Warming and the Water Cycle: More than Just a Faster Wetter Wet and Drier Dry

by |April 28, 2012

One of the most serious consequences of global warming is its predicted impact on the water cycle.  A new study, described below, presents evidence that the global water cycle is changing even faster than predicted.  A further concern is that future rainfall patterns may be extremely variable in both space and time.

Since the 1950s, parts of the world's ocean became saltier (red) and parts became fresher (blue) as the global water cycle intensified. The color scale refers to the observed change in salinity. The numbers on the scale correspond approximately to grams of salt per kilogram of seawater. Map Credit: Paul Durack/CSIRO/LLNL

As the atmosphere warms, its capacity to hold water vapor increases. This is quantified by the Clausius-Clapeyron relationship, which explains that the atmosphere will hold about about 7% more moisture for every degree Celsius of warming. That means more evaporation in areas that are already dry and increased precipitation in regions that already receive high rainfall.  Thus we can expect increasing droughts in dry areas and more floods in regions now prone to flooding.

In the 27 April 2012 issue of SCIENCE, Paul Durack of the Lawrence Livermore National Laboratory in California and coworkers explored a historical database of the saltiness of the surface ocean waters, or salinity, and found evidence that the global water cycle has intensified over the past half-century.  The observed shifts in ocean salinity are consistent with the Clausius-Clapeyron relationship and with the observed warming of the surface ocean by about 0.5°C.

Noting difficulties in obtaining accurate estimates of rainfall over land, Durack and coworkers used ocean salinity as a natural rain gauge.  Just as the long-term average evaporation and precipitation leave their imprint on land in terms of characteristic landscapes, they are recorded in the ocean by the saltiness of surface waters.  The saltiest ocean waters are found in the subtropics, where the dry descending air of the Hadley cell causes evaporation to exceed precipitation.  By contrast, surface waters at high latitudes tend to be less salty as precipitation is greater than evaporation.  This pattern has existed for much of Earth’s history.

Flood in Memphis, Tennessee, 10 May 2011. Photo Credit: Chris Wieland.

Durack and coworkers report that the difference between subtropical and high latitudes has increased over the past half century (see map), as predicted by climate models.  However, the authors also note that these changeshave occurred approximately two times faster than those predictions. It is disturbing that our best knowledge about future change, as expressed in climate models, may underestimate the full impact of global warming on the water cycle. This study has justifiably received much attention given the serious consequences of disruptions in the water cycle, especially for food production in subtropical regions, home to many of the world’s poorest people. Excellent discussions can be found in the general media, such as the UPIReutersNY TimesAustralian Broadcasting or Fox 8, or in more specialized blogs such as Climate Central or Nature.

Here, we’d like to call attention to extra wrinkle to the story.

Durack’s results are consistent with those of previous studies that also noted changes in surface ocean salinity.  But additional complexity within the general trends is also unsettling.  In a special issue of OCEANOGRAPHY (Vol. 21, No. 1, 2008) devoted to ocean salinity, Arnold Gordon and Claudia Giulivi of Columbia’s Lamont-Doherty Earth Observatory (LDEO) compared historical trends in surface salinity of the subtropical North Atlantic and North Pacific Oceans. They found that there were significant departures from the long-term trends on time scales of a decade or so; even more interestingly, the two oceans varied in the opposite direction.  Natural variability in atmosphere-ocean interactions occurs at the interannual scale, such as the El Niño Southern Oscillation (ENSO), or longer time scales, such as the Pacific Decadal Oscillation (PDO). Not surprisingly, the departures from the long-term salinity trends noted by Gordon and Giulivi were correlated with ENSO, as modified by the PDO.

Sonoran Desert, Sonora, Mexico. Photo Credit: Tomas Castelazo.

An important lesson from these studies is that the global intensification of the water cycle will be modified by a patchwork of regional fluctuations, and perhaps even extreme regional fluctuations. Richard Seager and colleagues, also at LDEO, have found that higher surface ocean temperatures will lead to greater year-to-year variability in the water cycle for much of the world because ocean temperates affect the atmospheric circulation patterns that carry moisture. Although circulation is often driven by known forcing, such as ENSO, future rainfall distributions will be difficult to anticipate. Increasing variability of precipitation may become the most serious consequence of warming over the remainder of this century.

Changes to the global water cycle will affect food availability, infrastructure, and the stability of governments.  As noted by Durack and coworkers:

“A change to freshwater availability in response to climate change poses a more important risk to human societies and ecosystems than warming alone.”

Considering the vital role of water for life on Earth, it is critical that we measure and understand these processes, including their spatial and temporal variability.

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