AGU journal highlights -- Oct. 16, 2008

Both natural and human-induced influences have changed twentieth-century climate, but their relative roles and regional impacts are still under debate. For example, most model-based studies point to increasing human-generated greenhouse gas and aerosol concentrations as the dominant cause of global surface warming after 1967, while some empirical analyses suggest that solar variability accounts for as much as 69 percent of warming seen in the past 100 years and 25-35 percent of recent warming. To help resolve this, Lean and Rind analyze the best available estimates of both natural and human-induced climate influences and compare them with observed surface temperatures across the globe from 1889 to 2006. They find that solar forcing contributed negligible long-term warming in the past 25 years and 10 percent of the warming in the past 100 years. Additionally, in contrast with recent model results by the Intergovernmental Panel on Climate Change, which estimates that anthropogenic warming has minimum values in the tropics and increases steadily from 30 degrees N to 70 degrees N, the authors find that the zonal surface temperature changes from the historical surface temperature record are more pronounced between 45 degrees S and 50 degrees N.

Title: How natural and anthropogenic influences alter global and regional surface temperatures: 1889 to 2006

Authors: Judith L. Lean: Space Science Division, Naval Research Laboratory, Washington D.C., U.S.A.;

David H. Rind: Goddard Institute for Space Studies, NASA, New York, New York, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL034864, 2008; http://dx.doi.org/10.1029/2008GL034864

2. Thinning sea ice bodes ice-free North Pole summers

In September of 2007, sea ice in the Arctic Ocean had a minimum monthly extent of 4.26 million square kilometers (1.64 million square miles), 23 percent lower than the previous minimum in 2005. Is this drastic reduction the result of natural variability superimposed on a general declining trend, or is Arctic sea ice cover shifting into a different climatic state characterized by completely ice-free summers? To help answer this, Haas et al. study the Arctic's Transpolar Drift, a current that carries ice from Siberia across the North Pole to the east coast of Greenland. Through helicopter-borne electromagnetic measurements, the authors calculate sea ice thicknesses over the Transpolar Drift during the late summers of 2001, 2004, and 2007, adding to a ground-based data set that extends to 1991. They find that average ice thickness has reduced by 44 percent since 2001. A model of ice ages shows that the area of older, thicker ice has decreased due to changed drift patterns. The authors suggest that the shift to younger and thinner ice could soon result in an ice-free North Pole during summer.

Title: Reduced ice thickness in Arctic Transpolar Drift favors rapid ice retreat.

Authors: Christian Haas: Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany; now at Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada;

Andreas Pfaffling: Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany; now at Norwegian Geotechnical Institute, Oslo, Norway;

Stefan Hendricks and Lasse Rabenstein: Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany;

Jean-Louis Etienne: Septieme Continent, Paris, France;

Ignatius Rigor: Polar Science Center, Applied Physics Laboratory, University of Washington, Seattle, Washington, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL034457, 2008; http://dx.doi.org/10.1029/2008GL034457

3. New model may explain slow earthquakes

Scientists have long recognized that in the absence of notable earthquakes, the Earth at plate boundaries can experience slow deformation, such as nonvolcanic low-frequency tremor and aseismic creep. New monitoring technologies have enabled a closer study of such deformation, but apart from an observed proportionality between seismic moment and slip duration, little is known about how slow slip differs from ordinary earthquakes. To help explain a wide variety of observed features, such as steady moment rates and scaled energies, characteristics of tremor signals, and the migration of source locations, Ide develops a simple model of slow earthquakes. In his model, slow earthquakes are represented as shear slip on circular faults whose radius is a random variable governed by specific parameters. His results show that varying the radius of these faults could explain differences in the behavior of slow slip events worldwide, suggesting that all slow slip phenomena fundamentally follow the same mechanism.

Title: A Brownian walk model for slow earthquakes

Authors: Satoshi Ide: Department of Earth and Planetary Science, University of Tokyo, Tokyo, Japan

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL034821, 2008; http://dx.doi.org/10.1029/2008GL034821

4. Understanding hops of persistent organic pollutants over the oceans

Persistent organic pollutants are toxic, bioaccumulable, and semivolatile compounds that have been detected in all environments, even in remote, pristine regions where they have never been produced or used. To get to remote regions, these pollutants undergo long-range atmospheric transport that introduces them to the oceans, where they become integrated into the food web. Studies have suggested that when persistent organic pollutants travel, they move in stages through a repeated process of volatilization and deposition. To learn more about this "grasshopper effect," Jurado and Dachs focus on a class of persistent organic pollutants called polychlorinated biphenyls (PCB). Through studies of the grasshopper effect's seasonal variability and long-range atmospheric transport potentials, the authors find that the number of hops taken by a chemical over the oceans is driven by seasonal biogeochemical processes occurring in the water column. For example, the transport of PCBs to the remote Arctic is maximized during seasons of low primary productivity. Such studies have important implications for understanding the distribution and fractionation processes of persistent organic pollutants on a global scale.

Title: Seasonality in the "grasshopping" and atmospheric residence times of persistent organic pollutants over the oceans

Authors: Elena Jurado and Jordi Dachs: Department of Environmental Chemistry, IIQAB-SCIS, Barcelona, Spain.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL034698, 2008; http://dx.doi.org/10.1029/2008GL034698

5. Antarctica ice-sheet loss modeled

The West Antarctic Ice Sheet has been rapidly losing mass to the oceans over recent decades. The most significant losses have occurred across the Amundsen Sea coastline, where dynamical changes in outlet glaciers have led to increased discharge. The synchronous response of several independent glaciers is generally considered an indicator that the changes are being forced by warming ocean waters. Scientists have documented deep flooding over the Amundsen Sea's continental shelf by almost unmodified Circumpolar Deep Water (CDW); though cold, this deep water is several degrees above freezing and drives rapid melting of the floating terminus of the Pine Island Glacier. To learn more, Thoma et al. model ocean circulation in the Amundsen Sea. They find that influx of CDW is related to regional wind forcing, which can drive seasonal on-shelf flow that varies in strength from year to year. A modeled period of warming following low CDW influx in the late 1980s and early 1990s coincides with a period of observed thinning and acceleration of Pine Island Glacier, suggesting that CDW fluctuations may be driving the observed glaciological changes.

Title: Modelling Circumpolar Deep Water intrusions on the Amundsen Sea continental shelf, Antarctica

Authors: Malte Thoma: British Antarctic Survey, Natural Environment Research Council, Cambridge, U.K.; now at Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany;

Adrian Jenkins: Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany;

David Holland: Courant Institute of Mathematical Sciences, New York University, New York, New York, U.S.A.

Stan Jacobs: Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL034939, 2008; http://dx.doi.org/10.1029/2008GL034939

6. Satellite maps tropospheric ozone

Ozone is a key species in the troposphere. A strong greenhouse gas and a main component of photochemical smog, its presence affects the lifetime of methane and many other hydrocarbons. Ozone, a powerful oxidizing agent that quickly reacts with organic compounds to produce toxic species, also is harmful to respiratory and cardiovascular systems. Measurements of ozone are therefore essential for air quality. Noting that satellite observations cover wide areas in short time, Eremenko et al. use data from the infrared atmospheric sounding interferometer (IASI), launched in October 2006 on board Europe's MetOp-A satellite, to compile the first day-by-day tropospheric ozone columns (from 0 to 6 kilometers (3.7 miles) in altitude) over Europe during the heat wave of July 2007. Comparing the results with selected balloonsonde measurements of ozone shows excellent agreement. Further, the ozone concentrations from IASI match well the predictions of a regional chemistry-transport model. This study demonstrates the capability of infrared satellite observations to monitor tropospheric ozone and to improve the forecasts of air quality and climate models.

Title: Tropospheric ozone distributions over Europe during the heat wave in July 2007 observed form infrared nadir spectra recorded by IASI

Authors: M. Eremenko, G. Dufour, G. Foret, C. Keim, J. Orphal, M. Beekmann, G. Bergametti, and J.-M. Flaud: Laboratoire Interuniversitaire des Systèmes Atmosphériques, Université Paris-Est and Université Denis Diderot, CNRS, Créteil, France.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL034803, 2008; http://dx.doi.org/10.1029/2008GL034803

7. Nordic currents shaped by topography

Surveys of the Nordic seas spanning more than 100 years reveal that warm waters from the North Atlantic flow along the eastern margin of the Nordic seas while cold waters from the Arctic flow south along the east coast of Greenland. However, how deep water circulates within these basins is not well known. Noting that the shape of the ocean bottom significantly constrains fluid motion in weakly stratified deep waters, Søiland et al. study the positions of neutrally buoyant subsurface drifting floats that were deployed across the northern slope of the Iceland–Faroe Ridge at a depth of 800 meters (2600 feet). They find that movement of the floats was tightly controlled by topography such that flow of intermediate waters within and the exchange between the basins took place along depth contours. By contrast, just upstream of the outflow of intermediate water from the Nordic seas to the North Atlantic, the flow is forced to cross depth contours. As a result, the flow becomes conspicuously unstable with strong eddy motion in the Faroe–Shetland Channel.

Title: Rigid topographic control of currents in the Nordic Seas

Authors: H. Søiland: Institute of Marine Research and Bjerknes Centre for Climate Research, Bergen, Norway;

M. D. Prater: Graduate School of Oceanography, University of Rhode Island, Kingston, Rhode Island, U.S.A.; now at WeatherPredict Consulting Inc., Narragansett, Rhode Island, U.S.A.;

T. Rossby: Graduate School of Oceanography, University of Rhode Island, Kingston, Rhode Island, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL034846, 2008; http://dx.doi.org/10.1029/2008GL034846

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