AGU Journal highlights — Aug. 30, 2010

The following highlights summarize research papers that have been recently published in Journal of Geophysical Research-Atmospheres (JGR-D), Water Resources Research (WRR), and Geophysical Research Letters (GRL).

1. Extensive relict coral reef found in southern Pacific

Coral reefs are sensitive to climate change and track sea level. New observations show that an extensive coral reef existed in the southern Pacific Ocean thousands of years ago. Woodroffe et al. used multi-beam sonar, coring, and dating to examine a relict reef discovered in water about 20󈞅 meters (65-82 feet) deep around Lord Howe Island in the southern Pacific Ocean.

They found that the reef thrived from about 9,000 to 7,000 years ago and covered an area 20 times larger than the modern reef, which is the southernmost Pacific coral reef. About 7,000 years ago, the reef was drowned, probably due to abrupt sea level rise, and then shrunk to its modern extent.

The observation shows the extent to which reefs grew 9,000 years ago. Today coral reefs exist mainly in shallow seawater with sea surface temperatures greater than 18 degrees Celsius (64 degrees Fahrenheit), at latitudes near the equator. The relict reef shows that corals previously existed at southern latitudes farther from the equator.

The researchers note that as ocean temperatures warm due to climate change, the relict reef could become a substrate for new coral reef growth.

Response of coral reefs to climate change: Expansion and demise of the southernmost Pacific coral reef

Colin D. Woodroffe, Michelle Linklater, Brian G. Jones: School of Earth and Environmental Sciences, University of Wollongong, Wollongong, New South Wales, Australia;

Brendan P. Brooke, Cameron Buchanan, Richard Mleczko: Geoscience Australia, Canberra, ACT, Australia;

David M. Kennedy, School of Geography, Environment and Earth Sciences, Victoria University of Wellington, Wellington, New Zealand;

Quan Hua, Australian Nuclear Science and Technology Organization, Kirrawee, New South Wales, Australia;

Jian-xin Zhao, Radiogenic Isotope Facility, Centre for Microscopy and Microanalysis,
University of Queensland, Brisbane, Queensland, Australia.


Geophysical Research Letters (GRL) paper 10.1029/2010GL044067, 2010

2. Heavy 2009-2010 snowfall due to colliding weather events

The winter of 2009� had anomalously large snowfall in central parts of the United States, particularly the mid-Atlantic states, and in northwestern Europe. The weather encouraged deniers of global warming to deride climate change science. To help better inform the public about weather and climate, it can be valuable to determine the causes of such an abnormally snowy season.

After studying 60 years of snowfall measurements, Seager et al. found that the anomalous winter was caused by two colliding weather events, El Niño and the negative North Atlantic Oscillation (NAO). El Niño is an anomalous warming of surface ocean waters in the eastern tropical Pacific. The NAO is a fluctuation of atmospheric pressure above the North Atlantic Ocean.

The researchers used two data sets: one for snowfall at ground stations in the United States from 1950 to 1999 and one for the post-2003 period. They also analyzed hemispheric, satellite-derived snow water equivalent data from 1979 to 2007. The researchers note that El Niño brings increased precipitation to the southern and central United States. The negative NAO has little if any connection to the total precipitation over the United States and causes reduced precipitation over northwestern Europe. However, a negative NAO brings cold temperatures to eastern North America and northern Europe, making it more likely that precipitation will fall as snow. Thus, the combination of the El Niño and the negative NAO resulted in the extremely snowy winter in parts of the United States and Europe.

The researchers conclude that the heavy snowfall of the past winter was an example of the kind of recurrent climate anomaly that can stem from the atmosphere-ocean system, which can be difficult to predict.

Northern Hemisphere winter snow anomalies: ENSO, NAO and the winter of 2009/10

R. Seager, Y. Kushnir, J. Nakamura, M. Ting, N. Naik, Lamont Doherty Earth Observatory, Earth Institute at Columbia University, Palisades, New York, USA.


Geophysical Research Letters (GRL) paper 10.1029/2010GL043830, 2010


3. Shifting ozone hole exposed South America to more ultraviolet light in 2009

The ozone layer, which protects humans, plants, and animals from potentially damaging ultraviolet (UV) light from the Sun, develops a hole above Antarctica in September that typically lasts until early December. However, in November 2009, that hole shifted its position, leaving the southern tip of South America exposed to UV light at levels much greater than normal.

To characterize this event and to evaluate satellite monitoring capabilities, de Laat et al. analyze satellite and ground-based measurements of ozone levels and the UV index (UVI). They find that the ozone column over southern South America was especially thin from 11 to 30 November 2009, and significantly higher UVI values were measured.

Such abnormally low ozone levels sustained during a continuous period of three weeks had not been observed above southern South American at any time in the past 30 years, the researchers say. The high UVI values occurred over populated regions, meaning that humans had been exposed to increased levels of UV light. The scientists also note that the satellite-based measurements agreed well with the ground-based measurements, suggesting that satellite measurements can be valuable for monitoring ozone and UV radiation levels.

Extreme sunbathing: three weeks of small total O3 columns and high UV radiation over the Southern tip of South America during the 2009 Antarctic O3 hole season

A. T. J. de Laat, R. J. van der A, M. A. F. Allaart, and M. van Weele: Royal Netherlands Meteorological Institute, de Bilt, Netherlands;

G. C. Benitez: Observatorio Central de Buenos Aires, Servicio Meteorológico Nacional, Buenos Aires, Argentina and PEPACG, Pontificia Universidad Católica Argentina, Buenos Aires, Argentina;

C. Casiccia: Laboratorio de Monitoreo de Ozono y Radiación Ultravioleta, University of Magallanes, Magallanes, Chile;

N. M. Paes Leme: Ozone Laboratory, National Institute for Space Research, São José dos Campos, Brazil;

E. Quel, J. Salvador, and E. Wolfram: División Lidar, CEILAP, CITEFA, CONICET, Río Gallegos, Argentina.


Geophysical Research Letters (GRL) paper 10.1029/2010GL043699, 2010

4. How does emissions mitigation policy affect ocean acidification?

The anthropogenic release of carbon dioxide has increased significantly in recent years. Some of this carbon dioxide 2 is absorbed by the ocean, increasing its acidity (lowering its pH). Studies have shown that since 1750, the absorption of carbon dioxide has caused a decrease of 0.1 in the ocean’s pH level. The biological impact of this increasing acidity is uncertain, but scientists believe that increasing acidity could adversely affect marine organisms and degrade ocean habitats.

To investigate the effects of climate mitigation policy on ocean acidification, Bernie et al. ran simulations to find out how the timing and rate of carbon dioxide emissions reduction could affect future pH levels.

According to the study, the global mean ocean surface pH in 2100 will be strongly influenced by the year in which emissions peak and by post-peak reduction rates. For instance, the researchers found that a large reduction starting soon, with emissions peaking in 2016 and decreasing by 5 percent per year thereafter, would limit the minimum pH to 8.02 by 2100, down from today’s level of about 8.07. Without any mitigation, pH is projected to fall to between 7.67 and 7.81 by 2100. The study showed that over the next 500 years, minimum pH was mainly dependent on the long-term level of carbon dioxide emissions.

Influence of mitigation policy on ocean acidification

D. Bernie: Met Office Hadley Centre, Exeter, UK;

J. Lowe: Met Office Hadley Centre, Department of Meteorology, University of Reading, Reading, UK;

T. Tyrrell, O. Legge: National Oceanography Centre, Southampton, University of Southampton, Southampton, UK.


Geophysical Research Letters (GRL) paper 10.1029/2010GL043181, 2010

5. Reducing soot emissions would cut Arctic ice loss

Fossil fuel soot, emitted during diesel fuel, jet fuel, and coal combustion, and biofuel soot, emitted mainly through burning of wood and organic waste for heating and cooking, can affect climate and air quality. To determine the effects of limiting fossil fuel and biofuel soot emissions, Jacobson used climate model simulations to investigate and compare the short-term effects of controlling fossil fuel and biofuel soot, as well as methane and carbon dioxide.

The author finds that eliminating fossil fuel soot, a combination of fossil fuel soot plus biofuel soot and gases, and methane individually reduced global air temperatures by 0.3𔂾.5 degrees Celsius (0.54𔂾.9 degrees Fahrenheit), 0.4𔂾.7 degrees C (0.72𔂿.26 degrees F), and 0.2𔂾.4 degrees C (0.36𔂾.72 degrees F), respectively, over 15 years. He notes that net global warming is due mainly to fossil fuel greenhouse gases, not fossil fuel soot.

Nonetheless, eliminating fossil fuel soot and fossil fuel soot plus biofuel soot could reduce gross warming by 13󈝼 percent and 17󈞃 percent, respectively. In addition, the study showed that although fossil fuel and biofuel soot both contribute to warming, fossil fuel soot causes more warming per unit mass emission, while biofuel soot contributes more to air pollution and mortality. Furthermore, after carbon dioxide, fossil fuel and biofuel soot is the second leading cause of global warming. The author concludes that limiting fossil fuel and biofuel soot could be a useful way to quickly reduce Arctic ice loss and control global warming, possibly even more effectively than limiting carbon dioxide.

Read a previous press release on this paper at: http://www.agu.org/news/press/pr_archives/2010/2010-20.shtml

Short-term effects of Controlling Fossil-Fuel Soot, Biofuel Soot and Gases, and Methane on Climate, Arctic Ice, and Air Pollution Health

Mark Z. Jacobson, Department of Civil and Environmental Engineering, Stanford University, Stanford, CA


Journal of Geophysical Research-Atmospheres (JGR-D) paper 10.1029/2009JD013795, 2010


6. Summer heat waves to be increase during next decades

This year has been the hottest on record globally, with many areas around the world experiencing extremely hot conditions this summer. According to Diffenbaugh et al., extreme heat waves can be expected to be a more common occurrence over the next 30 years as greenhouse gas concentrations continue to rise.

The scientists use a new high-resolution climate model simulation for the United States and compared their results with multiple global climate models. The models show that there are likely to be increasingly frequent hot extremes during the next three decades, including more record-breaking heat waves and hot seasons in many parts of the United States, especially in the west. These hot conditions pose a threat not only to the environment but also to human health. The results suggest that limiting global warming to the 2 degrees Celsius (3.6 degrees Fahrenheit) proposed by the 2009 Copenhagen Accord would not be enough to avoid an increase in potentially dangerous extremely hot conditions.

Title: Intensification of hot extremes in the United States

Noah S. Diffenbaugh, and Moetasim Ashfaq: Woods Institute for the Environment and Department of Environmental Earth System Science, Stanford University, Stanford, California, USA and Purdue Climate Change Research Center and Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana, USA.

Geophysical Research Letters paper 10.1029/2010GL043888, 2010


7. Using underground clues to determine past atmospheric heat

Before a global climate model can be used by scientists to predict future climate patterns, it must first successfully predict the climate of the past as known by historical records or as inferred by proxy data (for example, oxygen isotopes in ice cores and tree ring records). Because historical records are spatially and temporally scarce, many climate models rely on proxy data, which by definition introduce large amounts of error into model calibrations. Reducing these errors is of high importance to climate scientists.

The Earth’s continents soak up heat from the Earth’s atmosphere — this heat penetrates the subsurface with time such that progressively deeper regions hold signatures for the temperatures of progressively older times. Noting that this observation could provide an important check to climate models, MacDougall et al. compare records of subsurface temperatures with model-derived continental subsurface heat fluxes. They find that climate models do not adequately reflect the continental subsurface’s role in the Earth’s energy budget, and that models rather tend to simulate the effects of heat only from shallow depths. Because subsurface heat can influence soil processes, hydrology, and plant life, which in turn can affect the amount of carbon dioxide released into the atmosphere, the authors recommend that climate models factor deeper subsurface heat processes into their calculations.

Comparison of observed and general circulation model derived continental subsurface heat flux in the Northern Hemisphere

Andrew H. MacDougall: Environmental Sciences Research Centre, St. Francis Xavier University, Nova Scotia, Canada; also at Department of Earth Sciences, Simon Fraser University, Burnaby, British Columbia, Canada;

Hugo Beltrami and Evelise Bourlon: Environmental Sciences Research Centre, St. Francis Xavier University, Nova Scotia, Canada;

J. Fidel González Rouco, Departamento de Astrofísica y CC. de la Atmósfera, Universidad Complutense de Madrid, Spain;

M. Bruce Stevens: Environmental Sciences Research Centre, St. Francis Xavier University, Nova Scotia, Canada; also at Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia, Canada.


Journal of Geophysical Research-Atmospheres (JGR-D) paper 10.1029/2009JD013170, 2010


8. El Niño intensity increasing in the central equatorial Pacific

El Niño is the dominant year-to-year climate fluctuation on the planet, influencing patterns of weather variability worldwide. Recently, a new type of El Niño has been documented, the so-called “central Pacific” El Niño. These El Niño events exhibit maximum warming primarily in the central equatorial Pacific, in contrast to classical El Niño events, which have maximum warming in the eastern equatorial Pacific. It has been hypothesized that the increasing frequency of central Pacific El Niño events in recent decades may be due to anthropogenic greenhouse gas forcing.

Lee and McPhaden analyze satellite observations covering the past three decades, and they find that not only has the frequency of central Pacific El Niño events increased, but their intensity has increased as well. The magnitudes of El Niño events in the central Pacific have almost doubled since 1980, with the strongest warming occurring during the 2009� El Niño.

They also find that sea surface temperatures in the central Pacific have not significantly increased during neutral or La Niña years. They therefore concluded that a previously documented long-term warming trend in the central Pacific, ascribed by some to be the result of global warming, is primarily due to more frequent and intense central Pacific El Niño events rather than to gradually rising background sea surface temperatures. These results may help scientists better understand the relationship between El Niño and global climate change. They also have implications for long-range weather forecasting, because central Pacific and eastern Pacific El Niño events have different impacts on the general circulation of the atmosphere.

Increasing intensity of El Niño in the central equatorial Pacific

Tong Lee: Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA;

Michael J. McPhaden: Pacific Marine Environmental Laboratory, NOAA, Seattle, Washington, USA.


Geophysical Research Letters (GRL) paper 10.1029/2010GL044007, 2010


9. Dry lake bed salts promote cloud formation

One of the major uncertainties in climate modeling is the effect of aerosol particles on cloud formation. Sea salt in the air has been known to be important in cloud formation over oceans. A new study now provides the first direct measurements of clouds showing that wind-blown dust from dry lake beds (playas) can also act as cloud condensation nuclei, encouraging the formation of clouds over continents and thereby affecting climate.

Using aircraft-based measurements, Pratt et al. detected playa dust, which is mainly composed of salts, in clouds over Wyoming. The researchers also conducted laboratory studies that confirm that playa salts can act as cloud-condensation nuclei. Because climate change and land use changes could result in the formation of more dry lake beds and increasing frequency of dust storms, it is important to understand the effects of playa salts on cloud formation and climate.

Observation of playa salts as nuclei in orographic wave clouds


Kerri A. Pratt: Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California, USA; Now at Department of Chemistry, Purdue University, West Lafayette, Indiana, USA

For a full list of authors, please see http://dx.doi.org/10.1029/2009JD013606


Journal of Geophysical Research-Atmospheres (JGR-D) paper 10.1029/2009JD013606, 2010


10. Characterizing channels for transport of melt in mantle

Rock in the Earth’s mantle melts as it upwells toward the surface, as can be seen beneath mid-ocean ridge spreading centers. This buoyant melt rises through the mantle to the surface, where it solidifies and becomes part of the Earth’s crust. However, the pathways through which mantle melt migrates have not been well understood. It had been suggested that channels through highly porous dunite rock provided pathways for mantle melt, but the depth and other features of these channels have not been known.

To investigate how dunite channels would form and how melt would flow through them in an upwelling mantle, Liang et al. conduct numerical simulations. They find that interconnected dunite channels form the shallow part of the porous channels through which melt passes. However, deeper in the mantle, melt travels through channels composed of the rocks harzburgite and lherzolite. In addition, the simulation shows that primary channels deeper in the mantle can lead to the formation of shallower secondary channels created by reactions between melt and rock during melt migration. These results could help geologists interpret field measurements and improve models for mantle melt migration, shedding light on mantle dynamics and crust formation.


High-porosity channels for melt migration in the mantle: Top is the dunite and bottom is the harzburgite and lherzolite

Yan Liang, E. Marc Parmentier: Department of Geological Sciences, Brown University, Providence, Rhode Island, USA;

Alan Schiemenz: Department of Geological Sciences, Brown University, Providence, Rhode Island, USA and Division of Applied Mathematics, Brown University, Providence, Rhode Island, USA;

Marc A. Hesse: Department of Geological Sciences, Brown University, Providence, Rhode Island, USA, now at Department of Geological Sciences, University of Texas at Austin, Austin, Texas, USA;

Jan S. Hesthaven: Division of Applied Mathematics, Brown University, Providence, Rhode Island, USA.


Geophysical Research Letters (GRL) paper 10.1029/2010GL044162, 2010


11. Understanding volcanism and tectonic activity in Yellowstone and the Pacific Northwest

During the past 65 million years, the Pacific Northwest has experienced significant tectonic and volcanic activity, including volcanism in the Yellowstone region. The origins of volcanism in the area have been debated.

To learn more about the region’s tectonic activity and the underlying geologic structure, Obrebski et al. analyze high-resolution tomographic images of the Earth’s interior created using data from the EarthScope USArray. They find that there is a mantle plume (a column of hot rock rising upward within the mantle) beneath the Yellowstone Snake River Plain in Idaho. In addition, they found that the Juan de Fuca slab, which is subducting under the North American plate, is fragmented beneath Oregon.

The researchers suggest that the plume’s interaction with the slab contributed to the weakening and breakup of the Juan de Fuca slab. This unusual interaction between a subducting slab and a mantle plume could help explain tectonic and volcanic activity in the region.


Slab-plume interaction beneath the Pacific Northwest


Mathias Obrebski, Richard M. Allen: Department of Earth and Planetary Science, University of California, Berkeley, California, USA;

Mei Xue: School of Ocean and Earth Science, Tongji University, Shanghai, China;

Shu‐Huei Hung: Department of Geosciences, National Taiwan University, Taipei, Taiwan.


Geophysical Research Letters (GRL) paper 10.1029/2010GL043489, 2010


12. Complexities of aquifer systems impede reaction rate estimates

Contaminant concentrations in aquifers can change as chemical reactions occur during groundwater transport through the aquifer. For instance, denitrification, in which the contaminant nitrate is converted to molecular nitrogen, reduces nitratecontaminant loads. It is useful to understand the rates at which denitrification and other reactions occur in an aquifer to improve understanding and prediction of contaminant migration.

However, estimates of denitrification and other reaction rates are often based on simplified transport models, typically by assuming all water in a sample has the same travel time and reaction history, an unrealistic assumption in many cases because of mixing of water in complex, geologically heterogeneous natural systems. To investigate the effects of mixing during transport in a heterogeneous environment on reaction rate estimates, Green et al. study an aquifer in the San Joaquin Valley using field observations and numerical models.

They find that apparent isotope fractionation and reaction rate estimates derived from field data using simple models are quite different from intrinsic (true) values from more realistic models accounting for heterogeneity. In fact, the apparent and true rates can differ by an order of magnitude or more. Moreover, the true parameter values for isotope fractionation and oxygen inhibition are in much better agreement with laboratory data than field-based estimates that do not account for mixing. They conclude that the effects of mixing during transport through a heterogeneous aquifer are important and that models accounting for these effects can improve forecasts of reaction progress.

Title: Mixing effects on apparent reaction rates and isotope fractionation during denitrification in a heterogeneous aquifer


Christopher T. Green, Barbara A. Bekins: U.S. Geological Survey, Menlo Park, CA, USA

John Karl Böhlke: U.S. Geological Survey, Reston, VA, USA

Steven P. Phillips: U.S. Geological Survey, Sacramento, CA, USA


Water Resources Research (WRR) paper 10.1029/2009WR008903, 2010


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