RAINFOREST TO PASTURE: Deforestation, among other human impacts such as climate change, are having a rainforest-wide impact on the Amazon.
Image: Courtesy of Compton Tucker, NASA GSFC
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The Amazon rainforest is in flux, thanks to agricultural expansion and climate change. In other words, humans have "become important agents of disturbance in the Amazon Basin," as an international consortium of scientists wrote in a review of the state of the science on the world's largest rainforest published in Nature on January 19. (Scientific American is part of Nature Publishing Group.) The dry season is growing longer in areas where humans have been clearing the trees—as has water discharge from Amazon River tributaries in those regions. Multiyear and more frequent severe droughts, like those in 2005 and 2010, are killing trees that humans don't cut down as well as increasing the risks of more common fires (both man-made and otherwise).
The trees are also growing fast—faster than expected for a "mature" rainforest—according to a network of measurements.
The exact cause or causes of this accelerated growth—which means the Amazon's 5 million square kilometers of trees are now sucking in and sequestering some 400 million metric tons of carbon per year, or enough to offset the annual greenhouse gas emissions of Japan—"remains unknown," the researchers wrote in the review.
"When we measure that a particular stand of mature forest is accumulating carbon, it is difficult to say whether that might be due to recovery from some unrecognized disturbance long ago or whether it is due to more recent changes in climate and CO2," explained Woods Hole Research Center Senior Scientist and Executive Director Eric Davidson, lead author of the review, in an e-mail. Candidates include recovery from the potential wide-scale disturbance by pre-Columbian human societies now beginning to be uncovered or the increasing availability of some formerly limiting factor, such as atmospheric carbon dioxide.
In fact, increasing levels of CO2 in the atmosphere—now roughly 392 parts per million and rising—may be fertilizing the rainforest and preventing even greater impacts from reduced rainfall, although this question, Davidson and his colleagues wrote in the review, "may be one of the largest unknowns for the future of the Amazon forests."
What is known is that the forest clearing that has already gone on is decreasing forest rainfall. The Amazon produces roughly a third of its own precipitation—trees release moist air that then falls back as rain to nourish other trees (the rest comes from the Atlantic Ocean). But the air above cleared land warms faster and therefore rises more quickly, drawing the moist air from surrounding forested areas away. In fact, the conjunction of cleared and forested lands actually creates wind known as a vegetation breeze. But that breeze tends to blow rainfall away from the forest and over the surrounding pastures instead. It also weakens the continental-scale low-pressure system that draws rainfall over the Amazon.
The southern and eastern portions of the Amazon are the most affected, according to this review. For example, the southeastern Amazon around one of the local tributary rivers—the Tocantins—has seen pasture and cropland increase from 30 percent to 50 percent of the land between 1955 and 1995. As a result, that river now carries 25 percent more water. Another southeastern tributary, the Araguaia, now carries 28 percent more sediment—precious soil lost during downpours from surrounding, expanded agricultural fields.
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Add CommentSee - geoengineering does work, albeit in unintended ways...
Reply | Report Abuse | Link to thisAs population grows, and supplies dwindle. All of the laws in the world will come to nothing.
Reply | Report Abuse | Link to thisFor those of us waiting for the tipping point, I think they missed it.
Exploitation is not in everyone's best interest.
It won't be long before the region emits more carbon than it absorbs; some reports indicate this has already crossed the threshold.
Reply | Report Abuse | Link to this@Docspot,
Reply | Report Abuse | Link to thisYou speak of hysteria but you seem to be exhibiting large quantities of it in your post. I am sorry but your incredulity does not an argument make.
@Docspot,
Reply | Report Abuse | Link to thisEnvironmentalists may be prone to alarmism - but preservation of the environment has tangible economic benefits that are often more profitable than the gains made by immediate, short term exploitation and deforestation.
Obviously nature will recover and out live humans by millions of years - it's the potential harm to our own ability to survive people are worried about.
Sir:
Reply | Report Abuse | Link to thisIt is truly unbelievable the author thinks it's "unknown" that old forests
are now growing faster because of increased CO2 in the atmosphere.!
There are dozens of research-driven articles on this very subject.
One spot to start: CO2science.org.
A skeptic saying thanks for the rest of the 'study.'
Tree growth requires fuel and energy. Could the decline in rainfall mean the amount of sunlight reaching the trees is increasing? Clouds and highly humid air tend to deflect sunlight. Less rainfall may reduce the leaching of nutrients from the soil even though most are tied up in the biota anyways.
Reply | Report Abuse | Link to thisThe source of the "study" is the Woods Hole Research Center. They are an environmental advocacy group, not to be confused with the Woods Hole Oceanographic Institute. Of course, confusion is the objective here.
Reply | Report Abuse | Link to thisOnce big areas of the Amazon forest become free of dense vegetation, the local climate is likely to change. Powerful tornados will become commonplace destroying the settlements and fields that were the very reason for deforestation to take place. We better leave the Amazon as it is.
Reply | Report Abuse | Link to thisWhen farming began to develop in northern Africa, exactly the same misfortune seems to have happened to what is now the Sahara (cf. rock-pictures in southern Libya: The oldest (more than 10 000 years) show rhinos and the youngest (some 5 000 years ago) show only house-animals). In between it seems that a preexisting rain-forest changed into a savanna and later became a desert. The interesting point is that this had nothing to do with AGW nor CO2.
Reply | Report Abuse | Link to thisWhat happened to the forests covering Australia when early man burnt them down to promote grasslands that their kangeroo game could preferentially feed on?
Reply | Report Abuse | Link to thisWasn't there an article in SciAm a while back that claimed that abandoned clear cut rainforrest farms were transforming back into rainforrest?
Reply | Report Abuse | Link to thisI saw aerial photographs of that. I think we might stop Amazon rainforest depletion in a year.
Reply | Report Abuse | Link to thisBest will be to cut the Amazone forest off from Brasil and bring it under international controll like Antarctica for undisturbed wild flora and fauna. The same regarding the Himalaya, the Sahara, the North Pole region, part of the north-american prairies, etc.
Reply | Report Abuse | Link to thisOn smaller scale borderlines can be changed into humanfree borderzones. Thus the bear will become master again on the Golan while the lion roars in the Sinai. In Holland the IJselmeer-lake is far bigger than fish and birds need, so an artificial hilly ring surrounding a new "polder" may provide habitat to bear and (Siberian) tiger. "The corner of the field (its products) belong to the stranger and since man tries to take everything also to the wild animal".