AGU journal highlights - 10 February 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. Two large subglacial lakes are identified in Antarctica

Since the first evidence for subglacial Antarctic lakes was found in 1970, more than 145 such lakes have been identified in Antarctica, ranging in length from one to 280 kilometers [0.6 to 170 miles]. The deepest of these lakes are believed to contain diverse exotic ecosystems that were sealed when ice flowed over them 10-35 million years ago. Bell et al. discovered the size and hypothesized on the origins of two large subglacial lakes in East Antarctica, on the flanks of the Gamburtsev Subglacial Mountains. Based on evidence from satellite imagery, aerogeophysical data, laser-based ice surface altimetry, radar studies, and ground truth studies, they found that one lake, at 90 degrees East, is second only to subglacial Lake Vostok in extent--with a surface area of 2000 square kilometers [800 square miles], similar in area to the state of Rhode Island. The other lake lies beneath Sovetskaya Station, and covers a 1600 square kilometer[620 square mile] area. Both lakes are at least 900 meters[3,000 feet] deep. Their great depths, along with their orientation, elongated shapes, and their position on the western edge of the foreland basin abutting Lake Vostok, suggest that like Lake Vostok, they are of tectonic origin.

Title: Tectonically controlled subglacial lakes on the flanks of the Gamburtsev Subglacial Mountains, East Antarctica

Authors: Robin E. Bell and Michael Studinger: Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York., U.S.A.; Mark A. Fahnestock: Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, Durham, New Hampshire., U.S.A.; Christopher A. Shuman: NASA Goddard Space Flight Center, Greenbelt, Maryland, U.S.A.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL025207, 2006

2. A 20th century acceleration in global sea level rise

Past estimates of 20th century sea level rise have depended on tide-gauge records, some of which extend back before 1900. However, these records contain significant decadal variability and their sparse coverage areas back through time produce significant uncertainties in estimates of 20th century sea level trends. Satellite altimeters have produced high quality measurements of sea level since 1993. To combine tide-gauge and satellite data sets, Church and White used reductions of large matrices of satellite spatial data into fundamental mathematical relationships which reflect variability seen in the original observations. They then used tide-gauge data to determine changes in amplitude between consecutive months of a selected number of these reductions. Extrapolating from these trends, the authors reconstructed estimates of global mean sea level back to 1870, and found that between 1870 and 2004, sea level likely rose 195 millimeters [7.68 inches]. This yields a 20th century rate of sea level rise of 1.7 +/- 0.3 millimeters [0.067 +/- 0.01 inches] per year. If the rate of acceleration acceleration were to remain constant, the sea level would rise about 300 millimeters [10 inches] above current values by 2100.

Title: A 20th century acceleration in global sea-level rise

Authors: John A. Church and Neil J. White: CSIRO Marine and Atmospheric Research, and the Antarctic Climate and Ecosystems Cooperative Research Centre, Hobart Tasmania, Australia.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL024826, 2006

3. Manmade aerosols may delay ocean thermohaline circulation weakening

The thermohaline circulation (THC) in the North Atlantic is expected to weaken in response to increasing greenhouse gases. Models predict that this weakening will occur sometime in the 21st century, but it can be difficult to distinguish a climate change signal from the natural variability of the THC. Using an ensemble of simulations from a global coupled model, Delworth and Dixon examined the temporal evolution of the simulated North Atlantic THC from 1861-2000. They found that when the model was forced with a complete set of climate change forcing agents, no decrease in the THC was simulated. However, when subsets of these agents were used, they found that high levels of manmade aerosols, which cool the North Atlantic and reduce the poleward transport of water vapor, delayed greenhouse gas-induced weakening of the THC by several decades. The lack of change simulated in the 20th century was a result of a balance reached between greenhouse gas forcing and manmade aerosols. The authors caution that because aerosols reside in the atmosphere only for a few weeks, and greenhouse gases remain for centuries, the balance between aerosols and greenhouse gases could change rapidly.

Title: Have anthropogenic aerosols delayed a greenhouse gas-induced weakening of the North Atlantic thermohaline circulation?

Authors: Thomas L. Delworth and Keith W. Dixon: NOAA Geophysical Fluid Dynamics Laboratory, Princeton University, Princeton, New Jersey, U.S.A.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL024980, 2006

4. Abrupt increase in permafrost degradation in Arctic Alaska

Permafrost in Arctic zones has typically been considered stable because of low mean annual temperatures. However, the risk of thaw subsidence in Arctic lowlands is high, because of large volumes of ground ice at the top of the permafrost, which can melt in response to changes in the surface energy balance. To analyze the stability of Arctic permafrost, Jorgenson et al. studied ground ice wedges in northern Alaska, using field surveys conducted in 2003 and 2004, aerial photographs taken in 2001, and spectral analysis of the aerial photographs. By comparing these data to aerial photographs taken in 1945 and 1982, the authors identified temporal changes in various stages of ground ice within their field study. They found that an abrupt, large increase in permafrost degradation occurred in northern Alaska since 1982 to areas that had previously been stable for thousands of years. The authors inferred that this degradation was associated with record high temperatures observed in the area between 1989 and 1998, suggesting that even modest warming can have detrimental effects on permafrost.

Title: Abrupt increase in permafrost degradation in Arctic Alaska

Authors: M. Torre Jorgenson and Erik R. Pullman: ABR Inc., Fairbanks, Alaska, U.S.A.; Yuri L. Shur: Department of Civil and Environmental Engineering, University of Alaska, Fairbanks, Alaska, U.S.A..

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL024960, 2006

5. Suction of liquid iron into the mantle could perturb Earth's rotation

Perturbations in Earth's rotation rate at decadal time periods strongly suggest the existence of dissipative coupling at the core-mantle boundary to facilitate angular momentum transfer between the core and mantle. One way this transfer can occur is through Lorentz forces exerted by Earth's magnetic field on a thin conducting layer near the core-mantle boundary . Previous studies have hypothesized such a layer at the core-mantle boundary as a thin, buoyant, iron-bearing strip at the top of the liquid outer core. Kanda and Stevenson explored an alternate model, where a thin mantle-side conducting layer is maintained above the core-mantle boundary . This conducting layer is generated by the percolation of liquid iron from the outer core into the mantle in regions of mantle downwelling, because the liquid iron in the core is at a higher pressure than the lowermost mantle in such regions. The authors note that their model maintains a stable mantle-side conducting layer that enables core-mantle mass transfer and that such a layer could be seismically detectable. However, they caution that their model might not provide all of the coupling required at the core-mantle boundary to explain Earth's rotational perturbations.

Title: Suction mechanism for iron entrainment into the lower mantle

Authors: Ravi V.S. Kanda: Seismological Laboratory, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, U.S.A.; David J. Stevenson: Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, U.S.A.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL025009, 2006

6. Variations in El Nino events influence the predictability of the global climate

Changes in the El Nino Southern Oscillation (ENSO), associated with global warming, have been a major focus of climate research, due to ENSO's extensive impact on the global climate system. Changes in ENSO intensity may modulate global climate circulation anomalies and influence global climate predictability, whose signal comes mainly from fluctuations in sea surface temperature. The long-term changes of seasonal predictability are, however, difficult to quantify. Kang et al. examined the seasonal predictability of global surface air temperature for the 20th century, using the Climate of the 20th Century (C20C) AGCM experiment. Their studies found that since 1920, the seasonal mean predictability of global climate has increased. The signal-to-noise ratio of this parameter has also increased. The authors suggest that the increase of seasonal predictability for the 20th century is associated with an increase in ENSO amplitude and that the increase in ENSO amplitude may be related to increased surface temperature.

Title: Secular increase of seasonal predictability for the 20th century

Authors: In-Sik Kang, Emilia Kyung Jin, and Kyong-Hee An: School of Earth and Environmental Sciences, Seoul National University, Seoul, Korea.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL024499, 2006

7. Mantle plumes heat the lithosphere through small-scale eddies

Mantle plumes, believed to be thermal and/or chemical instabilities ascending through the convecting mantle, exist in all ocean bases where magmatism cannot be driven by plate tectonics. Yet despite their prevalence, little is known about the interactions that occur between the plume and the lithosphere [solid Earth]. To study this, Thoraval et al. performed numerical experiments, using a two-dimensional convection code that modeled the plume-lithosphere interactions beneath a fast moving plate like the Pacific. They found that plumes behave according to their buoyancy potentials and the region's Rayleigh number, which measures the likelihood of a fluid to convect. These small-scale convections help to make areas of the lithosphere significantly hotter, increasing the likelihood of melting events. Rather than eroding the lithosphere, fast moving plates flatten around these instabilities, leading to plate-like subsidence downstream of the plumes.

Title: Plume-lithosphere interaction beneath a fast moving plate

Authors: Catherine Thoraval and Andréa Tommasi: Laboratoire de Tectonophysique, Université de Montpellier 2, Montpellier, France; Marie-Pierre Doin: Laboratoire de Géologie, Ecole Normale Supérieure, Paris, France.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL024047, 2006

8. Air from Asia pollutes North America's upper troposphere

As economic growth expands rapidly in Asia, the transpacific transport of pollutants to North America will likely increase. Of particular concern is that influxes of nitrogen oxides (NOx) and other pollutants might increase ozone production, which could subside to ground levels and affect ambient air quality over North America. To study patterns of transpacific pollution, Wang et al. analyzed data collected during the Tropospheric Ozone Production about the Spring Equinox (TOPSE) experiment, conducted from February through May, 2000 over North America. These data showed large enhancements of nitrogen oxides and other pollutants in the upper troposphere during late spring, after low-lying transpacific pollutants usually peak. Using a model to simulate TOPSE observations, the authors showed that the increased flux of nitrogen oxides elevated the levels of photochemical oxidation and ozone production. The presence of chlorofluorocarbons and Halon-1211, which were phased out in developed countries, confirmed that the pollutants came from across the Pacific. The authors suggest that current global chemical transport models underestimate the long-range transport of pollutants, and thus their impact on air quality.

Title: Late-spring increase of trans-Pacific pollution transport in the upper troposphere

Authors: Yuhang Wang, Yunsoo Choi, and Tao Zeng: School of Earth and Atmospheric Science, Georgia Institute of Technology, Atlanta, Georgia, U.S.A.; Brian Ridley and Frank Flocke: Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, Colorado, U.S.A.; Nicola Blake and Donald Blake: Department of Chemistry, University of California, Irvine, California, U.S.A.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL024975, 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."
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The Highlights and the papers to which they refer are not under AGU embargo.

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