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AGU Journal highlights -- July 16

The following highlights summarize research papers that have been recently published in Journal of Geophysical Research-Biogeosciences (JGR-G), Journal of Geophysical Research-Atmospheres (JGR-D), Journal of Geophysical Research-Planets (JGR-E), Geochemistry, Geophysics, Geosystems (G3), and Geophysical Research Letters (GRL).

1. Droughts threaten Bornean rainforests

At 130 million years old, the rainforests of Southeast Asia are the oldest in the world and home to thousands of plant and animal species, some endemic to these forests. The rainforests also play important roles in modulating regional rainfall as well in the global carbon cycle.

However, since the 1960s, increased warming in the Indian Ocean and frequent El Nino events have reduced rainfall in the region by approximately 1 percent per decade. Further, the Intergovernmental Panel for Climate Change predicts that over the 21st century, Southeast Asia will experience higher land temperatures, more droughts, and increased seasonality -- wet seasons during the fall will get wetter, and dry seasons during the spring will get drier. However, few studies in the past have investigated how trees in the southeastern Asian rainforests respond to droughts and climate change.

In a new study, Kumagai and Porporato combine extensive field observations, historical records, and global climate models to investigate the potential impact of rainfall shifts and droughts on tree mortality in the Bornean rainforests of Southeast Asia. They find that as El Nino events become more frequent in the future in response to warming in the tropical oceans, even the species of trees that can adapt to drought conditions will be at increased risk of dying off. The small number of species that cannot adapt well to drought conditions will be at even greater risk of dying off.

Their study has implications for predictions of ecological changes, regional rainfall patterns, and global climate as well as direct applications for policies aimed at reducing additional human impacts on these ecosystems, which are not only vulnerable to climate change but also have the highest rates of deforestation in the whole world.

Source: Journal of Geophysical Research-Biogeosciences, doi:10.1029/2011JG001835, 2012

Title: Drought-induced mortality of a Bornean tropical rainforest amplified by climate change

Authors: Tomo'omi Kumagai: Hydrospheric Atmospheric Research Center, Nagoya University, Nagoya, Japan, and Department of Civil and Environmental Engineering, Duke University, Durham, North Carolina, USA;

Amilcare Porporato: Department of Civil and Environmental Engineering, Duke University, Durham, North Carolina, USA.

2. Replacing coal with natural gas would reduce warming

A debate has raged in the past couple of years as to whether natural gas is better or worse overall than coal and oil from a global warming perspective. The back- and-forth findings have been due to the timelines taken into consideration, the details of natural gas extraction, and the electricity-generating efficiency of various fuels. An analysis by Cathles, which focuses exclusively on potential warming and ignores secondary considerations, such as economic, political, or other environmental concerns, finds that natural gas is better for electricity generation than coal and oil under all realistic circumstances.

To come to this conclusion, the author considered three different future fuel consumption scenarios: (1) a business-as-usual case, which sees energy generation capacity continue at its current pace with its current energy mix until the middle of the century, at which point the implementation of low-carbon energy sources dominates and fossil fuel-derived energy production declines; (2) a gas substitution scenario, where natural gas replaces all coal power production and any new oil-powered facilities, with the same midcentury shift; and (3) a low- carbon scenario, where all electricity generation is immediately and aggressively switched to non-fossil fuel sources such as solar, wind, and nuclear.

The author finds that the gas substitution scenario would realize 40 percent of the reduction in global warming that could be achieved with a full switch to low- carbon fuel sources. The benefit for mitigating warming revolves around the fact that to produce an equivalent amount of electricity burning natural gas would release less carbon dioxide than burning oil or coal. Though atmospheric methane traps more outgoing radiation than carbon dioxide does, at reasonable leakage rates its atmospheric concentration is much lower and what is released decomposes much more quickly. The author suggests that over timescales relevant to large-scale warming-decades to centuries-the effect of any methane released during natural gas extraction would be inconsequential.

Source: Geochemistry, Geophysics, Geosystems, doi:10.1029/2012GC004032, 2012

Title: Assessing the greenhouse impact of natural gas

Authors: L. M. Cathles: Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York, USA.

3. New findings expand Apollo observations of lunar atmosphere

In December 1972 the astronauts of Apollo 17-the last manned mission to the moon-deployed the Lunar Atmospheric Composition Experiment (LACE), a spectrometer designed to measure and characterize the thin lunar atmosphere. Forty years later, Stern et al. built upon those initial measurements, providing the first remotely-sensed measurement of the Moon's gaseous environment from lunar orbit. Using the Lyman Alpha Mapping Project's (LAMP's) far ultraviolet spectrograph aboard the Lunar Reconnaissance Orbiter, the authors determined the atmospheric concentration of helium.

By angling LAMP's sensors towards the lunar limb and comparing those observations against measurements of the interstellar background, the authors were able to estimate the helium concentration of the near-surface lunar environment. They calculate a density of 7,000 atoms per cubic centimeter at 120 degrees Kelvin (-244 degrees Fahrenheit), the assumed atmospheric temperature. The previous LACE observations ranged between 10,000 - 20,000 and 50,000 atoms per cubic centimeter depending on the time of day, increasing at nighttime and decreasing during the day. The nighttime decrease occurs because the atmosphere cools and contracts, yielding an increased density.

The authors suggest that the next steps should involve looking for spatial or temporal variations in lunar atmospheric helium. Such observations could help to determine whether the helium is produced locally by radioactive decay of lunar material or if it is formed from trapped and neutralized solar wind.

Source: Geophysical Research Letters, doi:10.1029/2012GL051797, 2012

Title: Lunar atmospheric helium detections by the LAMP UV spectrograph on the Lunar Reconnaissance Orbiter

Authors: S. A. Stern and C. C. C. Tsang: Southwest Research Institute, Boulder, Colorado, USA;

K. D. Retherford and G. R. Gladstone: Southwest Research Institute, San Antonio, Texas, USA;

P. D. Feldman: Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland, USA;

W. Pryor: Department of Astronomy and Department of Geology, Central Arizona College, Coolidge, Arizona, USA.

4. No evidence of polar warming during penultimate interglacial

The Atlantic Meridional Overturning Circulation (AMOC), driven by temperature and salinity gradients, is an important component of the climate system; it transfers an enormous amount of heat via ocean currents and atmospheric circulation to high northern latitudes and hence has bearing on climate in the region.

Freshening of the surface ocean could weaken the AMOC. But during warm interglacial periods the effect of a fresh surface ocean on the AMOC may be muted. In fact, climate models predict that heat transfer from the North Atlantic to the Arctic may increase over the 21st century. A series of interconnected processes in the North Atlantic, known as polar amplification, could cause the Arctic to warm up faster compared to the rest of the world. It could even lead to ice-free conditions in the Arctic.

Previous paleoclimatic reconstructions indicate that the sub-Arctic may have been warmer by about 5 degrees Celcius (9 degrees Fahrenheit) with little summer sea ice cover during the Eemian, the penultimate interglacial centered around 125,000 years ago. Climate models favoring polar amplification use the Eemian as an analog of the present. In a new study, Bauch et al. compare reconstructed temperatures and water masses from two sediment cores that record the flow of meltwater in the subpolar and polar North Atlantic over the past 135,000 years. They do not find evidence of extreme warmth in the sub-Arctic during the Eemian interglacial period.

In fact, the Arctic may have been colder during the Eemian, with lower heat transfer from the North Atlantic. On the basis of their finding, the authors suggest that previous records may reflect other phenomena and caution against the use of the Eemian as an analog of the present. Their finding also challenges climate models that predict extreme warmth and ice-free conditions in the Arctic in response to greenhouse gas warming in the 21st century.

Source: Geophysical Research Letters, doi:10.1029/2012GL051800, 2012

Title: Contrasting ocean changes between the subpolar and polar North Atlantic during the past 135 ka

Authors: Henning A. Bauch: Akademie der Wisssenschaften und der Literatur Mainz, Helmholtz-Zentrum fur Ozeanforschung, Kiel, Germany;

Evguenia S. Kandiano: Helmholtz-Zentrum fur Ozeanforschung, Kiel, Germany;

Jan P. Helmke: Institute for Advanced Sustainability Studies, Potsdam, Germany.

5. Gas from pollutants, forest fires at potentially toxic levels

Forest fires and emission of air pollutants, which include fumes from vehicles running on diesel and slow burning of coal and charcoal, release isocyanic acid in the troposphere. In 2011, scientists first detected isocyanic acid in the ambient atmosphere at levels that are toxic to human populations; at concentrations exceeding 1 parts-per-billion by volume (ppbv), human beings could experience tissue decay when exposed to the toxin.

For the first time, using a chemical transport model designed to estimate the distribution and budget of isocyanic acid in the troposphere, Young et al. show that in several parts of the world, local emissions may increase the concentration of isocyanic acid in ambient atmosphere, thereby exposing large populations to potentially toxic levels of the acid.

Their research shows that regions that experience large forest fires, such as tropical Africa, Southeast Asia, Siberia, Canada, and the Amazon, or are heavily polluted, like China, are particularly vulnerable. In these regions, concentrations of isocyanic acid in the atmosphere exceeded the 1 ppbv limit for about 7-90 days per year. Their model also predicts that doubling the rate of air pollutant emission, particularly in heavily polluted regions of China, could increase the exposure of humans in the region to more than 170 days per year to isocyanic acid levels exceeding 1 ppbv.

On the basis of their study, the authors recommend more observations to improve estimates on global distribution of isocyanic acid in the atmosphere, particularly in regions experiencing large wild fires, where their model predicts the highest acid concentrations. Further, the authors suggest that scientists need to conduct research into indoor air pollution from the use of cooking stoves, which likely expose women and children to high levels of isocyanic acid.

A GeoSpace guest blog post on this study is available at: .

Source: Journal of Geophysical Research-Atmospheres, doi:10.1029/2011JD017393, 2012

Title: Isocyanic acid in a global chemistry transport model: Tropospheric distribution, budget, and identification of regions with potential health impacts

Authors: Paul. J. Young: Cooperative Institute for Research in the Environmental Sciences, University of Colorado Boulder, Boulder, Colorado, USA, and Chemical Sciences Division, Earth System Research Laboratory, NOAA, Boulder, Colorado, USA;

Louisa K. Emmons: National Center for Atmospheric Research, Boulder, Colorado, USA;

James M. Roberts: Chemical Sciences Division, Earth System Research Laboratory, NOAA, Boulder, Colorado, USA;

Jean-Franois Lamarque: National Center for Atmospheric Research, Boulder, Colorado, USA;

Christine Wiedinmyer: National Center for Atmospheric Research, Boulder, Colorado, USA;

Patrick Veres: Chemical Sciences Division, Earth System Research Laboratory, NOAA, Boulder, Colorado, USA, and Department of Chemistry and Biochemistry, University of Colorado Boulder, Boulder, Colorado, USA, and Max Planck Institute for Chemistry, Mainz, Germany;

Trevor C. VandenBoer: Department of Chemistry, University of Toronto, Toronto, Ontario, Canada.

6. Asteroid strikes cause the Moon's surface to smooth

The lunar surface is marred by impact craters, remnants of the collisions that have occurred over the past 4.5 billion years. The Orientale basin, the Moon's most recently formed sizeable crater, stands out from the rest. The crater, which lies along the southwestern boundary between the near and far sides of the moon, appears as a dark spot ringed by concentric circles of ejecta that reach more than 900 kilometers (560 miles) from the impact location. Though other craters have similar rings, the lunar surface surrounding the Orientale basin is unusually rough with reduced concavity. The anomalous features were identified by Kreslavsky and Head after they produced a map of the lunar surface topographic roughness using observations from the Lunar Orbiter Laser Altimeter aboard the Lunar Reconnaissance Orbiter.

The fact that other craters-even those of similar size and age-lack similar features suggests to the authors that mechanisms such as weathering or gravitational settling cannot explain the anomaly. Instead, the authors suggest that the Orientale basin, which formed about 3.8 billion years ago, stands out simply because it is the youngest large crater. They propose that whenever a large body slams into the Moon, seismic waves produced during the impact travel through the solid lunar material, inducing seismic shaking that causes landslides and surface settling. They estimate that the impactor would need to be at least 100 km (62 mi) across to cause sizeable seismic shaking. Unfortunately, the authors may need to wait more than a little while to conclusively test their hypothesis-until the Moon is next rocked by a massive asteroid, an event not expected to occur in the foreseeable future.

Source: Journal of Geophysical Research-Planets, doi:10.1029/2011JE003975, 2012

Title: New observational evidence of global seismic effects of basin-forming impacts on the Moon from Lunar Reconnaissance Orbiter Lunar Orbiter Laser Altimeter data

Authors: M. A. Kreslavsky: Department of Earth and Planetary Sciences, University of California-Santa Cruz, Santa Cruz, California, USA;

J. W. Head: Department of Geological Sciences, Brown University, Providence, Rhode Island, USA.


Contact: Mary Catherine Adams
American Geophysical Union

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