Up is Down, Brown is Green (with apologies to Orwell)

In the alternate universe of Fox News, Anthony Watts, and many others, up is down. Now, it appears, brown is green. Following the total confusion over the retraction of a paper on sea level, claims of another “mistake” by the IPCC are making the rounds of the blogosphere. This time, the issue is the impact of rainfall changes on the Amazon rainforest.

A study in 2007 showed that the forest gets greener when it rains less. A new study, by Samanta et al. in Geophysical Research Letters shows that the earlier work was flawed. Aided by an apparently rather careless press release, this is being used as evidence that the Amazon is less sensitive to rainfall changes than the IPCC claimed. But the Samanta et al. paper actually does not address the central questions at all. It only addresses whether a single anomalous rainfall year had an impact that is measureable and interpretable from a satellite sensor. The conclusion is that they could not detect a change. As noted in a commentary from Simon Lewis, University of Leeds, “the critical question is how these forests respond to repeated droughts, not merely single-year droughts.”

Lewis – a broadly published expert on tropical forests – makes a number of additional important points in his commentary below. Bottom line: IPCC gets it right as usual.

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Guest Commentary by Simon Lewis, University of Leeds, UK

The new Samanta et al. study uses sensors on satellites to assess the colour of the rainforest canopy in the dry season of the year 2005, compared to the dry seasons of the years 2003 and 2004. More detected green colour in 2005 may suggest that the forest is being more productive (more green leaves photosynthesising), or more brown colours may suggest leaves dying and less productivity, than the previous years. The results show that 2005 was little different to the previous years, despite the strong drought.

This is important new information, as in 2007, a paper using broadly the same satellite-based method showed a strong ‘greening-up’ of the Amazon in 2005, suggesting tolerance to drought (Saleska et al. 2007, Science). The new study shows that those results were not reproducible, but also highlight the extreme caution that should be attached to satellite studies generally in this field, with instruments in space collecting data which is then used to infer subtle changes in the ecology of tropical forests.

In contrast to the 2007 paper, Oliver Phillips, myself, and others, published a paper in Science last year, using ground observations from across the Amazon, showing that while the 2005 drought did not dramatically change the growth of the trees compared to a normal year, as Samanta et al. also show, the deaths of trees did increase considerably. The new study of Samanta et al. does not contradict the Phillips et al. study, which itself shows the Amazon is vulnerable to drought via impacts on tree mortality. The Phillips et al. paper showed that remaining Amazon forest trees changed from absorbing nearly 2 billion tonnes of CO2 from the atmosphere annually over recent decades, as tree growth has been exceeding mortality, to being a large, but temporary, source of over 3 billion tonnes, from the elevated tree mortality associated with the 2005 drought event.

The evidence for the possibility of a major die-back of the Amazon rainforest is due to two factors,

1. That climate change induced decreases in rainfall in the dry season occur, and

2. The trees cannot tolerate these reductions in rainfall.

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