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AGU journal highlights -- Aug. 18

American Geophysical Union

The following highlights summarize research papers that have been recently published in Geophysical Research Letters (GRL), Journal of Geophysical Research-Earth Surface (JGR-F), Journal of Geophysical Research-Biogeosciences (JGR-G), and Journal of Geophysical Research-Oceans (JGR-C).

1. Slowly but steadily, a stormier Europe

One anticipated consequence of global warming is a rise in the strength and frequency of wind storms striking Europe, bringing about associated increases in property damage, choppy seas, and coastal flooding. Previous research, mostly based on long-term pressure observations, has sought, to no avail, the signal of a persistent increase in European storms. Using the 20th Century Reanalysis, a recently developed atmospheric reconstruction stretching back to 1871, Donat et al. identify a significant increase in both the strength and frequency of wintertime storms for large parts of Europe.

The researchers calculate two different measures of storminess: the magnitude of extreme wind speeds and the frequency of storm events for six regions spread across Europe. They find a distinct increase in wind storm activity since the late 19th century. The authors also find that regional increases in storm frequencies and extreme wind speeds showed a gradient across Europe, with the largest increases in the northwest, near western Norway, decreasing towards the edge of the study area in central Germany. They find an average increase in storm frequency of between 0.1 and 0.5 storm days per year per decade for the different regions, corresponding to 1.4𔃄.8 additional storm days per year over the course of the study period, which they suggest could be attributed to global warming or natural variability.

Geophysical Research Letters,
doi:10.1029/2011GL047995, 2011

Title: Reanalysis suggests long-term upward trends in European storminess since 1871

Authors: M. G. Donat: Climate Change Research Centre, University of New South Wales, Sydney, New South Wales, Australia; and Institute of Meteorology, Freie Universität Berlin, Berlin, Germany;

L. V. Alexander: Climate Change Research Centre, University of New South Wales, Sydney, New South Wales, Australia;

S. Wild and U. Ulbrich: Institute of Meteorology, Freie Universität Berlin, Berlin, Germany;

D. Renggli: Institute of Meteorology, Freie Universität Berlin, Berlin, Germany; and Swiss Reinsurance Company, Zurich, Switzerland;

G. C. Leckebusch: Institute of Meteorology, Freie Universität Berlin, Berlin, Germany; and School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham, UK.

2. Martian soil oxidizing properties not too extreme for life

Ever since the NASA Viking mission, which reached Mars in 1976, there has been considerable interest in the composition of Martian soils. Some Viking measurements indirectly suggested that the soils contained highly oxidizing compounds, which could present extremely harsh conditions for life. Recent observations from the Phoenix Mars Mission pointed to evidence of perchlorate, a potentially highly oxidizing compound, in the Martian soils. However, some studies have noted that because perchlorate is highly stable, its presence in Martian soils cannot explain the Viking measurements.

Quinn et al. present a new analysis of Mars soil samples using the Wet Chemistry Laboratory, a component of the Microscopy, Electrochemistry, and Conductivity Analyzer on the NASA Mars Phoenix Lander. They find that although low levels of oxidizing compounds may be present, the oxidation-reduction potential of the soil is moderate and well within the range expected for habitable soils.

Geophysical Research Letters,
doi:10.1029/2011GL047671, 2011

Title: The oxidation-reduction potential of aqueous soil solutions at the Mars Phoenix landing site

Authors: Richard C. Quinn: Carl Sagan Center, SETI Institute, NASA Ames Research Center, Moffett Field, California, USA;

Julie D. Chittenden: Postdoctoral Program, NASA Ames Research Center, Moffett Field, California, USA;

Samuel P. Kounaves: Department of Chemistry, Tufts University, Medford, Massachusetts, USA;

Michael H. Hecht: Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA.

3. Global cyclone activity historically low

A new research study shows that overall global tropical cyclone activity has decreased to historically low levels during the past 5 years. Maue analyzes global tropical cyclone data from 1970 through May 2011 to examine the considerable interannual variability of the accumulated cyclone energy (ACE) metric. Since 2006, global and Northern Hemisphere ACE have decreased significantly, reaching the lowest levels since the late 1970s. Also, during 2010-2011, the overall global frequency of tropical cyclones reached a historical low. The researcher demonstrates that much of the variability in tropical cyclone energy during the past 40 years is clearly associated with natural large-scale climate oscillations such as the El Niño-Southern Oscillation and the Pacific Decadal Oscillation.

Geophysical Research Letters,
doi:10.1029/2011GL047711, 2011

Title: Recent historically low global tropical cyclone activity

Author: Ryan N. Maue: Center for Ocean and Atmosphere Studies, Department of Earth, Ocean and Atmospheric Science, Florida State University, Tallahassee, Florida, USA;

4. Heat-driven expansion not a major source of sea level rise

With the power to drown low-lying nations, destroy infrastructure, and seriously affect sensitive coastal ecosystems, sea level rise may be one of the most readily apparent consequences of global warming that is already under way. However, the sources of the rising waters, and the dynamics driving them, are not so clear. Melting land-locked glaciers, shrinking ice sheets over Greenland and Antarctica, and the ocean's thermal expansion will all play a part, but the expected contribution from each of these sources is still up for debate. Previous studies have suggested that thermal expansion driven by rising sea surface temperatures will account for up to 70 percent of sea level rise in the near future, but research by McKay et al. suggests this may be a drastic overestimate.

The authors draw on paleoclimate records and model simulations of the last interglacial period, when the sea level rose by more than 6 meters (19.7 feet), to isolate the contribution of thermal expansion to sea level rise during a previous period of global warming. The authors found that during the last interglacial period, between 130,000 and 120,000 years ago, the global average sea surface temperature changed between 𔂾.4 and 1.3 degrees Celsius (-0.7 and 2.3 degrees Fahrenheit). On the basis of research into the temperature sensitivity of thermal expansion, the authors suggest that between 𔂾.2 and 0.7 m (-0.66 and 2.29 ft) of ocean rise would have been attributable to thermal expansion. With thermal expansion playing such a small role in the pronounced sea level rise during the last interglacial, the authors suggest that the Greenland and, in particular, Antarctic ice sheets may be more sensitive to increasing temperatures than previously thought, with important implications for estimates of future sea level rise.

Geophysical Research Letters,
doi:10.1029/2011GL048280, 2011

Title: The role of ocean thermal expansion in Last Interglacial sea level rise

Authors: Nicholas P. McKay: Department of Geosciences, University of Arizona, Tucson, Arizona, USA;

Jonathan T. Overpeck: Department of Geosciences, Institute of the Environment, and Department of Atmospheric Science, University of Arizona, Tucson, Arizona, USA;

Bette L. Otto-Bliesner: National Center for Atmospheric Research, Boulder, Colorado, USA.

5. Bedrock cracks affect landslide susceptibility

The likelihood of landslides on an exposed bedrock hill is dependent on both the strength of the bedrock and the slope of the hill. In general, stronger rocks provide increased resilience against landslides and are capable of supporting steeper slopes. But for the hills of Fiordland and the Southern Alps, regions in New Zealand that experience repeated landslides, these simple proxy measures do not tell the whole story. The hills in these two regions have similar slopes, despite the fact that they are formed from rocks with significantly different inherent strengths, and the landslides in the Southern Alps are both more powerful and more common than those in Fiordland.

To find the cause of this disparity, Clarke and Burbank conduct a series of seismic refraction surveys at numerous sites in both regions to determine the distribution and density of subsurface fractures. The researchers find that at most sites in the Southern Alps, the destabilizing cracks were evenly distributed throughout the bedrock. At the majority of Fiordland sites, however, fracture patterns reveal two distinct subsurface zones: a heavily fractured surface layer in which fracture density diminishes with depth and a uniformly fractured lower layer. According to the authors, the uniform fracturing observed in both the Southern Alps and the lower layer of the Fiordland rock is due to tectonic forces, whereas the more intense fracturing in the surface layer of Fiordland is caused by geomorphic processes focused within the upper few meters of the bedrock. The authors suggest that the dual-layer pattern only develops on hillslopes that remain stable over timescales long enough to allow the geomorphic processes to act.

By comparing the tectonically induced fracture densities that delineate between the single- and dual-layer sites in each region, the authors identify the threshold densities above which the bedrock becomes unstable and susceptible to landslides. The research demonstrates one of the first techniques to relate subsurface structure to landslide occurrence and the potential for a powerful tool in landslide prediction.

Journal of Geophysical Research-Earth Surface,
doi:10.1029/2011JF001987, 2011

Title: Quantifying bedrock-fracture patterns within the shallow subsurface: Implications for rock-mass strength, bedrock landslides, and erodibility

Authors: Brian A. Clarke: Institute for Earth and Environmental Sciences, Universität Potsdam, Brandenburg, Germany;

Douglas W. Burbank: Department of Earth Science, University of California, California, US.

6. Sources and sinks of methane in an African lake

Africa's Lake Kivu, which borders Rwanda and the Republic of the Congo, contains huge amounts of methane (an estimated 60 cubic kilometers, or 14.39 cubic miles) as well as carbon dioxide (an estimated 300 cu km or 72 cu mi). Although the methane could provide a source of energy if it could be tapped, the high concentration of gases poses a risk of dangerous gas eruptions, threatening the local population. Furthermore, it has been reported that concentrations of methane have risen by as much as 15 percent in the past 30 years. Scientists need to learn more about the sources and sinks of methane in Lake Kivu to better assess the risk of catastrophic outgassing and sustainability of methane harvesting.

Pasche et al. study present-day formation and oxidation of methane. By analyzing isotopes of carbon in samples from the lake, the researchers determine that an unusually high proportion of methane (65 percent) in the deep parts of the lake is derived from geologic sources or produced biologically from geogenic carbon dioxide entering the lake through subaquatic springs. A significant amount of methane also comes from degrading organic material. Accumulation of such material has been increasing in recent decades as a result of factors such as the introduction of nonnative fish, enhanced internal recycling of nutrients due to increased discharges of subaquatic springs, and increased levels of nutrients caused by rising human populations. In addition, researchers find that oxidation of methane by aerobic bacteria is the main process preventing methane from escaping into the atmosphere, with anaerobic methane oxidation playing a minor role.

Journal of Geophysical Research-Biogeosciences,
doi:10.1029/2011JG001690, 2011

Title: Methane sources and sinks in Lake Kivu

Authors: Natacha Pasche: Eawag: Swiss Federal Institute of Aquatic Science and Technology, Surface Waters-Research and Management, Kastanienbaum, Switzerland; and Institute of Biogeochemistry and Pollutant Dynamics, Swiss Federal Institute of Technology, Zürich, Switzerland. Now at Lake Kivu Monitoring Program, Ministry of Infrastructure, Gisenyi, Rwanda;

Martin Schmid, Francisco Vazquez, Carsten J. Schubert, and Helmut Bürgmann: Eawag: Swiss Federal Institute of Aquatic Science and Technology, Surface Waters-Research and Management, Kastanienbaum, Switzerland;

Alfred Wüest: Eawag: Swiss Federal Institute of Aquatic Science and Technology, Surface Waters-Research and Management, Kastanienbaum, Switzerland; and Institute of Biogeochemistry and Pollutant Dynamics, Swiss Federal Institute of Technology, Zürich, Switzerland;

John D. Kessler: Department of Oceanography, Texas A&M University, College Station, Texas, USA;

Mary A. Pack and William S. Reeburgh: Department of Earth System Science, University of California, Irvine, California, USA.

7. First map of sea ice production in Arctic coastal areas

In Arctic coastal polynyas, persistent areas of thin ice, a large amount of new ice forms during the winters. Significant heat is lost through these regions of thin ice. Knowing how much ice is produced in polynyas is important for determining overall cold saline water formation in the Arctic Ocean. However, because of the difficulty of making in situ measurements, there have been few studies of ice production in coastal Arctic polynyas.

Tamura and Ohshima developed an algorithm for detecting polynyas and estimating sea ice thickness using data from satellite microwave sensors. The researchers use their method to create the first map of sea ice production in coastal polynyas over the entire Arctic Ocean. They also investigate the interannual variability of sea ice production in 10 Arctic coastal polynyas from 1992 to 2007. They find that sea ice production was better correlated with polynya extent than with surface air temperature.

Journal of Geophysical Research-Oceans,
doi:10.1029/2010JC006586, 2011

Title: Mapping of sea ice production in the Arctic coastal polynyas

Authors: Takeshi Tamura: Antarctic Climate and Ecosystems Cooperative Research Centre, University of Tasmania, Hobart, Tasmania, Australia; and National Institute of Polar Research, Tachikawa, Japan;

Kay I. Ohshima: Institute of Low Temperature Science, Hokkaido University, Sapporo, Japan.


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