Research at Boston University

BU Joins AAU

No Doubt about Drought

By Mark Dwortzan
Climate Change

Throughout the United States, extreme weather is becoming routine. According to the National Oceanic and Atmospheric Administration, the winter of 2010–2011 was one of the snowiest in recent years. The summer of 2011 was the second warmest on record, and the winter of 2011–2012 was the fourth warmest. Then came the icing—or lack thereof—on the cake: last March, the average temperature was 51.1 degrees Fahrenheit, a whopping 8.6 degrees above normal.

While natural weather patterns could be to blame for these developments, the recent proliferation of extreme weather is an expected outcome of global warming, which scientists have linked to the combustion of fossil fuels such as coal, oil, and natural gas. Through field and satellite observations, statistical modeling, and advanced data analysis, Boston University researchers are uncovering compelling evidence of global climate change—early warning signs that raise red flags about the future of the planet.

Threatened by clear-cutting and extreme weather events in recent years, the Amazonian rain­forest could become a tipping point for the future of global climate change. Scientists estimate that a total loss of the rainforest canopy would result in the release of about 100 billion tons of carbon dioxide into the atmosphere, significantly accelerating the pace of global warming. In the past decade, two severe droughts, in 2005 and 2010, have intensified concerns about the forest’s vulnerability to drought, leading Professor of Earth & Environment Ranga Myneni and graduate student Liang Xu to investigate.

Applying sophisticated analytical techniques to satellite remote sensing data of vegetation greenness—a measure of the forest’s capacity to absorb carbon dioxide from the atmosphere and store it within its constituent trees—the researchers discovered good and bad news. While the drought of 2005 had limited impact on the forest canopy, the far more dramatic sequel caused a widespread, severe, and persistent decline in the greenness of vegetation throughout the Amazon.

In a paper appearing in Geophysical Research Letters in April 2011, Myneni, Xu, and collaborators from Brazil and NASA reported that the 2010 drought resulted in record-low river levels and a decline in greenness that spanned an area four times greater and more severe than in 2005. Moreover, 51 percent of all drought-stricken forest acreage showed greenness declines in 2010 compared to only 14 percent in 2005. Unlike in 2005, those declines persisted following the end of the dry season drought and return of rainfall to normal levels. Myneni warns that if the drought-stricken regions do not recover, carbon dioxide concentrations in the atmosphere could rise, thus further warming the planet.

“The Amazon forest is about 70 percent of the area of the continental U.S. and is the largest undisturbed natural forest; it accounts for about 6 percent of the vegetated area on our planet and contains tremendous biodiversity. The drought of 2010 has stoked fears that once the forest is lost, it may not regenerate itself.”

Myneni, who pioneered the use of satellite data to monitor changes in vegetation from space, is quick to point out that both natural and man-made influences are at play. The El Niño/Southern Oscillation (ENSO), a five-year climate pattern in the equatorial Pacific region, causes floods, droughts, and other extreme weather events in the area regardless of human activities. On the other hand, climate models indicate that as the greenhouse effect intensifies, the Amazonian region will be subject to more frequent droughts.

“You could argue that the droughts of 2005 and 2010 are part of the natural variability in climate,” says Myneni. “But if you accept that our climate is already changing due to human activity, these events could be an effect.”

To obtain a clear picture of the impact of the two droughts on Amazonian vegetation, Myneni and Xu obtained and processed daily images of Earth’s surface from the MODIS instrument on board a NASA satellite. They compared levels of vegetation greenness before and after each drought and mapped the resulting decrease in greenness, and then corroborated their findings with other satellite data showing marked reductions in rainfall in the exact same areas where greenness declined.

While Myneni and Xu’s findings were widely accepted by the scientific community in 2011, Myneni’s analysis of the 2005 drought provoked a much more skeptical response when in 2009 he issued a controversial press release disputing the results of two groups of scientists. The first group had used MODIS data to conclude that greenness levels had actually increased during the 2005 drought; the second were scientists in the field who, noting an uptick in sightings of dead trees, reported just the opposite. Using an updated version of the MODIS data, Myneni determined that the first scientific group had made mistakes in their analysis and that greenness levels had decreased in 2005, but not significantly. When the far more severe drought occurred in 2010, he and Xu pinpointed areas of clearly reduced vegetation and rainfall. The results were unmistakable.

“People called the 2005 event a ‘once-in-a-century drought,’ but our data showed limited damage,” notes Myneni. “If that was a ‘once-in-a-century drought,’ then the 2010 event was the mother of all droughts.”

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