THE world has been suffering more droughts in recent decades, and climate change will bring many more, according to received wisdom. Now it is being challenged by an analysis that questions a key index on which it is based. Predictions of megadroughts affecting Africa and the western side of North America may be wrong. We could even be headed for wetter times, says Justin Sheffield of Princeton University.
In the article, Dr. Steve Running at the University of Montana comments on the new paper:
"If global drought is not increasing, if warmer temperatures are accompanied by more rainfall and lower evaporation rates, then a warmer wetter world would [mean] a more benign climate."
A prior paper also published by lead author Justin Sheffield examined the period from 1950-2000 and found, "Globally, the mid-1950s showed the highest drought activity and the mid-1970s to mid-1980s the lowest activity."In the article, Dr. Steve Running at the University of Montana comments on the new paper:
"If global drought is not increasing, if warmer temperatures are accompanied by more rainfall and lower evaporation rates, then a warmer wetter world would [mean] a more benign climate."
Link between global warming and drought questioned
- 14 November 2012 by Fred Pearce NEW SCIENTIST
THE world has been suffering more droughts in recent decades, and climate change will bring many more, according to received wisdom. Now it is being challenged by an analysis that questions a key index on which it is based.
Predictions of megadroughts affecting Africa and the western side of North America may be wrong. We could even be headed for wetter times, says Justin Sheffield of Princeton University.
This potential handbrake turn for climate forecasts hangs on the accuracy of our main measure of drought, the Palmer Drought Severity Index. The Intergovernmental Panel on Climate Change's 2007 science assessment cited studies using the PDSI to conclude that "droughts have become more common since the 1970s" as the world has warmed - a position we take to be true in this week's cover story (see "Climate downgrade: Arctic warming"). The report predicted droughts will increase with global warming.
The problem with the PDSI, says Sheffield, is that it does not directly measure drought. Instead, it looks at the difference between precipitation and evaporation. But since evaporation rates are hard to determine, it uses temperature as a proxy, on the assumption that evaporation rises as it gets hotter.
Sheffield points out that temperature is only one factor influencing evaporation. He inferred evaporation rates using the Penman-Monteith equation, which includes factors such as wind speed and humidity, and found "little change in global drought over the past 60 years" (Nature, DOI: 10.1038/nature11575). His new calculations back up his own previous analysisthat the most significant of recent droughts mostly occurred in the 1950s and 60s, before global warming got going.
The PDSI was created in the 1960s by US meteorologist Wayne Palmer to help allocate aid to drought-hit farmers, and was then widely adopted by climate scientists for its simplicity. Sheffield says he finds its continued use "a little strange".
Roger Pielke Jr of the University of Colorado in Boulder says that since the PDSI uses a formula that assumes higher temperatures cause more droughts, it was hardly surprising that it finds a link.
Simon Brown of the UK Met Office in Exeter says Sheffield's analysis is probably right. "There has been a growing acknowledgement that the PDSI should not be trusted when doing climate change studies," he says. But one of the lead authors of parts of the 2007 IPCC report, Kevin Trenberth of the US National Center for Atmospheric Research in Boulder, is sceptical. He backs work by Aiguo Dai of the State University of New York, Albany, who reported last year that using the Penman-Monteith equation "only slightly reduces the drying trend".
Sheffield's findings raise important questions, says Steve Running at the University of Montana in Missoula. "If global drought is not increasing, if warmer temperatures are accompanied by more rainfall and lower evaporation rates, then a warmer wetter world would [mean] a more benign climate."
Sheffield, J., K. M. Andreadis, E. F. Wood, D. P. Lettenmaier, 2009: Global and Continental Drought in the Second Half of the Twentieth Century: Severity–Area–Duration Analysis and Temporal Variability of Large-Scale Events. J. Climate, 22, 1962–1981. doi: http://dx.doi.org/10.1175/2008JCLI2722.1
Global and Continental Drought in the Second Half of the Twentieth Century: Severity–Area–Duration Analysis and Temporal Variability of Large-Scale Events
J. Sheffield
K. M. Andreadis
E. F. Wood
D. P. Lettenmaier
K. M. Andreadis
E. F. Wood
D. P. Lettenmaier
| Abstract |
|---|
Using observation-driven simulations of global terrestrial hydrology and a cluster algorithm that searches for spatially connected regions of soil moisture, the authors identified 296 large-scale drought events (greater than 500 000 km2 and longer than 3 months) globally for 1950–2000. The drought events were subjected to a severity–area–duration (SAD) analysis to identify and characterize the most severe events for each continent and globally at various durations and spatial extents. An analysis of the variation of large-scale drought with SSTs revealed connections at interannual and possibly decadal time scales. Three metrics of large-scale drought (global average soil moisture, contiguous area in drought, and number of drought events shorter than 2 years) are shown to covary with ENSO SST anomalies. At longer time scales, the number of 12-month and longer duration droughts follows the smoothed variation in northern Pacific and Atlantic SSTs. Globally, the mid-1950s showed the highest drought activity and the mid-1970s to mid-1980s the lowest activity. This physically based and probabilistic approach confirms well-known droughts, such as the 1980s in the Sahel region of Africa, but also reveals many severe droughts (e.g., at high latitudes and early in the time period) that have received relatively little attention in the scientific and popular literature.
Little change in global drought over the past 60 years
- Nature
- 491,
- 435–438
- (15 November 2012)
- doi:10.1038/nature11575
- Received
- Accepted
- Published online
Drought is expected to increase in frequency and severity in the future as a result of climate change, mainly as a consequence of decreases in regional precipitation but also because of increasing evaporation driven by global warming1, 2, 3. Previous assessments of historic changes in drought over the late twentieth and early twenty-first centuries indicate that this may already be happening globally. In particular, calculations of the Palmer Drought Severity Index (PDSI) show a decrease in moisture globally since the 1970s with a commensurate increase in the area in drought that is attributed, in part, to global warming4, 5. The simplicity of the PDSI, which is calculated from a simple water-balance model forced by monthly precipitation and temperature data, makes it an attractive tool in large-scale drought assessments, but may give biased results in the context of climate change6. Here we show that the previously reported increase in global drought is overestimated because the PDSI uses a simplified model of potential evaporation7 that responds only to changes in temperature and thus responds incorrectly to global warming in recent decades. More realistic calculations, based on the underlying physical principles8 that take into account changes in available energy, humidity and wind speed, suggest that there has been little change in drought over the past 60 years. The results have implications for how we interpret the impact of global warming on the hydrological cycle and its extremes, and may help to explain why palaeoclimate drought reconstructions based on tree-ring data diverge from the PDSI-based drought record in recent years9, 10.
http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-12-00449.1?af=R
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