The climate change theory that dry regions are getting drier and wet regions are getting wetter is not as universal as previously thought. According to a study by Swiss researchers, this rule strictly applies to only about 10% of the earth’s land surface.
Based on models and observations, climate scientists have devised a simplified formula to describe one of the consequences of climate change: regions already marked by droughts will continue to dry out in the future, and regions that already have a moist climate will experience additional rainfall. In short: dry gets drier; wet gets wetter – a theory known as DDWW.
However, the DDWW formula is less universally valid than previously assumed. This was demonstrated by a team of researchers from the federal technology institute ETH Zurich who published their study in the journal Nature Geoscience.
“One should be careful about using oversimplified catchphrases like DDWW when communicating with policymakers, especially when it comes to regional assessments and risk evaluation,” lead study author Peter Greve told swissinfo.ch. “This can be misleading, as it cannot do justice to the complexity of the underlying systems.”
Differentiating between land and sea
Traditional analyses relied on climate characteristics above the ocean, but this method is problematic over land, researchers found. It does not take into account the specific climatic properties of land surfaces, where the amount of available water is limited when compared with the ocean. Even though this has been acknowledged in the past, scientific and public discourse has largely neglected this aspect.
The ETH researchers analysed data sets of historical land dryness changes covering the period from 1948 to 2005. This was benchmarked against data compiled solely on land, such as rainfall, actual evaporation and potential evaporation.
While the DDWW principle still applies to the oceans, the evaluation shows no obvious trend towards a drier or wetter climate across three-quarters of the earth’s land area.
Only 10.8% of the global land area conformed to the DDWW pattern. The “wet gets wetter” rule was largely confirmed for the eastern United States, northern Australia and northern Eurasia. And the “dry gets drier” pattern was observed in the Sahel region, the Arabian Peninsula and parts of central Asia and Australia.
On the other hand, 9.5% of global land area shows the exact opposite trend to the DDWW pattern, wherein dry areas get wetter and wet get drier. Some regions which should have become wetter according to the simple DDWW formula have actually become drier in the past – these include parts of the Amazon, Central America, tropical Africa and Asia.
Conversely, certain dry areas have become wetter, such as parts of Patagonia, central Australia and the Midwestern region of the United States.