The problem with droughts

Climate models fail to ‘predict’ US droughts 

It's a bit surprising to see that climate models are still not good enough to replicate US mega-droughts.

The results were puzzling. Although the simulation produced a number of pronounced droughts lasting several decades each, these did not match the timing of known megadroughts. In fact, drought occurrences were no more in agreement when the model was fed realistic values for variables that influence rainfall than when it ran control simulations in which the values were unrealistically held constant. “The model seems to miss some of the dynamics that drive large droughts,” says study participant Jason Smerdon, a researcher at Lamont-Doherty who studies historical climate patterns.

Other climate models tested by the team fared no better, he says. In particular, the models failed to reproduce a series of multi-decadal droughts that occurred in the southwest during the Medieval Climate Anomaly, a period between AD 900 and 1200 when global temperatures were about as high as they are today.

The problem may lie in the models’ inability to reproduce the cycling between the ENSO’s El Niño and La Niña phases, especially given that many scientists think that La Niña is the major driver of drought in the southwest. The ENSO “behaves much messier in the real world than in climate models”, says Jessica Tierney, a climate scientist at the Woods Hole Oceanographic Institution in Massachusetts who has investigated the role of the ENSO in East African rainfall variability². “We’re not sure how it has varied in the past, and we don’t know how it might change in response to climate change. This is really one of the big uncertainties we’re facing.”

In addition to their failure to reproduce El Niño and La Niña, existing models do not fully capture other factors that influence rainfall, such as clouds and vegetation. But Smerdon adds that the atmospheric and oceanic dynamics that inhibit rainfall and favour prolonged drought may be essentially random and so almost unpredictable.

Last week’s findings highlight the broader challenge of predicting how precipitation patterns will change as the global climate warms. Models are often at odds over the very direction of regional changes. For example, different projections prepared for the Colorado Water Conservation Board disagree on whether mean precipitation in the state will increase or decrease by 2050.

 Presumably, there is therefore legitimate reason to be somewhat skeptical of climate model predictions for changes in rainfall patterns in Australia due to climate change.  But then again, it may be that some climate effects in some parts of Australia are easier to model than those affecting the US.  The long term decline in rainfall in the southern corner of Western Australia is, I think I have read, consistent with climate modelling, for example.

But in any event, are these uncertainties reason to let what is essentially the biggest experiment ever devised to continue?   I mean, if some relatively small, random things determine US megadroughts, doesn't it mean that some clear non-random things (say, a 2 degree global temperature increase, and increased humidity) is likely to have a very big effect indeed?