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Dry Amazon rainforest could contribute to climate change

Sun, 6th Feb 2011

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Back in 2005, the Amazon, which covers nearly 7 million km squared of South America, suffered a drought which was billed as a ‘once in a century’ event. Droughts like this are caused by increased sea surface temperatures in the atlantic ocean.

Now a team led by Simon Lewis from the University of Leeds have analysed data from another drought, in 2010, and concluded that this was in fact even more serious, and that these successive droughts could start causing big global problems.Amazonian rainforest, upper Amazon basin, Loreto region, Peru.

They found 57% of the Amazon region had low rainfall in 2010 compared to 37% in 2005, and also that the water stress on trees was more severe. They worked this out using a measure of drought severity called the maximum climatological water deficit, that correlates with how likely trees are to die from a drought, which they calculated by taking away the estimated transpiration of the trees - that’s the amount of water they lose from their leaves - from the lowest amount of water input to the region. So doing this they estimated 3.2km2 of forest in 2010 would have suffered a level of drought enough to cause significant tree death compared to 2.5km2 in 2005.

So why is this important? Well the Amazon acts as a giant carbon sink - sucking CO2 out of the atmosphere and locking it away in plant matter, which has helped to act like a buffer against all the extra CO2 humans have been pumping up into the atmosphere. But if droughts like the 2005 and 2010 events keep happening, and they will if sea surface temperatures continue to rise as they have been in recent years, more trees will die. This means that not only will the trees stop taking in CO2, but they will actually start to release it as a result of being broken down by microbes. Another effect of increasing temperatures, combined with a build up of dead plant material is an increase in the likelihood of forest fires, which can release huge amounts of CO2.

Simon Lewis and his colleagues suggest that this could become a positive feedback cycle, resulting in major forest loss and the loss of an important carbon buffering system, which could have major global implications.



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My first question is rain that is not falling in the Amazon...  is it just not falling... or is it being displaced elsewhere.  In particular, is there any displacement of the water into dryer regions?

2010 was listed as having the highest global rainfall in a century.  In fact some of the Aussies were complaining about having too much rainfall in 2010.  2005 is also listed as having an above average global annual rainfall, although not as intense.

There is a correlation between the low rainfall and the El Niño ocean currents.

So, the 2010 trend was likely brief... and likely has already been reversed.

There seems to be somewhat more prevalent El Niño currents in since about 1980.  But, it is perhaps early to tell if it is a statistical anomaly, and/or whether they have Anthropogenic causes.

One would also have to ask if these are actually "100 year droughts"...  as there are many indications of a significant Amazon drought in 1926, and perhaps a somewhat smaller drought in 1936, a decade later, as well as significant river flow fluctuations throughout the past century.  1906?  1912?  1963?  1992?  1995?  1998?

There will likely be some benefits if there is a displacement of water from the Equator towards the Tropic of Cancer and Tropic of Capricorn.

In fact, there seems to be somewhat of an anti-correlation between the Amazon River and the Parana River (into southern Brazil and Northern Argentina.

In the 2010 Global map, you can see the Parana basin was wet. 
Also compare:
50's and 60's, low flow Rio Negro, high flow Parana
Late 60's...  very distinctive high flow Amazon, low flow Parana

It would also be interesting to see a comparison of the flows between the Nile and the Amazon, for example. CliffordK, Wed, 9th Feb 2011

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