A 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 U.N. 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 co-author 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 co-author 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.