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
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 waterdischarge 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.
Agroforestry and other techniques for better environmental management of such agriculture remain rare, despite their proven ability to help balance increased food production with ecosystem services like carbon sequestration. On the whole, cutting down trees so that the Amazon covers only roughly 80 percent of the land it once did seems to have tipped the rainforest from being a sink for global CO2 emissions to a net source, although this calculation remains highly uncertain, the scientists noted. In addition, the entire rainforest may be transitioning from a relatively undisturbed ecosystem to what scientists like to call a "disturbance-dominated regime," or a biome that has become an "anthrome"—a landscape dominated by human impact.
The good news is that Brazil has in recent years begun to restrain such deforestation: annual rates fell from 28,000 square kilometers in 2004 to less than 7,000 in 2011. "Brazil is poised to become one of the few countries to achieve the transition to major economic power without destroying most of its forests," the researchers wrote in their conclusion. New laws currently under consideration may put that potential in peril, however, by allowing a return of previously banned forest-clearing practices. "There is considerable progress toward improved management of the impacts of development in the region," Davidson noted in his e-mail, "but there is still much work to be done."
The debate over climate change often takes the form of “tit-for-tat” blogs, conflicting commentary, and dogmatic ideological statements. Lost in this verbal debate are often the simple facts and data of climate change and the immense and definitive global observations of the ways in which our climate is actually changing around us.
So, without much commentary, here are just a few simple and clear pictures (and links) showing how the planet continued to warm and change around us in 2011. And these facts are just part of why all national academies of science on the planet and every major geophysical scientific society agree that humans are fundamentally changing the climate.
CO2 in the atmosphere continues its inexorable rise
The heart of the climate problem is that our burning of fossil fuels along with other human activities have thrown the atmospheric levels of greenhouse gases out of balance, and their concentration in the atmosphere is growing faster and faster. This classic record from Mauna Loa in Hawai’i shows the growth in the CO2 concentration in the past half century. But it’s worse than that: CO2 concentrations in our atmosphere are now higher than at any time in the past million years, and perhaps higher than in the past 15 million years.
The concentration of carbon dioxide is higher today than in a million years.
Higher concentrations of greenhouse gases leads to a hotter planet
As CO2 (and other heat-trapping gas) concentrations have risen, so have planetary temperatures, just as basic physics and models predict. Here are the land and ocean surface temperatures for the past 130 years as measured by scientists at NASA (first figure) and NOAA/NCDC (second figure), confirmed by independent scientists internationally. While temperatures go up and down, the long-term trend is indisputably up. 2011 was the 35th consecutive year since 1976 that the yearly global temperature was above average. La Niña cooled the global average temperature in 2011 by quite a bit, but it was still abnormally hot. Indeed, 2011 was the hottest “La Niña” year ever recorded.
A hotter planet means an intensification of the hydrological cycle
One of the consequences of global warming, predicted from basic physics and models, is more water in the atmosphere and more rainfall globally (with increases in both wet and dry extremes). And observations of the real world have shown these projections to be happening.
Globally, 2011 was the 2nd wettest year over land on record since 1900 (and the wettest year was just one year earlier: 2010). Drier than average conditions were widespread across much of French Polynesia, the Solomon Islands, Hawaiian Islands, northwestern Canada, and southwestern China. The wettest regions included much of Central America, Micronesia, northern Brazil, and the northeastern U.S.
A hotter planet means disappearing glaciers and ice, especially in the Arctic.
Ice in the Arctic continues to thin and disappear, even faster than anticipated. Arctic sea ice extent during September 2011 (the month when ice is at a minimum) was nearly 35 percent below the 1979-2000 average — the second smallest September Arctic sea ice extent since precise records began in 1979,according to the National Snow and Ice Data Center. And it wasn’t just September: overall, 2011 Arctic ice extent (that bottom blue line) was far below the long-term average for every single month.
The extent of Arctic ice in 2011 was lower, in every single month, than the long-term average.
A warming planet also means more extremes of climate.
Anyone watching or reading the news or looking out the window probably had a sense that 2011 was a weird year with one bad, extreme weather disaster after another. It was. As the climate changes, scientists anticipate more extremes, and the reality around us confirms that. Here are two graphs: the first from NOAA shows the fraction of the United States in either extreme drought or extreme flood over the past century: 2011 had the highest fraction ever recorded – nearly 60% of the country. The second, also from NOAA, shows that 2011 had the largest number of weather/climate disaster events exceeding a billion dollars ever – 14 of them, even adjusting for inflation.