A new study suggests the southern portion of the Amazon rainforest is at
a much higher risk of dieback due to stronger seasonal drying than
projections made by the climate models used in the latest report by the
Intergovernmental Panel on Climate Change.
Using ground-based rainfall measurements from the past three decades,
a research team led by Rong Fu, professor at The University of Texas at
Austin's Jackson School of Geosciences, found that since 1979, the dry
season in southern Amazonia has lasted about a week longer per decade.
At the same time, the annual fire season has become longer. The
researchers say the most likely explanation for the lengthening dry
season is global warming.
"The dry season over the southern Amazon is already marginal for
maintaining rainforest," says Fu. "At some point, if it becomes too
long, the rainforest will reach a tipping point."
The new results are in stark contrast to forecasts made by climate
models used by the IPCC. Even under future scenarios in which
atmospheric greenhouse gases rise dramatically, the models project the
dry season in the southern Amazon to be only a few to 10 days longer by
the end of the century, and therefore the risk of climate change-induced
rainforest dieback should be relatively low.
The report appears this week in the journal Proceedings of the National Academy of Sciences.
"The length of the dry season in the southern Amazon is the most
important climate condition controlling the rainforest," says Fu. "If
the dry season is too long, the rainforest will not survive."
To see why the length of the dry season is such a limiting factor,
imagine there is heavier than usual rainfall during the wet season. The
soil can only hold so much water and the rest runs off. The water stored
in the soil at the end of the wet season is all that the rainforest
trees have to last them through the dry season. The longer the dry
season lasts, regardless of how wet the wet season was, the more
stressed the trees become and the more susceptible they are to fire.
The researchers say the most likely explanation for the lengthening
dry season in the southern Amazon in recent decades is human-caused
greenhouse warming, which inhibits rainfall in two ways. First, it makes
it harder for warm, dry air near the surface to rise and freely mix
with cool, moist air above. And second, it blocks cold front incursions
from outside the tropics that could trigger rainfall. The climate models
used by the IPCC do a poor job representing these processes, which
might explain why they project only a slightly longer Amazonian dry
season, says Fu.
The Amazon rainforest normally removes the greenhouse gas carbon
dioxide from the atmosphere, but during a severe drought in 2005, it
released 1 petagram of carbon (about one-tenth of annual human
emissions) to the atmosphere. Fu and her colleagues estimate that if dry
seasons continue to lengthen at just half the rate of recent decades,
the Amazon drought of 2005 could become the norm rather than the
exception by the end of this century.
"Because of the potential impact on the global carbon cycle, we need
to better understand the changes of the dry season over southern
Amazonia," says Fu.
Some scientists have speculated that the combination of longer dry
seasons, higher surface temperatures and more fragmented forests
resulting from ongoing human-caused deforestation could eventually
convert much of southern Amazonia from rainforest to savanna.
Earlier studies have shown that human-caused deforestation in the
Amazon can alter rainfall patterns. But the researchers didn't see a
strong signal of deforestation in the pattern of increasing dry season
length. The dry season length increase was most pronounced in the
southwestern Amazon while the most intense deforestation occurred in the
southeastern Amazon.
Because the northwestern Amazon has much higher rainfall and a
shorter dry season than the southern Amazon, Fu and others think it is
much less vulnerable to climate change
This story is taken from Science Daily
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