Surprise! World’s largest rivers drying UP!

Climate change drying up world’s 925 largest ocean-reaching rivers

About 72 percent of the world’s 925 largest ocean-reaching rivers are drying up, most of them because of the climate change, according to a report by National Center for Atmospheric Research, Boulder, Colorado.

The Horseshoe Bend of the ‘sewage-green’ Colorado River located near the city of Page, Arizona (dated April 13, 2008). Photo:  Christian Mehlführer, User:Chmehl. Licensed under Creative Commons Attribution 2.5 license.

The largest rivers affected include the Colorado in the United States, the Yellow River in China, the Ganges in India and the Niger in West Africa.

Lower water levels combined with the impact of exponentially growing demands on water for damming, irrigation and other uses could pose a threat to future supplies water and food globally, the researchers said in the American Meteorological Society’s Journal of Climate.

Highlights of the report:

Rivers in some of the world’s most populated regions are losing water, many because of climate change, researchers reported on Tuesday.

• Some 925 largest ocean-reaching rivers are drying up,mostly because of climate change

• The largest affected rivers include the Colorado in the southwestern United States, the Yellow River in northern China, the Ganges in India and the Niger in West Africa.

• “About one-third of the top 200 rivers (including the Congo, Mississippi, Yenisey, Paraná, Ganges, Columbia, Uruguay, and Niger) show statistically significant trends during 1948–2004, with the rivers having downward trends (45) out-numbering those with upward trends (19).”

• Reduced water levels due to climate change combined with the impact of exponentially growing demands on water for damming, irrigation and other uses could pose a threat to future supplies of water and food globally, the researchers said in the American Meteorological Society’s Journal of Climate.

• “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.” Said the report’s lead author.

• About one-third of the top 200 rivers  show significant changes during 1948–2004, some 45 rivers showing downward trends, while only 19 show upward trends.

• Annual freshwater discharge into the world’s oceans decreased during the 1948–2004 research period as follows

• Pacific Ocean: down by about 6 percent, or 526 km³

• Indian Ocean: down by about 3 percent, or 140 km³

• In the case of Arctic Ocean annual discharge (from melting ice) rose about 10 percent, or 460 km³

• The Columbia River in the northwestern U.S. lost about 14 percent of its streamflow during the 54-year period because of reduced precipitation and higher water demands, while the Mississippi River flow rose by 22 percent because of increased precipitation in the U.S. Midwest.

• “[T]here is evidence that the rapid warming since the 1970s has caused an earlier onset of spring that induces earlier snowmelt and associated peak streamflow in the western United States and New England and earlier breakup of river-ice in Russian Arctic rivers and many Canadian rivers.”

The following is a free abstract. Full report is sold online at the American Meteorological Society website.

Changes in Continental Freshwater Discharge from 1948–2004

A new data set of historical monthly streamflow at the farthest downstream stations for world’s 925 largest ocean-reaching rivers has been created for community use. Available new gauge records are added to a network of gauges that covers ~80 × 10⁶ km² or ~80% of global ocean-draining land areas and accounts for about 73% of global total runoff. For most of the large rivers, the record for 1948–2004 is fairly complete. Data gaps in the records are filled through linear regression using streamflow simulated by a land surface model (CLM3) forced with observed precipitation and other atmospheric forcings that is significantly (and often strongly) correlated with the observed streamflow for most rivers. Compared with previous studies, the new data set has improved homogeneity and enables more reliable assessments of decadal and long-term changes in continental freshwater discharge into the oceans. The model-simulated runoff ratio over drainage areas with and without gauge records is used to estimate the contribution from the areas not monitored by the gauges in deriving the total discharge into the global oceans.

Results reveal large variations in yearly streamflow for most of world’s large rivers and for continental discharge, but only about one-third of the top 200 rivers (including the Congo, Mississippi, Yenisey, Paraná, Ganges, Columbia, Uruguay, and Niger) show statistically significant trends during 1948–2004, with the rivers having downward trends (45) out-numbering those with upward trends (19). The interannual variations are correlated with the El Niño-Southern Oscillation (ENSO) events for discharge into the Atlantic, Pacific, Indian, and global ocean as a whole. For ocean basins other than the Arctic, and for the global ocean as a whole, the discharge data show small or downward trends, which are statistically significant for the Pacific (−10.1 km³ yr⁻¹) and Indian Ocean (−5.4 km³ yr⁻¹). Precipitation is a major driver for the discharge trends and large interannual to decadal variations. Comparisons with the CLM3 simulation suggest that direct human influence on annual streamflow is likely small compared with climatic forcing during 1948–2004 for most of world’s major rivers. For the Arctic drainage areas, upward trends in streamflow are not accompanied by increasing precipitation, especially over Siberia, based on available data, although recent surface warming and associated downward trends in snow cover and soil-ice content over the northern high-latitudes contribute to increased runoff in these regions. Our results are qualitatively consistent with climate model projections, but contradict an earlier report of increasing continental runoff during the recent decades based on limited records. ( Copyrighted material.)