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AGU journal highlights -- September 20, 2011

Tue, 09/20/2011 - 11:23am
EurekAlert

AGU Journal Highlights -- 20 September 2011

Highlights, including authors and their institutions

The following highlights summarize research papers that have been recently published in Geophysical Research Letters (GRL), Paleoceanography (PA), Journal of Geophysical Research-Earth Surface (JGR-F), Journal of Geophysical Research-Atmospheres (JGR-D) and Space Weather (SW).

In this release:

1. Study suggests Arctic sea ice loss is not irreversible

2. Radiation hazards likely to rise for spacefarers, air travelers

3. Earthquake-generated landslides are an important control of riverbed erosion

4. Arctic air may become cleaner as temperatures rise

5. Reefs may have triggered a bout of global cooling

6. Ultraviolet solar irradiance was low during recent solar minimum

7. Geomagnetic storms may harm satellites nearer Earth than was thought

8. Observing changes in atmospheric heat content

Anyone may read the scientific abstract for any already-published paper by clicking on the link provided at the end of each Highlight. You can also read the abstract by going to http://www.agu.org/pubs/search_options.shtml and inserting into the search engine the full doi (digital object identifier), e.g. 10.1029/2011GL048739. The doi is found at the end of each Highlight below.

Journalists and public information officers (PIOs) at educational or scientific institutions who are registered with AGU also may download papers cited in this release by clicking on the links below. Instructions for members of the news media, PIOs, and the public for downloading or ordering the full text of any research paper summarized below are available at http://www.agu.org/news/press/papers.shtml.

1. Study suggests Arctic sea ice loss is not irreversible

The Arctic has been losing sea ice as Earth's climate warms, and some studies have suggested that the Arctic could reach a tipping point, beyond which ice would not recover even if global temperatures cool down again. However, a new study by Armour et al. using a state-of-the-art atmosphere-ocean global climate model finds no evidence of such irreversibility. In their simulations, the researchers increase atmospheric carbon dioxide levels until Arctic sea ice disappears year-round and then watch what happens as global temperatures are brought back down. They find that sea ice steadily recovers as global temperatures drop. An implication of this result is that future sea ice loss will occur only as long as global temperatures continue to rise.

Source: Geophysical Research Letters, doi:10.1029/2011GL048739, 2011 http://dx.doi.org/10.1029/2011GL048739

Title: The reversibility of sea ice loss in a state-of-the-art climate model

Authors: K. C. Armour: Department of Physics, University of Washington, Seattle, Washington, USA;

I. Eisenman: Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, USA, and Department of Atmospheric Sciences, University of Washington, Seattle, Washington, USA;

E. Blanchard-Wrigglesworth, K. E. McCusker, and C. M. Bitz: Department of Atmospheric Sciences, University of Washington, Seattle, Washington, USA.

2. Radiation hazards likely to rise for spacefarers, air travelers

Galactic cosmic rays and solar energetic particles can be hazardous to humans in space, damage spacecraft and satellites, pose threats to aircraft electronics, and expose aircrew and passengers to radiation. A new study shows that these threats are likely to increase in coming years as the Sun approaches the end of the period of high solar activity known as the "grand solar maximum," which persisted through the space age. High solar activity can help protect the Earth by repelling incoming galactic cosmic rays.

Understanding the past record can help scientists predict future conditions. Barnard et al. analyze a 9,300-year record of galactic cosmic ray and solar activity based on cosmogenic isotopes in ice cores as well as on neutron monitor data. They use this to predict future variations in galactic cosmic ray flux, near-Earth interplanetary magnetic field, sunspot number, and probability of large solar energetic particle events. The researchers find that the risk of space weather radiation events will likely increase noticeably over the next century compared with recent decades, and that lower solar activity will lead to increased galactic cosmic ray levels.

Source: Geophysical Research Letters, doi:10.1029/2011GL048489, 2011 http://dx.doi.org/10.1029/2011GL048489

Title: Predicting space climate change

Authors: L. Barnard: Space Environment Physics, Department of Meteorology, University of Reading, Reading, UK;

M. Lockwood: Space Environment Physics, Department of Meteorology, University of Reading, Reading, UK, and RAL Space, Rutherford Appleton Laboratory, Chilton, UK;

M. A. Hapgood: RAL Space, Rutherford Appleton Laboratory, Chilton, UK;

M. J. Owens: Space Environment Physics, Department of Meteorology, University of Reading, Reading, UK;

C. J. Davis: Space Environment Physics, Department of Meteorology, University of Reading, Reading, UK, and RAL Space, Rutherford Appleton Laboratory, Chilton, UK;

F. Steinhilber: EAWAG, Swiss Federal Institute of Aquatic Science and Technology, Dubendorf, Switzerland.

3. Earthquake-generated landslides are an important control of riverbed erosion

River erosion is a powerful shaper of topography, cutting through bedrock and over time converting smooth terrain to rolling hills or jagged cliffs. The rate of bedrock incision is influenced by the stream's slope and width, the water's flow rate, and the presence of sediment, among other factors. Researchers know sediment can play two conflicting roles. If the river's sediment load is low, it acts like sandpaper, increasing the rate of riverbed erosion. If there is plentiful sediment in the river, it forms a protective barrier along the riverbed, limiting erosion. What remains uncertain, however, is the influence of large, periodic inputs of sediment that enter rivers because of landslides or other extreme events.

In 1999, the Chi-Chi earthquake in central Taiwan triggered thousands of landslides along the Peikang River, providing Yanites et al. with an opportunity to investigate the effect of a spike of sediment on long-term incision rates. The authors collected measurements of riverbed sediment depth at eight locations along the river, combining their results with previous results of long-term incision rates and morphological data. Using estimates of sediment flux, the researchers estimate the extent to which the riverbed bedrock is exposed and thus susceptible to erosion. They find that it could take decades or even centuries to flush the plug of sediment from the landslides out of the river system, trumping the influence of other river parameters. So while properties like flow rate and slope are important for the river's maximum potential erosion rate, the sediment's buffering capacity is a stronger determinant of long-term average rates.

Source: Journal of Geophysical Research-Earth Surface, doi:10.1029/2010JF001933, 2011 http://dx.doi.org/10.1029/2010JF001933

Title: The influence of sediment cover variability on long-term river incision rates: An example from the Peikang River, central Taiwan

Authors: Brian J. Yanites: Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan, USA;

Gregory E. Tucker: Department of Geological Sciences, University of Colorado at Boulder, Boulder, Colorado, USA; and CIRES, Boulder, Colorado, USA;

Han-Lun Hsu and Chien-chih Chen: Institute of Geophysics, National Central University, Jhongli, Taiwan;

Yue-Gau Chen: Department of Geosciences, National Taiwan University, Taipei, Taiwan;

Karl J. Mueller: Department of Geological Sciences, University of Colorado at Boulder, Boulder, Colorado, USA.

4. Arctic air may become cleaner as temperatures rise

The air in the Arctic is cleaner during summer than during winter. Previous studies have shown that for light-scattering pollutants, this seasonal cycle is due mainly to summer precipitation removing pollutants from the air during atmospheric transport from midlatitude industrial and agricultural sources.

With new measurements from Barrow, Alaska, and Alert, Nunavut, Canada, Garrett et al. extended previous research to show that light-absorbing aerosols such as black carbon are also efficiently removed by seasonal precipitation.

Precipitation removes these particles from the air most efficiently at high humidities and relatively warm temperatures, suggesting that as the Arctic gets warmer and wetter in the future, the air and snow might also become cleaner. If Arctic aerosols have a net warming effect, as is believed to be the case, precipitation removing these particles from the air would represent a negative climate feedback, mitigating anticipated Arctic warming.

Source: Geophysical Research Letters, doi:10.1029/2011GL048221, 2011 http://dx.doi.org/10.1029/2011GL048221

Title: The role of scavenging in the seasonal transport of black carbon and sulfate to the Arctic

Authors: Timothy J. Garrett: Department of Atmospheric Sciences, University of Utah, Salt Lake City, Utah, USA;

Sara Brattstrom: Meteorologiska Institutionen, Stockholms Universitet, Stockholm, Sweden;

Sangeeta Sharma and Douglas E. J. Worthy: Environment Canada, Toronto, Ontario, Canada;

Paul Novelli: GMD, ESRL, Boulder, Colorado, USA.

5. Reefs may have triggered a bout of global cooling

Earth's history shows a steady stream of extreme climatic change, spanning everything from planet-engulfing glaciers to tropical conditions near high latitudes. These large climate shifts are often attributed to permanent changes: the opening of a new waterway, the evolution of photosynthesizing plants, or massive bouts of erosion. Punctuating each long stretch of roughly steady climate are smaller transient shifts in environmental conditions commonly associated with changes in atmospheric carbon dioxide concentration. Yet the initial change in the climate system that led to the shift in carbon dioxide concentration for each period often remains unknown.

Focusing on the Middle Late Jurassic Transition (MLJT), a million-year-long bout of cool temperatures that took place roughly 160 million years ago, Donnadieu et al. find that changes in the growth rate and spatial extent of carbonate platforms may explain the temporary climate shift. Carbonate platforms are reefs laid down by carbonate-precipitating organisms. The authors find that leading up to the MLJT, carbonate platforms became concentrated at low latitudes before eventually ceasing production. They suggest that the drop in carbonate platform growth would have increased the oceanic concentration of carbonate ions, shifting the equilibrium for carbonate chemistry and increasing the ocean's ability to act as a sink for carbon dioxide. The changes in carbonate platform activity decreased atmospheric carbon from 700 to between 200 and 350 parts per million by volume, with a corresponding 9.3 degree to 4.5 degree Celsius (16.7 degree to 8.1 degree Fahrenheit) drop in atmospheric temperature.

The researchers are unable to pin down the specific trigger for the sudden change in carbonate platform activity, suggesting it could be attributable to temperature changes, tectonic activity, nutrient fluxes, or other processes that could affect the community of carbonate-precipitating organisms. However, by tracking the effect on historical climate, the research shows that carbonate platform functionality could be an important feedback system in Earth's currently changing climate.

Source: Paleoceanography, doi:10.1029/2010PA002100, 2011 http://dx.doi.org/10.1029/2010PA002100

Title: A mechanism for brief glacial episodes in the Mesozoic greenhouse

Authors: Yannick Donnadieu, Guillaume Dera and Christophe Dumas: LSCE, IPSL, CEA -CNRS -UVSQ, Gif-sur-Yvette, France;

Gilles Dromart and Nicolas Olivier: UMR 5570, Universite de Lyon, CNRS, ENS de Lyon, France;

Yves Godderis: LMTG, CNRS - Universite Paul Sabatier, Toulouse, France;

Emmanuelle Puceat: Universite de Bourgogne, CNRS Biogeosciences, Dijon, France;

Benjamin Brigaud: UMR 8148, Universite Paris -Sud 11, CNRS, Orsay, France.

6. Ultraviolet solar irradiance was low during recent solar minimum

Solar irradiance, which varies with the 11-year solar cycle and on longer time scales, can affect temperature and winds in the atmosphere, influencing Earth's climate. As the Sun currently wakes up from a period of low sunspot activity, researchers want to know how irradiance during the recent solar minimum compares to historical levels. In addition to understanding the total received power, it is important to know how various spectral bands behave, in particular the ultraviolet, which causes heating and winds in the stratosphere.

Lockwood analyzes solar ultraviolet spectral irradiance data from May 2003 to August 2005 from both the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) instrument on board the Upper Atmosphere Research Satellite (UARS) and the Solar Stellar Irradiance Comparison Experiment (SOLSTICE) instrument on the Solar Radiation and Climate Experiment (SORCE) satellite. Using several different methods to intercalibrate the data, he develops a data composite that can be used to determine differences between the recent solar minimum and previous minima. He finds that solar irradiance during the recent sunspot minimum has been especially low.

Source: Journal of Geophysical Research-Atmospheres, doi:10.1029/2010JD014746, 2011 http://dx.doi.org/10.1029/2010JD014746

Title: Was UV spectral solar irradiance lower during the recent low sunspot minimum?

Authors: Mike Lockwood: Space Environment Physics Group, Department of Meteorology, University of Reading, Reading, Berkshire, UK; and Space Science and Technology Department, Rutherford Appleton Laboratory, Didcot, Oxfordshire, UK.

7. Geomagnetic storms may harm satellites nearer Earth than was thought

Potentially hazardous space weather could affect satellites traversing the inner radiation zone, closer to Earth than previously thought, a new study shows. The Earth's radiation belts have a two-layer structure, and it had been thought that most geomagnetic storms, which are triggered by solar activity, affected only the outer belt. However, simulations by Shprits et al. show that during a very large geomagnetic storm, electrons could actually be accelerated in the inner radiation belt layer. Changes in the near-Earth radiation environment could then persist for several years after a large geomagnetic storm. High-energy electrons can damage satellites, and the altered radiation environment near Earth could significantly reduce the lifetime of satellites in that zone. The authors suggest that a large geomagnetic storm that reached the inner radiation belt could have potentially devastating economic impacts.

Source: Space Weather, doi:10.1029/2011SW000662, 2011 http://dx.doi.org/10.1029/2011SW000662

Title: Profound change of the near-Earth radiation environment caused by solar superstorms

Authors: Yuri Shprits: Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California, USA and Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, California, USA;

Dmitriy Subbotin and Binbin Ni: Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, California, USA;

Richard Horne: British Antarctic Survey, Cambridge, UK;

Daniel Baker: LASP, University of Colorado at Boulder, Boulder, Colorado, USA;

Patrick Cruce: Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California, USA.

8. Observing changes in atmospheric heat content

Globally, air temperatures near the surface over land have been rising in recent decades, and this has been presented as solid evidence of global warming. However, some scientists have argued that total heat content (energy), rather than temperature, should be used as a metric of warming trends. Surface air temperature is only one component of the energy content of the surface atmosphere - kinetic energy and latent heat also contribute.

Peterson et al. present the first study to use observational data to estimate global changes in surface energy of the atmosphere over time. They include temperature, kinetic energy, and latent heat in their analysis. The researchers find that total global surface atmospheric energy and heat content have increased since the 1970s, even though kinetic energy decreased slightly and in some regions latent heat declined while temperature increased. They conclude that surface air temperature provides a good estimate of heat content changes and thus is a good metric to describe global warming trends.

Source: Geophysical Research Letters, doi:10.1029/2011GL048442, 2011 http://dx.doi.org/10.1029/2011GL048442

Title: Observed changes in surface atmospheric energy over land

Authors: Thomas C. Peterson: National Climatic Data Center, NOAA, Asheville, North Carolina, USA;

Katharine M. Willett: Met Office Hadley Centre, Exeter, UK;

Peter W. Thorne: National Climatic Data Center, NOAA, Asheville, North Carolina, USA; and Cooperative Institute for Climate and Satellites, North Carolina State University, Asheville, North Carolina, USA.

SOURCE

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