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Highlights, including authors and their institutions

The following highlights summarize research papers that have been published or are "in press" (accepted, but not yet published) in Geophysical Research Letters (GRL) or the Journal of Geophysical Research - Atmospheres (JGR-D).
You may read the scientific abstract for any already-published paper (not papers "in press") by clicking on the link provided at the end of each Highlight. You can also read the abstract by going to and inserting into the search engine the full doi (digital object identifier), e.g.

10.1029/2008GL033707. The doi is found at the end of each Highlight below.

Instructions for journalists and the public for downloading or ordering the full text of any research paper summarized in this release are available at . Please note that papers not yet published (i.e. "in press") are available only to journalists and to public information officers at educational or scientific institutions.

1. Fire suppression may have reduced carbon storage in western U.S. forests

Active fire suppression since the early twentieth century has caused a widespread increase in fire-intolerant trees, smaller trees, and the density of stems growing on trees within western U.S. forests. These factors have created thicker forests and are thought to account for much of North America's carbon sink. To better quantify changes in aboveground biomass, Fellows and Goulden compare California forest inventories from the 1930s with those from the 1990s. To compare these data, interpolation measures are used that result in underestimations of stem density and biomass estimates for data from the 1930s. Nonetheless, the authors find that stem density in these conifer forests increased by 34 percent between 1930 and 1990, reflecting an increase in the number of small trees. However, aboveground carbon stocks decreased by 26 percent because large trees, which contain a disproportionate amount of carbon, experienced a net loss between the surveys. The authors conclude that twentieth-century fire suppression and the resulting increase in stand density may have decreased, rather than increased, the amount of biomass stored in western U.S. forests.

Has fire suppression increased the amount of carbon stored in western U.S. forests?
Aaron W. Fellows and Michael L. Goulden: Department of Earth System Science, University of California, Irvine, California, U.S.A..
Geophysical Research Letters (GRL) paper 10.1029/2008GL033965, 2008;

2. New tracking method reveals giant volcanic clouds' paths

On 17 August 1980, Iceland's Hekla volcano erupted, spewing a sulfur dioxide cloud into the north polar stratosphere that reached roughly 15 kilometers (9 miles) in altitude. Although satellites recorded this event, techniques that exploit the strong absorption of infrared radiation by sulfur dioxide have only recently emerged, allowing scientists to reanalyze old data to track volcanic gas clouds. Using the ultraviolet data collected by the Nimbus 7 Total Ozone Mapping Spectrometer and infrared data collected by the High Resolution Infrared Radiation Sounder (on NOAA's Television Infrared Observation Satellite (TIROS) Operational Vertical Sounder), Carn et al. tracked sulfur dioxide released by Hekla. They find that the eruption emitted about 0.5 to 0.7 teragrams (trillions of grams) of sulfur dioxide, which later split into three distinct clouds, one of which circled the North Pole for 6 days. The others drifted across eastern Russia, Alaska, and Canada. Through this analysis, the authors show that integrated satellite sulfur dioxide measurements may be used to test air parcel trajectory models used for aviation hazard mitigation. This study also highlights the potential impacts of Icelandic volcanic eruptions on the polar atmosphere and Arctic ozone loss.

Circumpolar transport of a volcanic cloud from Hekla (Iceland)
S. A. Carn: Joint Center for Earth Systems Technology
(NASA/University of Maryland Baltimore County), University of
Maryland Baltimore County, Baltimore, Maryland, U.S.A;
A. J. Prata: Atmosphere and Climate Department, Norwegian
Institute for Air Research (NILU), Kjeller, Norway;
S. Karlsdottir: Icelandic Meteorological Office, Reykjavík, Iceland.
Journal of Geophysical Research-Atmospheres (JGR-D) paper
10.1029/2008JD009878, 2008;
This paper is "in press".
3. Frost risks to plants up, and down, in changing climate
As climate warms, scientists expect that the bud-break and blooming
cycles of plants will start progressively earlier each year. Most studies
of the risk associated with early blooming use simple increases in
monthly mean temperatures to represent future climate scenarios.
However, both the average and the variation of daily temperatures
are forecast to increase in future climate scenarios; such variation in
temperatures increases the risk that early bud-breaks are followed by
damaging frost. To study such frost risk to vegetation, Rigby and
Porporato developed a probabilistic model that represents bud-break
in the context of fluctuating but warming temperatures. After
calibrating this model to temperature data from Durham, N. C., the
authors find that model results show that frost risk is equally
sensitive to increases in daily temperature fluctuations (which serves
to increase frost risk) as to increases in average temperatures (which
serves to decrease frost risk).
Spring frost risk in a changing climate
J. R. Rigby and Amilcare Porporato: Department of Civil and
Environmental Engineering, Duke University, Durham, North
Carolina, U.S.A.
Geophysical Research Letters (GRL) paper 10.1029/2008GL033955,
4. Martian mineral layers offer tempting clues
Clay minerals such as montmorillonite and other smectites have been
previously detected in layered outcrops in and around the Martian
outflow channel Mawrth Vallis. Wray et al. additionally identify
kaolinite and oxide minerals such as hematite in the Mawrth Vallis
outcrops and find that these diverse minerals occur in distinct
stratigraphic horizons, implying either that they formed over time
under different environmental conditions or that they have distinctly
different sediment sources. The authors observe that this pattern of
layers occurs on both sides of the outflow channel and on its floor,
with aluminum-rich clay-bearing layers typically overlying iron-rich
clay deposits. This, combined with high-resolution topographic data,
suggests that the aluminum-rich clay-bearing layers are younger than
the outflow channel and may represent a later sedimentary or altered
volcanic ash deposit that drapes the topography. Because of Mawrth
Vallis's distinct layering history, the authors expect that this would
make a good location for future surface missions to Mars to study
geologic history and ancient habitable environments on Mars.
Compositional stratigraphy of clay-bearing layered deposits at
Mawrth Vallis, Mars
J. J. Wray and S. W Squyres: Department of Astronomy, Cornell
University, Ithaca, New York, U.S.A.;
B. L. Ehlmann and J. F. Mustard: Department of Geological
Sciences, Brown University, Providence, Rhode Island, U.S.A.;
R. L. Kirk: Astrogeology Program, U.S. Geological Survey,
Flagstaff, Arizona, U.S.A.
Geophysical Research Letters (GRL) paper 10.1029/2008GL034385,
5. Uruguay River flow responds to climate, land-use changes
The Uruguay River basin has experienced extensive land change
during the second half of the twentieth century as agricultural area
expanded. Concurrent with this has been an increase of streamflow
and precipitation due to atmospheric dynamics. To help determine
which factor--land use change or atmospheric dynamics--has
contributed more to fluctuations in Uruguay River discharge, Saurral
et al. study streamflow along the Uruguay River using a hydrology
model run between 1960 and 2000 that explicitly accounts for the
role of land cover. The authors find that increases in average
streamflow are more likely attributable to climatic variations,
implying that land use changes were not large enough to produce
appreciable changes in basin runoff. This is perhaps because most
land changes did not result from deforestation but instead involved
converting grassland (pasture) to crops. However, the authors note
that basin response, namely, that flows at the basin outlet now occur
about 2 days sooner than in the 1960s, appears to be attributable
solely to land cover change between the 1960s and the 1990s.
Land use impact on the Uruguay River discharge
Ramiro I. Saurral and Vicente R. Barros: Center for Atmospheric
and Oceanic Research, National Scientific and Technical Research
Council, University of Buenos Aires, Buenos Aires, Argentina; also
at Department of Atmospheric and Ocean Sciences, University of
Buenos Aires, Buenos Aires, Argentina;
Dennis P. Lettenmaier: Department of Civil and Environmental
Engineering, University of Washington, Seattle, Washington, U.S.A.
Geophysical Research Letters (GRL) paper 10.1029/2008GL033707,
6. Reexamining stratosphere effects on lower-atmosphere
It is well established that the troposphere, the atmospheric layer
closest to the Earth's surface, significantly influences the circulation
of the stratosphere, the layer above the troposphere. The alternate
possibility, that the stratosphere can have significant downward
influence on tropospheric circulation, is less well established.
Sigmond et al. investigate the potential for such downward influence
to alter current predictions of global warming. Comparing the
predicted warming response in two general circulation models--one
with a well-resolved stratosphere (high-top version) and one without
a well-resolved stratosphere (low-top version)--they find significant
differences. While similar results in the past have been taken as
evidence that a well-resolved stratosphere is essential for modeling
future climate projections, the authors question this conclusion.
Instead, they show that further analysis demonstrates that the
differing warming responses in the two models are not related to the
differing model lid height, but are due to differing treatments of
parameterized gravity waves, which have a large influence on the
climatological winds in the lower stratosphere.
Impact of the stratosphere on tropospheric climate change
Michael Sigmond and Paul J. Kushner: Department of Physics,
University of Toronto, Toronto, Ontario, Canada;
John F. Scinocca: Canadian Centre for Climate Modelling and
Analysis, Meteorological Service of Canada, Victoria, British
Columbia, Canada;
Geophysical Research Letters (GRL) paper 10.1029/2008GL033573,
7. How are human-made aerosols changing clouds?
Human-generated aerosol particles affect solar radiation by direct
scattering and absorption, but also change cloud properties through
particles acting as cloud condensation nuclei (CCN) and ice nuclei
(IN), a pathway referred to as the "indirect aerosol effect." This
effect is likely the manifestation of two different aerosol-cloud
interaction mechanisms. One encompasses aerosols' effect on cloud
water, specifically how a decrease in cloud particle size decreases
precipitation efficiency, thereby increasing cloud lifetimes.
Oreopoulos and Platnick study the other mechanism, called the
Twomey effect, which involves the radiative effect of cloud
microphysical changes only (no change in cloud water amount). Here
the greater availability of CCN or IN yields clouds with more
numerous but smaller cloud particles, and therefore larger optical
thickness. The authors seek to quantify the spatial and temporal
sensitivity of liquid clouds to the Twomey effect by studying data
from NASA's satellite-based Moderate Resolution Imaging
Spectroradiometer for 4 months in 2005. Through the global maps
generated, they find that the detailed nature of cloud microphysical
perturbations, as well as the unperturbed cloud properties, is
important for determining the radiative forcing associated with the
Twomey effect.
The radiative susceptibility of cloudy atmospheres to droplet number
perturbations, part 2: Global analysis from MODIS
Lazaros Oreopoulos and Steven Platnick: Joint Center for Earth
Systems Technology, University of Maryland Baltimore County,
Baltimore, Maryland, U.S.A.; and Laboratory for Atmospheres,
NASA Goddard Space Flight Center, Greenbelt, Maryland, U.S.A.
Journal of Geophysical Research-Atmospheres (JGR-D) paper
10.1029/2007JD009655, 2008;
This paper is "in press".
8. Taking sharper look at key atmospheric region
The middle atmosphere is composed of the stratosphere and
mesosphere and extends from about 12 to 90 kilometers (7.5 to 56
miles) in altitude. This region is important because it houses ozone,
which shields the Earth from ultraviolet light, and gravity and
planetary waves, which influence weather close to the Earth's
surface. Many atmospheric general circulation models (GCMs)
currently used for climate studies do not have sufficiently high
spatial resolution to resolve small-scale gravity waves. To understand
the roles of such small-scale phenomena in the Earth's climate,
Watanabe et al. develop a new GCM that uses very high spatial
resolution. This model has horizontal resolution of 0.5625 degrees in
latitude and longitude, and covers a region that extends from the
surface to a height of about 85 km (53 mi) with uniform vertical
resolution of 300 meters (980 feet) throughout the middle
atmosphere. This GCM successfully simulates the spontaneous
generation of gravity waves by convection, topography, instability,
and adjustment processes, as well as their propagation and
dissipation, resulting in a realistic reproduction of general
circulation in the midlatitude and polar stratosphere and
General aspects of a T213L256 middle atmosphere general
circulation model
Shingo Watanabe, Yoshio Kawatani, and Kazuyiki Miyazaki:
Frontier Research Center for Global Change, Japan Agency for
Marine-Earth Science and Technology, Yokohama, Japan;
Yoshihiro Tomikawa: National Institute of Polar Research, Tokyo,
Masaaki Takahashi: Frontier Research Center for Global Change,
Japan Agency for Marine-Earth Science and Technology, Yokohama,
Japan; Also at Center for Climate System Research, University of
Tokyo, Kashiwa, Japan;
Kaoru Sato: Department of Earth and Planetary Science, Graduate
School of Science, University of Tokyo, Tokyo, Japan.
Journal of Geophysical Research-Atmospheres (JGR-D) paper
10.1029/2008JD010026, 2008;
Peter Weiss
American Geophysical Union
2000 Florida Avenue, N.W.
Washington, DC 20009, U.S.A.
Phone (direct): +1 (202) 777 7507
Phone (toll free in North America): +1 (800) 966 2481 x507
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Peter Weiss | American Geophysical Union
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