AGU journal highlights -- Jan. 28, 2008

The Antarctic Peninsula has experienced a dramatic change in climate over the past 50 years (the period of observational record), with annual temperatures increasing and many ice shelves on the fringes of the peninsula disintegrating. Despite this, some records from the peninsula show that the number of days with snowfall has increased by at least 62 days since 1950. To extend knowledge of climate dynamics beyond the 50-year observation record, Thomas et al. analyze data from a new ice core drilled at Gomez, a site of high snow accumulation on the Antarctic Peninsula. On the basis of these data, the authors find that snow accumulation has doubled since the 1850s, representing the largest increase observed across the region. Through comparing Gomez records with the behavior of the Southern Annular Mode (SAM), a dominant cycle of atmospheric variability in the Southern Hemisphere, the authors suggest that SAM circulation patterns have shifted to bring warm moist air to the region, causing an increase in snow accumulation as this air cools.

Title: A doubling in snow accumulation in the western Antarctic Peninsula since 1850

Authors: Elizabeth R. Thomas and Gareth J. Marshall: British Antarctic Survey, Cambridge, U.K.;

Joseph R. McConnell: Desert Research Institute, Reno, Nevada, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL032529, 2007; http://dx.doi.org/10.1029/2007GL032529

As temperatures continue to warm due to anthropogenic climate change, scientists are focusing on how plants are beginning to respond, especially in northern areas. Kharuk et al. studied larch, a deciduous pine widespread in Russia from the tundra zone in the north to steppes in the south, specifically focusing on wildfire history and the long-term trends in fire occurrence in larch-dominated communities. Through statistical analysis of burn scars present in tree-ring data, the authors calculate fire return intervals for zones where larch dominate and those called "larch-mixed taiga," where larch are less prevalent. They find that fires return on average every 65 years for the zone of larch dominance and every 50 years for the larch-mixed taiga zone. By comparison, in the nineteenth century fires returned on average every 101 years for the zone of larch dominance and every 97 years in the larch-mixed taiga zone. Because larch seeds require the extreme heat of fires to germinate, this decrease in fire return intervals may help sustain larch as competitor species migrate north as a result of warmer climates.

Title: Wildfires dynamic in the larch dominance zone

Authors: Viacheslav I. Kharuk and Maria L. Dvinskaya: V. N. Sukachev Institute of Forest, Krasnoyarsk, Russia;

K. Jon. Ranson: NASA Goddard Space Flight Center, Greenbelt, Maryland, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL032291, 2007; http://dx.doi.org/10.1029/2007GL032291

Incoming visible solar radiation heats the surface of terrestrial planets; infrared photons emitted by the surface are absorbed in the low atmosphere by greenhouse gases. By contrast, incoming ultraviolet solar radiation is absorbed by upper atmospheric particles, causing heating and expansion of the exosphere, the highest level of a planet's atmosphere. This expansion is modulated by carbon-dioxide infrared cooling to space, which occurs since the atmosphere is too thin to absorb infrared photons at these levels. Noting that comparative studies of exospheric properties can yield a deeper understanding of atmospheric properties unique to each planet, Forbes et al. study the response of Mars's exosphere to solar cycle variation by analyzing data collected between 1999 and 2005 from Mars Global Surveyor. Those data include periods of maximum and minimum solar activity. The authors find that exosphere temperatures on Mars change only 36 to 50 percent as much as those at Earth as solar activity increases from solar minimum to solar maximum. The response at Venus is one fifth that of Mars. The authors suggest that these differences may be strongly influenced by different general circulations and carbon-dioxide cooling efficiencies in these planets' high atmospheres.

Title: Solar flux variability of Mars' exosphere densities and temperatures

Authors: Jeffrey M. Forbes and Xiaoli Zhang: Department of Aerospace Engineering Sciences, University of Colorado, Boulder, Colorado, U.S.A.;

Frank G. Lemoine and Michael D. Smith: NASA Goddard Space Flight Center, Greenbelt, Maryland, U.S.A.;

Sean L. Bruinsma: Department of Terrestrial and Planetary Geodesy, Centre Nationale D'Etudes Spatiales, Tolouse, France.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL031904, 2007; http://dx.doi.org/10.1029/2007GL031904

Driven by the wind, the highly energetic circulation patterns of the upper ocean play important roles in the global distribution of heat and nutrients. Despite this importance, many aspects of upper ocean dynamics are poorly known, especially how its variability is characterized by the interaction of different types of motion and scales. To investigate the properties of upper ocean variability, Griffa et al. study data from instrument-laden buoys, called surface drifters, which are carried by ocean currents while collecting data. The authors focus on the subtropic and subpolar regions (10 to 60 degrees latitude), an area well sampled by drifters. They compute the ocean surface cyclonic and anticyclonic motion for scales ranging from large vortices to smaller eddies. The authors find two zonal bands of small-scale motion: one a known wind-induced anticyclonic band at 30 to 40 degrees latitude, and the other an unexpected cyclonic band at 10 to 20 degrees latitude. The latter might be due to fine-scale processes related to salinity front instabilities. These results provide a first global view of the upper ocean through drifter data.

Title: Cyclonic and anticyclonic motion in the upper ocean

Authors: Annalisa Griffa: Instituto de Scienze Marine, Consiglio Nazional delle Ricerche, Pozzuolo di Lerici, Italy; also at Division of Meteorology and Physical Oceanography, Rosentiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida, U.S.A.;

Rick Lumpkin: Physical Oceanography Division, Atlantic Oceanographic and Meteorological Laboratory, National Oceanic and Atmospheric Administration, Miami, Florida, U.S.A.;

Milena Veneziani: Ocean Science Department, University of California, Santa Cruz, California, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL032100, 2007; http://dx.doi.org/10.1029/2007GL032100

Many models predict that the rate of ocean overturning will slow as a response to global warming. Recent observations support this, with some showing that oceanic biogeochemical conditions are also changing on decadal scales. To better understand what causes such changes, Watanabe et al. study four 50-year time series of biogeochemical properties from across the northern Pacific Ocean. They find that due to surface stratification caused by an influx of cold water from the Arctic, several key nutrients were changing on linear scales. Noting that stratified conditions would create iron deficiencies at the surface, the authors predicted that the prevalence of diatoms (phytoplankton characterized by cell walls made of silica) would create silica-rich but fixed-nitrogen-poor environments. However, they find decreasing trends of dissolved oxygen paired with increasing phosphate levels. This suggests that biological matter is becoming remineralized, increasing levels of fixed nitrogen in subsurface and surface waters. Surface waters also showed a decrease in the ratio of silicon to nitrogen. Thus a shift away from diatoms has already occurred in the study region due to ocean stratification.

Title: Evidence of a change in oceanic fixed nitrogen with decadal climate change in the North Pacific subpolar region

Authors: Yutaka W. Watanabe and Masahito Shigemitsu: Faculty of Earth and Environmental Science, Hokkaido University, Sapporo, Japan;

Kazuaki Tadokoro: Tohoku National Fisheries Research Institute, Miyagi, Japan.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL032188, 2007; http://dx.doi.org/10.1029/2007GL032188

Human-induced climate change has warmed global temperatures for at least the past 50 years, leaving Arctic areas particularly vulnerable. Noting that the plateau ice caps of Baffin Island, Arctic Canada, are sensitive indicators of climate, Anderson et al. analyze aerial imagery and find that ice cover has diminished by more than 50 percent since 1958. Linear projections suggest that all ice caps on the island's plateau will disappear by 2070 A.D. From radiocarbon dating of dead vegetation that has emerged as ice margins have receded, the authors find that ice caps are now the smallest that they have been since at least 350 A.D. Cosmogenic radiocarbon accumulating in exposed rocks, which reveals their postglacial exposure history, shows that the plateau supported ice caps for most of the past 2800 years, accentuating the anomalous nature of twentieth-century warmth. Periods of widespread ice cap expansion during the Little Ice Age coincide with peak levels of volcanic aerosols in the stratosphere and reduced solar luminosity, suggesting a trigger mechanism for the Little Ice Age. The current warming exceeds any sustained warm episode in this area for at least the past 1600 years.

Title: A millennial perspective on Arctic warming from ¹⁴C in quartz and plants emerging form beneath ice caps

Authors: Rebecca K. Anderson, Gifford H. Miller, and Stephen B. DeVogel: Institute of Arctic and Alpine Research and Department of Geological Sciences, University of Colorado, Boulder, Colorado, U.S.A.;

Jason P Briner: Department of Geology, State University of New York at Buffalo, Buffalo, New York, U.S.A.;

Nathaniel A. Lifton: Geosciences Department, University of Arizona, Tucson, Arizona, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL032057, 2007; http://dx.doi.org/10.1029/2007GL032057

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