Another Alarmist Claim Bites The Dust

By Paul Homewood

 

 

A new paper in Nature, “Little change in global drought over the past 60 years” by Justin Sheffield, Eric Wood and Michael Roderick, casts doubt on claims that global drought has been increasing in frequency and severity over recent decades.

Many of these claims have been based on the Palmer Drought Severity Index (PDSI), which uses a simplistic approach based on temperature and precipitation. In essence, the model assumes that as the climate warms, evaporation increases and, therefore, drought increases unless offset by higher precipitation.

The Palmer index was originally developed by Wayne Palmer in the 1960’s, for use in the corn belt in the American Mid West and Palmer himself said

“The method was specifically designed to treat the drought problem in semi arid and dry sub humid regions. Extrapolation beyond the circumstances for which it was designed may lead to unrealistic results”.

The new paper makes the following points:-

• 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 change.
• Here we show that the previously reported increase in global drought is overestimated because the PDSI uses a simplified model of potential evaporation 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 principles 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.

 

According to the 2007 Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC), there have been more intense and longer droughts observed over wider areas since the 1970s, particularly in the tropics and subtropics.

"Increased drying linked with higher temperatures and decreased precipitation has contributed to changes in drought," the report says, quoted by Roderick and colleagues.

Roderick says such assessments of historical drought trends have relied on what is known as the Palmer Drought Severity Index (PDSI), developed in the US.

This index has also been used by government agencies, for example in the US, to estimate the severity of drought and to allocate financial aid.

But, he says, the index oversimplifies the effect of global warming by incorrectly assuming that increasing temperatures will cause more evaporation that will dry out the soil.

"Water doesn’t necessarily evaporate faster as the temperature goes up. It depends on the details. And the details are all important here," says Roderick, a land surface specialist and biophysicist.

Roderick says drought occurs when there is a net loss of water from the soil. This not only depends on the supply of water to the soil (from rainfall) but also on the demand for water by the atmosphere (due to evaporation).

Evaporation in turn, is influenced by more factors than temperature, says Roderick.

"What determines the evaporation rate of water predominantly is solar radiation, the humidity of the air and the wind," he says.

Roderick and colleagues reanalysed the data used in the 2007 IPCC report and found that when evaporation was assumed only to be a function of temperature global drought was shown to have been increasing.

But, when the researchers used an updated index, which included solar radiation, air humidity and wind, they found little overall global change in drought in the past 60 years.

 

The full abstract is below, but, before that, I will give the final comment to Wayne Palmer, himself :-

”In conclusion, this method of climatic analysis must be regarded as only a step in measuring and describing meteorological drought. Real understanding can only follow measurement and description.”

 

ABSTRACT

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 warming^, . 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 warming. 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 change. Here we show that the previously reported increase in global drought is overestimated because the PDSI uses a simplified model of potential evaporation 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 principles 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 years.

 

 

http://www.nature.com/nature/journal/v491/n7424/full/nature11575.html