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Planetary Watch: Good News From Africa – Tropical Forests Inhale

by Bill Chameides | February 23rd, 2009
posted by Erica Rowell (Editor)

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Now even more evidence suggests a large tropical forest sink. (Source:Wikimedia)

A heretofore unrecognized ally has emerged in the global warming fight: tropical forests.

A favorite topic among climate scientists is the “missing sink” – a sort of geek-code not for “where’s the bathroom?” but for “where’s the carbon?”

Every year, humans emit about 9 billion tons of carbon as carbon dioxide (CO2) into the atmosphere. Some 7.5 billion tons come from burning fossil fuels, and the remainder mostly from tropical deforestation (source). However, only about 45 percent (called the airborne fraction) stays or accumulates in the atmosphere. Some 55 percent or almost 5 billion tons is removed within a year of its being emitted.

The fact that only 45 percent of what we emit accumulates in the atmosphere is great; unfortunately, what does accumulate hangs around for decades to centuries. That’s why we need to know what determines the airborne fraction and whether it will change in the future.

Just because the airborne fraction is 45 percent today doesn’t mean it will always be. If it increases, atmospheric CO2 will accumulate faster, effectively increasing CO2 emissions and necessitating decreases of our actual emissions that much more. If, on the other hand, the airborne fraction becomes lower, our job of avoiding dangerous climate change gets a little easier.

Whither the 55 Percent?

For quite some time we’ve known the oceans remove about 25 percent of the 55 percent from the atmosphere. This CO2 transfer is a good thing because it prevents CO2 from building up in the atmosphere. But that “good thing” is diminished somewhat by the fact that heightened CO2 levels make oceans more acidic, thus posing potentially serious consequences for coral and other calcifying organisms.

Whither the Missing Sink?

The remaining 30 percent is what scientists call the “missing sink” – missing because it was not clear where the sink went to. For more than a decade it has been pretty clear that the missing sink resides in the so-called terrestrial biosphere, a fancy term for forests. The suspicion has been that much of this forest sink is located in the northern mid-latitudes, perhaps as a result of:

  • inadvertent nitrogen fertilization of the forests from deposition of nitrogen oxide pollutants,
  • the fertilization effect of CO2 itself, and/or
  • the regrowth of forests that had previously been under cultivation.

But the data supporting that suspicion have been less than definitive.

Since the late 1990s, scientists reported tantalizing data suggesting a larger-than-expected portion of the sink may reside in tropical forests. These forests comprise the Earth’s dominant carbon engine. While covering only about 6 percent of the land, they already hold about 25 percent of the carbon stored in all the planet’s forests (see here). That these forests were sucking up some of our CO2 pollution did not seem unreasonable although many were unconvinced.

Oliver Phillips of the University of Leeds and colleagues first documented that this was indeed the case for Amazonian forests back in 1998. An analysis of airborne CO2 concentrations by Britton Stephens of the National Center for Atmospheric Research and colleagues elaborated on this and subsequent work (see here and here). The researchers concluded that:

  • the size of the terrestrial tropical sink was underestimated and
  • undisturbed tropical forests were soaking up atmospheric CO2 at a rate of about 1 billion tons of carbon per year.

New Paper: Trees Taking Up More Carbon Than Previously Thought

Now a new set of measurements in African forests, by Simon Lewis of the University of Leeds and colleagues, provides even more convincing evidence for a large tropical forest sink. In their new paper published in Nature, Lewis and his colleagues analyzed a time-series of measurements of the total carbon content of trees and soils in 79 plots distributed across a tropical forest network spanning ten countries. Because Lewis’s analysis is based on the forests’ actual carbon content, it is far more definitive than that of Stephens et al., which could only infer the uptake of the carbon by forests from atmospheric CO2 measurements.

The data reported by Lewis at al. are striking and, when extrapolated to all undisturbed tropical forests, suggest a total sink of 1.3 billion tons of carbon per year. To rationalize this result with the global carbon budget, it likely requires that the mid-latitude forest sink is about half as large as has been assumed or that the rate of deforestation is significantly smaller.

The fact that undisturbed tropical forests appear to be sucking up our CO2 pollution is good news.

The catch is that for this sink to be an effective warrior in the fight against global warming, these undisturbed tropical forests have to stay intact. A global regime that incentivizes the conservation of these forests is all the more critical.

Interestingly, economic development in the tropical forest nations may actually be good for conserving their forests. It appears that rural populations moving to cities is leading to forest regrowth in some areas and presumably an even greater rate of carbon uptake (see news article).  All good for keeping the airborne fraction down.

filed under: climate change, faculty, forests, global warming, Planetary Watch
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