NASA is using satellite data to paint a detailed global picture of the interplay among natural disasters, human activities and the rise of carbon dioxide in the Earth's atmosphere during the past 20 years.
According to a new scientific study that appears in the July issue of the journal Global Change Biology, scientists used satellite observations to estimate the amount of leafy cover worldwide and sudden decreases in 'greenness.' Greenness is a measure of the amount of chlorophyll in live plants.
"Green leaf cover is probably the most fragile and vulnerable piece of Earth's ecosystem that scientists can easily monitor during ecological disturbances," said Christopher Potter, a scientist at NASA Ames Research Center, located in California's Silicon Valley, and the principal author of the technical paper.
"The new results come from a technique called 'data mining,' which sorts through a huge amount of satellite and scientific data to detect patterns and events that otherwise would have been overlooked," added Kumar, the principal investigator of a joint project of the University of Minnesota, California State University and NASA Ames to develop data-mining techniques to help Earth scientists discover changes in the global carbon cycle and climate system.
The Earth's land cover is so vast that much of it in the tropics and the tundra is inaccessible to regular ground observations there, according to the study's scientists. "Many years of satellite observations of remote areas have revealed completely new pictures of ecological changes and disasters, but we have had to develop new formulas to clearly reveal sudden changes in greenness over extensive areas," said Potter.
Detecting sudden changes from large amounts of global data required the development of automated techniques that take into account the timing, location and magnitude of such changes, according to Tan.
Researchers then matched abrupt changes in plant greenness with records of large wildfires or massive crop losses to validate the study's conclusions. "The majority of the potential disturbance events that caused carbon to go into the atmosphere occurred in tropical savanna and shrub lands or in cold forest ecosystems," Klooster said.
Scientists define an ecological disturbance as an event that disrupts the physical make-up of an ecosystem and how it works for longer than one growing season of native plants. Natural disturbances may include fires, hurricanes, floods, droughts, lava flows and ice storms. Other natural disturbances are due to plant-eating insects and mammals, and disease-causing microorganisms.
Human-caused disturbances could happen as a result of logging, deforestation, draining wetlands, clearing, chemical pollution and introducing non-native species to an area, according to scientists.
"Ecosystem disturbances can contribute to the current rise of carbon dioxide in the atmosphere," Potter said. Nine billion metric tons of carbon may have moved from the Earth's soil and surface life forms into the atmosphere in 18 years beginning in 1982 due to wildfires and other disturbances, according to the study.
A metric ton is 2,205 pounds, equivalent to the weight of a small car. In comparison, fossil fuel emission of carbon dioxide to the atmosphere each year was about seven billion metric tons in 1990.
Some of the carbon dioxide that goes into the air reenters the Earth's biosphere when plants recover this gas during 'natural recycling.'
Scientists used the Advanced Very High Resolution Radiometer aboard National Oceanic and Atmospheric Administration satellites to measure monthly changes in leafy plant cover worldwide. Boston University used unique NASA computer codes to produce global greenness values.
These codes removed interfering data from atmospheric effects. When statistics showed there was much less greenness in specific areas that lasted more than a year, scientists also found a high probability of ecological disturbances.
"Watching for changes in the amount of absorption of sunlight by green plants is an effective way to look for ecological disasters," Potter said. "This study was literally a proof of concept because we learned how to use data mining to bring new knowledge out of existing Earth observation data," Klooster added.
Follow-up studies using much higher resolution satellite images are likely to reveal more localized events, such as floods, hurricanes and major logging operations, according to the study's scientists. "This is important because many natural disasters in remote areas are not noticed and never recorded," Potter explained.
"In the new era of worldwide carbon accounting and management, we need an accurate method to tell us how much carbon dioxide is moving from the biosphere and into the atmosphere," Potter said. "Global satellite images go beyond the capability of human eyesight. All we need to do is look at the data with the proper formulas to filter out just what we need," he concluded.
Co-authors of this study include Pang-Ning Tan, Michael Steinbach and Vipin Kumar, all of the University of Minnesota, Minneapolis; Steven Klooster and Vanessa Genovese, both of California State University-Monterey Bay, Seaside, Calif.; and Ranga Myneni, Boston University, Boston.
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