Rainforests may be more resilient to climate change
- 07 Feb 2013, 12:10
- Roz Pidcock
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