Out in the Ama-zone

Guest Post by Willis Eschenbach

There have been lots of articles lately discussing the retraction by the UK Sunday Times of their claims about Amazongate. Folks like George Monbiot are claiming that their point of view has been vindicated, that Amazongate is “rubbish” and that skeptics have been “skewered”. So I decided to follow the tortuous trail through the Amazon jungle, to see where the truth lies.

Figure 1. The long, twisted, rainy jungle trail leading to the facts …

First, what did the IPCC say that caused all of the furor? Here’s the quote:

Up to 40% of the Amazonian forests could react drastically to even a slight reduction in precipitation; this means that the tropical vegetation, hydrology and climate system in South America could change very rapidly to another steady state, not necessarily producing gradual changes between the current and the future situation (Rowell and Moore, 2000). It is more probable that forests will be replaced by ecosystems that have more resistance to multiple stresses caused by temperature increase, droughts and fires, such as tropical savannas.  (IPCC, PDF, p. 596)

Scary stuff, climates tipping to a new steady state, 40% of the Amazon rainforest changing to savanna …

Now, this is referenced to Rowell and Moore (PDF). The first problem that arises is that this is a World Wide Fund for Nature (WWF) overview piece, and is as far from peer-reviewed science as one can imagine. The WWF says:

Up to 40% of the Brazilian forest is extremely sensitive to small reductions in the amount of rainfall. In the 1998 dry season, some 270,000 sq. km of forest became vulnerable to fire, due to completely depleted plant-available water stored in the upper five metres of soil. A further 360,000 sq. km of forest had only 250 mm of plant-available soil water left. 46

Note that already we see a difference between the citation (such as it is) and the IPCC statement. The WWF says that the forest is “extremely sensitive” to “small reductions” in rainfall. The IPCC has upped the ante, saying the forest could “react drastically” to “even a slight reduction” in rainfall. In addition, the IPCC has added an uncited claim that the South American “vegetation, hydrology and climate system” could suddenly change to a new “steady state” … be very afraid.

Now, the WWF paragraph has a citation (46). This is:

46 D. C. Nepstad, A. Veríssimo, A. Alencar, C. Nobre, E. Lima, P. Lefebvre, P. Schlesinger, C. Potter, P. Mountinho, E. Mendoza, M. Cochrane, V. Brooks, Large- scale Impoverishment of Amazonian Forests by Logging and Fire, Nature, 1999, Vol 398, 8 April, pp505

The problem is that their citation only supports the second half of the paragraph, the part that relates to the 1998 dry season. It says nothing about the extreme sensitivity of the Amazon. It says nothing about a new “steady state.” Even Dr. Lewis, who convinced the Times to issue the retraction, admits this:

The 40% claim is not actually referenced in the Rowell & Moore 2000 report (they use Nepstad to reference the specific figures in the next sentence). The Nepstad Nature paper is about the interactions of logging damage, fire, and periodic droughts, all extremely important in understanding the vulnerability of Amazon forest to drought, but is not related to the vulnerability of these forests to reductions in rainfall. I don’t see how that can be the source of Rowell’s 40% claim. Its more likely an unreferenced statement by Rowell.

And there, the trail stops. Despite Pachauri’s oft-repeated claim that the IPCC is based 100% on peer reviewed science, the IPCC has referenced a WWF document which:

1. Is not peer reviewed, and

2. Has no further citation for the claim.

So why did the Times have to retract their claim? It was the result of a letter sent to the Times by Dr. Simon Lewis, who claimed that a) he had been misquoted, and b) the IPCC claim was scientifically accurate.

From Dr. Lewis’s statement, I do believe he was misquoted. However, that does not mean that the IPCC statement was correct. Dr. Lewis defends it, saying:

The IPCC statement itself is poorly written, and bizarrely referenced, but basically correct. It is very well known that in Amazonia tropical forests exist when there is more than about 1.5 meters of rain a year, below that the system tends to ‘flip’ to savanna, so reductions in rainfall towards this threshold could lead to rapid shifts in vegetation.

Indeed, some leading models of future climate change impacts show a die-off of more than 40% Amazon forests, due to projected decreases in rainfall. The most extreme die-back model predicted that a new type of drought should begin to impact Amazonia, and in 2005 it happened for the first time: a drought associated with Atlantic, not Pacific sea-surface temperatures. The effect on the forest was massive tree mortality, and the remaining Amazon forests changed from absorbing nearly 2 billion tonnes of CO2 from the atmosphere a year, to being a massive source of over 3 billion tonnes.

The Amazon drought impacts paper was written by myself and colleagues in Science (attached). Here is the press release explaining the sensitivity: http://www.leeds.ac.uk/news/article/36/amazon_carbon_sink_threatened_by_drought

Now, there’s a couple of things to note about this claim. First, other than a paper by Dr. Lewis himself about Amazon carbon sinks, there are no citations. The paper about carbon sinks is interesting, but it does not show anything about a “flip” to savannah, and doesn’t mention the 40% claim.

Second, he does not present any evidence that the 40% statement is correct. Instead, he says that climate models show that the statement is correct … Now, climate model results are interesting, but they are not evidence of anything but the assumptions of the programmers of the models.

And in fact, the 40% claim is called into question by another paper by the same Nepstad cited by the WWF document. It says:

During the severe drought of 2001, PAW10m [plant-available soil water to 10 metres depth] fell to below 25% of PAWmax in 31% of the region’s forests and fell below 50% PAWmax in half of the forests.

Now, if the Amazon were so sensitive, if it “could react drastically” to even a “slight reduction” in rainfall, certainly such a large reduction would make a big difference … but that didn’t happen. There was no “flip” to savannah mentioned in the paper.

Third, Dr. Lewis seems to want us to think that some fraction of the rainforest becoming savannah is supportive of the IPCC claim that:

… the tropical vegetation, hydrology and climate system in South America could change very rapidly to another steady state …

That’s just misdirection. Dr. Lewis does not provide any evidence in support of the alarmist claim that the South American climate is in danger of a rapid change to some other steady state. Which is no surprise to me, since I know of no historical evidence of such a rapid large-scale change in the tropical climate to a much dryer state.

And finally, even Dr. Lewis recognizes that there is no scientific certainty about this question, saying:

This is not to say this there isn’t much uncertaintly as to exactly how vulnerable how much of the Amazon is to moving to a savanna system.

Well … yeah. Given that uncertainly, his claim that the IPCC statement is “basically correct” is unsupportable. “Much uncertainty” means that we cannot make scary statements like the IPCC has done, and we certainly can’t say that they are “basically correct”. All we can say is that they are uncertain.

Before going on to look at some actual data, lets review the story so far:

1. The IPCC made a claim that “Up to 40% of the Amazonian forests could react drastically to even a slight reduction in precipitation”, and that the South American climate could change rapidly to a new steady state.

2. This was referenced to a WWF review paper which was not peer reviewed.

3. The WWF paper had no citation for that claim.

4. Dr. Lewis says the claims are correct. However, like the IPCC, he does not provide a citation for his claim that the 40% statement is correct. He points us to a 2009 paper, of which he is a co-author. It doesn’t contain any support for the 40% claim. He refers to a few climate models, but shows no evidence.

5. Dr. Lewis says that there is “much uncertainty” about the question.

6. Dr. Lewis does not provide any evidence to support the idea that the South American climate is likely to change rapidly to a new steady state.

Now, having reviewed the story so far, lets think about this a bit dispassionately. First, is it theoretically possible for the Amazon to “flip” from rainforest to savannah?

Certainly it can. If the Amazon rainfall went to a tenth of the current value, it would all be savannah. So how much would a “slight reduction” affect the Amazon rainforest?

To investigate this, we can look at the amounts of rainfall around the Amazon. Figure 2 compares the vegetation and the rainfall:

Figure 2. Vegetation map of central South America. The Amazon rainforest is dark green. Violet rectangle show area of measured rainfall shown below in Fig. 3. Red lines show rainfall in millimetres per year.

There are several things we can see from this map. First, rainfall is not the only thing that is limiting the Amazon rainforest. There are areas with less than 1600 mm which are rainforest, and areas with more than 1600 mm which are not rainforest.

Second, at the left edge of the rainforest, we have the Andes mountains. In these areas, the Amazon is limited by elevation rather than by rainfall.

Now, suppose that the rainfall drops by 10%. I’d call that a “slight reduction” in rainfall. Will that affect 40% of the rainforest? No way. If we were to shrink all of the red lines by 10%, we’d only get about a 20% reduction in area … but there are large areas which are not rainfall limited in that sense. So a 10% reduction in rainfall might, and I emphasize might, give us a maximum of a 20% reduction in rainforest area. To get to 40% rainforest loss, we’d need a large reduction in rainfall, not a slight reduction.

But who is claiming that there will be a large reduction in Amazon rainfall? That is a model prediction, and not even one that appears in all of the models. Dr. Lewis says:

Indeed, some leading models of future climate change impacts show a die-off of more than 40% Amazon forests, due to projected decreases in rainfall.

This, of course, also means that some leading models do not show a die-off. Even the models don’t all agree with the IPCC claim.

However, all of this, all of the claims and counterclaims, and the models, and Dr. Lewis’s letter, and the cited scientific documents, all run aground on one ugly fact:

The data shows no change in Amazon rainfall in a century of measurements.

Figure 3 shows three different ground-based observational datasets, along with the recent Tropical Rainfall Measuring Mission (TRMM) satellite data.

Figure 3. Four Amazon rainfall datasets, covering the rectangular area shown in violet in Fig. 2 (2.5°N–12.5°S, 72.5°W–50°W). Note the generally good agreement between the four datasets (including the TRMM satellite data)

The main feature of this dataset is its stability. Note the lack of any trend over the last century, and the lack of any large excursions in the rainfall. It stays between two and two and a half metres per year. There are no really wet years, and no really dry years. 95% of the years are within ± 10% of the average rainfall. There are individual dry years, but no prolonged periods of drought.

So while Dr. Lewis says (correctly) that rainforest can change to savannah, he is not correct that 40% of the Amazon is at risk from a “slight reduction” in rainfall. More to the point, there is no evidence to indicate that we are headed for a reduction in Amazon rainfall, “slight” or otherwise. That is a fantasy based on climate models.

The reality is that despite the globe warming by half a degree or so over the last century, there has been no change in the Amazon rainfall. As usual, the IPCC is taking the most alarmist position possible … and Dr. Lewis is doing all he can to claim that the IPCC alarmism is actually good science.

Unfortunately for both the IPCC and Dr. Lewis, here at the end of a long, twisted, and rainy jungle trail, we find that the facts inconveniently disagree with their claims.

[UPDATE] Credit where credit is due. I love writing here because I always learn something. The Amazongate story was originally broken by Richard North, whose blog is EUReferendum. Give it a look, lots of good stuff.