'Warming Hole' Delayed Climate Change Over Eastern United States
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| Observed change in surface air temperature between 1930 and 1990. Observations are from the NASA GISS Surface Temperature Analysis. (Credit: Image courtesy of Eric Leibensperger) |
ScienceDaily — Climate scientists at the Harvard School of Engineering
and Applied Sciences (SEAS) have discovered that particulate pollution in the
late 20th century created a "warming hole" over the eastern United
States -- that is, a cold patch where the effects of global warming were
temporarily obscured.
While greenhouse gases
like carbon dioxide and methane warm Earth's surface, tiny particles in the air
can have the reverse effect on regional scales.
"What we've shown is
that particulate pollution over the eastern United States has delayed the
warming that we would expect to see from increasing greenhouse gases,"
says lead author Eric Leibensperger (Ph.D. '11), who completed the work as a
graduate student in applied physics at SEAS.
At this point, most of
the "catch-up" warming has already occurred.
The findings, published
in the journal Atmospheric
Chemistry and Physics, present a more complete picture of the processes
that affect regional climate change. The work also carries significant
implications for the future climate of industrial nations, like China , that have not yet implemented air quality
regulations to the same extent as the United States .
Until the United States
passed the Clean Air Act in 1970 and strengthened it in 1990, particulate
pollution hung thick over the central and eastern states. Most of these
particles in the atmosphere were made of sulfate, originating as sulfur
emissions from coal-fired power plants.
Compared to greenhouse gases, particulate pollution has a very short lifetime (about 1 week), so its distribution over Earth is uneven.
Compared to greenhouse gases, particulate pollution has a very short lifetime (about 1 week), so its distribution over Earth is uneven.
"The primary driver
of the warming hole is the aerosol pollution -- these small particles,"
says Leibensperger. "What they do is reflect incoming sunlight, so we see
a cooling effect at the surface."
This effect has been
known for some time, but the new analysis demonstrates the strong impact that
decreases in particulate pollution can have on regional climate.
The researchers found
that interactions between clouds and particles amplified the cooling. Particles
of pollution can act as nucleation sites for cloud droplets, which can in turn
reflect even more sunlight than the particles would individually, leading to
greater cooling at the surface.
The researchers' analysis
is based on a combination of two complex models of Earth systems. The pollution
data comes from the GEOS-Chem model, which was first developed at Harvard and,
through a series of many updates, has since become an international standard
for modeling pollution over time. The climate data comes from the general
circulation model developed by NASA's Goddard Institute for Space Studies. Both
models are rooted in decades' worth of observational data.
Since the early 20th
century, global mean temperatures have risen by approximately 0.8 degrees
Celsius from 1906 to 2005, but in the U.S. "warming hole,"
temperatures decreased by as much as 1 degree Celsius during the period
1930-1990. U.S.
particulate pollution peaked in 1980 and has since been reduced by about half.
By 2010 the average cooling effect over the East had fallen to just 0.3 degrees
Celsius.
"Such a large
fraction of the sulfate has already been removed that we don't have much more
warming coming along due to further controls on sulfur emissions in the
future," says principal investigator Daniel Jacob, the Vasco McCoy Family
Professor of Atmospheric Chemistry and Environmental Engineering at SEAS.
Jacob is also a Professor
of Earth and Planetary Sciences at Harvard and a faculty associate of the Harvard University Center
Besides confirming that
particulate pollution plays a large role in affecting U.S. regional
climate, the research emphasizes the importance of accounting for the climate
impacts of particulates in future air quality policies.
"Something similar
could happen in China ,
which is just beginning to tighten up its pollution standards," says
co-author Loretta J. Mickley, a Senior Research Fellow in atmospheric chemistry
at SEAS. "China
could see significant climate change due to declining levels of particulate
pollutants."
Sulfates are harmful to
human health and can also cause acid rain, which damages ecosystems and erodes
buildings.
"No one is
suggesting that we should stop improving air quality, but it's important to
understand the consequences. Clearing the air could lead to regional
warming," Mickley says.
Leibensperger, Jacob, and
Mickley were joined by co-authors Wei-Ting Chen and John H. Seinfeld
(California Institute of Technology); Athanasios Nenes (Georgia Institute of
Technology); Peter J. Adams (Carnegie
Mellon University
The research was
supported by the Electric Power Research Institute (EPRI) and the U.S.
Environmental Protection Agency (EPA); neither EPRI nor the EPA has officially
endorsed the results. The work also benefited from resources provided by
Academic Computing Services at SEAS.
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Journal Reference:
1.
E. M. Leibensperger,
L. J. Mickley, D. J. Jacob, W.-T. Chen, J. H. Seinfeld, A. Nenes, P. J. Adams,
D. G. Streets, N. Kumar, D. Rind. Climatic
effects of 1950–2050 changes in US anthropogenic aerosols – Part 2: Climate
response. Atmospheric
Chemistry and Physics, 2012; 12 (7): 3349 DOI: 10.5194/acp-12-3349-2012
