Drying in the Amazon rainforest - what could it mean for climate change?
- 22 Oct 2013, 17:00
- Freya Roberts
CC - Cajie on Flickr
The southern reaches of the Amazon rainforest
are drying up - a little bit more each year. That's according to a
new study which finds that since 1979, the region's dry season has
got about a week longer each decade.
If these outer edges of the rainforest spend too much
time each year exposed to dry conditions, they could be prone to
more forest fires and could ultimately end up changing into an
entirely different habitat - one that locks up much less carbon
Even a partial loss of rainforest would
substantially increase carbon levels in the atmosphere, say the
authors, with consequences for the climate.
Longer dry seasons
new research shows that since 1979, the dry season
in southern parts of the Amazon rainforest has grown by about seven
days per decade. The authors can't definitively link the changes to
any one factor, but say the trend they observed resembles the
effects of climate change.
Overall, scientists know rainforests are
resilient to some level of climate change.
But a longer dry season is particularly problematic for trees in
areas where conditions are only just wet enough for rainforest
species to survive. Like on the southern edge of the Amazon, for
Lead author of the study, Rong Fu,
"The dry season over the
southern Amazon is already a marginal for maintaining rainforest.
At some point, if it becomes too long, the rainforest will reach a
The tipping point is like a climatic limit
beyond which conditions would be too dry for rainforest species to
survive. Once the limit is passed, the rainforest would die back,
and be replaced by other habitats like seasonal forests and
Since tropical tree species lock up lots of
carbon as they grow, losing parts of the rainforest would limit
earth's ability to buffer humans' rising greenhouse gas emissions.
But even before the problem reaches that point, longer dry seasons
could have serious consequences for the climate.
The chances of droughts increase with longer dry
seasons, raising the risk of forest fires. During droughts and
fires, the forest turns from a sink to a temporary source of carbon
dioxide. Under normal conditions the Amazon removes
billions of tonnes of carbon dioxide from the
atmosphere each year, but in 2005, for example, a major drought
caused the release of more than one
gigatonne of carbon - that's about about
one-tenth of humans' annual carbon
As the forest has dried over the last 30 years,
the authors observed an increasing number of forest fires. They
suggest that if even if the dry season doesn't continue to lengthen
at the same pace in the future, drought years like 2005 will become
the norm rather than the exception by the end of the 21st
Is more drying due?
The problem, according to the study, is that
climate models, including those used by the Intergovernmental Panel
on Climate Change (IPCC), don't predict the sort of dry season
changes seen in this study.
In its latest report reviewing the science
underpinning climate change, the IPCC
notes that many climate models predict the
Amazon will experience "an increase in dry season length" under a
high emissions scenario. It also acknowledges models which suggest
that "future climate change would increase the risk of tropical
Amazon forest being replaced by seasonal forest or even
At this point in time, the IPCC hasn't made any
new predictions quantifying things like the length of dry seasons
in the Amazon - that sort of detail is likely to be contained in
its next report on climate change impacts due in March.
But the authors have done their own analysis,
using the emissions scenarios and climate models the IPCC uses.The
results suggest that even under a high emissions scenario, the dry
season would only be a maximum of
10 days longer by the end of the century - a
much smaller change than already witnessed in the last 30 years.
The authors suggest this could mean that the models are
underestimating how sensitive the region is to drying.
Simulating tropical rainfall isn't easy and
predicting how it will change in the future is a challenge for
scientists. It's an area of ongoing research - just a few days ago
new study was published suggesting rainfall
bands over the tropics could shift south, altering where most rain
falls in the tropics.
But with considerable consequences for the
climate at stake, making accurate predictions about changes in the
Amazon is important, they say.
Fu et al. (2013) Increased dry-season length over
southern Amazonia in recent decades and its implication for future
climate projection. Proceedings of the National Academy of
Sciences. DOI: 10.1073/pnas.1302584110