April 19, 2010

Amazing Amazon Analysis

Filed under: Droughts, Floods, Precipitation

If you really want to hit a home run with a global warming story, manage to link climate change to the beloved rainforest of the Amazon. The rainforest there is considered by many to be the “lungs of the planet,” the rainforest surely contains a cure for any ailment imaginable, all species in the place are critical to the existence of life on the Earth, and the people of the Amazon are surely the most knowledgeable group on the planet regarding how to care for Mother Earth.

The global warming alarmists have taken full advantage of the Amazon and they are very quick to suggest that the Amazon ecosystem is extremely sensitive to climate change. Furthermore, not only can climate change impact the Amazon, but global climate itself is strongly linked to the state of the Amazon rainforest.

But, as usual, there is more to this story than meets to eye (or, rather, the press).

For instance, a headline last year from USA Today sounded the alarm declaring “Amazon hit by climate chaos of floods, drought”. In the first few sentences, we learn that “Across the Amazon basin, river dwellers are adding new floors to their stilt houses, trying to stay above rising floodwaters that have killed 44 people and left 376,000 homeless. Flooding is common in the world’s largest remaining tropical wilderness, but this year [2009] the waters rose higher and stayed longer than they have in decades, leaving fruit trees entirely submerged. Only four years ago, the same communities suffered an unprecedented drought that ruined crops and left mounds of river fish flapping and rotting in the mud. Experts suspect global warming may be driving wild climate swings that appear to be punishing the Amazon with increasing frequency.”

This piece is typical of thousands of other news stories about calamities in the Amazon that are immediately blamed on global warming. Other headlines quickly found include “Ocean Warming - Not El Niño - Drove Severe Amazon Drought in 2005” or “Amazon Droughts Will Accelerate Global Warming” or “Amazon Could Shrink by 85% due to Climate Change, Scientists Say.” Notice that climate change can cause droughts and floods in the Amazon PLUS droughts in the Amazon can cause global warming (by eliminating trees that could uptake atmospheric carbon dioxide). Throughout many of these stories, the words “delicate” and “irreversible” are used over and over.

As we have discussed countless times in other essays, climate models are predicting the greatest warming in the mid-to-high latitudes of the Northern Hemisphere during the winter season. The Amazon is not located in a part of the Earth expected to have substantial warming due to the buildup of greenhouse gases. Somewhat surprisingly, the IPCC Technical Summary comments “The sign of the precipitation response is considered less certain over both the Amazon and the African Sahel. These are regions in which there is added uncertainty due to potential vegetation-climate links, and there is less robustness across models even when vegetation feedbacks are not included.” Basically, the models are not predicting any big changes in precipitation in the Amazon due to the change in atmospheric composition, nor are the models predicting any big change in temperature. Should the people of the Amazon deforest the place down to a parking lot, there is evidence that precipitation would decrease. There is a lot going on in the Amazon – deforestation, elevated carbon dioxide levels, global warming, and all these reported recent droughts and floods. One would think that the entire place is a wreck!

A recent article in Hydrological Processes might come as a huge surprise to the climate change crusade. The first two sentences of the abstract made this one an immediate favorite at World Climate Report. The author has the nerve to write “Rainfall and river indices for both the northern and southern Amazon were used to identify and explore long-term climate variability on the region. From a statistical analysis of the hydrometeorological series, it is concluded that no systematic unidirectional long-term trends towards drier or wetter conditions have been identified since the 1920s.” We should leave it at that!

The author is José Marengo with Brazil’s “Centro de Ciéncia do Sistema Terrestre/Instituto Nacional de Pesquisas Espaciais”; the work was funded by the Brazilian Research Council and the “UK Global Opportunity Fund-GOF-Dangerous Climate Change”. Very interesting – we suspect the “Dangerous Climate Change” group was not happy with the first two sentences of the abstract.

José Marengo begins the piece noting “The main objective of this study is the assessment of long-term trends and cycles in precipitation in the entire Amazon basin, and over the northern and southern sections. It was addressed by analysing rainfall and streamflow indices, dating from the late 1920s”. The Figure 1 shows his subregions within the greater Amazon basin.

Figure 1. Orientation map showing the rainfall network used on this study for (a) northern Amazonia (NAR) and (b) southern Amazonia (SAR) (from Marengo, 2009).

The bottom line here is amazing. The author writes “The analysis of the annual rainfall time series in the Amazon represented by the NAR and SAR indices indicates slight negative trends for the northern Amazon and positive trends for the southern Amazon. However, they are weak and significant at 5% only in the southern Amazon” (Figure 2). So, nothing is happening out of the ordinary in the north and the south is getting wetter. There is definitely variability around the weak trends, but it all seems to be related to natural variability, not deforestation or global warming.

Figure 2. Historical hydrometeorological indices for the Amazon basin. They are expressed as anomalies normalized by the standard deviation from the long-term mean, (a) northern Amazonia, (b) southern Amazonia. The thin line represents the trend. The broken line represents the 10-year moving average (from Marengo, 2009).

Marengo notes “Since 1929, long-term tendencies and trends, some of them statistically significant, have been detected in a set of regional-average rainfall time series in the Amazon basin and supported by the analysis of some river streamflow time series. These long-term variations are more characteristic of decadal and multi-decadal modes, indicators of natural climate variability, rather than any unidirectional trend towards drier conditions (as one would expect, due to increased deforestation or to global warming).” [emphasis added]

José – nice work, have a Cuervo on us!!!


Marengo, J.A. 2009. Long-term trends and cycles in the hydrometeorology of the Amazon basin since the late 1920s. Hydrological Processes, 23, 3236-3244.

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