Tropical Ecosystems Boost Carbon Dioxide as Temperatures Rise

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By Glynn Wilson

NASA scientists and an international team of researchers have found tropical ecosystems can generate significant carbon dioxide when temperatures rise, unlike ecosystems in other parts of the world, further exacerbating global warming and climate change, according to a new study.

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The researchers discovered a temperature increase of just 1 degree Celsius in near-surface air temperatures in the tropics leads to an average annual growth rate of atmospheric carbon dioxide equivalent to one-third of the annual global emissions from combustion of fossil fuels and deforestation combined. In tropical ecosystems carbon uptake is reduced at higher temperatures, a finding that provides scientists with a key diagnostic tool to better understand the global carbon cycle.

“What we learned is that in spite of droughts, floods, volcano eruptions, El Niño and other events, the Earth system has been remarkably consistent in regulating the year-to-year variations in atmospheric carbon dioxide levels,” said Weile Wang, a research scientist at NASA’s Ames Research Center in Moffett Field, Calif., and lead author of a paper published Wednesday in the Proceedings of the National Academy of Sciences.

The study provides support for the “carbon-climate feedback” hypothesis proposed by many scientists for years, one that asserts a warming climate will lead to accelerated carbon dioxide growth in the atmosphere from vegetation and soils.

Multiple Earth system processes, such as droughts and floods, also contribute to changes in the atmospheric carbon dioxide growth rate, but the new finding demonstrates observed temperature changes are a more important factor than rainfall changes in the tropics.

Wang, the lead scientist, explained further in an e-mail interview that the research looked at anomalies of temperature or atmospheric CO2 level.

“By ‘anomaly’ we mean the deviation of temperature (or CO2) from its normal level,” he said. “Therefore, in the tropics if the near-surface air temperature is 1 degree (Celsius) higher than normal, the forests will be affected by increases in respiration (which releases carbon dioxide to the atmosphere) and decreases in photosynthesis (which absorbs carbon dioxide from the atmosphere).”

As a result, he said, the net uptake of carbon dioxide by the forests will be less than normal. It does not necessarily mean that the net uptake will become zero or negative. It’s just that the tropical forests release more-than-usual carbon dioxide with warmer temperature, which leads to further global warming and climate change.

“In an analogy, the tropical forests are sometimes called the ‘lungs of Earth,'” he said. “If the temperature is higher than normal in the tropics (put another way, if the Earth gets a fever), the efficiency of the ‘lungs’ in recycling atmospheric carbon dioxide into oxygen (and biomass) will decrease.”

The tropical forests are still a key component of the global carbon cycle.

Nemani responded to the question, “is this good or bad?”

“Any process that puts more CO2 in the air cannot be good in the long run,” he said.

The team used a state-of-the-art, high-performance computing and data access facility called NASA Earth Exchange (NEX) at Ames to investigate the mechanisms underlying the relationship between carbon dioxide levels and increased temperatures. The NEX facility allowed scientists to analyze widely available data of atmospheric carbon dioxide concentrations and global air temperatures between 1959 and 2011, while studying outputs from several global dynamic vegetation models.

“Climate warming is what we know with certainty will happen under climate change in the tropics,” said Josep G. Canadell, executive director of the Global Carbon Project in Canberra, Australia, and co-author on the paper. “This implies the release of carbon dioxide from the tropical ecosystems will very likely be accelerated with future warming.”

Events that can temporarily influence climate, such as volcanic eruptions, may disturb the strength of the relationship between annual temperature and carbon dioxide growth for a few years, but the coupling always recovers after such events.

“The study really highlights the importance of long-term Earth observations for improving our understanding of the Earth system,” said Rama Nemani, principal scientist at Ames for the NEX project. “Conclusions drawn from analysis of shorter records could be misleading.”

The study was supported by the Earth Science Division in the Science Mission Directorate at NASA Headquarters in Washington, D.C.

For more information about the NEX program, click here.

For more information about NASA and agency programs, click here.

© 2013, Glynn Wilson. All rights reserved.

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