Further Evidence Of The Diversity Of Human Climate Forcings

There is a news report today by Eryn Brown of the Los Angeles Times titled

“Dust cuts Colorado River flow, scientists say – Dark particles settle on Rocky Mountain snowfields and alter the melting rate, a study shows.”

The article starts with the text

“The dark dust thrown up by human activity in the deserts of the Southwest hastens the melting of Rocky Mountain snow and ultimately reduces the amount of water flowing into the upper Colorado River by about 5%, scientists reported Monday.”

Excerpts include

The lost water amounts to more than 250 billion gallons — enough to supply the Los Angeles region for 18 months, said study leader Thomas H. Painter, a snow hydrologist at the Jet Propulsion Laboratory in La Cañada Flintridge.

Researchers had already shown that dust emissions in the Southwest have increased fivefold since the mid-19th century, when settlers and their livestock poured across the frontier, breaking up the fragile crusts atop desert soils. That extra dust absorbs more sunlight, melting the snowpack sooner and shortening the duration of snow cover each year by three to four weeks, Painter said.

To quantify the effect on runoff, Painter and his colleagues plugged historical data into a computer model that projected what annual runoff would have been from 1916 to 2003 under the cleaner snow conditions that existed before 1880.

Accelerated melting due to dust exposes surface vegetation earlier in the year, and the growing plants suck water out of the soil. As a result, the team calculated, there is 5% less runoff available to flow into rivers.

The model did not factor in the likelihood that a longer-lasting snowpack also cools the atmosphere, probably resulting in less evaporation and more runoff, Painter said. This means the 5% figure is a minimum estimate of the amount of Colorado River water that is lost, he said.

As we have discussed in our paper

Pielke Sr., R., K. Beven, G. Brasseur, J. Calvert, M. Chahine, R. Dickerson, D. Entekhabi, E. Foufoula-Georgiou, H. Gupta, V. Gupta, W. Krajewski, E. Philip Krider, W. K.M. Lau, J. McDonnell,  W. Rossow,  J. Schaake, J. Smith, S. Sorooshian,  and E. Wood, 2009: Climate change: The need to consider human forcings besides greenhouse gases. Eos, Vol. 90, No. 45, 10 November 2009, 413. Copyright (2009) American Geophysical Union

“…the 2007 Intergovernmental Panel on Climate Change (IPCC) assessment did not sufficiently acknowledge the importance of these other human climate forcings in altering regional and global climate and their effects on predictability at the regional scale. It also placed too much emphasis on average global forcing from a limited set of human climate forcings. Further, it devised a mitigation strategy based on global model predictions. For example, although aerosols were considered as a global average forcing, their local effects were neglected (e.g., biomass burning, dust from land use/land cover management and change, soot from inefficient combustion).”

This new report on the role of dust on the climate in the western United States is an example of the range of influences of humans on the climate system. This is also another issue that the 2007 IPCC failed to adequately assess.

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