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AGU Journal highlights -- February 23, 2007

February 23, 2007

1. Ground rising again at volcano near Naples, Italy

Ground deformation data indicates that the Campi Flegrei caldera, near Naples, Italy, is undergoing renewed uplift. Troise et al. report that the volcanic area, which had its last eruption in 1538, started a new uplift episode in November 2004. This uplift began at a low rate, but has since slowly and steadily increased. According to previous studies, the 16th century eruption occurred after decades of uplift coupled with brief periods of subsidence. Within the past 40 years, the caldera experienced a huge uplift phase until 1985. The new data indicate that a subsequent period of subsidence has now ended. The ratio of maximum horizontal to vertical displacement, determined from Global Positioning System data, suggests that the uplift is associated with input of magma from a shallow chamber, the authors conjecture. They expect that future uses of this displacement method will help scientists monitor magma intrusion processes at this and other volcanoes and thus help quantify volcanic hazards.

Title: Renewed ground uplift at Campi Flegrei caldera (Italy): New insight on magmatic processes and forecast.

Authors: C. Troise, G. De Natale, F. Pingue, F. Obrizzo, P. De Martino, U. Tammaro, and E. Boschi: Instituto Nazionale de Geofisica e Vulcanologia-Osservatario Vesuviano, Naples, Italy.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL028545, 2007

2. New satellite method unclouds water-vapor measurements

A new satellite method overcomes an obstacle to measurements from space of atmospheric water vapor. When viewing the atmosphere above land obscured by clouds, satellites often have trouble measuring how much water vapor is present from the Earth's surface to the top of the atmosphere--a quantity known as the precipitable water vapor. Merritt Deeter has now used a microwave radiometer aboard NASA's Aqua spacecraft to determine the precipitable water vapor. The new approach has little sensitivity to clouds, and it works during day and night and over ocean and land surfaces, Deeter reports. For the study of climate and weather, water vapor is important because of its role as a greenhouse gas and its relationship to clouds and precipitation. Deeter tested his method against precipitable-water-vapor data collected from a network of Global Positioning System receivers. The comparison showed good agreement, except over regions with dense vegetation.

Title: A new satellite retrieval method for precipitable water vapor over land and ocean

Authors: Merritt N. Deeter: Research Applications Laboratory, National Center for Atmospheric Research, Boulder, Colorado, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL028019, 2007

3. Irrigation may cause cooling that masks greenhouse-gas warming

By running a climate model with and without irrigation, researchers find that irrigation may cause regional cooling. The magnitude of the simulated effect varies strongly with season, evidenced by large dry-season decreases in monthly mean and maximum temperatures, but little change in rainy-season temperatures. In their case study of California, which has intense irrigation, Kueppers et al. also show that irrigation-induced cooling may produce changes in regional air circulation. The authors hypothesize that past expansion of irrigation may have masked regional warming signals due to greenhouse gas increases. Many climate recording stations are in areas with irrigated agriculture, they note. Moreover, greenhouse gas emissions and irrigated agricultural area have both increased in the last century.

Title: The irrigation cooling effect: Regional climate forcing by land-use change

Authors: Lara M. Kueppers: Department of Earth and Planetary Sciences, University of California, Santa Cruz, California, U.S.A.; now at School of Natural Sciences, University of California, Merced, California, U.S.A.;

Mark A. Snyder and Lisa C. Sloan: Department of Earth and Planetary Sciences, University of California, Santa Cruz, California, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL028679, 2007

4. Assessing methane release from a large submarine landslide

Undersea landslides may play a role in releasing the potent greenhouse gas methane into the ocean and atmosphere. Yet new evidence suggests that a huge, ancient, submarine landslide belched little if any methane. Paull et al. studied the 8,000-year-old Storegga Slide, the largest known of many landslide scars within continental margins. Located off Norway, the Storegga Slide is believed to have occurred in sediments that may have initially contained gas hydrates. Those ice-like solids composed of water and gases, such as methane, form under low-temperature, high-pressure conditions and are common within continental margins. Using analyses of sulfate concentrations to model the relative amounts of methane present in sediments within and around the landslide, the authors find that considerable methane exists in sediments adjacent to and unaffected by the Storegga Slide. However, methane was absent from sampled landslide debris and from sediments exposed by the slide. The authors hypothesize that either methane was lost during slope failures prior to the Storegga Slide event, or that it was never present in significant concentrations within the sediments that failed.

Title: Assessing methane release from the colossal Storegga submarine landslide

Authors: C. K. Paull and W. Ussler III: Monterey Bay Aquarium Research Institute, Moss Landing, California, U.S.A.; W. S. Holbrook: University of Wyoming, Laramie, Wyoming, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL028331, 2007

5. When sea heights reveal deep pressures

A new analysis illuminates when and where sea-level height provides an accurate gauge of ocean-bottom pressure. Using simulations, Vinogradova et al. studied the relationship between those ocean characteristics as a function of timescale, bottom topography, and geographic location. Because the correct interpretation of sea level height and bottom pressure fluctuations is important to climate and ocean circulation studies, the authors caution that modeling and data processing procedures should carefully consider particular conditions in the regions and timescales studied. Specifically, conditions of ocean density and pressure known as "barotropic" allow accurate determination of ocean bottom pressure conditions from sea level height. Those known as "baroclinic" may not. On month-long timescales, barotropic conditions are widespread, except in the tropics, the authors find. At longer timescales, barotropic conditions persist only at high latitudes and in shallow depths. Elsewhere and at even longer timescales, bottom pressure and sea level fields can differ significantly.

Title: The relation between sea level and bottom pressure and the vertical dependence of oceanic variability

Authors: Nadya T. Vinogradova and Rui M. Ponte: Atmospheric and Environmental Research Inc., Lexington, Massachusetts, U.S.A.; Detlef Stammer: Institute für Meereskunde, Universität Hamburg, Germany.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL028588, 2007

6. Turbulent mixing roils ocean near Japan

Field studies find an important feature of ocean circulation at a location where simulations indicated it might occur. In numerical simulations of global ocean circulation, the convection that pushes surface waters downward and brings deep waters to the surface depends on the ability of a water parcel to mix vertically with surrounding waters. Tidal currents that interact with undersea topography to form waves called internal tides supply the energy for such mixing. Nagasawa et al. conducted field studies at three locations--the Aleutian Ridge (off Alaska), the Hawaiian Ridge (in the center of the Pacific Ocean), and the Izu-Ogasawara Ridge (off Japan)--where previous modeling indicated that internal tides should be strong. Through analysis of vertical profiles of microscale velocity distribution, the authors detect intense, turbulent mixing throughout the observed water depth along the Izu-Ogasawara Trough, which runs parallel to the Izu-Ogasawara Ridge. In contrast, the Aleutian and Hawaiian ridges hosted only light to moderate mixing.

Title: Microstructure measurements in the mid-depth waters of the North Pacific

Authors: Maki Nagasawa, Toshiyuki Hibiya, Kanako Yokota, and Yuki Tanaka: Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, Tokyo, Japan; Shogo Takagi: Faculty of Fisheries, Hokkaido University, Hakodate, Japan.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL028695, 2007

7. Deep low-frequency earthquakes are generated by shear slip on the plate interface

Deep, non-volcanic low-frequency tremors, which consist of swarms of intermittent weak signals at relatively low frequencies (1 to 8 hertz) and last from days to a few weeks once activity begins, were first discovered along Japan's Nankai trough, a location known to generate great earthquakes. Previous research has suggested that these low-frequency earthquakes (LFEs) represent shear slip on the boundary between the Pacific and Philippine Sea Plate and the Eurasian Plate. Ide et al. examined the mechanisms of low-frequency earthquakes along Nankai trough by comparing compilations of LFE waveforms with waveforms of earthquakes rupturing under known mechanisms within the Philippine Sea Plate. Through analyses of focal mechanisms and moment tensors from the two groups of earthquakes, they found that LFEs indeed represent shear slip on a low angle thrust fault at the plate interface that dips to the northwest. The authors suggest that deep tremor is generated directly by shear slip on the plate interface, and thus represents a seismic signature of the accompanying slow-slip events.

Title: The mechanism of deep low frequency earthquakes: Further evidence that deep non-volcanic tremor is generated by shear slip on the plate interface

Authors: Satoshi Ide: Department of Earth and Planetary Science, University of Tokyo, Tokyo, Japan; David R. Shelley and Gregory C. Beroza: Department of Geophysics, Stanford University, Stanford, California, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL028890, 2007

8. Slowdowns of ocean circulation expected from increased greenhouse gas emissions

The formation rate of North Atlantic Deep Water (NADW) and the intermediate-depth Labrador Sea Water (LSW) tightly governs the strength of the Atlantic meridional overturning circulation (AMOC) and thus the circulation of heat in the world's oceans. Through complex ocean atmosphere circulation models, much past research has sought to observe whether global warming from increasing manmade greenhouse gas emissions will melt ice caps and introduce freshwater to the North Atlantic, thereby slowing NADW and LSW formation. Weaver et al. ran several models with varying initial conditions and found that the strength of the AMOC always declines when greenhouse gases are increased by one percent per year, with largest declines in those experiments with the strongest initial AMOC. Further, they confirm that changes in surface heat flux, rather than changes in surface freshwater flux (i.e., increased precipitation, evaporation, and river runoff), slow down the AMOC, an idea suggested by past research. Finally, the authors note that their models were strongly influenced by water vapor and snow/ice feedbacks, and were thus sensitive to mean climate conditions.

Title: The response of the Atlantic meridional overturning circulation to increasing atmospheric CO2: Sensitivity to mean climate state

Authors: Andrew J. Weaver and Michael Eby: School of Earth and Ocean Sciences, University of Victoria, Victoria, British Columbia, Canada; Markus Kienast: Department of Oceanography, Dalhousie University, Halifax, Nova Scotia, Canada; Oleg A. Saenko: Canadian Centre for Climate Modelling and Analysis, University of Victoria, British Columbia, Canada.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL028756, 2007

9. Internal tidal mixing in Indonesian seas

When ocean tidal currents encounter undersea topography, waves called internal tides can be generated. Once generated, these waves propagate into the ocean interior and can contribute significantly to oceanic mixing when they break. The Indonesian archipelago is characterized by strong oceanic mixing induced by internal tides, which are trapped and forced to dissipate in semi-enclosed seas of the archipelago. This region is the only low-latitude passage between two oceans and hence plays an important role in the global ocean and climate regulation, yet it has been little-studied. Koch-Larrouy et al. investigated the role of internal tidal mixing on water and energy transfers between the ocean basins. They developed a three-dimensional mixing parameterization of internal tides that they then introduced into an oceanic general circulation model. As a result, water mass characteristics generated from the model better matched the temperature and salinity observations in all of Indonesia's seas. From this, the authors were able to better characterize exported Indonesian throughflow waters as they propogate into the Indian Ocean.

Title: On the transformation of Pacific water into Indonesian throughflow water by internal tidal mixing

Authors: Ariane Koch-Larrouy, Gurvan Madec, Pascale Bouruet-Aubertot, and Robert Molcard: Laboratoire d'Océanographie Dynamique et de Climatologie, Paris, France; Theo Gerkema: Royal Netherlands Institute for Sea Research, Den Burg, The Netherlands; Laurent Bessières: Laboratoire d'Etudes en Géophysique et Océanographie Spatiale, Tolouse, France.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL028405, 2007

10. Daily wind patterns in the coastal ocean

In most coastal areas, daily sea/land breeze (SLB) circulation occurs, due to differential heating of adjacent land and water masses, where land heats up more rapidly than water during the day and cools off more rapidly at might. The cycling of cold and warm air creates rotating pockets of high and low pressures, forming a circulation cell that propagates both onshore and offshore. Hunter et al. studied SLB circulation using data collected from February through May 2005 on the Hudson River's outflow into the New York Bight, an area off the coast of the United States' mid-Atlantic region. The authors find that daily SLB circulation can dominate motion over most of the continental shelf, both near shore and as far as 100 kilometers [60 miles] offshore, accounting for nearly half coastal waters' kinetic energy during the spring. Because continental margins are among the most productive ecosystems on Earth, the authors say that further quantification of the SLB system in the New York Bight and in other areas may help predict changes in global biogeochemical cycles.

Title: Spatial and temporal variability of diurnal wind forcing in the coastal ocean

Authors: Eli Hunter, Robert Chant, Louis Bowers, Scott Glenn and Josh Kohut: Institute for Marine and Coastal Sciences, Rutgers University, New Brunswick, New Jersey, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL028945, 2007

11. Modeling volcanic hazards on Italy's Mt. Vesuvius

Mt. Vesuvius, an Italian volcano with a history of explosive eruptions, poses a serious threat to the nearly one million people who live near its flanks, but this hazard is difficult to quantify. Neri et al. used numerical simulations based on flow transport models to analyze possible scenarios of the geographic distribution of volcanic debris as it would evolve during a medium-scale eruption. In particular, they focused on hazards posed by pyroclastic density flows, which are rivers of hot volcanic debris and gas that course down a volcano during an eruption and are the most destructive and deadliest of volcanic phenomena. The simulations showed that local topography, such as nearby Mt. Somma, significantly influences the flow around Vesuvius. Their results also reveal that low-fountaining, boiling-over events appear to be the most hazardous scenarios.

Title: 4D simulation of explosive eruption dynamics at Vesuvius

Authors: Augusto Neri, Tomaso E. Ongaro, Gianluca Menconi, and Mattia D. Vitturi: Instituto Nazionale de Geofisica e Vulcanologia, Pisa, Italy; Carlo Cavazzoni and Giovanni Erbacci: High Performance Computing Group, CINECA. Bologna, Italy; Peter J. Baxter: Institute of Public Health, University of Cambridge, Cambridge, United Kingdom.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL028597, 2007

12. Typhoons and hurricanes are the dominant cause of mixing between the troposphere and stratosphere

Deep air convection influences climate through injecting water-vapor-rich, ozone-poor air from Earth's near surface to the upper troposphere and lower stratosphere and displacing water-vapor-poor, ozone-rich air downward. However, the mechanisms that drive this convection are poorly understood, especially in the tropics, where, instead of being marked by a sharp transition in temperature, the boundary between the troposphere and the stratosphere is a diffuse region of nearly constant temperature. Rossow and Pearl conducted a 22-year survey of tropical convection, using subsets of data from the International Satellite Cloud Climatology Project. The authors show that deep convection occurs mostly in larger, more organized convective systems, but that smaller, unorganized convective systems rarely penetrate into the stratosphere. Durations of penetration are longest for the larger systems, such as hurricanes and typhoons, which generally exceed one day. The authors suggest that the role of tropical storms should be examined more closely, since, though intermittent, they dominate stratosphere-troposphere exchanges. Further, obtaining adequate statistics on stratosphere-troposphere mixing will rely on generating and maintaining long-term data records.

Title: 22-year survey of tropical convection penetrating into the lower stratosphere

Authors: William B. Rossow: Electrical Engineering, The City University of New York/The City College, New York City, New York, U.S.A.; Cindy Pearl: NASA Goddard Institute for Space Studies, Columbia University, New York City, New York, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL028635, 2007

13. Polar mesosphere summer echoes over Antarctica

Polar mesosphere summer echoes (PMSE) are strong signal power enhancements at very high radar frequencies that occur between about 80 and 95 kilometers [50 and 60 miles] in altitude (the mesosphere) near the poles during summer. These phenomena are thought to provide information on mesospheric temperatures, a parameter which may shed light on whether the polar atmospheres between the northern and southern hemisphere are different. Most studies have focused on PMSE in the northern hemisphere. Morris et al. collected the first complete season of southern hemisphere PMSE data above Davis, Antarctica, using a 55-megahertz atmospheric radar during the 2004-2005 southern hemisphere summer. They supplemented their findings with Aura satellite temperature measurements and ground-based partial reflection observations to investigate the thermal and dynamical state of the polar mesosphere during conditions of PMSE occurrence. The authors found that peak occurrence of PMSEs corresponded with temperature lows. Further, the PMSEs are linked with the typical summer equatorward flow of mesospheric winds, but were perturbed when this wind flowed poleward.

Title: The first complete season of PMSE observations above Davis, Antarctica, and their relation to winds and temperatures

Authors: Ray J. Morris, Damian J. Murphy and Andrew R. Klekociuk: Australian Government Antarctic Division, Kingston, Tasmania, Australia; David A. Holdsworth: Atmosphere Radar Systems, Thebarton, South Australia, Australia; also at Department of Physics, University of Adelaide, Adelaide, South Australia, Australia.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL028641, 2007

14. Rapid volume loss from two Greenland outlet glaciers

The Greenland ice sheet's contribution to sea level rise increased from 0.23 millimeters [0.0091 inches] per year in 1996 to 0.57 millimeters [0.022 inches] per year in 2005. Several large outlet glaciers accelerated during this same period, and snow accumulation increases cannot compensate for the enhanced mass discharge. Stearns and Hamilton studied ice dynamics on the coastal portions of southeast Greenland's Kangerdlugssuaq and Helheim glaciers, which have each lost at least 51 cubic kilometers [13 trillion U.S. gallons] of ice each year since 2001. Through analysis of digital elevation models derived from satellite images taken over the last five years, the authors show that the main contribution to this ice loss was dynamic thinning caused by the acceleration in flow of these glaciers. This thinning destabilized the ice mass, making it more susceptible to calving and melting. Based on their data, the authors estimate that together these glaciers have actually lost about 122 cubic kilometers [32.2 trillion U.S. gallons] each year since 2001, accounting for half the total ice mass lost from the entire Greenland ice sheet over this time.

Title: Rapid volume loss from two East Greenland outlet glaciers quantified using repeat stereo satellite imagery

Authors: Leigh A. Stearns and Gordon S. Hamilton: Climate Change Institute, University of Maine, Orono, Maine, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL028982, 2007

15. Carbon dioxide-caused ocean acidification may reduce shellfish populations

In recent experiments, the calcification rates of two types of mollusks kept in closed laboratory environments declined linearly with increasing levels of carbon dioxide in the air. Such creatures grow their shells by a process of calcification, or deposition of calcium carbonate. Data from the third assessment of the Intergovernmental Panel on Climate Change (IPCC) predict that the partial pressure of carbon dioxide in the atmosphere could more than double by 2100, causing ocean acidity to increase. Because increased acidity may have an adverse effect on calcareous marine life, Gazeau et al. studied calcification rates of edible mussels and Japanese oysters. Based on the researchers' laboratory findings, mussel and oyster calcification may decrease by 25 percent and 10 percent, respectively, by the end of the century if IPCC predictions come to pass. Because these species are important ecosystem engineers and represent a large part of worldwide aquaculture production, the resulting population decrease will potentially influence coastal biodiversity and ecosystem function, as well as lead to significant economic loss.

Title: Impact of elevated carbon dioxide on shellfish calcification

Authors: Frédéric Gazeau, Christophe Quiblier, Jeroen M. Jansen, Jack J. Middelburg, and Carlo H. R. Heip: Netherlands Institute of Ecology, Center for Estuarine and Marine Ecology, Yerseke, The Netherlands; Jean-Pierre Gattuso: Laboratoire d'Océanographie de Villefranche, Centre National de la Recherche Scientifique and Université Pierre et Marie Curie, Villefranche-sur-Mer, France.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL028554, 2007
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