AGU Journal Highlights -- Aug. 29, 2006

1. Statistical causality tests provide evidence that climate change has increased hurricane intensity

Atlantic tropical cyclones are increasing in intensity, a trend that has been previously correlated with an increase in the late summer and early fall sea-surface temperature over the North Atlantic. Some studies attribute increases in hurricane intensity to a natural climate fluctuation, known as the Atlantic Multidecadal Oscillation; others suggest that climate change related to manmade greenhouse gases emissions is the cause. Noting that the only difference between these two hypotheses is the causal connection between global mean near-surface air temperature (GT) and Atlantic sea-surface temperature (SST), James Elsner analyzed GT, SST, and hurricane power dissipation data taken over the past 50 years. Using statistical causality tests, which determine whether time series values of one variable can predict future values of another variable, Elsner shows that GT is useful in predicting Atlantic SST. SST, however, is not useful in predicting GT. Thus, GT "causes" SST, supporting the hypothesis that climate change influences hurricane intensity. He inferred that future hurricane hazard mitigation efforts should reflect that hurricane damage will continue to increase, in part due to greenhouse warming.

[See also AGU Press Release 06-29: http://www.agu.org/sci_soc/prrl/prrl0629.html]

Title: Evidence in support of the climate change: Atlantic hurricane hypothesis

Author: James B. Elsner: Department of Geography, Florida State University, Tallahassee, Florida, U.S.A.

Source: Geophysical Research Letters (GL) paper 10.1029/2006GL026869, 2006


2. Changes in the pace of the water cycle indicate that summer is bleeding into spring

The water cycle is linked with natural nutrient cycles, and it is influenced by agriculture and human society, which in turn influences the ecosystem's sustainability. Thus, a key question regarding climate change concerns whether the hydrologic cycle is accelerating. Dirmeyer and Brubaker applied a water tracing algorithm, using atmospheric analyses and observed precipitation for the period of 1979 and 2003. Using past atmospheric conditions, including winds, temperature, and moisture content, the authors estimated how much evaporated water over all land regions of the globe could have fallen back as precipitation over the same areas. They compared this to total precipitation measured over the region to give the water recycling ratio. They found that seasonal trends of this ratio are changing over northern latitudes, consistent with an expansion into spring of the warmer season's regime of water vapor recycling. This trend is also consistent with observed vegetation-related changes often attributed to global climate change.

Title: Evidence for trends in the Northern Hemisphere water cycle

Authors: Paul A. Dirmeyer: Center for Ocean-Land-Atmosphere Studies, Calverton Maryland, U.S.A.;

Kaye L. Brubaker: Department of Civil and Environmental Engineering, University of Maryland, College Park, Maryland, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL026359, 2006


3. The equatorial ionosphere is influenced by atmospheric tides and weather in the tropics

The ionosphere, the region of highest plasma density in Earths space environment, supports a host of plasma instability processes that can adversely affect global communications and geo- location applications. In daytime, the equatorial ionosphere rises and spreads away from the region of maximum production, forming two bands of high plasma density on either side of the magnetic equator. Through observations of these two bands made from the Imager for Magnetosphere-to-Aurora Global Exploration (IMAGE) and the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellites from March and April 2002, Immel et al. discovered a 1000-kilometer [600 mile] scale longitudinal variation in ionospheric densities that co-varies with the magnitude of upward propagating atmospheric tides around the planet. These are generated by tropospheric weather systems in the tropics. This strong connection between tropospheric and ionospheric conditions was unexpected, but can be understood by considering the effects of upward propagating tides on the lowest daytime ionospheric layers. The authors note that further research into this connection will involve collaboration among scientists studying the atmosphere at all altitudes.

Title: The control of equatorial ionospheric morphology by atmospheric tides

Authors: T. J. Immel, S. L. England, S. B. Mende, and H. U. Frey: Space Sciences Laboratory, University of California Berkeley, Berkeley, California, U.S.A.;

E. Sagawa: National Institute of Information and Communications Technology, Tokyo, Japan;

S. B. Henderson and C. M. Swenson: Department of Electrical and Computer Engineering, Utah State University, Logan, Utah, U.S.A.;

M. E. Hagan: High Altitude Observatory, National Center for Atmospheric Research, Boulder, Colorado, U.S.A.;

L. J. Paxton: Applied Physics Laboratory, The Johns Hopkins University, Laurel, Maryland, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL026161, 2006


4. The equatorial ridge on Saturn's moon Iapetus: Formation through ring collapse?

The most recent observations made by the Imaging Science Subsystem (ISS) device aboard the Cassini spacecraft showed that the third largest Saturnian satellite, Iapetus, has a curious ridge system exactly aligned with the equator. Some scientists think that this ridge system is the result of tectonic stress caused by despinning of the proto-Iapetus from a rotational period of a few hours to the present configuration, where it spins in synchronous rotation with Saturn over a period of 79 days. W.-H Ip proposes a different hypothesis, suggesting that because Iapetus has large gravitational influence despite perturbations from Saturn, a ring system might have been present during its formation. He further hypothesizes that as the rotational period of Iapetus lengthened, this ring collapsed and ring particles collided onto Iapetus, forming the equatorial ridge system. Ip says that if correct, this hypothesis could potentially shed new light on the origin of Iapetus as well as satellite formation in general.

Title: On a ring origin of the equatorial ridge of Iapetus

Author: W.-H. Ip: Institute of Astronomy and Space Science, National Central University, Chung-Li, Taiwan.

Source:Geophysical Research Letters (GRL) paper 10.1029/2005GL025386, 2006


5. Ice core records are used to extract Antarctic temperatures over the past two centuries

The climate of the Antarctic continent is the most poorly observed of anywhere on Earth. Meteorological records are few and far between, and routine observations began only in the late 1950s. Schneider et al. sought to extend the instrumental record of Antarctic temperature with a reconstruction using water-stable isotope records from high-resolution, precisely dated ice cores. Both direct and reconstructed temperatures indicate a large interannual-to-decadal scale variability, with temperature anomalies on the Antarctic continent out of phase with those on the Antarctic Peninsula. Comparisons between the records of mean temperature from the Southern Hemisphere and the authors' reconstruction suggest that at longer timescales, temperature changes in Antarctica have paralleled changes in the Southern Hemisphere as a whole. The authors' reconstruction indicates that Antarctic temperatures have increased by about 0.2 degree Celsius [0.4 degree Fahrenheit] since the late nineteenth century. Moreover, because Antarctic temperatures are in-phase with Southern Hemisphere mean temperature trends, the authors propose that as global temperatures rise, so too will Antarctic temperatures, in accordance with model-based predictions.

Title: Antarctic temperatures over the past two centuries from ice cores

Authors: David P. Schneider and Eric J. Steig: Department of Earth and Space Sciences, University of Washington, Seattle, Washington, U.S.A.;

Tas D. van Ommen: Department of the Environment and Heritage, Australian Antarctic Division and Antarctic Climate and Ecosystem CRC, Hobart, Tasmania, Australia;

Daniel A. Dixon and Paul A. Mayewski: Climate Change Institute, University of Maine, Orono, Maine, U.S.A.;

Julie M. Jones: Institute for Coastal Research, GKSS Research Centre, Geesthacht, Germany;

Cecilia M. Bitz: Department of Atmospheric Sciences, University of Washington, Seattle, Washington, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL027057, 2006


6. Climate signals in corals might instead originate in coral biology

Seasonal cycles in the abundance of several trace-elements are seen in the skeletons of reef corals. These patterns are often attributed to seasonal variations in sea surface temperature or another physical parameter, which affects the uptake and incorporation of elements into the skeleton. Trends in these cycles provide highly resolved records of ocean temperature. Implicit in such methods is the assumption that physiological processes within the corals do not bias these records. Sinclair et al questioned this through their analysis of magnesium, strontium, and uranium abundances in reef and deep-sea corals. Noting that deep-sea corals should show no trends attributable to seasonality, because they are shielded from sea-surface temperatures, the authors found a large inverse relationship between magnesium and uranium within deep-sea corals that was dependent on skeletal type. Reef corals exhibit similar, but smaller, inverse relationships between magnesium and uranium, suggesting that biological fractionation occurs in these corals. The authors hypothesize that seasonal cycles therefore derive from a variation in a biological parameter, not a physical one, and note that physiological disturbances have the potential to significantly affect the accuracy of environmental records.

Title: A biological origin for climate signals in corals--Trace element "Vital Effects" are ubiquitous in Scleractinian coral skeletons

Authors: D. J. Sinclair: Center for Research in Isotopic Geochemistry and Geochronology (GEOTOP), University of Québec at Montréal, Montréal, Québec, Canada; now at Jackson School of Geosciences, University of Texas at Austin, Austin, Texas, U.S.A.;

B. Williams: Department of Geological Sciences, Ohio State University, Columbus, Ohio, U.S.A.;

M. Risk: School of Geography and Geology, McMaster University, Hamilton, Ontario, Canada.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL027183, 2006

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