in a recent issue of the Bulletin of the American Meteorological Society.

Unlike scattered ground-based weather observing stations that measure the temperature two to three meters above land, satellites measure ground surface temperature, or skin temperature, that is essential in assessing regional and global climate change. Other supplementary satellite measurements, such as land cover, cloud, precipitation and sea surface temperatures can then be used to give scientists many of the tools they need to determine the causes of land surface warming.

The study used data from the National Oceanic and Atmospheric Administration (NOAA)/NASA Pathfinder Advanced Very High Resolution Radiometer (AVHRR) land dataset; NASA's Moderate Resolution Imaging Spectroradiometer (MODIS) skin temperature measurements; and NOAA's TIROS Operational Vertical Sounder (TOVS) data, for validation.

A Wealth of Data

The 18-year Pathfinder data in the study showed that average global temperatures increased 0.43 Celsius (C) or 0.77 Fahrenheit (F) per decade, while ground station data indicated a rise of 0.34 C (0.61 F). A re-analysis of skin temperature data completed by the National Center for Environmental Prediction (NCEP) showed a similar trend, with temperatures rising 0.28 C (0.50 F) per decade.

Not surprisingly, however, the research did find variations at the regional level. "While many regions were warming, central continental regions in North America and Asia were actually cooling," Jin said.

Land surface temperatures Image to Left: This image shows the differences in daytime land surface temperatures collected between July 2001 and July 2003 by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Terra satellite. A blanket of deep red across southern and eastern France (left of image center) shows where temperatures were 10 degrees Celsius (18 degrees Fahrenheit) hotter this summer. White areas show where temperatures were similar, and blue shows where temperatures were cooler in 2003 than 2001. Credit: NASA

The satellite dataset's coverage and high resolution also allowed Jin to look at daily, seasonal and annual temperature trends on global and regional scales. The greatest daily temperature range, largely dependent on vegetation cover, was 35.0 C (63.0 F), in tropical and sub-tropical desert areas, with decreasing daily ranges toward the poles. The daily skin temperature range showed a decreasing global mean trend over the 18-year period, resulting from greater temperature increases at nighttime compared to daytime.

Although this technology can't detect surface temperatures over snow, it can see the extent of snow coverage and its variations in mountainous areas that are typically hard to monitor. The study also corrected any skewed skin temperature data created by cloud cover, aerosols, volcanic eruptions, and extreme climate events, such as El Nino.

While limited to a few years, skin temperature measurement data are extremely useful since they are improvements on existing data. Such satellite-based measurements are also necessary in improving climate analyses and computer models. Consistency with other research and technology, including TOVS, its global coverage, and easy data access suggest skin temperature observations will continue to be valuable in future climate change studies.

Related Links:

NASA Goddard Space Flight Center
NOAA/NASA Pathfinder Land Data Sets
NASA Moderate Resolution Imaging Spectroradiometer skin temperature data

Mike Bettwy
Goddard Space Flight Center