There is evidence that suggests that certain areas of the globe have had more pronounced and longer lasting droughts over the past century.  The effects of increases in atmospheric CO2 have been shown to have a greater effect on temperature in arid regions such as the southwestern United States, the African Sahel, Australia, South Africa, and the Arctic. Over the past several decades, these regions have warmed more significantly that the rest of our planet, as shown by the table below from the 2001 IPCC report. From 1976 to 2000, warming in desert areas ranged from 0.5-2°C, which is much larger than the average increase of 0.45°C for the entire planet (as of 2000). The amount of warming predicted under two different CO2 increase scenarios (see table) for the latter part of the 21st century (2071-2100) is significant for all deserts.
Precipitation change predictions are more complex, however. Rainfall has decreased since 1976 in many deserts. The predicted rainfall trends for two scenarios are shown in the table to either increase or decrease, depending on locations. An important factor seems to be the season in which the desert normally receives rainfall. Desert areas where summer rainfall is prevalent have dramatically less precipitation, while the opposite is true for deserts with winter rainfall. The reason for this is believed to be the expected increase of El Nino events (described earlier) which enhance cool season rainfall.
The term “megadrought,” first proposed in 1998, describes prolonged drought conditions that are as severe as, but longer lasting than the worst 20th century droughts in a given region.  Since there were no surface or satellite observations of rainfall and soil conditions prior to the late 19th century, proxy data must be used to infer those conditions. Examples of proxy data are tree rings, stalagmites and stalactites found in caves, and even the types of fossilized microorganisms found in lake beds. Analysis of this data had found that modern droughts pale in comparison to those observed during the late first millennium to the middle of the second millennium (i.e., 900 to 1500 A. D.) (Figure 1).  Droughts in the latter period persisted for more than 35 years. One of those, in the late 1200s, may have resulted in the relocation of the ancient Pueblo Indians from their homes in the Four Corners region of the Southwest U. S. Coincidentally, this period coincides with the Medieval Warm Period, which was described in an earlier blog (the Hockey Stick Controversy).
Figure 1. Drought history of the American west reconstructed from tree rings including (a) the percentage of the western U. S. experiencing a megadrought, (b) the average soil moisture index in the southwestern U. S. and (c) the water flow rate in the Colorado River, just downstream of the Lake Powell reservoir. The vertical brown bars mark intervals when soil moisture indices or river flow were 0.5 standard deviations below the long term mean for 35 years or more.
The presence of spectral channels on modern environmental satellites that monitor chlorophyll associated with plant life allows global estimates of changes in vegetation. Recent findings from at least two studies indicate that there has been an increase in desert vegetation due to (1) an effect known as “CO2 fertilization,” and (2) an increase in moisture and thus, precipitation due to the warmer temperatures.  These measurements were supported by computer model calculations. This “greening” represents an important feedback mechanism not previously accounted for. It is estimated that 16 out of the 20 most important crops will benefit from increased CO2.
A more recent study that also used satellite data from 1982-2009 (Figure 1) found that while a small area of the Earth (<4%) experienced leaf loss (browning), 25-50% of the global surface showed a greening during growing seasons.  By comparing the results with numerical ecosystem model simulations, it was estimated that 70% of the greening was likely due to “CO2 fertilization,” while 9% was due to increased nitrogen.
Figure 2. Change in percentage of vegetation cover (expressed as Leaf Area Index (LAI)) from 1982-2009 derived from NASA’s MODIS instrument on polar-orbiting satellites. Observed LAI was compared with a 28-year average LAI assuming IPCC scenario S1. (From Zhu et al. 2016)
In the crop-growing regions of the world, droughts and resulting crop failures are strongly related to the short range El Niño and La Niña cycles. But additional effects due to global warming could produce added stress, according to computer models that assume a rise of about 1.8°C by 2030. The most hard-hit areas are expected to be southern Asia and southern Africa where principal crops (wheat and corn) may be reduced by 10 to 15%. Unfortunately, these are among the most poverty-stricken regions of the world. One unknown variable in this projection is rainfall. The computer models don’t agree on how rainfall will change in the coming decades, nor do they consider extremes in temperature and rainfall, which create major stress on plants. Developing drought-resistant varieties of crops or growing different crops entirely would help alleviate this concern.
Warming temperatures, if they occur as predicted, will also likely lead to an increase in insects and plant diseases, which are certain to reduce crop yields in many areas. Increasing CO2 will have some positive effects on crop yields however, helping to compensate for effects of rising temperature. In addition to being an essential compound in photosynthesis, more CO2 will help plants by increasing their efficiency in the use of water.
In summary, droughts seem to be lasting longer and are becoming more severe in many parts of the globe, although compared to the medieval period, they are not as severe by comparison. The effects of predicted global warming on drought cycles and crops are expected to be mixed, resulting in more pronounced drought and crop failures in some areas, and increased rainfall in others, including some desert areas where winter rains are relied upon. However, predictions of cataclysmic drought and crop failures seem to be contradicted by recent satellite observations of increased plant area coverage globally over the past few decades due to the positive effects of CO2.
- Intergovernmental Panel on Climate Change (IPCC) Report, 2001: https://www.ipcc.ch/report/ar3/wg1/
- Woodhouse, C. A. and J. T. Overpeck, 1998: Bulletin of the American Meteorological Soc., 79, page 2693.
- Ault, T. and S. St. George, 2018: Unraveling the mysteries of megadrought. Physics Today, August issue, pages 44-50.
- Donohue, R., M. Roderick, T. McVicar and G. Farquhar, 2013: Impact of CO2 fertilization on maximum foliage across the globe’s warm, arid environments. Geophysical Research Letters, Vol. 40, pages 3031-3035.
- Zhu and co-authors, 2016: Greening of the Earth and its drivers. Nature Climate Change, doi:10.1038/nclimate3004.
- Lobell, D. B., Burke, M. B., Tebaldi, C., Mastrandrea, M. D., Falcon, W. P., Naylor, R. L., 2008: Prioritizing climate change adaptation needs for food security in 2030. Science, Vol.319 (5863): 607-610
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