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 dioxide.
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
The 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 example.
Lead author of the study, Rong Fu, explained:
“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 tipping point.”
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 savannahs.
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 emissions.
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 century.
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 savannah.”
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 a 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