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As temperatures rise, tropical forests absorb less CO2

Rising temperatures are linked to a decrease in carbon dioxide (CO2) absorption by tropical forests, according to a 50-year…

Tropical forests such as this in Borneo remove large quantities of atmospheric carbon dioxide. H-D Viktor Boehm

Rising temperatures are linked to a decrease in carbon dioxide (CO2) absorption by tropical forests, according to a 50-year study published today.

Greenhouse gases, such as CO2, contribute to global warming, sea level rises and extreme weather events, previous studies have shown.

Forests absorb CO2 during photosynthesis and release it during respiration.

The new NASA-lead study, conducted with the CSIRO and published in the journal Proceedings of the National Academy of Sciences, builds on a growing body of evidence that suggests global warming will accelerate as time goes by.

The researchers analysed data on global atmospheric carbon dioxide concentration and climate variability between 1959 and 2011, which included El Niño years characterised by higher temperatures and lower average rainfalls.

The researchers found that a tropical land surface temperature rise of one degree Celsius led to an average extra 3.5 Petagrams of CO2 being pushed into the atmosphere per year. A Petagram is a billion tonnes.

“Tropical forests are carbon sinks but when it gets hotter, they become less efficient in absorbing CO2 from the atmosphere. We are learning very clearly that tropical forests do not like to be any hotter than they are. As soon as you increase the temperature, they perform less well as carbon sinks,” said study co-author Pep Canadell, Executive Officer of the Global Carbon Project.

“Many processes involved in this response are the same as what is known as the carbon-climate feedback, which it is thought will lead to an acceleration of carbon emissions from vegetation and soils and into the atmosphere under future climate change.”

Exacerbating the problem

Steve Sherwood, Director of the Climate Change Research Centre at the University of New South Wales, said the findings were important.

“The signals they see are a good way to test climate models that include an interactive carbon cycle. Also, their findings help explain the well-known, natural, year-to-year variations in the rate of carbon dioxide buildup, and in so doing, they increase the likelihood that global warming in the future will cause more of the carbon dioxide now stored in tropical plants and soils to go into the atmosphere,” said Professor Sherwood, who was not involved in the study.

“This will exacerbate our own emissions of this gas, and therefore will make human-caused climate change and ocean acidification a bit worse than these problems would otherwise have been. This is not a new conclusion, but their results increase our confidence in it.”

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  1. John Newlands

    tree changer

    Not just the tropics but temperate forests as well. In Tasmania old timers tell me they have never before seen the sight of dessicated trees after hot spells. Those heat events were in January and March and many of those trees are still dead. The trees were kilometres from any fire front and can be seen driving past some bluegum plantations as well as old growth. I therefore speculate that Tasmania's biomass is now less in 2013 than it was this time in 2012.

    If this is correct it means that Aussies forests are not helping with carbon uptake, at odds with the received wisdom a few months ago. If summer 2014 is another shocker we'll need a major rethink about the role of forests.

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    1. Liam J

      logged in via email @gmail.com

      In reply to John Newlands

      I'll second that observation John, Januarys heatwave killed many many trees & shrubs in NE Vic, on forested hillsides, road reserves, creeklines, gardens, everywhere.

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  2. Chris O'Neill

    Victim of Tony Abbotts Great Big New Tax

    "The researchers found that a tropical land surface temperature rise of one degree Celsius led to an average extra 3.5 Petagrams of CO2 being pushed into the atmosphere per year. A Petagram is a billion tonnes."

    This is a fraction of the CO2 that humans dump in the space above the ground every year but it doesn't help.

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  3. ian cheong

    logged in via email @acm.org

    So what happens to the subtropical forests that are now the temperature that the tropical forests were?

    Our friend who is a coral scientist says corals grow faster with more CO2.

    The earth is a complex thing and all the models have bugs. There is still a lot we don't know. In 100 years we'll know a lot that we didn't know now. Geologists have a longer term perspective (hundreds of millions of years).

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    1. Alice Kelly
      Alice Kelly is a Friend of The Conversation.

      sole parent

      In reply to ian cheong

      Sub-tropical forests?. They burn well when extra heat subjects them to drought.
      More CO2 absorbed by the ocean, and coral will start to dissolve. at the present the extra heat in the ocean results in the coral spitting out the algae (zooxanthellae) in it. They can grow again if the water temperature lowers again, otherwise they starve and die.
      CO2 also impacts their formation genetically, it is not enough to say this is "good for them"
      http://www.sciencealert.com.au/news/20121104-23300.html

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    2. Rotha Jago

      concerned citizen

      In reply to Alice Kelly

      That warming is happening is not in dispute, but
      What if we looked at the soil instead of at the atmosphere?
      The Photo at the head of this article shows a tropical forest in Burma.
      Those whitened tree trunks tell me to examine the soil beside the forest.

      Has that soil been sprayed with Herbicide? If it has, then the ability of the forest to use CO2 will be inhibited. Soils within the forest will be drier than before herbicide use. The trees will also be less hydrated.

      No weed could have…

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    3. Alice Kelly
      Alice Kelly is a Friend of The Conversation.

      sole parent

      In reply to Rotha Jago

      I doubt very much that you could assume that the tree species in the photo have been impacted by herbicide because they have white trunks. Those trees look healthy to me. I suppose trees have white trunks when the outer bark wood has been shed, but in that case they'd be dead with no leaves. Many forest trees have white or pale grey trunks.
      Yes tropical forests can burn.
      What interests me is that trees are very good at storing CO2, however close their stomates to a smaller size when they've had enough (so they can maintain a level of CO2 compatible and equal in their leaves), but that this means the tree uses less water. Good for the tree, not good for evaporation and the formation of clouds (moving water from the earth to the atmosphere). So climate models have to be fine-tuned again. There's another article about the same subject in the brit. conversation today, also interesting.

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    4. Rotha Jago

      concerned citizen

      In reply to Alice Kelly

      That forest is sick,really. You should not be able to see the trunks so clearly if at all, because the canopy should be too thick. Also if you look closely you can see trees which are greyer or browner, not quite straight. These trees will be beside water courses.

      In a Glyphosate afflicted landscape the trees beside the water fare the worst. For a person used to looking at landscapes that is the most striking thing because it is exactly opposite from what you would expect.

      They are absorbing less CO2, but the reason is in the soil and water.
      It could be verified if tests were taken. A few local enquiries would provide the names of the herbicides to look for.

      Climate Change is never the only influence on the landscape.

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    5. David Arthur

      resistance gnome

      In reply to ian cheong

      Mr Cheong asks: "So what happens to the subtropical forests that are now the temperature that the tropical forests were?"

      Trouble is, Mr Cheong, there is negligible subtropical forests relative to tropical forests. This is due to the Hadley circulation, which means that the equatorial (tropical) high rainfall belt is surrounded (~30 degrees north and south) by warm arid lands. There's relatively little subtropical forest, much more subtropical savannah and desert. It's all explained for you in the 20 July 2013 edition of New Scientist (No2926) in Anil Ananthaswamy's article.

      Sure, there's a lot we don't know, but we don't know a whole lot less than Mr Cheong might believe. That said, I cannot comment on the limits to Mr Cheong's own lack of knowledge.

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    6. Liam J

      logged in via email @gmail.com

      In reply to ian cheong

      Ian Cheong "Our friend who is a coral scientist says corals grow faster with more CO2." - you can't be serious, low-mid latitude coral reefs are in decline around the world thanks to ocean acidification & coral bleaching (+overfishing).

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  4. William Jago

    Project Manager

    Is it possible that the link between temperature and carbon takeup applies not only to tropical forests, but to many other ecosystems?

    I observe that Australian state governments have in recent years started to subsidise councils, farmers and their contractors to use herbicides such as Glyphosate to reduce or eliminate ground cover on road verges, river banks and railway cuttings. This must surely be increasing the temperature of these areas and the adjacent land, not to mention the effect of…

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