Air pollution reduces energy to power the hydrological cycle

Minute particles primarily made up of black carbon can reduce sunlight reaching the oceans, leading to a weaker hydrological cycle, and possibly impacting water availability and quality, according to new research.

The study, carried out by researchers from Scripps Institution of Oceanography at the University of California, US, is based on results obtained during the US$25 million international Indian Ocean Experiment (INDOEX), which documented a human-produced brown haze layer of about 10 million square kilometres over the Indian/Asian region. The study involved more than 150 scientists from a number of countries, and revealed that Asia had joined the West as a major air polluter (see related story).

Particles within the brown haze, which consist of a mix of sulphates, nitrates, organic particles, fly ash and mineral dust from fossil fuel and rural biomass burning, have been found to cause a three-fold decrease in solar radiation reaching the earth’s surface.

“Initally, we were seeing aerosols as mainly a cooling agent, offsetting global warming,” said co-author of the study, Professor V Ramanathan. “Through INDOEX we found that aerosols are cutting down sunlight going into the ocean. The energy for the hydrological cycle comes from sunlight.”

Aerosols may also be having a second, serious impact of water availability, say the researchers. The particles may also be limiting rainfall over polluted areas, explained co-author of the research Daniel Rosenfeld. Within clouds, aerosols can limit the size of water droplets, stifling the development of the larger droplets required for efficient production of raindrops.

“By combining a unique set of field measurements with models, INDOEX scientists have provided strong evidence that human-produced atmospheric pollution may be having a profound effect on the Earth’s water cycle, weakening it as pollution increases,” said Jay Fein, Director of the climate dynamics programme at the US government research body, the National Science Foundation, which part funded the research. “The large extent and magnitude of the effect is an unexpected discovery which will have important implications for environmental policy.”

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