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expert reaction to study on drought

Simplified models of drought have led climate researchers to overestimate its severity for the last 60 years, according to a study published in Nature

 

Dr Ben Lloyd-Hughes, Walker Institute, University of Reading, said:

“There’s no doubt that we’ve seen warming and changes in rainfall over the 20th century as a result of human induced climate change, but quantifying what this means for “drought” is problematic.

 “That Sheffield et al see different results for different drought indices doesn’t surprise me at all – the sensitivity of the Palmer Drought Severity Index to its formulation and calibration have been of concern since the original critique of Alley (1984).

“The article raises the interesting, much broader, problem of how indices of drought actually relate to ‘real world’ drought impacts experienced by people on the ground. There is nothing wrong per se with the Palmer Drought Severity Index as a measure of drought within the narrow confines of its definition.  However, care needs to be taken when using a particular index as a proxy for drought in the broader sense. Numerous review articles have sought to compare and contrast the abilities of various indices to describe various drought impacts.  As is to be expected, the consensus is that no single index is sufficient to characterise the peril. This motivates the subjective combination of drought indicators into products, such as the United States Drought Monitor, that attempts to assess the `total environmental moisture status’. The sooner such an approach, as difficult as it may be, is taken in the context of climate change the better!”

 

Prof Richard Harding, Director of the Biogeochemistry Programme, Centre for Ecology and Hydrology (CEH), said:

“The IPCC AR4 report concluded it was likely that there had been an increase in droughts since 1970.  This conclusion was based on a study of the Palmer Drought Severity Index.  This paper  revisits this study with a more physically realistic version of this Index.  The previous version used a commonly used empirical regression between evaporation and temperature – in fact in a changing climate such empirical relationships may change with time.  The new analysis suggests the previous study overestimated the trend in severity and area of droughts across the globe.  This is likely to lead to a reassessment of the level of confidence in the AR4 conclusion on changing droughts.  This study is a good example of the way scientists are continually questioning and refining our assessment of past and future climate changes.  This study in no way invalidates the overall conclusions of the IPCC AR4 study.”

 

Prof Piers Forster, Professor of Physical Climate Change at the University of Leeds, said:

“This study is a really comprehensive look at how we calculate drought trends. It shows that a much used previous estimate based only on temperature is flawed; in reality we don’t have good enough measurements to determine drought trends accurately. This re-evaluation finds no long-term globally averaged increase in drought since 1950. This revision principally affects the Americas, Russia and Australia.  Droughts in Europe, Africa and China are still believed to have become more severe since 1950.

“It needs to be understood, however, that this study looks at physical drought only and not crop yields.  In terms of staple harvests of wheat and maize, high temperatures at certain times of the growing season (e.g. temperatures above 35C at the time of wheat flowering) can kill off crops.  We know that these extreme temperatures have been increasing , causing harvest loss – e.g.  the French harvest of maize in 2003 was 20% smaller due to the summer heatwave that year.  

“This study is an important contribution highlighting the complexity of drought prediction  but  it does not make me downgrade the substantial threat to harvests posed by climate change.”

 

 

‘Little change in global drought over the past 60 years’ by Justin Sheffield et al.,  published in Nature on Wednesday 14 November.

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