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Rainforests may be more resilient to climate change

  • 07 Feb 2013, 12:10
  • Roz Pidcock

Cajie - rainforest

Tropical rainforests like the Amazon may not be as vulnerable to rising carbon dioxide levels as previously thought, says a new study. This doesn't mean global warming is a good thing for rainforests, the scientists warn, but projections of how much we're likely to lose during the 21st century could be too high.

As part of the earth's natural carbon cycle, vast amounts of carbon dioxide are taken out of the atmosphere and absorbed by the land each year. Tropical rainforests, the extremely productive forest ecosystems found gathered around the equator, are responsible for most of that exchange.

Tropical rainforests store a lot of carbon as living biomass. If the size of global rainforest changes, it can have a big impact on the amount of carbon dioxide in the atmosphere.

Tropical dieback

According to the new study, published this week in Nature, most scientific models predict that tropical rainforests will degrade rapidly this century because of climate change - a process known as rainforest dieback.

Dieback is predicted due to changes in temperature patterns and reduced rainfall. As rainforests die, they absorb less carbon dioxide, leading to further warming - and so on. So dieback is important because it can add to climate change.

That's the theory. But just how vulnerable tropical rainforests are to manmade climate change is not yet settled. Different computer models predict different responses. The Nature paper notes that there is a factor of seven difference between the likely impacts of rainforest dieback on atmospheric warming - a considerable degree of uncertainty.

Model matching

The new research aims to reduce some of that uncertainty using a new technique, to try and pin down more precisely how sensitive rainforests are to climate change.

The scientists look at how the amount of carbon dioxide absorbed by rainforests changes year-to-year when the tropical climate is warmer or cooler than normal. They found the computer models that best represent the year-to-year variations are also the ones in which rainforests were less sensitive to climate change.

From this, the scientists suggested that many existing models overestimate the severity of forest dieback in response to climate change. Professor James Randerson, author of a summary article accompanying the paper, explains:

"As a result, the authors argue, the likelihood of a tropical dieback event is considerably lower than might be inferred from previous work".

Carbon dioxide fertilisation

What's more, the scientists suggest that any losses in rainforests' ability to store carbon - leading to more carbon dioxide emissions - is balanced out by the fact that tree growth is likely to be enhanced by higher carbon dioxide levels.

This is known as carbon dioxide fertilisation - and it's not a new concept in the science world. Some studies show that it is possible to increase the growth of some plants with extra carbon dioxide, under controlled conditions inside greenhouses. But when scientists have experimented with real outdoor conditions, the outcome is less promising.

But the new findings suggest that the rainforest can use carbon dioxide to speed up growth, enough to offset the extra emissions resulting from dieback. The net result, according to the scientists, is that the forests continue to store carbon overall.

Less vulnerable than thought

This research is not evidence that climate change is a good thing for rainforests. Lead author Professor Peter Cox from the University of Exeter told Carbon Brief:

"Our findings suggest that climate change is bad for the tropical rainforest - so much so that it would lead to 50 billion tonnes of carbon being released per degree of warming in the tropics."

Cox continues in the press release:

"Fortunately this carbon release is counteracted by the positive effects of carbon dioxide fertilisation on plant growth under most scenarios of the 21st century."

What the research does suggest is that rainforests may not suffer the effect of carbon-dioxide induced warming as seriously as scientists thought. Cox told Reuters:

"I am no longer so worried about a catastrophic die-back due to carbon dioxide induced climate change"

Some complexities

There are some important caveats in the study. For example, the new prediction only applies to carbon-dioxide induced warming. Temperature rise caused by other greenhouse gases in the atmosphere, such as methane, or a reduction in aerosols won't have the same positive effect on plant growth.

Also, carbon dioxide fertilisation only balances dieback in terms of emissions if carbon dioxide fertilises tree growth as strongly as climate models suggest. If it's less, the negative effects of forest dieback may overtake the positive effects of carbon dioxide fertilisation after all. The study reduces uncertainty, but doesn't eliminate it. As Professor Cox explained to Carbon Brief:

"[I]f the carbon dioxide fertilization effect saturates (for example because of nutrient limitations) then we are expecting big carbon losses from the tropics under climate change."

This study shows how advances in scientific understanding can make computer models more sophisticated - good news for better understanding the climate role rainforests may play in the future, even if reality may turn out to be more complex.

Cox et al., (2013) Sensitivity of tropical carbon to climate change constrained by carbon dioxide variability. Nature, doi:10.1038/nature11882

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