In a recent post here at RealClimate, Simon Lewis wrote regarding a 2010 paper by Samanta et al. on the effect of single-year drought conditions on the Amazon. Samanta et al. claimed to have contradicted a 2007 paper by Scott Saleska et al., and to have thereby overturned some IPCC conclusions.
Lewis showed why Samanta’s paper did not contradict the IPCC, even if it may have correctly identified an error in Saleska et al. Now Saleska has written to say that, actually, Samanta et al.’s results do not identify any error in their work: the results agree completely. With our apologies for the journalistic whiplash, Simon Lewis and I are convinced he’s right. The more general point though, is that the the balance of evidence shows that the Amazon is sensitive to drought, and the IPCC’s statements about it remain valid.
Here is Saleska’s commentary in full
Guest Commentary by Scott Saleska, University of Arizona
The title of the Lewis post (“Up is Down, Brown is Green”) is perhaps even more true than the insightful commentary by my colleague Simon Lewis indicates! The Samanta et al paper says brown, but in fact their own data (when you dig it out of the supplement) shows green, consistent with (and indeed virtually indistinguishable from) our original findings published in Science (Saleska et al., 2007).
Samanta et al. misrepresents our work on many levels (one of which is to assert, falsely, that we did not filter out atmosphere-corrupted observations when in fact we did), and we intend, of course, to present an appropriate response in the peer reviewed literature, where the technical details of our differences may be evaluated by anyone who wishes. But for the moment we will, for the sake of argument, accept their analysis at face value and ask: even if Samanta et al. are 100% correct in their critique of our methods (which we of course dispute), what are the implications? Does the alternative to our method which Samanta et al. advocate, or the recent update in the MODIS satellite data (to version 5 from version 4), make any difference for the main conclusion of our paper? With due respect to our friends and colleagues at Boston University, the answer is no, it does not.
First: the actual relevant Samanta et al data (which comes from their Supplement, Table S3) is this:
|Table S3 (Samanta et al. 2010, supplement)|
|Year||Rain defecit (%)||Area Green (%)||Area Brown (%)||Area unchanged(%)||Area with valid pixels (%)|
Note that the green area in the drought region increases to its maximum (10.8% of the total area = 10.8/33.68 = 32% of the valid area) in 2005. In other words, the Samanta et al data contradict the Samanta et al text and title (which states that Amazon forests did not green up): not only do forests in the drought region green up, they green up alot, more than any other year since the MODIS satellite sensor was launched.
Second, how does this compare to Saleska et al. (2007), which Samanta et al claim to rebut? Here are the numbers (again, taken directly from Samanta et al, Table S3 and Saleska et al., 2007):
Fraction of valid pixels in the 2005 drought region that are “green” (> + 1 Standard deviation)
Saleska et al. (2007): 34% (p<0.000001)
Samanta et al. (2010): 32% (p<0.004)
The bottom line is that their observed 2005 result (32% greenness) is indistinguishable from ours (34%). I.e. Samanta et al effectively reproduce the results of Saleska et al.
This summary response, of course, begs some very interesting questions about tropical forest function under climatic variability and change (indeed the most interesting questions of all!): what caused the anomalously disproportionate green-up in the drought region? And, even if satellite “green up” does in fact represent an increase in photosynthesis (as we think), could this in fact be a symptom of the trees compensating for the increased stress of the drought? The bottom line “carbon balance” of a tree depends on both photosynthetic uptake and respiratory losses, and it is almost certainly the case that those losses (which were not seen by the satellite) increased under the hotter and drier conditions of the drought as well.
Thus, the most intriguing idea to me is that the short-term satellite-detected green-up, and the longer term increase in net carbon loss reported in the Phillips et al paper (discussed by Simon Lewis) are not in conflict at all. It might well be that they represent different parts of a coherent forest response to drought, in which the longer term losses are larger than the satellite-detected attempt to compensate for them by increasing photosynthesis, and in the end, increased tree mortality is the result.
In conclusion I would like to reinforce Simon’s point about Samanta et al and the IPCC. More important than whatever they say about our one short paper, Samanta et al. truly and egregiously misrepresent the implications, of both their work and ours, when they claim that a single paper on short term vegetation response somehow rebuts the IPCC’s review of the large scientific literature on how Amazonia might respond to long-term shifts in the mean climate state. It is an illogical and misguided claim on many levels, one that is already and deservedly attracting the opprobrium of many of my colleagues, talented scientists who study Amazon forests and climate (see Scientists speak: Amazon “myths” are not debunked).
– Samanta et al data show a drought region green up that is on average indistinguishable from Saleska et al (but they call it NO green up).
– Samanta et al data almost exactly reproduce Saleska et al’s most salient bottom-line result (but they say what we did was not reproducible).
– the Samanta et al paper, based on a three-month drought response, says not one word about long-term climate change scenarios reviewed in IPCC (but they advertise their analysis as “reject[ing] claims” put forward by the IPCC).