Source: Geophysical Research Letters
One of the most difficult questions regarding climate change is how the warming of the Earth’s atmosphere will affect water availability around the world. Climate models present a range of possible scenarios, some more extreme than others– which can complicate planning for cities, states and countries. Now, however, in a new study, Padron et al. suggest a way to reduce uncertainty by using past precipitation patterns.
A rule of thumb for the impact of global warming on Earth’s water availability that was sometimes proposed in the past was that dry regions would get drier and wet regions would get wetter, also known as DDWW hypothesis. But growing evidence suggests the reality is more complicated. In 2014, for example, one of the authors and colleagues of the study found that the DDWW hypothesis verifies less than half the time when applied to historic records for precipitation.
Climate models may generate different results due to inherent climate variability or due to flaws in the models. To reduce the models’ contribution to this uncertainty, the authors assigned 36 climate models to accurately reproduce historical records of precipitation from 1976 to 2005. The best-performing historical models, they hypothesized, would converge in their predictions of future water availability. , defined as precipitation (rain and snow) minus evapotranspiration (water consumed by plants and evaporated by the Sun).
The assumption turned out to be correct. When the team only used historically accurate models to simulate water availability from 2006 to 2100, they produced more consistent predictions. All models were run under a business-as-usual scenario in which nothing is done to reduce greenhouse gas emissions.
The historically accurate models lacked many of the extreme changes that the full set of climate models predicted, the team found. Although the possibility of extreme drying in Europe, western North America and South Africa remains, for example, these models predicted that it would be 5 times less likely to occur compared to prediction of the full set of 36 models. Overall, the core group of models showed that previous projections of very extreme future changes in water availability were less likely to occur over more than 70% of Earth’s land surface.
The historically adept models yielded other surprises, including greater confidence in Amazon drying, where ensemble predictions from 36 models are inconclusive. Atmospheric circulation patterns are complex, making it extremely difficult to accurately predict how changes in the water cycle will unfold as the atmosphere warms. But the new study suggests that models that agree with historical observations are the most likely to produce reliable predictions. (Geophysical Research Letters, https://doi.org/10.1029/2018GL0805212019)
—Emily Underwood, freelance writer
Underwood, E. (2019), What climate models are wrong about future water availability, Eos, 100, https://doi.org/10.1029/2019EO119325. Published on April 05, 2019.
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