NASA Finds Drought May Take Toll on Congo Rainforest

A View of the entire African rainforest area (green) transitions into a view of the region included in the Nature study, primarily in the Congo forest (mostly brown). The study area represents intact areas in the Congo rainforest where satellite data are high quality. Credit: NASA/Goddard Space Flight Center Scientific Visualization Studio
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from NASA's Jet Propulsion Laboratory, April 23, 2014

A new analysis of NASA satellite data shows Africa's Congo rainforest, the second-largest tropical rainforest in the world, has undergone a large-scale decline in greenness over the past decade.

The study, led by Liming Zhou of University at Albany, State University of New York, shows between 2000 and 2012 the decline affected an increasing amount of forest area and intensified. The research, published Wednesday in Nature, is one of the most comprehensive observational studies to explore the effects of long-term drought on the Congo rainforest using several independent satellite sensors.

"It's important to understand these changes because most climate models predict tropical forests may be under stress due to increasing severe water shortages in a warmer and drier 21st century climate," Zhou said.

Scientists use the satellite-derived "greenness" of forest regions as one indicator of a forest's health. While this study looks specifically at the impact of a persistent drought in the Congo region since 2000, researchers say that a continued drying trend might alter the composition and structure of the Congo rainforest, affecting its biodiversity and carbon storage.

Previous research used satellite-based measurements of vegetation greenness to investigate changes in the Amazon rainforest, notably the effects of severe short-term droughts in 2005 and 2010. Until now, little attention has been paid to African rainforests, where ground measurements are even sparser than in the Amazon and where droughts are less severe but last longer.

To clarify the impact of long-term drought on the Congo rainforest, Zhou and colleagues set out to see whether they could detect a trend in a satellite measure of vegetation greenness called the Enhanced Vegetation Index. This measure is developed from data produced by the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on NASA's Terra satellite. The scientists focused their analysis on intact, forested regions in the Congo Basin during the months of April, May and June each year - the first of the area's two peak rainy and growing seasons each year.

The study found a gradually decreasing trend in Congo rainforest greenness. The decrease, sometimes referred to as "browning," suggests a slow adjustment to the long-term drying trend. This is in contrast to the more immediate response seen in the Amazon, such as large-scale tree mortality, brought about by more episodic drought events.

The browning of the forest canopy is consistent with observed decreases in the amount of water available to plants, whether that is in the form of rainfall, water stored in the ground, water in near-surface soils, or water within the vegetation.

These changes in available water were detected in part with NASA satellites including the NASA/JAXA Tropical Rainfall Measuring Mission, NASA's Quick Scatterometer (QuikScat), and NASA's Gravity Recovery and Climate Experiment, a joint mission with the German Aerospace Center. The latter two missions are managed by NASA's Jet Propulsion Laboratory, Pasadena, Calif.

"Combining measurements from different sensors has given us more confidence in the results of the MODIS data and provided us with insights into the environmental and physiological mechanisms of the browning observed by the MODIS data," said co-author Sassan Saatchi of JPL.

Climate factors known to affect vegetation growth were also in line with the observed browning. Land surface temperatures, for example, were observed to increase over most of the study area. Decreased cloudiness allowed more solar radiation to reach the plants, which typically promotes photosynthesis, but in this case it likely posed an extra stress on the plants from the resulting depletion of soil moisture.

"Forests of the Congo Basin are known to be resilient to moderate climate change because they have been exposed to dry conditions in the past few hundred years," Saatchi said. "However, the recent climate anomalies as a result of climate change and warming of the Atlantic Ocean have created severe droughts in the tropics, causing major impacts on forests."

How the changes affect individual plant species in the area remains to be seen. For example, drier conditions may favor deciduous trees at the expense of evergreen trees.

"Our assessment is a step toward an improved understanding of how African rainforests respond to increasing drought," Zhou said. "We need to consider the complex range of processes affecting different tropical rainforest species before we can fully assess the future resilience of tropical forests."

The other authors for this research include Yuhong Tian at I.M. Systems Group, Inc. at the Center for Satellite Applications and Research, the science arm of National Oceanic and Atmospheric Administration's Satellite and Information Service, College Park, Md.; Ranga Myneni at Boston University in Massachusetts; Philippe Ciais at Laboratoire des Sciences du Climat et de l'Environnement, Gif sur Yvette Cedex, France; Yi Y. Liu at University of New South Wales, Australia; Shilong Piao at Peking University, China; Haishan Chen at Nanjing University of Information Science and Technology, China; Eric Vermote of NASA's Goddard Space Flight Center, Greenbelt, Md.; and Conghe Song and Taehee Hwang at the University of North Carolina at Chapel Hill.

NASA monitors Earth's vital signs from land, air and space with a fleet of satellites and ambitious airborne and ground-based observation campaigns. NASA develops new ways to observe and study Earth's interconnected natural systems with long-term data records and computer analysis tools to better see how our planet is changing. The agency shares this unique knowledge with the global community and works with institutions in the United States and around the world that contribute to understanding and protecting our home planet.

http://www.jpl.nasa.gov/news/news.php?release=2014-124

Congo forest feels bigger climate impact

by Tim Radford, Climate News Network, April 25, 2014

Researchers say there is increasing evidence that climate change is among the factors causing serious damage to the rainforest of the Congo basin.

LONDON, 25 April - The Congo – one of the world’s greatest rainforests – is getting steadily less green. The slow change in colour during this century, recorded by a series of US satellites, has been matched by a rise in temperature and lower precipitation. And, researchers think, it could reflect a forest’s response to climate change.

Scientists from Australia, China, the US and France report in the journal Nature that they examined optical, thermal, microwave and gravity data collected by orbiting sensors between 2000 and 2012.

They concentrated on intact forested regions during the months of April, May and June each year, which span the peaks of growth and rainfall. They detected an intensification in the forest’s decline. This decline was consistent with lower rainfall, poorer water storage below the canopy and a gradual change in the composition of species.

“It is important to understand these changes because most climate models predict tropical forests may be under stress due to increasing severe water shortages in a warmer and drier 21st century climate,” said Liming Zhou, of Albany State University of New York. But other factors could accelerate this “browning” of one of the world’s greatest rainforests.

Half as hot again

A team from the Catholic University of Louvain in Belgium - also known in Belgium’s other language as KU Leuven - predicts in the Journal of Climate that explosive population growth and inefficient agricultural practices are likely to make things a great deal hotter for the region and a great deal worse for the rainforest.

By 2050, according to their computer models, Central Africa will be on average 1.4 °C hotter than it is today just because of greenhouse gas emissions. And the steady destruction of the forest will add an extra 0.7 °C to that figure.

Temperature increases on such a scale will harm plant and animal species and even bring about some extinction. Where the forests have been cleared, there will be increased levels of evaporation, and consequent rises in temperature.

Across the Atlantic, things also look bleak for the Amazon rainforest. Paulo Brando of the Amazon Environmental Research Institute in Brazil and colleagues from the US report in the Proceedings of the National Academy of Sciences that the piecemeal clearing of the rainforest, along with drought, has begun to create “tinderbox” conditions  and an ever more destructive cycle of burning.

Non-average menace

Over the course of 8 years, in one of the longest-running experiments of its kind, the researchers burned 50-hectare plots of forest in the south-eastern Amazon, a region vulnerable to climate change. They compared the tree deaths each year to measure the impact of drought on fire intensity.

“Drought causes more intense and widespread fires,” said Dr Brando. “Four times more adult trees were killed by fire during a drought year, which means that there was also more carbon dioxide released into the atmosphere, more tree species loss and a greater likelihood of grasses invading the forest.”

This research, too, was backed up by satellite observation. In 2007, a year of drought, fires in south-east Amazonia burned 10 times more forest than in an average year - an area equivalent to a million soccer fields, according to Douglas Morton of the US space agency NASA, a co-author.

Climate change is expected to bring shorter, more intense rainy seasons and longer dry seasons in the region. Michael Coe of Woods Hole Research Center, another author, said “We tend to think only about average conditions, but it is the non-average conditions we have to worry about.”