Back in 2007 a paper, Amazon Forests Green-Up During 2005 Drought, was published in Science:
Coupled climate-carbon cycle models suggest that Amazon forests are vulnerable to both long- and short-term droughts, but satellite observations showed a large-scale photosynthetic green-up in intact evergreen forests of the Amazon in response to a short, intense drought in 2005. These findings suggest that Amazon forests, although threatened by human-caused deforestation and fire and possibly by more severe long-term droughts, may be more resilient to climate changes than ecosystem models assume.
Now a new paper contradicting the previous paper, Amazon forests did not green-up during the 2005 drought has been published:
We find no evidence of large-scale greening of intact Amazon forests during the 2005 drought – approximately 11%-12% of these drought-stricken forests display greening, while, 28%-29% show browning or no-change, and for the rest, the data are not of sufficient quality to characterize any changes. These changes are also not unique – approximately similar changes are observed in non-drought years as well.
So how does this get reported? Here’s Terence Corcoran in the National Post:
But this week new research supports the original Amazongate version of the science. The Amazon may not be at risk from climate change. Researchers at Boston University, headed by Ranga B. Myneni, professor of geography and environment, found that satellite readings used by other scientists were based on contaminated data. In a paper published by Geophysical Research Letters, Prof. Myneni and associates say they found no evidence that the Amazon suffers extreme tree mortality, excessive forest greening or other trauma under extreme climate conditions.
The Myneni paper examined the impact on the Amazon of a major 2005 drought. Some scientists have argued that the 2005 drought caused significant rainforest disturbances. But Prof. Myneni says that science is based on satellite data that cannot be reproduced because much of it is “atmosphere corrupted.”
Don’t you love the way Corcoran describes greening as “significant disturbances” and implies that the earlier study found “extreme tree mortality” rather than geening?
But while Corcoran is being deceitful, you can’t blame this all on him.
The Boston University press release also misrepresents the paper:
[Amazon rain forests] may be more tolerant of droughts than previously thought
(Boston) — A new NASA-funded study has concluded that Amazon rain forests were remarkably unaffected in the face of once-in-a-century drought in 2005, neither dying nor thriving, contrary to a previously published report and claims by the Intergovernmental Panel on Climate Change.
Simon Lewis comments on the new paper and the press release:
The new Samanta study uses 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 the satellite-based same method showed a strong ‘greening-up’ of the Amazon in 2005, suggesting tolerance to drought. 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, using ground observations from across the Amazon, that while the 2005 drought did not dramatically change the growth of the trees compared to a normal year, as Samanta also show, but the deaths of trees did increase considerably. The new study of Samanta et al., supports the Phillips et al. study, which itself shows the Amazon is vulnerable to drought. The Phillips paper showed that remaining Amazon forests changed from absorbing nearly 2 billion tonnes of CO2 from the atmosphere a year, to being a massive committed source of over 3 billion tonnes, from tree mortality.
The evidence for the possibility of a major die-back of the Amazon rainforest is due to two factors,
That climate change induced decreases in rainfall in the dry season occur, and
The trees cannot tolerate these reductions in rainfall.
The Samanta paper does not directly address the first point, this is addressed using modelling. The second point is only addressed in a limited way. The critical question is how these forests respond to repeated droughts, not merely single-year droughts. The forests are of course able to withstand these single droughts (otherwise there would be no rainforest!) – it is their ability to survive an increased frequency of the most severe droughts that is critical to answer. Drought experiments, where a roof is built under the forest canopy, show that most forest trees survive a single year’s intense drought, but can’t persist with repeated years of drought. The Samanta study does not address this point at all.
In conclusion the new study lends further weight to the emerging picture of the 2005 drought, that tree growth was relatively unaffected, but tree mortality increased, contributing to temporarily accelerating the rate of climate change, rather than as usual reducing it via additions of carbon to the atmosphere from the dead trees. Furthermore, the climate change model results suggesting decreasing rainfall in the dry season over Amazonia in the coming decades are unaffected by the new study, thus overall the conclusions in the IPCC 2007 Fourth Assessment Report are strengthened (because the anomalous result of the Saleska 2007 science paper appear to be at fault), not weakened, by the new study as the press release implies.
The press release also states:
The IPCC is under scrutiny for various data inaccuracies, including its claim – based on a flawed World Wildlife Fund study — that up to 40% of the Amazonian forests could react drastically and be replaced by savannas from even a slight reduction in rainfall. …
“The way that the WWF report calculated this 40% was totally wrong, while [the new] calculations are by far more reliable and correct,” said Dr. Jose Marengo, a Brazilian National Institute for Space Research climate scientist and member of the IPCC.
I contacted Marengo to see if this fairly represented his position, and, no it didn’t, having been taken out of context. He wrote:
I did not know that Sangram would pass my comments for a blog.
I have exchanged few emails with him, and I agree with him about
his position on the greening of Amazonia as shown by Saleska et
al (2007). However, I have questioned him few times about his
conclusions on the IPCC 40% value. In his paper he does not show
anything that go against the 40%, and he did not mention IPCC at
all. So this comparison is out of of context considering the
finding of this high quality paper.
What I said is that the 40% was obtained qualitatively from a
map from Nepstad et al (2004), comparing the area burn during the
El Nino 1998 and the mean area. Nepstad considered the El Nino 1998
situation as an analogue of what the future could be, which may
not be entirely realistic. I said that between an eye calculation
to get the 40% reported by the WWF document and the calculations
from Samanta et al (2010), even though they refer to different things,
Samanta et al did more correct and reliable work.
Yes, I believe that the Amazon forests are vulnerable to rainfall
reduction, and high temperatures, and this would lead to what some
studies call the Amazon die back. However, the die back is still
somewhat uncertain, but without reaching a level in which the forest
would replaced by savanna, the forest is highly vulnerable to drought.
And is it any suprise that Jonathan Leake is also pushing this story?
Hat tip: Ben Stewart for sending me the comments from Simon Lewis.