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Water: too much, too little

Climate change may accentuate seasonal differences in moisture availability

Water: too much, too little - Image shows effects of major drought on plants across U.S. on June 24, 2011

A major U.S. drought in 2011 put intense stress on vegetation in different parts of the nation at different times. This image shows satellite-estimated plant stress on June 24, 2011. (Image courtesy NASA/Goddard Scientific Visualization Studio/USDA-ARS.)

April 10, 2014 | Climate change will reduce water availability during dry seasons and increase it during wet seasons around the globe, new research suggests. It also finds there will be large regional variations in water-related impacts. Some areas may see an increase in both droughts and floods by the later part of the 21st century.

“Climate models have been saying for quite a while now that wet regions would get wetter and dry regions would get drier,” said NCAR scientist David Lawrence, who participated in the study. “This research provides a more nuanced view, because we find that there are lots of places around the world where overall precipitation is projected to go up, but at the same time the dry season would still get drier.”

To reconcile previous research suggesting that climate change will bring both increased precipitation and increased drought, researchers developed a new metric for measuring monthly water fluctuations. The Available Water (AW) index accounts for seasonal changes in both precipitation and evapotranspiration (evaporation from water bodies as well as water put into the air by plants). AW allows for interseasonal comparisons that are unavailable from other indices, such as the Palmer Drought Severity Index or the Standardized Precipitation Index.

“Evapotranspiration becomes increasingly important in climate projections because global warming affects humidity much more than precipitation,” said NCAR visiting scientist Sanjiv Kumar, the lead author on an article recently published in Earth’s Future describing the research. Kumar participated in the research as a multi-institutional post-doctoral fellow at NCAR and COLA (the Center for Ocean-Land-Atmosphere Studies).

The researchers calculated AW for global regions using results from 20 climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5), which simulated 20th-century climate and projected future climate conditions.

When they compared the historical available water from 1961–1990 with model projections of the world in 2070–2099, 46 percent of the global land areas showed a significant decrease in dry season AW, whereas only 15 percent had significantly more water available in the dry season. Northern North America, central Europe, Russia, parts of China and India, and east central Africa were among the areas the researchers found would see significantly drier dry seasons.

Similarly, 60 percent of the global land area showed a significant increase in wet season AW, while 10 percent had significant decreases, including Mexico, southwestern Africa, the Mediterranean region, and parts of South America. The researchers did not find significant changes in wet season AW for the Amazon region and Australia.

“It’s important to note that we have only considered biophysical constraints on water availability,” Kumar said. “Growing human populations will increase water demands to meet their rising agricultural, industrial, and domestic water needs, putting added stress on water resources.”

Kumar, S., D. M. Lawrence, P. A. Dirmeyer, and J. Sheffield (2013), Less reliable water availability in the 21st century climate projections, Earth’s Future, doi:10.1002/2013EF000159.