November 11, 2010
A new study in Science is likely to reopen the contentious debate about the impact of climate change on tropical rainforests. Scientific modeling of future climate conditions in tropical rainforests, such as the Amazon, has shown that climate change—combined with deforestation and fire—could create a tipping point whereby a significant portion of the Amazon could turnover to savannah, pushing untold species to extinction and undercutting the many ecosystem services provided by tropical rainforests. Yet, a new study headed by Carlos Jaramillo, staff scientist at the Smithsonian Tropical Research Institute (STRI), has found a tropical forest ecosystem thriving in much warmer conditions than today.
While the study is notable, a conservation biologist at James Cook University who has spent decades working in the tropics, Dr. William Laurance, told mongabay.com that he "urge[s] people to be very cautious in extrapolating the lessons [of the study] to today."
According to the study's findings, some 56 million years ago, rainforests thrived in temperatures 3-5 degrees higher than today. Conservative estimates expect the tropics to see a 3-degree-rise by the end of the century. In addition, atmospheric carbon dioxide levels at the time were 2.5 times higher than today’s levels.
Examining prehistoric pollen in the Amazon from the global warming event dubbed the Paleocene-Eocene Thermal Maximum, researchers found that biodiversity exploded during this time with new species forming at a faster than species went extinct. Both passionflower plants and chocolate first evolved during this time.
But what does this really mean for the likely impacts of current global climate change on the world’s tropical rainforests?
"It is remarkable that there is so much concern about the effects of greenhouse conditions on tropical forests," says Klaus Winter, staff scientist at STRI, in a press release.
Yet Winter adds that, "however, these horror scenarios probably have some validity if increased temperatures lead to more frequent or more severe drought as some of the current predictions for similar scenarios suggest."
Laurance goes further, explaining that there are considerable differences between current climate change impacts and those highlighted in the study.
"The doubling of carbon dioxide at the Paleocene-Eocene boundary occurred over a period of some 10,000 years. Our current doubling of carbon dioxide is occurring far faster, with most of the increase happening in just a century or so—a rate a hundred times faster. This gives species far less time to migrate or adapt to changing conditions," he says.
Like Winter, Laurance says that the key to rainforest survival in the future may not be temperature, but rather shifts in precipitation. This year, the Amazon experienced its worst drought in decades, pushing the Amazon River to its lowest point in a century.
"Rainfall regimes could change with global warming, and quite possibly in unpredictable ways. You can’t have a healthy rainforest without fairly consistent rainfall. In general atmospheric humidity will likely rise, but beyond this our capacity to predict local- and regional-scale changes is often appalling poor."
In addition, while the study looks at species in lowland rainforests, Laurance points out that it neglects "higher-elevation specialists in the tropics."
"It is these species which are adapted for living in wet, cloudy, cool conditions, that are probably most vulnerable to global warming," he explains. "As the world heats up, these species literally have nowhere to go—aside from heaven, which is where a lot of them could end up."
Finally, Laurance points out another significant difference between climate change 56 million years ago and climate change today: humans. Tropical forests are not only facing warmer temperatures, higher carbon levels, and perhaps drier conditions, but also have been undergoing vast environmental changes by people—including unprecedented deforestation.
"Species today are often besieged by an array of environmental insults—habitat loss and fragmentation, overhunting, invading species, and manifold other environmental changes—and these could greatly exacerbate the pressures resulting from climate change," Laurance explains. "If this were a boxing match, nature isn’t just being jabbed once, but is suffering a flurry of blows arriving all at once."
The theory that a significant portion of the Amazon is susceptible to flip over to savannah under climate change was a part of a recent journalistic scandal. Jonathan Leake, Science and Environment Editor of the Sunday Times, claimed that the theory was put forth by "green campaigners" with conservation group WWF and not scientists, since this was how it was cited in an IPCC report. However the Sunday Times was forced to retract the column when they realized the theory had robust scientific studies behind it and that Leake had misrepresented the opinions of scientists he spoke with and never contacted WWF.