Science Briefs

Human Impacts on the Global Water Cycle: Effects on Sea-Level and Climate

Large-scale human manipulation of water has significantly altered global patterns of streamflow. Resulting changes in sea level, ocean salinity, and in biophysical properties of the land surface could ultimately generate climate feedbacks.

According to a study by Vivien Gornitz (GISS/Columbia), Cynthia Rosenzweig (GISS), and Dan Hillel (University of Massachusetts), human regulation of river flow and vegetation clearing has reduced river runoff by around 324 km3 per year, representing nearly 1% of the total annual streamflow (41,022 km3/yr) and around 10% of the yearly volume of fresh water used by people (3240 km3/yr).

Cartoon showing human processes affecting the water cycle
Major human processes affecting the water cycle on land. R = storage of water in reservoirs; G = groundwater mining; I = irrigation; U = urbanization; C = combustion; D = deforestation; W = wetlands.

This annual reduction in runoff corresponds to a sea level lowering of 0.8 mm/yr. This rate represents a significant fraction of the observed sea-level rise of 1-2 mm/yr, but is opposite in direction. Thus, were it not for human diversion of runoff, sea level would be rising faster than it is.

Close to 5000 km3 of water — nearly 12% of the total annual river runoff — are presently stored in large reservoirs. Between 9-12% of the earth's land area is presently cultivated, and around 17% of this farmland is irrigated. Almost 2800 km3 of water are evaporated from both irrigated fields and from reservoirs each year — a volume equivalent to nearly 7% of the global annual river runoff.

A 14-meter drop in the Aral Sea since the 1960s and high rates of coastal erosion and land inundation in Louisiana are just some of the major environmental problems associated with upstream water diversion and dam-building. In summary, human processes are now on a large enough scale to significantly perturb the global hydrological cycle on land. One of the interesting, but as yet unanswered questions is whether this human diversion and redistribution of water has any noticeable effect on climate.

Reference

Gornitz, V., C. Rosenzweig, and D. Hillel 1997. Effects of anthropogenic intervention in the land hydrologic cycle on global sea level rise. Global Planet. Change 14, 147-161.

Contact

Please address all inquiries about this research to Dr. Vivien Gornitz.