In order to complement my previous post, I have written this one to deal with the topic of extreme droughts in the Amazon Rainforest. Only a few years ago in 2005, the Amazon experienced an unusually dry season caused in part due to the 2002-2003 El Niño. The El Niño/Southern Oscillation is known to influence the climate of the forest, and as I mention in my previous post, is responsible for magnifying the effects of the flooding season as well as the drought season (I will write more about the ENSO in a future post). The difference in 2005 was that in addition to the ENSO affecting the Amazon from the Pacific Ocean, there was also a warming of the waters in the Atlantic Ocean.
The result was an “extreme drought event in the Amazon river basin, regarded as the worst in over a century”, as outlined by a paper by Chen et al. This is the same Chen from my previous post, and once again he uses GRACE satellite measurements to back his findings. There was in fact a widespread deficit in terrestrial water storage in August/September (apex of the drought) compared to the average TWS changes in the values they had for the years 2002-2007. The regions that were hit the hardest saw a decrease in 20-30 cm of water. Also, the average accumulated precipitation from June to September in 2005 was almost 150mm lower than the previous year.
What does all this mean? First of all, the Brazilian government announced a “state of public calamity” in the populated areas. Significant amounts of crops were lost, access to water was minimal if not nonexistent, and the level of forest fires greatly rose. In fact, the state of Acre noted three times as many forest fires compared to 2004. These forest fires are not only dangerous to the local population for obvious reasons, but are a major threat to the forest itself.
Chen’s paper graphed the water levels of four river gauge stations throughout the length of the Amazon River:
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| Figure 6. Nonseasonal daily water-level change at 4 selected river gauge stations marked in Figure 5. Annual and semiannual variations have been removed from these time series using unweighted least squares fit. |
A multiple meter drop in water levels means that several branches of the Amazon River, both those usually intermittent and not, dry up. For the vast majority of the population in the region, the River and its branches are integral to most aspects of daily life, ranging from transport to livelihood.
The Forest itself does not do well with this degree of dryness. Some studies have argued that when the Amazon experiences an extreme drought, there is actually a “greening-up” of the canopy, as in there is an increase in foliage. This is attributed to an increase in sunlight availability, which outweighs the lack of access to water. This claim is widely disputed; for example, a paper by Phillips et al. states there was actually extensive aboveground biomass loss. In fact, there was aboveground biomass loss in most of the monitored regions, outweighing the amount of gains, as the following graph shows:
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| Fig. 3. Aboveground biomass change in the Amazon Basin and contiguous lowland moist forests. The 2005 drought reversed a multidecadal biomass carbon sink across Amazonia. Symbols represent magnitude and direction of measured change and approximate location of each plot. (A) Annual aboveground biomass change before 2005. (B) Annual aboveground biomass change during the 2005 interval. (C) Difference in rates of change in aboveground biomass, 2005 versus pre-2005, for those plots monitored throughout. Grayscale shading in (A) and (B) represents proportion of area covered by forests. Colored shading in (C) indicates the intensity of the 2005 drought relative to the 1998–2004 mean as measured from space using radar-derived rainfall data [Tropical Rainfall Measuring Mission (TRMM)]. |
The process of greening-up does in fact follow periods of decreased rainfall. This is a normal part of the Amazonian climactic cycle, as every year there is a dry and a rainy season. Yet this greening-up only goes to a certain extent, after which the plants begin to die off. As was shown in the case of 2005, the drought pushed the forest to and beyond that point. Another potential problem that researchers fear is that, with extreme droughts and plant death, natural selection will make it so that drought-resistant plants compose more and more of the forest. This not only means a decrease in biodiversity as the drought-susceptible species would cease to exist, but also, the drought-resistant species generally capture less carbon from the atmosphere.
Finally, I would like to bring the reader’s attention to the fact that the 2005 drought, due to the convergence of abnormal temperatures from both the Atlantic and the Pacific, was considered to be a once in a century occurrence. Until it happened again in 2010, for the same reasons, and affected an area larger than the 2005 drought (large rainfall anomalies over 3 million km2 in 2010, and 1.9 million km2 in 2005). Since this happened only last year, data on its full effects are still unknown, but what is clear is that this kind of drought should not be happening this often.
Sources: (2009), 2005 drought event in the Amazon River basin as measured by GRACE and estimated by climate models, J. Geophys. Res., 114, B05404, doi:10.1029/2008JB006056.
Rohter, Larry. "Record Drought Cripples Life Along the Amazon ." New York Times. N.p., 11 Dec. 2005 Web. 3 May 2011. <http://www.nytimes.com/2005/12/11/international/americas/11amazon.html>.
Drought Sensitivity of the Amazon Rainforest Science 6 March 2009: 1344-1347. [DOI:10.1126/science.1164033]
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