An
overview of projects throughout the organization
A team
of researchers in ESSL/CGD
is developing a model
to study the effects of
global warming on cities.
The model appears to do
a good job of simulating
solar radiation, heat
fluxes, and temperatures
in Mexico City, shown
here.
Tropospheric
ozone and climate. Helen Worden
(ESSL/ACD) and colleagues at the Jet
Propulsion Laboratory (JPL) recently
published a satellite-based analysis
of the role played by tropospheric
ozone in boosting the greenhouse effect.
The results, which appeared online
in Nature Geoscience on April 20, help
clarify a significant uncertainty in
the climate-change picture.
Along with its ill effects on human
health, ozone in the troposphere—the
lowest atmospheric layer—also
acts as an important greenhouse gas,
ranking third behind carbon dioxide
and methane. However, it varies dramatically
by region and over time, and it’s
unclear how much tropospheric ozone
existed before the industrial era.
In 2007, the Intergovernmental Panel
on Climate Change estimated that, on
average, changes in tropospheric ozone
from human activities have increased
the radiation trapped by the atmosphere
anywhere from 0.25 to 0.65 watts per
square meter, roughly the same as the
radiative forcing from methane and
several times less than that from carbon
dioxide.
Helen and her collaborators drew
on recent data from the Tropospheric
Emissions Spectrometer, launched in
2004 on NASA’s Aura satellite.
They found that the radiative forcing
from tropospheric ozone (from both
natural and human sources) between
the latitudes 45°S and N averaged
between 0.34 and 0.62 watts per square
meter. The analysis focused on clear-sky
radiances observed over the ocean in
these latitudes. Although not directly
comparable to IPCC estimates for anthropogenic
radiative forcing, this measurement
provides confirmation that the IPCC
understanding of the impact of tropospheric
ozone on climate change is essentially
correct.
“These are the first global observations
to partition the role of tropospheric
ozone in the greenhouse effect,” says
Helen, who spent 14 years at JPL before
joining ACD last year. “These
measurements provide new observational
constraints for climate models, and
we are working to extend the analysis
beyond clear-sky ocean radiances to
cloud and land scenes, where models
have the largest uncertainties.”
Cities and climate
change. Because
of the large number of people who live
in urban areas, scientists want to examine
the likely impacts of global warming
on cities. But cities are not represented
in climate models, both because they
are too small for the coarse resolution
of these models and because urban surfaces
respond to the atmosphere in different
ways than undeveloped areas.
A team of scientists, led by Keith
Oleson in ESSL/CGD, is making important
progress toward adding an urban component
to the Community Climate System Model.
The team has created a model that represents
an urban canyon, consisting of roofs,
sunlit and shaded walls, and a canyon
floor that is divided into both pervious
surfaces such as lawns and impervious
surfaces such as roads. One of the most
difficult aspects of creating such a
model is to correctly account for the
trapping and reflection of solar radiation
by various urban surfaces.
Testing has revealed that the model
appears to do a good job of simulating
solar radiation, heat fluxes, and temperatures
in two cities—one based on the
architecture of Mexico City, and the
other on Vancouver. The team is continuing
to refine the model, focusing on the
height-to-width ratios of city buildings
and the thermal properties of various
surfaces. In the next year or so, they
hope to integrate the model into CCSM,
enabling researchers to study the effect
of a warming world on different urban
areas.
The research is summarized in two recent
papers that appeared in the April issue
of the Journal of Applied Meteorology
and Climatology.
Permafrost and
sea ice. A study led
by David Lawrence (ESSL/CGD) has found
that the rate of climate warming over
northern Alaska, Canada, and Russia
could more than triple during periods
of rapid sea ice loss. The research
raises concerns about the thawing of
permafrost (permanently frozen soil),
which has potential consequences for
sensitive ecosystems, human infrastructure,
and greenhouse gas emissions.
“Our study suggests that if sea
ice continues to contract rapidly over
the next several years, Arctic land
warming and permafrost thaw are likely
to accelerate,” David says.
The research was spurred in part by
events last summer, when the extent
of Arctic sea ice shrank to more than
30% below average, setting a modern-day
record. From August to October, air
temperatures over land in the western
Arctic were also unusually warm, reaching
more than 3.6°F (2°C) above
the 1978–2006 average and raising
the question of whether or not the unusually
low sea-ice coverage and warm land temperatures
were related. To investigate the question,
David and colleagues generated climate
change simulations with the Community
Climate System Model.
The image below shows simulated autumn
temperature trends during periods of
rapid sea-ice loss, which can last for
5 to 10 years. The accelerated warming
signal reaches nearly 1,000 miles inland.
In contrast, the image at right shows
the comparatively milder but still substantial
warming rates associated with rising
amounts of greenhouse gases in the atmosphere
and the moderate sea-ice retreat that
is expected during the 21st century.
Most other parts of the globe still
experience warming, but at a lower rate
of less than 0.9°F (0.5°C) per
decade. (Image by Steve Deyo, COMET).
