Volume 11, Number 13: 26 March 2008
In a review article recently published in Science, Malhi et al. (2008) note that the forest biome of Amazonia is facing the dual threat of climate change and a number of direct anthropogenic disturbances, such as deforestation. In the case of the former of these challenges, however, the literature they cite suggests that the region may easily weather whatever comes its way under the umbrella of climate change.
Stating that there are "risks of a drying climate" in response to CO2-induced global warming, the six researchers report that even the climate models employed in the IPCC Fourth Assessment Report "show no consistent trend in annual, Amazon-wide rainfall over the 21st century," additionally noting that they suggest only a tendency "to less dry-season rain in the east," and that they actually predict more rain "in the west and in the wet season." On top of this good news, they report "there is mounting evidence from artificial drought experiments (Fisher et al., 2007), flux towers, and satellite remote sensing of forest greenness (Huete et al., 2006) that intact Amazonian forests are more resilient to climatic drying than is currently represented in vegetation-climate models."
Explaining why this is so, Malhi et al. state that dry season water supply in Amazonia "is greatly enhanced by root systems accessing deep soil water and redistributing it into the surface soil through the process of hydraulic lift, enabling the whole forest ecosystem to maintain high transpiration and photosynthesis rates (Oliveira et al., 2005)." In addition, they say that "plant acclimation to higher temperatures may limit detrimental effects below 45°C," and that "rising CO2 may improve plant water use efficiencies and offset the negative transpiration effects of rising temperatures."
Looking backwards in time, the UK and US scientists report that "southern Amazonia was considerably drier as recently as the early-mid Holoocene, yet the region seems to have remained largely forested (Mayle et al., 2004)." This observation is truly amazing in light of the fact that the atmosphere's CO2 concentration at that time was only about two-thirds of what it is today. This much-reduced CO2 content should have severely handicapped the forest biome in terms of its ability to produce biomass, especially in a water-stressed environment. Nevertheless, Amazonia still prospered.
All things natural considered, the forest biome of Amazonia should thus be doing well today and should continue to do well throughout the foreseeable future. However, we know that both of these prospects are questionable. Why? Because of the direct assaults of humanity locally, which are discussed in detail by Malhi et al. This latter venue is the context within which our efforts must be focused if we are to preserve this incomparable biome. To futilely fight a misguided war against rising atmospheric CO2 concentrations -- especially in ways that actually encourage anthropogenic encroachments upon Amazonia (Laurance, 2007) -- is to push it ever closer to the ignominious end the world's climate alarmists claim they are seeking to prevent.
Sherwood, Keith and Craig Idso
Fisher, R.A., Williams, M., da Costa, A.L., Malhi, Y., da Costa, R.F., Almeida, S. and Meir, P. 2007. The response of an Eastern Amazonian rain forest to drought stress: results and modelling analyses from a throughfall exclusion experiment. Global Change Biology 13: 2361-2378.
Huete, A. R., Didan, K., Shimabukuro, Y.E., Ratana, P., Saleska, S.R., Hutyra, L.R., Yang, W., Nemani, R.R. and Myneni, R. 2006. Amazon rainforests green-up with sunlight in dry season. Geophysical Research Letters 33: 10.1029/2005GL025583.
Laurance, W.F. 2007. Switch to corn promotes Amazon deforestation. Science 318: 1721.
Malhi, Y., Roberts, J.T., Betts, R.A., Killeen, T.J., Li, W. and Nobre, C.A. 2008. Climate change, deforestation, and the fate of the Amazon. Science 319: 169-172.
Mayle, F.E., Beerling, D.J., Gosling, W.D. and Bush, M.B. 2004. Responses of Amazonian ecosystems to climatic and atmospheric carbon dioxide changes since the last glacial maximum. Philosophical Transactions of the Royal Society of London B 359: 499-514.
Oliveira, R.S., Dawson, T.E., Burgess, S.S.O. and Nepstad, D.C. 2005. Hydraulic redistribution in three Amazonian trees. Oecologia 145: 354-363.