AGU Journal highlights — Oct. 26, 2009

The following highlights summarize research papers that have been published or accepted for publication (paper in press) in Geophysical Research Letters (GRL).

Anyone may read the scientific abstract for any of these papers by clicking on the link provided at the end of each Highlight. You can also read the abstract by going to and inserting into the search engine the full doi (digital object identifier), e.g. 10.1029/2009GL039209. 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

1. Second volcano helped chill coldest decade on record

The extremely cold temperatures during the latter part of the decade from 1810 to 1819 — the coldest decade on record during the past 500 years — have been attributed mainly to the enormous 1815 eruption of the Tambora volcano in Indonesia. Such large volcanic eruptions cool the planet by spewing ash and gases into the stratosphere, where they form sulfate aerosols that block sunlight. But what accounts for the abnormally chilly temperatures earlier in the decade, between 1810 and 1815? Some recent studies have suggested that an unrecorded large volcanic eruption occurred around 1809. To investigate further, Cole-Dai et al. analyze ice cores from Greenland and Antarctica. In the 1809?to-1810 snow layers, the authors find anomalous sulfur isotopes that must have resulted from chemical reactions that could only have occurred in the stratosphere following a very large volcanic eruption. The results, which help improve understanding of volcanoes’ effects on climate, provide the first compelling evidence that an unknown large volcanic eruption occurred in the tropics in early 1809 and contributed to the coldest decade in recorded history.

Cold Decade (AD 1810 to 1819) Caused by Tambora (1815) and Another (1809) Stratospheric Volcanic Eruption

Jihong Cole-Dai, David Ferris, and Alyson Lanciki: Department of Chemistry and Biochemistry, South Dakota State University, Brookings, South Dakota, USA;

Joël Savarino: Laboratoire de Glaciologie et Géophysique de l´Environnement, CNRS/Université Joseph Fourier — Grenoble 1, 38400 Saint Martin d´Hères, France;

Mélanie Baroni: CEREGE, Collège de France, Université Paul Cézanne, UMR6635, CNRS, Aix-en-Provence, France;

Mark H. Thiemens: Department of Chemistry and Biochemistry, University of Califronia, San Diego, La Jolla, California, USA.

Geophysical Research Letters (GRL) paper in press;

2. Stalagmite climate record disputes ice-core findings

During the last glacial period, a number of rapid climate variations known as Greenland interstadials (GIs; also known as Dansgaard-Oeschger events) took place. These climate shifts have been observed in ice core records, but the precise timing of these events has been uncertain. To assign precise ages to some of the GI events, Fleitmann et al. obtain a new, well-dated carbon and oxygen isotope record from stalagmites in the Sofular Cave in northwestern Turkey. The authors note that the new stalagmite record, which covers the past 50,000 years, differs from the most recent Greenland ice core chronology by as much as several centuries at some points. Furthermore, although some scientists had suggested that GIs occurred on a 1470-year cycle, the new stalagmite record does not support that interpretation, the authors find. They also note that the new record indicates that the climate and ecosystem in the eastern Mediterranean changed rapidly in response to the GIs; these changes could have affected Neanderthal populations living in the area.

Timing and climatic impact of Greenland interstadials recorded in stalagmites from northern Turkey

D. Fleitmann, S. Badertscher, and O. M. Göktürk: Institute of Geological Sciences, University of Bern, Bern, Switzerland and Oeschger Centre for Climate Change Research, University of Bern, Bern, Switzerland;

H. Cheng: Department of Geology and Geophysics, University of Minnesota-Twin Cities, Minneapolis, Minnesota, USA;

R. L. Edwards: Department of Geology and Geophysics, University of Minnesota-Twin Cities, Minneapolis, Minnesota, USA;

M. Mudelsee: Climate Risk Analysis, Hannover, Germany;

A. Fankhauser, R. Pickering, A. Matter, and J. Kramers: Institute of Geological Sciences, University of Bern, Bern, Switzerland;

C. C. Raible: Oeschger Centre for Climate Change Research, University of Bern, Bern, Switzerland, and Climate and Environmental Physics, Physics Institute, University of Bern, Bern, Switzerland;

O. Tüysüz: Eurasia Institute of Earth Sciences, Istanbul Technical University, Istanbul, Turkey.

Geophysical Research Letters (GRL) paper 10.1029/2009GL040050, 2009;

3. Model explains Colorado plateau uplift

The mechanisms driving the evolution of the Colorado plateau in the past 30 million years have been the focus of debate among geologists. To address the issue, Moucha et al. create a numeric model that reconstructs mantle flow during the past 30 million years and track the induced topographic changes due to a mantle upwelling. The study confirms that the tectonic evolution of the southwestern United States has been influenced by mantle upwelling associated with the ancient northern portion of the East Pacific Rise. The authors note that uplift of the southern Colorado plateau totaled about 1 kilometer (0.6 mile) in the past 20 million years. In the past 10 million years, the center of uplift moved northeastward from the southwestern rim of the plateau. Furthermore, they suggest that the uplift of 100 to 300 meters (330 to 980 feet) in the past 5 million years may have played an important role in the formation of the Grand Canyon by establishing a gradient in the flow direction of the Colorado River.

Deep mantle forces and the uplift of the Colorado Plateau

Robert Moucha and Alessandro M. Forte: GEOTOP, Université du Québec à Montréal, Montreal, Quebec, Canada;

David B. Rowley: Department of the Geophysical Sciences, University of Chicago, Chicago, Illinois, USA;

Jerry X. Mitrovica: Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, USA;

Nathan A. Simmons: Atmospheric, Earth, and Energy Division, Lawrence Livermore National Laboratory, Livermore, California, USA;

Stephen P. Grand: Jackson School of Geological Sciences, University of Texas at Austin, Austin, Texas, USA.

Geophysical Research Letters (GRL) paper 10.1029/2009GL039778, 2009

4. Australian lake deposits weigh against Mars climate shift

In recent years the discovery of two different types of mineral deposits — sulfates and phyllosilicates?in close proximity on Mars led scientists to suggest that a significant global climate change must have occurred. However, others have suggested that these minerals could actually have formed under the same climatic conditions. To help resolve the issue, Baldridge et al. examine chemical and mineral data from acidic saline lakes in Western Australia, which have been recognized as a useful chemical analog for mineral formation on Mars. They note that Western Australian lakes have large pH differences separated by only a few tens of meters (tens to more than hundred feet), which shows that variable chemistries can coexist. On the basis of Australian lake data, the authors suggest an alternative Martian mineral formation mechanism in which some phyllosilicates could have formed in the neutral or alkaline subsurface while sulfates formed near the surface in more acidic conditions. The authors conclude that Mars had a complex hydrological history and the phyllosilicates and sulfates may be separated by chemical gradients rather than by temporal boundaries.

Contemporaneous deposition of phyllosilicates and sulfates: Using Australian acidic saline lake deposits to describe geochemical variability on Mars

A. M. Baldridge, S. J. Hook, and N. T. Bridges: Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA;

J. K. Crowley: U.S. Geological Survey, Reston, Virginia, USA;

G. M. Marion: Desert Research Institute, Reno, Nevada, USA;

J. S. Kargel: Department of Hydrology and Water Resources, University of Arizona, Tucson, Arizona, USA;

J. L. Michalski: Institut d’Astrophysique Spatiale, Université Paris Sud, Orsay, France;

B. J. Thomson: Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA;

C. R. de Souza Filho: Department of Geology and Natural Resources, University of Campinas, Campinas, Brazil;

A. J. Brown: SETI Institute, Mountain View, California, USA.

Geophysical Research Letters (GRL) paper 10.1029/2009GL040069, 2009;

5. How bushfires and Indian Ocean conditions are linked

Southeastern Australia has been hit by several serious bushfires in recent years, including the devastating February 2009 “Black Saturday” fire, which killed more than 170 people. To improve understanding of how climate change may affect the occurrence of bushfires, Cai et al. examine the connection between bushfires and positive Indian Ocean Dipole (pIOD) events. Such events are a phase in which the eastern Indian Ocean is cooler than usual while the western Indian Ocean is warmer than usual. These conditions tend to lead to lower than average rainfall and higher temperatures over southeastern Australia. The authors find that pIODs reduce the soil moisture, increasing the fuel load leading into summer. Furthermore, they show that of 16 pIOD events since 1950, 11 were followed by major bushfires, and of the past 21 major bushfires, 11 were preceded by pIOD. The authors also found that bushfires are more strongly associated with pIOD events than with El Niño events. Because global warming is likely to increase the frequency of pIOD events, the authors suggest that bushfire risk will also increase.

Positive Indian Ocean Dipole events precondition southeast Australia bushfires

W. Cai and T. Cowan: Wealth from Oceans Flagship, CSIRO Marine and Atmospheric Research, Aspendale, Victoria, Australia;

M. Raupach: CSIRO Marine and Atmospheric Research, Canberra, ACT, Australia.

Geophysical Research Letters (GRL) paper 10.1029/2009GL039902, 2009;

6. Benefits, risks, and costs of geoengineering

Stratospheric geoengineering, in which the precursors of sulfate aerosols are injected into the atmosphere, has been suggested as a possible way to reduce global warming. Aerosols would block sunlight from reaching the surface, thereby cooling the planet. Although such projects are impossible using current technology, geoengineering schemes are being considered as an option in case efforts to mitigate global warming fail. Robock et al. evaluate the benefits, risks, and costs of stratospheric geoengineering and find that the cost of using existing U.S. military planes to inject aerosol precursors into the atmosphere would be several billion dollars per year; other methods, including artillery and weather balloons, would cost more. The authors point out some of the benefits of geoengineering: It would cool the planet, reduce the melting of sea ice, reduce sea level rise, and increase plant productivity. However, they note that there are also many risks, including potential drought in some regions, continued ocean acidification, less sunlight for solar power, possible unexpected consequences, rapid warming if geoengineering were stopped, potential moral hazards, and problems for terrestrial astronomy.

Benefits, risks, and costs of stratospheric geoengineering

Alan Robock, Allison Marquardt, Ben Kravitz, and Georgiy Stenchikov: Department of Environmental Sciences, Rutgers University, New Brunswick, New Jersey, USA.

Geophysical Research Letters (GRL) paper 10.1029/2009GL039209, 2009;

7. Detecting water vapor trend would take 50 years

Water vapor in the upper troposphere contributes to the greenhouse effect, and scientists predict that humidity will increase in the future along with rising levels of atmospheric carbon dioxide. However, there is currently no observing program that could detect the predicted trends. To determine instrumental needs to measure long-term changes in upper tropospheric water vapor, Boers and van Meijgaard analyze how frequently and for how long observations would need to be made to clearly detect a trend in upper tropospheric water vapor. They used a regional climate model to simulate a perfect 150-year humidity record, and then sample from the model data to simulate realistic radiosonde water vapor observations with various observation frequencies. The analysis shows that it would take 30 years for a clear trend to show up in the perfect record; sampling every four days, it would take at least 50 years of observations to detect this trend. The authors suggest that these results, along with economic considerations, should be an important consideration for those planning an atmospheric water vapor monitoring program.

What are the demands on an observational program to detect trends in upper tropospheric water vapor anticipated in the 21st century?

R. Boers and E. van Meijgaard: Royal Netherlands Meteorological Institute, De Bilt, Netherlands

Geophysical Research Letters (GRL) paper 10.1029/2009GL040044, 2009;

8. Understanding distributions of energetic electrons

During geomagnetic storms, high-energy electrons from the Earth’s outer magnetosphere can enter the atmosphere, where they can affect atmospheric chemistry and may alter climate by destroying ozone. To improve knowledge of the special and temporal distribution of this energetic electron precipitation, Horne et al. analyze 9 years of data from low-altitude satellites for different phases of geomagnetic storms. They find that precipitation of electrons with energy greater than 300 keV (thousand electron volts) peaks during the main phase of geomagnetic storms, whereas precipitation of electrons with energy greater than 1 MeV (million electron volts) peaks later, after the geomagnetic storm peak. Furthermore, 300-keV electron precipitation occurs at all longitudes in both hemispheres, whereas 1-MeV electron precipitation occurs mainly in the Southern Hemisphere southward of the South Atlantic anomaly, a known weakness in Earth’s magnetic field. This indicates that different processes are responsible for scattering different energy electrons into the atmosphere. The authors suggest that these results should be considered in models of the atmospheric response to geomagnetic storms and solar variability.

Energetic electron precipitation from the outer radiation belt during geomagnetic storms

Richard B. Horne and Mai Mai Lam: British Antarctic Survey, Cambridge, UK;

Janet C. Green: Space Weather Prediction Center, National Oceanic and Atmospheric Administration, Boulder, Colorado, USA.

Geophysical Research Letters (GRL) paper 10.1029/2009GL040236, 2009;

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