When scientists think about climate change, we often focus on long term trends and multi-year averages of various climate measures such as temperature, ocean heat, sea level, ocean acidity, and ice loss. But, what matters most in our day-to-day lives is extreme weather. If human-caused climate change leads to more extreme weather, it would make taking action more prudent.
It is clear that human emissions have led to increased frequencies of heat waves and have changed the patterns of rainfall around the world. The general view is that areas which are currently wet will become wetter; areas that are currently dry will become drier. Additionally, rainfall will occur in heavy doses. So, when you look at the Earth in total, the canceling effects of wetter and drier hides the reality of regional changes that really matter in our lives and our economies.
Some of the precipitation changes are associated with large scale changes to atmospheric wind patterns that are moving climate zones. Other changes, however, are more local and relate to the ability of liquid water to evaporate. It is well known, and each of us knows this by experience, warm air can hold more water than cold air (although technically air doesn't "hold" water). As the Earth heats, there is the tendency for a more moist atmosphere – consequently, heavier downpours. All of this has been predicted and observed.
But, this added moisture has to come from somewhere and in regions where there is ample water (such as over oceans), a nearly unlimited supply means rising temperatures lead to increasing moisture. But, in dry regions where there is very little water to evaporate, this "moistening" effect doesn't exist. Here, increasing temperatures just dry things out. The fancy term for this moisture transfer is evapotranspiration (ET).
So on to droughts. A very recent study by Trenberth et al., "Global warming and changes in drought" published in Natural Climate Change has investigated the way droughts are measured. They discuss various drought metrics such as the Standardized Precipitation Index which is based entirely on precipitation, the Standardized Precipitation and Evapotranspiration Index which includes ET effects, and the Palmer Drought Severity Index (PDSI) which balances precipitation, evaporation, runoff, and includes local soil moisture and vegetation.
This most sophisticated metric has led different teams of researchers to very different conclusions about drought trends. One study reports "little change in drought over the past 60 years" while another, "increasing drought under global warming in observations and models". How could researchers come to such different conclusions? It turns out that various versions of the PDSI have differing algorithms for calculating ET which partially explains the differences.
First, quantifying evapotranspiration is very challenging; the observational spread is large as are regional uncertainties. Second, the choice of baseline period is crucial. The first study mentioned above used a 1950-2008 baseline which includes human impacts while the second study baseline is limited to 1950-1979 which, while having less human influence turned out to be a wetter than normal period.
Another issue is the quantification of precipitation. The two studies used different precipitation datasets which are characterized by their number of stations and geographic coverage.
So what are the recommendations for improving the agreement? First, the authors suggest that countries should make available more of their precipitation data. Secondly, higher resolution precipitation information is needed (such as hourly data) so that the amount of runoff can be more accurately known. Similar improvements in ET are needed. Overall, the study concludes,
Posted by John Abraham on Monday, 23 December, 2013
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