AGU Journal Highlights -- Aug. 30, 2013

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

In this release:

1. Parts of Amazon on the verge of forest-to-grassland shift
2. Hawaiian Islands formed through extrusive volcanic activity
3. Shifts of the Subtropical Shelf Front controlled by atmospheric variations
4. Atmosphere's emission fingerprint affected by how clouds are stacked
5. Loess landscapes could be major source of dust
6. Sediment wedges not stabilizing West Antarctic Ice Sheet

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://onlinelibrary.wiley.com/ and inserting into the search engine the full doi (digital object identifier), e.g. doi: 10.1002/grl.50570. The doi is found at the end of each Highlight below.

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1. Parts of Amazon on the verge of forest-to-grassland shift

The stability of the Amazon rainforest, and the ecosystem's resilience to widespread deforestation, may be much lower than previously thought. The replacement of stands of trees with grassland changes evapotranspiration rates and atmospheric moisture convergence, which in turn reduce regional rainfall, a feedback effect that could drive further deforestation. Previous research indicated that a dramatic shift from forest to grassland could overtake the Amazon when the total deforested area hits 40 to 50 percent of the forest's current size. New research by Pires and Costa, however, find that the deforestation needed to trigger this equilibrium shift is much lower, closer to just 10 percent.

Using a climate-biosphere model the authors calculated how different parts of the Amazon, such as the forest interior or the border regions, would stand up to deforestation-induced precipitation changes. They find that in different zones of the Amazon the precipitation responds to deforestation in different ways. In some places deforestation causes a linear decrease in precipitation. In some areas, it takes dramatic deforestation to induce a change in rainfall, while in others, slight deforestation results in sharp precipitation declines. The fact that the region's sensitivity to deforestation was found to be significantly higher than previously reported stems from the fact that in addition to the rainforest itself the authors also considered deforestation of nearby cerrado, a region of savanna-like vegetation in central Brazil.

The authors argue that to avoid an equilibrium shift, 90 percent of existing forest and 40 percent of cerrado land should be preserved. Presently, around 40 percent of the Amazon is protected area. They suggest that the forests of Bolivia and of Brazil's Pará state are most susceptible to such an equilibrium shift.

Source: Geophysical Research Letters, doi:10.1002/grl.50570, 2013 http://onlinelibrary.wiley.com/doi/10.1002/grl.50570/abstract

Title: Deforestation causes different subregional effects on the Amazon bioclimatic equilibrium

Authors: Gabrielle Ferreira Pires and Marcos Heil Costa: Department of Agricultural Engineering, Federal University of Viçosa, Viçosa, Brazil.



2. Hawaiian Islands formed through extrusive volcanic activity

Scientists generally believe that the Hawaiian Islands formed primarily through endogenous growth, or intrusion, in which hot magma intrudes into a rock and then solidifies before it reaches the surface. However, a new study suggests that the islands may actually have formed primarily through extrusion, which occurs when a volcano erupts and magma reaches the surface and flows away from the eruption site before cooling and solidifying.

Flinders et al. used gravity data from historical land surveys along with recently compiled marine gravity data to estimate the volumes of intrusive material, which can be identified by its higher density, below all of the known volcanoes throughout the Hawaiian Islands.

Contrary to previous studies, which had estimated that intrusions account for about 65 to 90 percent of the total volume of the islands, the authors find that the volcanos of the main Hawaiian Islands are composed of less than 30 percent dense intrusive material, on average. This suggests that the islands are not built primarily through endogenous growth, as had been thought, but rather through extrusive growth.

Source: Geophysical Research Letters, doi:10.1002/grl.50633, 2013 http://onlinelibrary.wiley.com/doi/10.1002/grl.50633/abstract

Title: Intrusive dike complexes, cumulate cores, and the extrusive growth of Hawaiian volcanoes

Authors: Ashton F. Flinders: Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island, USA;

Garrett Ito, Michael O. Garcia, John M. Sinton and Brian Taylor: Department of Geology and Geophysics, School of Ocean and Earth Science and Technology, University of Hawai'i at Mānoa, Honolulu, Hawaii, USA;

Jim Kauahikaua: U.S. Geological Survey, Hawai'i National Park, Hawaii, USA.



3. Shifts of the Subtropical Shelf Front controlled by atmospheric variations

In the western South Atlantic, off the coast of South America, a band of cold, fresh, nutrient-rich Sub-Antarctic Shelf Water (SASW) meets warm, salty, nutrient-poor Subtropical Shelf Water (STSW) to form the Subtropical Shelf Front (STSF). This front is the shallow-water expression of the major Brazil-Malvinas Confluence Zone and has moved northward and southward during the Holocene (the past ~12,000 years). Bender et al. reconstruct the latitudinal shifts of the STSF over the past 11,000 calendar equivalent years using records of oxygen and carbon stable isotope compositions of benthic foraminifera and total organic carbon and calcium carbonate content from a sediment record collected off Uruguay. These measurements serve as proxies for ocean water temperature and nutrient content, which can be used to distinguish the SASW and STSW.

The authors identify the latitudinal shifts in the location of the front. In general, they suggest that the movements are controlled by large-scale atmospheric forcings. Before about 9400 years ago the STSF was located south of 36 degrees south (the position of their sediment core) because of a southerly location of the Southern Westerly Winds (SWW). Between 9400 and 7200 years ago the STSF migrated northward, reaching north of 36 degrees south, driven primarily by northward movement of the SWW. After 4000 years ago the STSF oscillated around latitudes close to 36 degrees south, possibly because of an intensification of the El Niño–Southern Oscillation. Notably, during the past 200 years the STSF migrated southward, probably because of a southward shift of the SWW caused by global climate change.

Source: Paleoceanography, doi:10.1002/palo.20044, 2013 http://onlinelibrary.wiley.com/doi/10.1002/palo.20044/abstract

Title: Holocene shifts of the Subtropical Shelf Front off southeastern South America controlled by high and low latitude atmospheric forcings

Authors: Vera B. Bender: MARUM – Center for Marine Environmental Sciences, University of Bremen, Bremen, Germany;

Till J.J. Hanebuth: Center for Marine Environmental Sciences, University of Bremen, Bremen, Germany and Woods Hole Oceanographic Institution, Massachusetts, USA;

Cristiano M. Chiessi: Escola de Artes, Ciências e Humanidades, Universidade de São Paulo, São Paulo, SP, Brazil.



4. Atmosphere's emission fingerprint affected by how clouds are stacked

Clouds, which can absorb or reflect incoming radiation and affect the amount of radiation escaping from Earth's atmosphere, remain the greatest source of uncertainty in global climate modeling.

By combining space-based observations with climate models, researchers are able to derive baseline spectral signals, called spectral fingerprints, of how changes in the physical properties of the Earth's atmosphere, such as the concentration of carbon dioxide or the relative humidity, affect the amount of radiation escaping from the top of the atmosphere. Researchers can then use these spectral fingerprints to attribute changes in the observed top-of-atmosphere radiation to changes in individual atmospheric properties. However, recent research has shown that the way global climate models represent the interactions between clouds and radiation can complicate the process of making these spectral fingerprints. Researchers are finding that what matters is not only the presence or absence of clouds at each location represented in the model but also how the clouds are stacked vertically within each model grid.

Using a simulation experiment to mimic the future climate, Chen et al. tested how different approaches to parameterize cloud stacking affect the attributions of climate change signals in the longwave spectra recorded at the top of the atmosphere. The authors tested three approaches to parameterize cloud stacking and find that the differences in stacking assumptions affected the modeled global mean for outgoing longwave radiation by only a few watts per square meter. The global average for outgoing longwave radiation at the top of the atmosphere is around 240 watts per square meter. However, based on which parameterization is used, similar changes in the portion of the sky covered by clouds (especially the clouds in the middle and lower troposphere) can lead to spectral fingerprints that differ by up to a factor of two in the amplitude.

Source: Journal of Geophysical Research-Atmospheres, doi:10.1002/jgrd.50562, 2013 http://onlinelibrary.wiley.com/doi/10.1002/jgrd.50562/abstract

Title: Non-negligible effects of cloud vertical overlapping assumptions on longwave spectral fingerprinting studies

Authors: Xiuhong Chen and Xianglei Huang: Department of Atmospheric, Oceanic, and Space Sciences, University of Michigan, Ann Arbor, Michigan, USA;

Xu Liu: NASA Langley Research Center, Hampton, Virginia, USA.



5. Loess landscapes could be major source of dust

Dust, which affects weather and climate and can be hazardous to health, can be generated when sand or silt grains are either dislodged from the surface by other windblown grains (saltation) or lifted by wind directly (direct entrainment). Direct entrainment of silt has been thought to generate only minimal quantities of dust, and atmospheric models that include dust usually assume that dust generation occurs through saltation.

To test dust emission mechanisms, Sweeney and Mason conducted field and laboratory experiments with samples with varying dust grain size from the Peoria Loess in the central Great Plains of Nebraska. Loess is formed through accumulation of wind-blown dust and composed mainly of silt-sized particles. Scientists have believed that these landscapes should not be major sources of dust.

Through their experiments, the authors determine the threshold wind velocity needed to generate dust through direct entrainment of various size grains from the loess. They find that most dust generation from loess does occur by direct entrainment with fairly moderate wind velocities, while saltation was rare, contrary to expectations.

The results indicate that dust generation from the Peoria Loess could have been significant over geologic time scales. The authors suggest that a dry, windy climate, combined with lower vegetation density in the past, could have led to large-scale erosion of the Peoria Loess in Nebraska, causing the formation of wind-aligned ridges that have been observed.

Furthermore, the study shows that, in general, if loess landscape surfaces are exposed to wind, perhaps through changes in vegetation cover or land use, they could potentially become major sources of dust, with potential implications for climate, weather, and human health.

Source: Journal of Geophysical Research-Earth Surface, doi:10.1002/jgrf.20101, 2013 http://onlinelibrary.wiley.com/doi/10.1002/jgrf.20101/abstract

Title: Mechanisms of dust emission from Pleistocene loess deposits, Nebraska, USA

Authors: Mark R. Sweeney: Earth Sciences, University of South Dakota, Vermillion, SD, USA;

Joseph A. Mason: Department of Geography, University of Wisconsin-Madison, Madison, WI, USA.



6. Sediment wedges not stabilizing West Antarctic Ice Sheet

The stability of the West Antarctic Ice Sheet is uncertain as climate changes. An ice sheet such as the West Antarctic Ice Sheet that is grounded well below sea level on a bed that slopes toward the interior of the sheet is believed to be unstable: when the grounding zone, where the grounded sheet transitions into floating ice, retreats inland, it can potentially lead to massive amounts of ice becoming ungrounded and floating into the ocean, unless something such as a wedge of sediment stabilizes the ice sheet. It has been proposed that such a sediment wedge stabilizes the grounding zone of the Whillans Ice Stream, one of the large ice streams flowing from the West Antarctic Ice Sheet. However, most understanding of ice sheet stability at grounding zones comes from models and remote observation rather than field-based observations.

Horgan et al. looked at sediment deposition at the current grounding zone of Whillans Ice Stream using seismic and kinematic GPS methods. They find that sedimentary deposits are present, but these sediments are not forming the wedge-shaped deposits thought to stabilize the grounding zone. They suggest that some other mechanism may be creating stability.

Source: Geophysical Research Letters, doi:10.1002/grl.50712, 2013 http://onlinelibrary.wiley.com/doi/10.1002/grl.50712/abstract

Title: Sediment deposition at the modern grounding zone of Whillans Ice Stream, West Antarctica

Authors: Huw J. Horgan: Antarctic Research Centre, Victoria University of Wellington, Wellington, New Zealand;

Knut Christianson and Robert W. Jacobel: Department of Physics, St. Olaf College, Northfield, Minnesota, USA;

Sridhar Anandakrishnan and Richard B. Alley: Department of Geosciences, and Earth and Environmental Systems Institute, Pennsylvania State University, University Park, Pennsylvania, USA.

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