Manmade climate change linked to lower river levels

April 23, 7:56 AMDenver Weather ExaminerTony Hake

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Water deliveries through many major rivers, including the
Colorado River, have decreased.  (US Bureau of Reclamation)

Two studies released this week offer a dire picture of the Earth’s rivers.  Both reach the conclusion that the flows are decreasing and manmade climate change is the cause. 

Researchers led by scientists at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado have noted rivers in many of the more populous regions of the world are losing water.  Suggesting that the loss of the water is due to global warming, scientists warn that the decreased flows threaten food and water supplies for entire regions of the globe.

• See the map below for a look at river glows across the globe

The study to be published in the American Meteorological Society's Journal of Climate found significant changes in stream flow in one-third of the world’s largest rivers.  Of particular concern are the Yellow River in northern China, the Ganges in India, the Niger in West Africa, and the Colorado in the southwestern United States – all rivers that serve large populations.  During the period studied, the authors found that freshwater discharges into the Pacific Ocean decreased 6 percent while flow into the Indian Ocean decreased 3 percent. 

Lead author NCAR scientist Aiguo Dai said in a press release, "Reduced runoff is increasing the pressure on freshwater resources in much of the world, especially with more demand for water as population increases.  Freshwater being a vital resource, the downward trends are a great concern."

Interestingly enough, the study noted increased flows for much of the contiguous United States east of the Rocky Mountains.  The study found that the Mississippi River flow increased 22 percent due to increased precipitation in the Midwest.  Similarly, river discharge into the Arctic Ocean increased 10 percent. 

Is this the future for the Colorado River? (National Weather
Service)

For the Colorado River, researchers at the Scripps Institution of Oceanography, UC San Diego, released similar findings this week as well.  Their studies indicate that by the middle of this century, the Colorado River will be unable to provide its needed water supply 60 to 90 percent of the time.  For water thirsty and growing states such as Colorado, Arizona, Nevada and California, this could have a tremendous impact. 

Scripps research marine physicist Tim Barnett, a co-author of the report said, "We considered the question: Can the river deliver water at the levels currently scheduled if the climate changes as we expect it to. The answer is no." 

Even under conservative climate change scenarios, Barnett and Scripps climate researcher David Pierce found that reductions in the runoff that feeds the Colorado River mean that it could short the Southwest of a half-billion cubic meters (400,000 acre feet) of water per year 40 percent of the time by 2025. (An acre foot of water is typically considered adequate to meet the annual water needs of two households.) By the later part of this century, those numbers double.

Much like the study from NCAR, the Scripps researchers point the finger at climate change as the primary cause of the decreased river flows.  Manmade climate change will be evident during years when water delivery is below expectations, however it will be manageable through conservation.  They caution that there is however an increased chance of substantial shortfalls. 

Study co-author David Pierce said, “Fortunately, we can avoid such big shortfalls if the river's users agree on a way to reduce their average water use.  If we could do that, the system could stay sustainable further into the future than we estimate currently, even if the climate changes."

This map shows the change in runoff inferred from streamflow records worldwide between 1948 and 2004, with bluish colors indicating more streamflow and reddish colors less. In many heavily populated regions in the tropics and midlatitudes, rivers are discharging reduced amounts into the oceans. In parts of the United States and Europe, however, there is an upward trend in runoff. The white land areas indicate inland-draining basins or regions for which there are insufficient data to determine the runoff trends. (Graphic courtesy Journal of Climate, modified by UCAR.)