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Tuesday, August 14, 2001 New NASA Satellite Sensor and Field Experiment Shows Aerosols Cool the Surface but Warm the Atmosphere Scripps Institution of Oceanography / University of California, San Diego font face="Arial, Helvetica, sans-serif">SCRIPPS CONTACTS:Mario Aguilera or Cindy Clark 858/534-3624 scrippsnews@ucsd.edu NASA CONTACT: FOR RELEASE: August 14, 2001 NEW
NASA SATELLITE SENSOR AND FIELD EXPERIMENT SHOWS AEROSOLS COOL THE SURFACE
BUT WARM THE ATMOSPHERE New research based upon NASA satellite data and a multi-national field experiment shows that black carbon aerosol pollution produced by humans can impact global climate as well as seasonal cycles of rainfall. Because aerosols that contain black carbon both absorb and reflect incoming sunlight, these particles can exert a regional cooling influence on Earth’s surface that is about 3 times greater than the warming effect of greenhouse gases. But even as these aerosols reduce by as much as 10 percent the amount of sunlight reaching the surface, they increase the solar energy absorbed in the atmosphere by 50 percent–thus making it possible to both cool the surface and warm the atmosphere. Scientists are concerned that this heating may perturb atmospheric circulation and rainfall patterns. "When we combined the satellite measurements with surface measurements, we found that the reduction of sunlight reaching the surface was three times larger than the amount of sunlight reflected back to space," says V. Ramanathan, director of the Center for Clouds, Chemistry, and Climate at Scripps Institution of Oceanography at the University of California, San Diego. "Averaged over the entire northern Indian Ocean, the man-made pollutants reflected more solar radiation back to space (than pristine skies), but they absorbed up to twice as much radiation in the atmosphere." Together with K. Rajeev, of India’s Vikram Sarabhai Space Centre, the authors report their findings in the August 16 Journal of Geophysical Research. Data for their investigation were collected during the Indian Ocean Experiment (INDOEX)–an international, multi-agency measurement campaign conducted from January through March in the years 1997, 1998, and 1999. INDOEX used instruments on land and on aircraft together with measurements made by NASA’s Clouds and Earth’s Radiant Energy System (CERES) sensor as it flew overhead aboard the Tropical Rainfall Measuring Mission (TRMM) satellite. The experiment’s objective was to help scientists understand to what extent human-produced aerosols may offset global warming. Earlier global warming studies suggest aerosols make our world "brighter" by reflecting more sunlight back to space, thereby helping to counteract the greenhouse effect. The Indian subcontinent offered the architects of the INDOEX campaign an ideal setting for their field experiment. The region was chosen for its unique combination of meteorology, landscape (relatively flat plains framed by the towering Himalayan Mountains to the north and open ocean to the south), and the large Southern Asian population (roughly 1.5 billion) with a growing economy. "Together, these features maximize the effects of aerosol pollution," Ramanathan explains. As a result of human industry–automobiles, factories, and burning vegetation–particles build up in the atmosphere where they are blown southward over most of the tropical Indian Ocean. The Indo-Asian haze covered an area larger than that of the United States. Although the INDOEX team found atmospheric particles of natural origin–such as trace amounts of sea salts and desert dust–they also found that 75 percent of the aerosol over the region resulted from human activities–including sulfates, nitrates, black carbon, and fly ash. Most natural aerosols scatter and reflect sunlight back to space, thereby making our planet brighter. However, human-produced black carbon aerosol absorbs more light than it reflects, thereby making our planet darker. "Ultimately, we want to determine if our planet as a whole is getting brighter or darker," Ramanathan states. *"We could not answer that question until we could measure the sunlight reflected at the top of the atmosphere with an absolute accuracy of 1 percent. The CERES sensors provide that accuracy for the first time ever from a space-based sensor." CERES was first launched in 1997 aboard TRMM, which flies in a near-equatorial orbit. Two more CERES sensors were launched in December 1999 aboard NASA’s Terra satellite, which flies in a near-polar orbit. Terra’s polar orbit allows CERES to measure the Earth’s incoming and outgoing radiant energy on a global scale every day. Moreover, the Moderate-resolution Imaging Spectroradiometer (MODIS) aboard Terra makes precise global measurements of aerosols that greatly enhance scientists’ ability to study their effects. "A large reduction of sunlight at the surface has implications for the hydrological cycle because of the close tie between heat and evaporation," Ramanathan says. "It could change the heating structure of the atmosphere and perturb the climate system in ways we don’t understand now. We don’t know, for example, how this might affect the monsoon season." While Ramanathan admits that scientists don’t know the net effect of bright and dark aerosols on global climate, through INDOEX and CERES they have shown that aerosols have a net cooling effect on the surface and they now know the magnitude of that cooling over a large region. But, he says, the INDOEX campaign does not solve the greater mystery. The next step is to use the CERES and MODIS sensors aboard NASA’s Terra satellite to extend this study to the global scale. The study was funded by the National Science Foundation. ###
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available at: For more information see: Journalists may request a copy of the paper from Harvey Leifert at hleifert@agu.org. Please indicate whether you prefer PDF or fax and provide your contact information. # # # Note to broadcast and cable producers: University of California, San Diego provides an on-campus satellite uplink facility for live or pre-recorded television interviews. Please phone or e-mail the media contact listed above to arrange an interview. Scripps Institution of Oceanography, at University of California, San Diego, is one of the oldest, largest and most important centers for global science research and education in the world. The National Research Council has ranked Scripps first in faculty quality among oceanography programs nationwide Now in its second century of discovery, the scientific scope of the institution has grown to include biological, physical, chemical, geological, geophysical and atmospheric studies of the earth as a system. Hundreds of research programs covering a wide range of scientific areas are under way today in 65 countries. The institution has a staff of about 1,300, and annual expenditures of approximately $155 million from federal, state and private sources. Scripps operates one of the largest U.S. academic fleets with four oceanographic research ships and one research platform for worldwide exploration. |
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