Another blow to global warming doomsayers

Despite concerns over global warming, scientists have discovered
something that may have actually limited the impact of greenhouse gases
in the atmosphere in recent years by reducing the amount of sunlight
reaching the surface of the Earth. So-called "global dimming," will be
the focus of a NOVA special scheduled to air on April 18 and featuring
Lamont-Doherty researcher Beate Liepert.

In research they published last year in the journal Geophysical
Research Letters, a team led by Beate Liepert at the Lamont-Doherty
Earth Observatory reported that they found a measurable decline between
1950 and 1990 in the amount of sun reaching the Earth's surface. The
team concluded that solar radiation is being reduced by growing
quantities of man-made particles in the atmosphere-in particular
those produced by burning fossil fuels. This increase in small
particles of ash, soot and sulfates has also caused the cloud cover to
thicken in recent decades, further reducing solar irradiance. Only
recently have climate models begun to include them in predictions of
future global warming.

"Aerosols are highly variable in space and time, which is why aerosol
forcing of climate is generally very difficult to include in climate
studies," said Liepert. "Furthermore, aerosols are found near the
Earth's surface and affect mainly the fluxes of energy and water at the
surface. These new ideas on the affects of aerosols might open up many
avenues and solve more discrepancies in the climate change debate."

It is widely agreed that greenhouse gasses in the atmosphere are
causing temperatures to rise around the world. What has not been
considered until recently is how temperatures would respond if the
aerosol layer and cloud cover were not reflecting some of the suns
radiation away from the planet. Liepert and her co-authors also
concluded that the imbalance between a decline in solar radiation and
warming surface temperatures will lead to weaker turbulent heat fluxes
and reduced evaporation and precipitation, which could ultimately lead
to a more humid atmosphere in which it rains less.

Although rising temperatures should increase the amount of moisture in
the atmosphere, the research shows that human-produced airborne
aerosols can cause water to condense to form more, but smaller, cloud
droplets. This process is likely contributing to the observed
thickening of the Earth's cloud cover. However, smaller droplets are
not heavy enough to sink through the air as rain. As a result, the
cloud cover lasts longer and there is less rain.

"Water has a characteristic residence time in the atmosphere before it
gets rained out. In a warmer world, this residence time is longer
because a warmer atmosphere can hold more water," said Liepert.
"Aerosols affect clouds by suppressing rain and increasing its
residence time."

Examples of data supporting this new hypothesis include studies
indicating a steady decline of water evaporation in the Northern
Hemisphere over the past 50 years. Over the last 60 years, large
regions of Eurasia have seen soil moisture increase by more than one
centimeter per decade, yet no significant changes in precipitation are
being seen.

Global dimming also resulted in an observable difference in the
brightness of sunlight. Even in remote areas, the atmosphere was more
polluted and therefore darker. In the end, successful efforts to reduce
emissions and clean up air pollution around the world could cause the
full effect of global warming to become more noticeable in the near
future.

"We thought we live in a global warming world, but this is actually not
right," Liepert said in a BBC interview last year. "We lived in a
global warming plus a global dimming world and now we are taking out
global dimming. So we end up with the global warming world, which will
be much worse than we thought it will be, much hotter."

Source: The Earth Institute at Columbia University




This news is brought to you by PhysOrg.com