隨著人類燃燒化石燃料、碳排放越來越高，全球森林利用這些二氧化碳快速生長，減少大氣中的二氧化碳，這個過程叫做「二氧化碳肥料效應」（carbon dioxide fertilization）。
Tropical forests have a bigger appetite for the greenhouse gas carbon dioxide than many scientists have believed – even greater than that of temperate forests, finds a new study led by NASA and the National Center for Atmospheric Research, NCAR.
As human-caused emissions from burning fossil fuels add more carbon dioxide to the atmosphere, forests worldwide are using that CO2 to grow faster, reducing the amount of the greenhouse gas that stays airborne. This effect is called carbon dioxide fertilization.
Forests and other land vegetation now remove up to 30 percent of human carbon dioxide emissions from the atmosphere by absorbing CO2 during photosynthesis. If the rate of absorption were to slow down, the rate of global warming would speed up, the authors say.
Tropical forests absorb 1.4 billion metric tons out of a total global absorption of 2.5 billion metric tons, the authors estimate.
The question of whether temperate or tropical forests absorb the most CO2 “is not just an accounting curiosity,” said NCAR scientist Britton Stephens, a co-author on the paper. “It has big implications for our understanding of whether global terrestrial ecosystems might continue to offset our carbon dioxide emissions or might begin to exacerbate climate change.”
For about 25 years, most atmospheric inverse computer models have shown that mid-latitude forests in the Northern Hemisphere absorb more CO2 than tropical forests.
Now the scientists on this study say that result was based on an outdated understanding of global air flows and on limited data suggesting that deforestation was causing tropical forests to release more carbon dioxide than they were absorbing.
The new study is the first to make apples-to-apples comparisons of CO2 uptake estimates from many sources at different scales, such as computer models of ecosystem processes, atmospheric inverse models, satellite images and data from experimental forest plots, among other sources.
The researchers reconciled these analyses and obtained their new estimate of the tropical carbon absorption from the weighted average of atmospheric, ecosystem model, and ground-based data.
“Until our analysis, no one had successfully completed a global reconciliation of information about carbon dioxide effects from the atmospheric, forestry, and modeling communities,” said co-author Joshua Fisher of the Jet Propulsion Laboratory. “It is incredible that all these different types of independent data sources start to converge on an answer.”
In the mid-2000s, Stephens used measurements of CO2 made from aircraft to show that many atmospheric inverse models were not correctly representing flows of carbon dioxide in the air above ground level.
Models that matched the aircraft measurements better showed more carbon dioxide absorption in the tropical forests. But there were still not enough global data sets to validate the idea of large tropical-forest absorption.
Schimel said their new study takes advantage of a great deal of work other scientists have done since Stephens’ paper to pull together national and regional data of various kinds into robust, global data sets.
“What we’ve had up till this paper was a theory of carbon dioxide fertilization based on phenomena at the microscopic scale and observations at the global scale that appeared to contradict those phenomena,” said David Schimel of NASA’s Jet Propulsion Laboratory in Pasadena, California.
“Here, at least, is a hypothesis that provides of a consistent explanation that includes both how we know photosynthesis works and what’s happening at the planetary scale.”
Atmospheric models have improved over the past decade, but there is still disagreement among atmospheric inverse estimates of the distribution of carbon uptake, owing to remaining differences in modeling global air flows.
The scientists must also take wildfires into account. Climate change decreases water availability in some regions and makes Earth warmer, leading to more frequent droughts and larger wildfires. In the tropics, humans compound the problem by burning wood during deforestation. Fires stop carbon absorption by killing trees and also emit huge amounts of carbon into the atmosphere as the wood burns.