Tiny pollution particles may carry large consequences for earth's water supply
provided by Scripps Institution of Oceanography
new study by NASA researchers at Scripps Institution of Oceanography at the University of California, San Diego, argues that particles of human-produced pollution may be playing a significant role in weakening Earth's water cycle - much more than previously realized.
Tiny aerosols primarily made up of black carbon, the authors argue, can lead to a weaker hydrological cycle, which connects directly to water availability and quality, a major environmental issue of the 21st century.
A UN Population Fund report released Nov. 7, for example, noting that water use has grown six-fold over the past 70 years, states, Water may be the resource that defines the limits of sustainable development.
The research paper, based on results obtained during the international Indian Ocean Experiment (INDOEX), is published in the Dec. 7 issue of the journal Science.
Initially we were seeing aerosols as mainly a cooling agent, offsetting global warming. In this article we are saying that perhaps an even bigger impact of aerosols is on the water budget of the planet, said Scripps Professor V. Ramanathan, who along with Professor Paul Crutzen, a coauthor of the new study, led the INDOEX science team as co-chief scientists.
Through INDOEX we found that aerosols are cutting down sunlight going into the ocean. The energy for the hydrological cycle comes from sunlight. As sunlight heats the ocean, water escapes into the atmosphere and falls out as rain. So as aerosols cut down sunlight by large amounts, they may be spinning down the hydrological cycle of the planet, Ramanathan concluded.
Another coauthor of the paper, Daniel Rosenfeld of the Hebrew University of Jerusalem, also notes that these aerosol particulates may be suppressing rain over polluted regions.
Within clouds, aerosols can limit the size of cloud droplets, stifling the development of the larger droplets required for efficient raindrops. Rosenfeld has used data from NASA's Tropical Rainfall Measuring Mission to study aerosol phenomena over the Middle East.
The INDOEX project involved more than 150 scientists across several disciplines from Austria, France, Germany, India, Maldives, Netherlands, Sweden and the United States. The $25 million project, sponsored in part by the National Science Foundation and NASA, focused on the Indian Ocean region in a multiplatform-analysis approach of satellites, aircraft, ships, surface stations and balloons. The project was designed to assess the nature and magnitude of the chemical pollution over the tropical Indian Ocean and to assess the significance of the region's aerosols.
Early in the project, INDOEX researchers documented a human-produced brownish-gray haze layer of about 10 million square kilometers (slightly larger than the area of the entire United States) over the Indian-Asian region. The particles within the haze, the researchers discovered, were causing a three-fold decrease in solar radiation reaching the Earth's surface as compared with the top of the atmosphere. The aerosols, typically in the submicrometer- to micrometer-size range, were a mixture of sulfates, nitrates, organic particles, fly ash and mineral dust, formed by fossil fuel combustion and rural biomass burning.
One of the key revelations from INDOEX is that air pollution is not only an industrial phenomenon, said Scripps Professor Crutzen, a 1995 Nobel Laureate. The part of the atmosphere that you would expect to be the cleanest - the areas without a lot of industrialization - in fact can be very highly polluted, especially during the dry season.
In the new Science paper, the authors say the aerosol issues raised from INDOEX are a major environmental concern. Not only do the researchers question the role aerosols are playing on the regional and global hydrological cycle, but they say, globally averaged, the aerosols increase the solar heating of the atmosphere, accompanied by a reduction in the solar heating of the Earth's surface, and these effects may be quite comparable with those due to greenhouse gases.
At present these effects are not generally accounted for in climate-model prediction studies, but we will need to include the absorbing aerosols in future model predictions, the authors argue.