The Congo—one of the world’s greatest rainforests—is getting steadily less green. The slow change in color during this century, recorded by a series of U.S. 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 U.S. 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 twenty-first 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.
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 degrees Celsius hotter than it is today just because of greenhouse gas emissions. And the steady destruction of the forest will add an extra 0.7 degrees Celsius 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 U.S. 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.
Over the course of eight 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 U.S. 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.”