October 26, 2006

Relief for Africa

Droughts and/or floods – blame global warming, right? Time and Newsweek are all too quick to blame either one on global warming, Al Gore’s movie certainly gave us vivid images of both severe droughts and severe floods, and to this day, ongoing drought in the Southwest is blamed on global warming in some circles. The Summary for Policymakers in most recent (2001) Intergovernmental Panel on Climate Change (IPCC) Report claims “In the mid- and high latitudes of the Northern Hemisphere over the latter half of the 20th century, it is likely that there has been a 2 to 4% increase in the frequency of heavy precipitation events.” Also, IPCC notes “Over the 20th century (1900 to 1995), there were relatively small increases in global land areas experiencing severe drought or severe wetness.” However, IPCC claims “In some regions, such as parts of Asia and Africa, the frequency and intensity of droughts have been observed to increase in recent decades.” In the main body of the document, we find in Southern Africa “significant decreases in precipitation being observed since the late 1970s.”

A team of scientists from the University of Virginia and the University of Arkansas has recently produced a 200-year regional precipitation chronology from Zimbabwe in southern Africa based on tree-rings, and their results shed light on the predictions from the greenhouse advocates. Trees produce growth rings every year that preserve a year-to-year history of rainfall and/or temperature during their lifetime, and dendroclimatologists have over the past century developed a hundred clever and sophisticated techniques to extract the climate history from the trees. Virginia scientist Matthew Therrell and three associates studied the Pterocarpus angolensis trees of southern Africa (locally known as Mukwa trees or what we might call African teak) to examine what the trees have to tell us about the last 200 years. Mukwa is a deciduous tropical hardwood that is known for its medicinal properties and its wonderful hardwood products.

In case you are a bit rusty on your southern African geography, the map below (Figure 1) shows the spatial range of the trees and where they collected the core samples (relax, they core the living trees and do not cut them down). In addition to the tree samples, they also assembled temperature and precipitation data from gridded datasets widely used by the climate community. The team note that understanding rainfall in this part of the world is particularly important because “in Zimbabwe a large majority of the population is directly dependent on maize agriculture” which is going to fluctuate year-to-year with growing season rainfall. In reviewing other studies, they conclude “In recent decades severe droughts have caused drastic reductions in maize production in southern Africa.” These droughts are undoubtedly caused by global warming, right?

Figure 1. The natural range of Pterocarpus angolensis is shown by the gray shading. Tree-ring chronologies for this study came from sites denoted by the black and white triangles (from Therrell et al., 2006). Grid centers are for climate data used in the study.

One of the team members had previously established that “Mukwa trees growing in western Zimbabwe produce definitive annual rings and that ring width growth is significantly correlated with monthly and seasonal precipitation totals from December through February (DJF) over a large area of central southern Africa.” Recall that the study area is in the Southern Hemisphere, so the conclusion here is that the trees respond well to their summer rainfall.

The reconstruction of precipitation is shown below (Figure 2), and we at World Climate Report cannot help but notice the red areas depicting drought have curiously not appeared since 1960. Our skeptical eyes see a lot of red from 1840 to 1960, and no red since? We see no long-term drought in the past 45 years, but we see three significant dry periods from 1840 to 1890. The two droughts in the 20th century occurred in the 1920s and 1950s, and no drought of that magnitude has occurred since.

Agriculturalists realize the extreme wet periods can be just as harmful as droughts. Note that the three wettest periods were centered on 1830, 1900, and 1920. Therrell et al. comment that “A 6-year wet period at the turn of the nineteenth century (1897–1902) exceeds any wet episode during the instrumental era.” Furthermore, they write “The 1880s–1890s drought was followed by six unbroken years of high rainfall from 1897 to 1902. This wet period was unequalled by any similar event in the instrumental period, though similar episodes occurred in the 1910s and 1950s.”

Figure 2. Annual (thin line) and 10-year smoothing spline (heavy line) values of reconstructed
November–February regional rainfall from 1796 to 1996 (from Therrell et al., 2006).

It sure looks to us at World Climate Report that (a) droughts were far more severe and frequent over a 100 years ago in southern Africa, (b) wet periods were more frequent over 100 years ago, and (c) climate variability is far less today in southern African than 100+ years ago. Sorry, but the precipitation record in southern Africa provides no support for any prediction of increased drought or climate variability given the buildup of greenhouse gases.

The figure above suggests quite the opposite may be the reality!


Intergovernmental Panel on CLimate Change, 2001. Third Assessment Report.

Therrell, M.D., D.W. Stahle, L.P. Ries, and H.H. Shugart. 2006. Tree-ring reconstructed rainfall variability in Zimbabwe. Climate Dynamics, 26, 677–685.

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