TCS Daily

Raining on the Parade

By Willie Soon - December 6, 2002 12:00 AM

The November/December issue of Harvard Magazine features a cover article entitled "The Great Global Experiment." The article claims that climate models predicting increased concentrations of greenhouse gases (CO2) suggest there will be a warming and drying of the equatorial tropics and that "Brazil's rainforest could dry out and disappear" as early as the 2050s.

Sounds serious. So what do we know about climatic conditions over the Amazon and Brazilian rainforests? Is this prediction from Harvard Magazine of a dried out and disappearing rainforest accurate or is it hype and exaggeration?

It's worth finding out, because any commitment to the preservation and proper stewardship of rainforests or other eco-system resources need not be corrupted by false alarmism. Preservation efforts are already hard enough. They needn't be further complicated by the cheap manipulation of emotion. Especially when the source of that manipulation is a distinguished institution like Harvard University.

So let's start with some basic facts. A large part of the Brazilian rainforest has rainfall of about 2000-3000 mm per year. Let's assume that the predictions of rainfall for the Amazon rainforest in the current generation of climate models are meaningful. Predicted rainfall is expected to decrease by 730 mm per year in the region - that's a decrease in rainfall of about 29% by the 2050s. (The predicted precipitation trend is available here.)

Such a drop is significant, but that hardly means "Brazil's rainforest could dry out and disappear." Such an assertion replaces fact with fantasy

Prediction vs. Observation

Figure 1: Observed versus simulated precipitation trend in % per 100 years from 1901-1996

Next, consider Figure 1, adopted from a new paper in the June 2002 issue of Climate Dynamics, a respected scientific journal of climate science. Figure 1 presents the precipitation trends from 1901-1996 and includes observed data and compares that with simulated results. The comparison is made by a state-of-the-art coupled ocean-atmosphere general circulation model.

The comparison shows that there are large differences between the observed and simulated precipitation trends over many regions. There is a remarkable increase in precipitation predicted over a large-part of the Sahara desert. That contrasts sharply with the large drying trend observed. Over the tip of Northeast Brazil, a relatively large drying is predicted by the climate model. This, too, contrasts with the observed mild drying or small increase in rainfall. There is also a large mismatch between the simulated and observed precipitation trend over the west coast of United States.

The primary problem with trusting a model calculation of precipitation trend changes has always been the overall inconsistency of the model simulation over areas with differing rainfall conditions. Figure 1 demonstrates that climate model calculations of precipitation trends, especially on regional scales, are highly unreliable.

A group of researchers from Iowa State University recently produced a more focused assessment of rainfall patterns and trends in the Amazon Basin over the last 40 years. Using a variety of indicators, they confirmed that the hydrological cycle has actually intensified and rainfall has been increasing over the basin. But climate models that attempted to quantify the effects from deforestation and doubling of CO2 have simulated large drying over the Amazon Basin. Again, the observed record conflicts with the model predictions.

It is not surprising that there is great difficulty in ascertaining and understanding the nature of the changes now occurring in the Amazon Basin. The Basin is almost an autonomous region, immensely complex due to its rich biological, chemical and physical reservoirs. Of course, one should not arbitrarily claim that the increased rainfall trend observed in the Amazon region now will persist indefinitely. But so, too, should one avoid the alarmism that prevails in contemporary literature regarding the potential global and regional climatic changes induced by carbon dioxide or any other man-made greenhouse gases. We therefore recommend a total ban - on distasteful appeals to alarmism.

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