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AGU journal highlights -- July 23, 2008

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.

Title: Has fire suppression increased the amount of carbon stored in western U.S. forests?

Authors: Aaron W. Fellows and Michael L. Goulden: Department of Earth System Science, University of California, Irvine, California, U.S.A..

Source: 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.

Title: Circumpolar transport of a volcanic cloud from Hekla (Iceland)

Authors: 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. Karlsdttir: Icelandic Meteorological Office, Reykjavk, Iceland.

Source: 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).

Title: Spring frost risk in a changing climate

Authors: J. R. Rigby and Amilcare Porporato: Department of Civil and Environmental Engineering, Duke University, Durham, North Carolina, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL033955, 2008;

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.

Title: Compositional stratigraphy of clay-bearing layered deposits at Mawrth Vallis, Mars

Authors: 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.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL034385, 2008;

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 factorland use change or atmospheric dynamicshas 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.

Title: Land use impact on the Uruguay River discharge

Authors: 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.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL033707, 2008;

6. Reexamining stratosphere effects on lower-atmosphere warming

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 modelsone 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.

Title: Impact of the stratosphere on tropospheric climate change

Authors: 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;

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL033573, 2008;

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.

Title: The radiative susceptibility of cloudy atmospheres to droplet number perturbations, part 2: Global analysis from MODIS

Authors: 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.

Source: 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 mesosphere.

Title: General aspects of a T213L256 middle atmosphere general circulation model

Authors: 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, Japan;

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.

Source: Journal of Geophysical Research-Atmospheres (JGR-D) paper 10.1029/2008JD010026, 2008;


Contact: Peter Weiss
American Geophysical Union

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