RIO DE JANEIRO — The critical role that vast tropical forests such as Brazil’s Amazon play in suppressing climate change is well known: They store huge quantities of carbon, acting as “carbon sinks.”
But as a report out this week argues, scientists are making the case that cutting down these forests does more than simply release carbon into the atmosphere — it has a direct and more immediate effect on the climate, from changes in rainfall patterns to increases in temperatures. The amount of water that forests pump into the air is key to this. But scientists disagree on how that happens.
Complete deforestation of the Amazon would alter rainfall in much of the United States, according to the report, titled “Effects of Tropical Deforestation on Climate and Agriculture” and published Thursday in the journal Nature Climate Change.
“Deforestation is about much more than carbon dioxide. Forests regulate the climate in many ways, and storing CO2 is just one of them,” said Deborah Lawrence, a professor of environmental sciences at the University of Virginia and author of the report. “What this study shows is that there are additional, independent effects of deforestation on climate.”
Lawrence’s report is a peer-reviewed summary of existing research, and she found that deforestation, even at small, localized levels, can change the climate. “Farmers in one place are connected to farmers in another. Countries are connected to each other,” Lawrence said. “We don’t want to wait until the climate system has shifted so we can measure it on the ground.”
She said there is a possible “tipping point” of 30 to 50 percent deforestation for the Amazon and Central Africa. Deforestation beyond that could invite disaster.
“Tropical deforestation on many scales influences local, regional and even global climate. Deforestation-driven changes to water availability and climate variability could have strong implications for agricultural production systems and food security in some regions,” the report says.
“Significant large-scale deforestation in any of these regions could have impacts on agriculture — across the world there will be regions that suffer,” Lawrence said.
About 20 percent of the Amazon has been deforested. And scientists have noticed changes in its climate. “Among these effects are drastic, widespread decreases in forest transpiration, changes in the dynamics of clouds and rain, and the extended duration of the dry season,” said a report released in October by Brazilian scientist Antonio Nobre, who is associated with the National Institute of Space Research.
Lawrence also found evidence of an extended dry season in the Amazonian state of Rondonia. “There is no doubt that the warming due to deforestation is measurable,” she added. “Anyone who has walked from a forest into an adjacent field will tell you that. Models confirm that effect at larger scales as well.”
But where scientific opinion diverges is on how forests affect climate. In Brazil, where the country’s most economically important southeast region has been hit by a devastating drought, this question has particular resonance.
The “biotic pump” theory, developed by Anastassia Makarieva and Victor Gorshkov from the Petersburg Nuclear Physics Institute in St. Petersburg, Russia, and their colleagues, argues that forests with high levels of atmospheric condensation draw in air and water from the ocean — creating aerial rivers of moisture. If the forest is thought of as a well, deforestation makes that well shallower.
“As we make it more shallow, the first thing we will notice will be the largest fluctuations,” Makarieva said. “This picture is very much in line with what is happening in Sao Paulo right now.”
Antonio Nobre’s brother, Carlos Nobre, a climate scientist and official at Brazil’s Ministry of Science, Technology and Innovation, said that deforestation in the Amazon was not linked to the Sao Paulo drought.
He blamed an “atmospheric block” of high pressure air over the Atlantic — a common phenomenon but one that had lasted 45 days instead of the usual 10 to 15.