Study reveals the effects of deforestation on rainfall.
Deforestation in the Brazilian Amazon
Deforestation could cause rainfall across the Amazon rainforest to drop precipitously, warns a new study published in the journal Nature.
Using a computer model that accounts for forest cover and rainfall patterns, Dominick Spracklen of the University of Leeds and colleagues estimate that large-scale deforestation in the Amazon could reduce basin-wide rainfall 12 percent during the wet season and 21 percent in the dry season by 2050. Localized swings would be greater. Forest clearing in the Congo Basin would produce similar results.
The shift would primarily result from disrupting the forest’s water cycle. Trees absorb water from rain and then release moisture back into the air via the process of evapotranspiration. That moisture fuels further rainfall. When forests are cleared, evapotranspiration and more water runs off into rivers leaving less moisture for the formation of rain.
While the cycle has long been understood, the new study attempts to quantify the effects for a large forest area using newly available data from NASA.
Rainfall over the Amazon River in Colombia. Photo by Rhett A Butler.
“We looked at what had been happening to the air over previous days – where it came from and how much forest it had traveled over,” said Spracklen in a statement. “Our study implies that deforestation of the Amazon and Congo forests could have catastrophic consequences for the people living thousands of kilometers away in surrounding countries.”
Other research suggests that up to 70 percent of South America’s GDP is produced in areas that receive precipitation or surface water from the Amazon.
“The Amazon forest maintains rainfall over important agricultural regions of Southern Brazil, while preserving the forests of the Congo Basin increases rainfall in regions of Southern Africa where rainfed agriculture is important. Increased drought in these regions would have severe implications for their mostly subsistence farmers.”
However the results alone don’t provide a complete picture, at least for the Amazon. Other climate models indicate that warmer temperatures in the tropical Atlantic could reduce precipitation in the Southern Amazon, exacerbating the effects of drought in the region.
“Changes in regional climate could exacerbate drought-related tree mortality, which in turn would reduce carbon stocks, increase fire risk and lower biodiversity,” noted Luiz Aragao of the University of Exeter in a separate commentary also published in Nature. “Such changes might also directly threaten agriculture, which generates US$15 billion per year in Amazonia, and the hydropower industry, which supplies 65% of Brazil’s electricity. Society should therefore take urgent action now, to curb tropical deforestation and avert future environmental problems.”
There are positive signs that society — or at least Brazilian society — is responding to the deforestation threat. Forest clearing in the Brazilian Amazon has fallen by about 60 percent (7,000 sq km per year from 2009-2011) relative to the 1997-2002 baseline of 17,600 sq km used in the study. Should the trend hold, forest loss may be significantly lower than assumed by Spracklen and colleagues.
Nevertheless there remains cause for concern. Brazil’s Congress last week wrapped up language on a revision to the country’s Forest Code, which limits how much forest a landowner can clear. Environmentalists worry that the new Forest Code could usher in a new period of deforestation and reverse recent progress on saving the Amazon.
Effect of deforestation on rainfall in the tropics. (a) Much of the rainfall over tropical forests
comes from water vapour that is carried by the atmosphere from elsewhere. But a large component
is ‘recycled’ rain — water that is pumped by trees from soil into the atmosphere through a process
called evapotranspiration. Water exits from forests either as run-off into streams and rivers, or as
evapotranspirated vapour that is carried away by the atmosphere. The atmospheric transport of water
vapor into the forest is balanced by the exit of water in the form of vapor and run-off. (b) Spracklen
and colleagues’ analysis suggests that deforestation reduces evapotranspiration and so inhibits water
recycling. This decreases the amount of moisture carried away by the atmosphere, reducing rainfall in
regions to which the moisture is transported. Decreasing evapotranspiration may also increase localized
run-off and raise river levels. Image and caption courtesy of NATURE
Luiz Aragão (2012). Environmental science: The rainforest’s water pump. Nature 05 September 2012 doi:10.1038/nature11485
D. V. Spracklen, S. R. Arnold & C. M. Taylor (2012). Observations of increased tropical rainfall preceded by air passage over forests. Nature 05 September 2012 doi:10.1038/nature11390
(02/01/2012) New, radical theories in science often take time to be accepted, especially those that directly challenge longstanding ideas, contemporary policy or cultural norms. The fact that the Earth revolves around the sun, and not vice-versa, took centuries to gain widespread scientific and public acceptance. While Darwin’s theory of evolution was quickly grasped by biologists, portions of the public today, especially in places like the U.S., still disbelieve. Currently, the near total consensus by climatologists that human activities are warming the Earth continues to be challenged by outsiders. Whether or not the biotic pump theory will one day fall into this grouping remains to be seen. First published in 2007 by two Russian physicists, Victor Gorshkov and Anastassia Makarieva, the still little-known biotic pump theory postulates that forests are the driving force behind precipitation over land masses.
(08/09/2011) A recent push to revise Brazil’s forest code has emerged as one of the more contentious political issues in the country, pitting agribuisness against environmentalists trying to preserve the Amazon rainforest. Historically, the forest code has required private landowners to maintain a substantial proportion of natural forest cover on their properties, though the law has often been ignored. While both sides claim to be basing their recommendations on the ‘best science’ available, Brazilian scientists say they haven’t had much of a voice in the debate. In fact, says Antonio Donato Nobre, a researcher at the Amazon Research Institute and Brazil’s National Space Research Institute, ‘throughout the development of the said revisions, Congress has neither invited nor commissioned a coordinated and serious contribution from the scientific community.’
(02/03/2011) In 2005 the Amazon rainforest underwent a massive drought that was labeled a one-in-100 year event. The subsequent die-off of trees from the drought released 5 billion tons of CO2. Just five years later another major drought struck. The 2010 drought, which desiccated entire rivers, may have been even worse according to a new study in Science, adding on-the-ground evidence to fears that climate change may inevitably transform the world’s greatest rainforest.
(09/21/2010) Researchers recently traveled to the remote Brazilian Amazon to investigate how clouds are formed and rain falls in an atmosphere unburdened by human-caused pollution. Studying the atmospheric aerosol particles, which impact cloud formation and particles, above a pristine forests, researchers discovered that when left alone the Amazon acts as its own ‘bioreactor': clouds and precipitation are produced by the abundance of plant materials.
(03/03/2010) Twenty years of research has led Dr. Graham Jones of Australia’s Southern Cross University to discover a startling connection between coral reefs and coastal precipitation. According to Jones, a substance produced by thriving coral reefs seed clouds leading to precipitation in a long-standing natural process that is coming under threat due to climate change.
(06/25/2009) Between 1700 and 1850 forest cover in India and China plummeted, falling from 40-50 percent of land area to 5-10 percent. Forests were cut for agricultural use across Southeast Asia to feed a growing population, but the changes from forests to crops had unforeseen consequences. A new study published in the Proceedings of the National Academy of Sciences links this deforestation across Southeast Asia with changes in the Asian Monsoon, including significantly decreased rainfall.
(04/01/2009) Two Russian scientists, Victor Gorshkov and Anastassia Makarieva of the St. Petersburg Nuclear Physics, have published a revolutionary theory that turns modern meteorology on its head, positing that forests—and their capacity for condensation—are actually the main driver of winds rather than temperature. While this model has widespread implications for numerous sciences, none of them are larger than the importance of conserving forests, which are shown to be crucial to ‘pumping’ precipitation from one place to another. The theory explains, among other mysteries, why deforestation around coastal regions tends to lead to drying in the interior.
(04/18/2007) The rapid expansion of soybean cultivation in the Amazon may be having a larger impact on climate than previously believed, according to research published last week in Geophysical Research Letters. Using experimental plots in the Amazon, a team of scientists led by Marcos Costa from the Federal University of Vicosa in Brazil found that clearing for soybeans increases the reflectivity or albedo of land, reducing rainfall by as much as four times relative to clearing for pasture land.
(12/11/2006) A new study suggests that deforestation that follows a “fishbone” pattern may be less damaging from an environmental standpoint than traditional clear-cutting. The reason? Fishbone deforestation patterns may create conditions that increase precipitation levels which help cleared vegetation recover quicker.
(09/19/2006) A new study from NASA scientists shows that forest clearing for large-scale agriculture has recently become a significant cause of deforestation in the Brazilian Amazon. The researchers warn that this change in land use may affect the region’ climate and the Amazon’s ability to absorb carbon dioxide, a potent greenhouse gas.
(04/14/2005) Smoke from forest fires reduces rainfall and spells trouble for the Amazon rainforest