AGU journal highlights - 25 January 2006

I. Highlights, including authors and their institutions

The following highlights summarize research papers in Geophysical Research Letters (GL). The papers related to these Highlights are printed in the next paper issue of the journal following their electronic publication.

You may read the scientific abstract for any of these papers by going to http://www.agu.org/pubs/search_options.shtml and inserting into the search engine the portion of the doi (digital object identifier) following 10.1029/ (e.g., 2005GL987654). The doi is found at the end of each Highlight, below. To obtain the full text of the research paper, see Part II.

1. Aerosols delay manmade warming of the world's oceans
The global ocean, with its large heat capacity, is the dominant reservoir for storing heat gained or lost by Earth's climate system. Past studies suggested that increases observed in ocean temperature during the 20th century can be attributed to higher levels of greenhouse gases. To test this, Delworth et al. used a new climate model to separate simulated ocean temperature changes between 1861 and 2000 into components attributable to natural and manmade atmospheric influences. According to their model, manmade aerosols [fine particles], which block incoming solar radiation, delayed ocean warming by several decades, reducing its magnitude by half when compared to the response expected from increased greenhouse gases alone. When the effects of volcanic aerosols were also included, the simulated ocean warming was reduced by two-thirds. The cooling after strong volcanic eruptions persisted in subsurface ocean temperature for decades. Aerosols also lowered the simulated increases in sea level due to thermal expansion, from approximately seven centimeters [three inches] to 2.4 centimeters [0.94 inches]. The authors noted that the short residence time of aerosols in the atmosphere could cause a rapid change in the mitigating effect that aerosols play on global warming.

Title:The impact of aerosols on simulated ocean temperature and heat content in the 20th century

Authors: Thomas L. Delworth and V. Ramaswamy: NOAA Geophysical Fluid Dynamics Laboratory, Princeton University, Princeton, New Jersey, U.S.A.;Georgiy L. Stenchikov: Department of Environmental Sciences, Rutgers University, New Brunswick, New Jersey, U.S.A.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL024457, 2005

2. Ships make clouds brighter and higher close to European harbors
In the year 2001, ships consumed 280 million metric tonnes [310 million short tons] of fuel, compared to only 64.5 million tonnes [71.1 million short tons] in 1950. As one of the least regulated sources of air pollution, emissions from ships (exhaust gases, hydrocarbons, and particulate matter) are expected to rise. Devasthale et al. used data collected between 1997 to 2002 by the Advanced Very High Resolution Radiometer aboard the NOAA-14 satellite to find the effect of ship pollution on clouds around the English Channel and the top three polluting harbors in Europe (Rotterdam, Netherlands; Antwerp, Belgium; and Milford Haven, United Kingdom). By dividing the region into coastal zones, inland areas, and the sea, they found that cloud albedo [reflectivity] increased and cloud top temperature decreased over coastal zones, while the opposite was observed inland. The ship-related cloud albedo increase of about 1.5 percent and cloud top temperature decrease of about 1.4-1.9 degrees Celsius [2.5-3.4 degrees Fahrenheit] resulted in compensating radiative effects in the solar and the thermal range. The authors note that future studies should integrate ship emission inventories, chemical transport models, and remote sensing information to better quantify impacts on regional and global scales.

Title:Impact of ship emissions on cloud properties over coastal areas

Authors: Abhay Devasthale, Olaf Krüger, and Hartmut Graßl: Meteorological Institute, University of Hamburg, and Max-Planck Institute for Meteorology, Hamburg, Germany.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL024470, 2006

3. Onset conditions for ionospheric storms
Irregularities in the equatorial F region, the section of the ionosphere above about 160 kilometers [100 miles] at the equator, scatters radio signals, scrambling communication and navigational systems. This phenomenon, termed equatorial spread F (ESF), occurs at sunset when the direction of the electric field suddenly shifts due to sunlight no longer ionizing atmospheric particles and can lead to widespread ionospheric storms. To investigate the dynamics and layer structures of the post-sunset ionosphere prior to the onset of ESF, Hysell et al. deployed instrumental and chemical release rockets into the sky over Kwajalein Atoll in the Marshall Islands on 7 August and 15 August 2004, as part of the NASA EQUatorial Ionospheric Studies II (EQUIS II) campaign. Using radar to monitor coherent and incoherent scatter, the authors detected a strong plasma shear flow and periodic patchy layers where scattering was more pronounced. They also found that the large-scale plasma depletions that formed later during ESF reproduced the periodic patchy structure of the original layers. The authors concluded that the original layers that formed before the onset of ESF were predictive of the ESF layers that followed.

Title: Onset conditions for equatorial spread F determined using EQUIS II

Authors: D. L. Hysell: Earth and Atmospheric Science, Cornell University, Ithaca, New York, U.S.A.; M. F. Larsen: Physics and Astronomy, Clemson University, Clemson, South Carolina, U.S.A.; C. M. Swenson and A. Barjatya: Electrical and Computer Engineering, Utah State University, Logan, Utah, U.S.A.; T. F. Wheeler: Electrical Engineering, Pennsylvania State University, University Park, Pennsylvania, U.S.A.; M. F. Sarango, R. F. Woodman, and J. L. Chau: Jicamarca Radio Observatory, Istituto Geofísico del Perú, Lima, Perú.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL024743, 2005

4. Forecasting volcanic eruptions
Accelerations in seismicity are important precursors to eruptions at volcanoes reawakening after long periods of dormancy and have the potential to provide warning signals to populations at risk from volcanic hazards. In particular, the accelerations yield maximum warning times before eruptions in subduction-zone settings, where volcanoes have produced the largest explosive outbursts in historical time. Past studies have quantified seismic accelerations in subduction zone volcanoes in terms of the linkage of crustal faults by shearing. Kilburn and Sammonds sought to refine this technique by introducing a damage-mechanics criterion for weakening rock between major fractures in response to pressurization of a magma chamber. By modeling this system, the authors found that seismicity typically accelerates dramatically within the 2-3 weeks preceding the eruption, but that, since at least a week or more is required to identify an accelerating trend, seismic forecasts of eruptions after a long period of dormancy can only be reliable a few days in advance. Because of this, the authors conclude that improvements in forecasting will require data assessments of geophysical and geochemical properties beyond basic seismic monitoring.

Title: Maximum warning times for imminent volcanic eruptions

Authors: Christopher R. J. Kilburn: Benfield Hazard and Research Centre, Department of Earth Sciences, University College London, London, United Kingdom; Peter R. Sammonds: Benfield Hazard and Research Centre, Department of Earth Sciences, University College London, London, United Kingdom; Mineral, Ice, and Rock Physics Laboratory, Department of Earth Sciences, University College London, London, United Kingdom

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL024184, 2005

5. Severe near-surface permafrost degradation is possible during the 21st century
Continuous permafrost currently spans more than 10 million square kilometers [four million square miles], mostly over Earth's northern latitudes. Recent observations suggest that permafrost temperatures are warming and that the upper portion of the soil that thaws each summer and refreezes each winter is expanding downward. Seeking to predict future spatial extents of near-surface permafrost, Lawrence and Slater used the Community Climate System Model (CCSM), which includes soil freeze-thaw processes, to determine which areas would retain permafrost at each of 10 soil depths extending down to 3.5 meters [11 feet]. The authors found that within 100 years, just one to four million square kilometers [400,000 to two million square miles] of near-surface permafrost will remain, depending on whether high or low greenhouse emission scenarios are assumed. This permafrost degradation contributes to a slow drying of soil and a redistribution of runoff from surface networks to the subsurface. According to the model, freshwater discharge into the Arctic Ocean will increase by 28 percent, due to both permafrost melt and precipitation increases. The authors note that future studies should consider possible accelerations to climate change that may occur if thawing releases organic material from the soil as greenhouse gases, such as methane or carbon dioxide.

Title: A projection of sever near-surface permafrost degradation during the 21st century

Authors: David M. Lawrence: Climate and Global Dynamics Division, National Center for Atmospheric Research, Boulder, Colorado, USA; Andrew G. Slater: Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL025080, 2005

6. Localized erosion affects national carbon budget
Rivers transport carbon between land and the atmosphere, oceans, and sediments. Carbon lost from terrestrial ecosystems to rivers as dissolved or particulate organic carbon can be oxidized and returned to the atmosphere as carbon dioxide or buried in sediments. If dissolved and particulate organic carbon are buried and resist oxidation, they could introduce significant errors into terrestrial-atmosphere carbon dioxide exchange calculations. Such errors could be important, because carbon dioxide emissions are accounted for under the United Nations Framework Convention on Climate Change and traded under the Kyoto Protocol. To quantify the potential errors, Scott et al. studied New Zealand's rivers. They found total dissolved and particulate organic carbon exports equivalent to 40 percent of the country's fossil fuel carbon dioxide emissions. Small mountain rivers contributed to most of the particulate organic carbon export. From these rivers, the majority of the particulate organic carbon exported may be buried within ocean sediments rather than oxidized to carbon dioxide. If this is the case, rapid mountain building and human-induced sedimentation can be viewed as important carbon sinks. In light of this, the authors argue that localized erosion deserves increased attention in carbon budgets and accounting.

Title:Localized erosion affects national carbon budget

Authors: Durelle T. Scott: Landcare Research, Palmerston North, New Zealand; now at the Department of Geosciences, University of Nebraska-Lincoln, Lincoln, Nebraska, USA; Troy Baisden, Michael J. Page, and Kevin R. Tate: Landcare Research, Palmerston North, New Zealand; Rob Davies-Colley: National Institute of Water and Atmospheric Research, Hamilton, New Zealand; Basil Gomez: Geomorphology Laboratory, Indiana State University, Terre Haute, Indiana, USA; D. Murry Hicks and Ross A. Woods: National Institute of Water and Atmospheric Research, Christchurch, New Zealand; Nicholas J. Preston: Landcare Research, Palmerston North, New Zealand; now at the School of Earth Sciences, Victoria University, Wellington, New Zealand; Noel A. Trustrum: Landcare Research, Palmerston North, New Zealand; now at the Institute for Geologic and Nuclear Sciences, Lower Hutt, New Zealand.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL024644, 2006

II. Ordering information for science writers and general public
Journalists and public information officers of educational and scientific institutions (only) may receive one or more of the papers cited in the Highlights by sending a message to Jonathan Lifland [jlifland@agu.org], indicating which one(s). Include your name, the name of your publication, and your phone number. The papers will be e-mailed as pdf attachments.

Others may purchase a copy of the paper online for nine dollars:
1. Copy the portion of the digital object identifier (doi) of the paper following "10.1029/" (found under "Source" at the end of each Highlight).
2. Paste it into the second-from-left search box at http://www.agu.org/pubs/search_options.shtml and click "Go."
3. This will take you to the citation for the article, with a link marked "Abstract + Article."
4. Clicking on that link will take you to the paper's abstract, with a link to purchase the full text: "Print Version (Nonsubscribers may purchase for $9.00)."
5. On the next screen, click on "To log-in to your AGU member services or personal subscription, click here."
6. On the next screen, click on "Purchase This Article."
7. The next screen will ask for your name, address, and credit card information to complete the purchase.

The Highlights and the papers to which they refer are not under AGU embargo.

Contact:
Harvey Leifert
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): (800) 966-2481 x507
Fax: +1 (202) 328-0566
Email: hleifert@agu.org