AGU Journal Highlights -- Sept. 24, 2013

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

In this release:

1. Statistically linking extreme precipitation to global warming
2. The mixed mechanisms of large-earthquake nucleation
3. Evaluating solutions to the faint young Sun problem
4. Updated ice core record captures Industrial Era carbon variability
5. Mechanism could explain rapid, dramatic, cyclic Arctic warming
6. Reconstructing ancient ocean temperature from deep sea shells

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.50811. The doi is found at the end of each Highlight below.

Journalists and public information officers (PIOs) at educational or scientific institutions who are registered with AGU also may download papers cited in this release by clicking on the links below. If you are a reporter and have not yet registered for a complimentary press subscription, please fill out the form at http://sites.agu.org/sciencepolicy/agu-press-subscriptions/.

1. Statistically linking extreme precipitation to global warming

Extreme rainfall can have serious effects on societies and ecosystems. Increases in extreme precipitation events are predicted to occur as Earth's climate warms, in part because warmer air has greater capacity to hold moisture, leading to more precipitation in a warmer climate. However, directly attributing changes in rainfall to global warming is difficult because climate models have limited precisions and because extreme events are rare and occur at irregular intervals.

To put that link on firmer footing, Benestad used a statistical analysis to determine whether extreme precipitation is related to global mean temperatures. The author used an empirical relationship, showing that daily rainfall amounts follow an exponential distribution, to determine that slow variations in the observed heavy precipitation events (the wet day 95th percentile) on a global scale follow the changes in the global mean temperature. Using this relationship, the author conducted a multiple regression analysis on rain gauge data and global surface air temperature data to show statistically that recent trends in wet day 95th percentile precipitation amounts are influenced by global mean temperatures.

Source: Journal of Geophysical Research-Atmospheres, doi:10.1002/jgrd.50814, 2013 http://onlinelibrary.wiley.com/doi/10.1002/jgrd.50814/abstract

Title: Association between trends in daily rainfall percentiles and the global mean temperature

Authors: R.E. Benestad: The Norwegian Meteorological Institute, Oslo, Norway.

2. The mixed mechanisms of large-earthquake nucleation

An important open question in seismology is: Where do big earthquakes come from? High-energy earthquakes often come from faults under high strain, but the mechanism that underlies their onset is still debated.

The strength of an earthquake depends in part on the size of the region that first starts to slip, with a larger "nucleation" region generally spawning a stronger earthquake. There are two hypotheses for the source of major earthquakes. One is that large earthquakes are necessarily triggered by large nucleation regions. The second is that large earthquakes start from small, slip-prone regions within a fault, and the size of the earthquake can grow as small quakes trigger larger ones.

Noda et al. simulate how an earthquake ruptures when a small fragile zone is embedded within a larger, less slip-prone region. They find that both of the hypothesized mechanisms can occur, but which takes place depends on the size of the fragile patch compared to the size of the nucleation region required to trigger the whole fault. They find that if the whole fragile patch is smaller than the nucleation size required to trigger big earthquakes, then only small earthquakes will come from the fragile patch. Large earthquakes could still occur if there is a large slip in the tough region away from the fragile zone. If the fragile patch is bigger than the nucleation size of the big earthquake, then small earthquakes will cascade up into large ones. If the fragile patch and the nucleation size are comparable, then both types of large earthquakes will occur over time.

Source: Journal of Geophysical Research-Solid Earth, doi:10.1002/jgrb.50211, 2013 http://onlinelibrary.wiley.com/doi/10.1002/jgrb.50211/abstract

Title: Large nucleation before large earthquakes is sometimes skipped due to cascade-up—Implications from a rate and state simulation of faults with hierarchical asperities

Authors: Hiroyuki Noda and Takane Hori: Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, Yokohama, Kanagawa, Japan;

Masao Nakatani: Earthquake Research Institute, The University of Tokyo, Tokyo, Japan.

3. Evaluating solutions to the faint young Sun problem

During the Archean eon, between about 3.8 billion years ago and 2.5 billion years ago, the Sun was about 20 to 25 percent fainter than it is today. With less sunlight to warm the Earth, the oceans should have been frozen over, but geological evidence suggests that this was not the case.

Some proposed solutions to this problem, known as the faint young Sun problem, include an atmospheric composition with higher concentrations of greenhouse gases, higher atmospheric pressure, increased cloud droplet size, and changes in land distribution and Earth's rotation rate.

Charnay et al. used a three-dimensional global climate model coupled to a dynamic ocean model to examine these possible solutions. They find that an atmosphere that had 100 millibars of carbon dioxide and 2 millibars of methane 3.8 billion years ago, and 10 millibars of carbon dioxide and 2 millibars of methane 2.5 billion years ago—levels corresponding to 25 to 250 times the present level of carbon dioxide and 1000 times the present level of methane—would have made it possible for Earth to have had a temperate climate with a mean surface temperature between 10 and 20 degrees Celsius (50 and 68 degrees Fahrenheit), close to the current climate. The authors suggest that these levels of greenhouse gases are consistent with geological data, making such an atmospheric composition a viable solution to the faint young Sun problem. Cloud feedbacks were also shown to prevent a full snowball Earth from developing during that time period. The authors find that some of the other potential solutions could have produced some warming during the Archean, but none individually produced enough warming to avoid widespread glaciation.

Source: Journal of Geophysical Research-Atmospheres, doi:10.1002/jgrd.50808, 2013 http://onlinelibrary.wiley.com/doi/10.1002/jgrd.50808/abstract

Title: Exploring the faint young Sun problem and the possible climates of the Archean Earth with a 3D GCM

Authors: B. Charnay, F. Forget, J. Leconte, E. Millour, F. Codron, and A. Spiga: Laboratoire de M´et´eorologie Dynamique, Universit´e P&M Curie (UPMC), Paris;

R. Wordsworth: Department of the Geophysical Sciences, University of Chicago, Chicago, Illinois.

4. Updated ice core record captures Industrial Era carbon variability

In 1999, researchers published data from ice cores collected at Law Dome, a research site in East Antarctica. These data are distinguished by their high time resolution, and by their overlap with modern measurements, providing one of the most important records of how the atmosphere's chemical composition changed over the past 1000 years. Air trapped in bubbles in the ice core let researchers measure the concentration of carbon dioxide and other gases, and analyze the ratio of carbon-13 to carbon-12 isotopes in the atmospheric carbon dioxide. Fossil fuel burning releases carbon dioxide that is depleted in carbon-13 isotopes, and the Law Dome record provided evidence that modern increases in atmospheric carbon dioxide are due to anthropogenic activity. In their new study, Rubino et al., a team that includes some of the authors from that original analysis, use novel tools and techniques to update their ice core record.

The shallowest portions of the Law Dome core contain air which overlaps in age with direct marine boundary layer samples collected since 1978 in Cape Grim, Tasmania, and with samples collected from firn at Law Dome and at the South Pole, providing a bridge from paleoclimate measurements to direct modern observations. Firn is compressed snow that forms beneath the snow surface, and the air within it contains a record of recent atmospheric composition. Previous research had shown inconsistencies between the records of the carbon dioxide isotope ratio derived from these various locations. However, using modern techniques to reanalyze ice core and firn samples that were previously collected, the authors find that the records of atmospheric carbon dioxide isotope ratio could be brought into agreement. Further, the authors' new analysis improves the sampling density of the past 150 years, capturing decadal variability of atmospheric carbon fluxes during the Industrial Period.

Source: Journal of Geophysical Research-Atmospheres, doi:10.1002/jgrd.50668, 2013 http://onlinelibrary.wiley.com/doi/10.1002/jgrd.50668/abstract

Title: A revised 1000 year atmospheric δ13C-CO2 record from Law Dome and South Pole, Antarctica

Authors: M. Rubino and D. M. Etheridge: Centre for Australian Weather and Climate Research, CSIRO Marine and Atmospheric Research, Aspendale, Australia and Centre for Ice and Climate, Niels Bohr Institute, University of Copenhagen, Denmark;

C. M. Trudinger, C. E. Allison, R. L. Langenfelds, L. P. Steele and R. J. Francey: Centre for Australian Weather and Climate Research, CSIRO Marine and Atmospheric Research, Aspendale, Australia;

M. O. Battle: Department of Physics and Astronomy, Bowdoin College, Brunswick, Maine, USA;

M. Curran: Department of the Environment and Heritage, Australian Antarctic Division, Antarctic Climate and Ecosystem Cooperative Research Centre, Private Bag 80, Hobart, Tasmania 7001, Australia;

M. Bender: Department of Geosciences, Princeton University, Princeton, New Jersey, USA;

J. W. C. White: Institute of Arctic and Alpine Research, University of Colorado, Boulder, Colorado, USA;

T. M. Jenk and T. Blunier: Centre for Ice and Climate, Niels Bohr Institute, University of Copenhagen, Denmark.

5. Mechanism could explain rapid, dramatic, cyclic Arctic warming

Multiple times in the Earth's past, the air over Greenland has warmed by 10 degrees Celsius (18 degrees Fahrenheit) or more in just a few decades before slowly resetting over centuries. First discovered in the 1990s, these so-called Dansgaard-Oeschger (D-O) events are spurred by an unknown cause. Dokken et al. propose that D-O events are driven by cyclic interactions between sea ice and ocean circulation patterns in Nordic seas.

During cold phases in the D-O cycle, Nordic seas are covered by ice, the authors suggest. Warm Atlantic subsurface water flowing into the region, trapped beneath the sea ice and a freshwater cap, warms the subsurface. Eventually, this warming breaks the halocline, a barrier to vertical mixing caused by sharp differences in salinity, allowing the warm water to rapidly melt the sea ice cap and vent the heat to the atmosphere—the warming spike of a D-O event. The high temperatures melt water from the nearby Fennoscandian ice sheet, rebuilding the freshwater cap, promoting sea ice formation, and resetting the mechanism. The authors note that their hypothesis avoids ad hoc freshwater forcings that other hypotheses rely on and is supported by observations from high-resolution marine sediment cores.

The authors suggest that a similar mechanism could be active in the Arctic right now—the observed subsurface warming of Arctic waters could drive a sudden burst of warming, rapidly pushing the Arctic into an ice-free state and heating the surrounding area.

Source: Paleoceanography, doi:10.1002/palo.20042, 2013 http://onlinelibrary.wiley.com/doi/10.1002/palo.20042/abstract

Title: Dansgaard-Oeschger cycles: interactions between ocean and sea ice intrinsic to the Nordic Seas

Authors: Trond M. Dokken: UNI Research AS, Allegaten 55, 5007 Bergen, Norway and Bjerknes Centre for Climate Research, 5007 Bergen, Norway;

Kerim H. Nisancioglu: Department of Earth Science, University of Bergen, 5007 Bergen, Norway and UNI Research AS, Allegaten 55, 5007 Bergen, Norway and Bjerknes Centre for Climate Research, 5007 Bergen, Norway;

Camille Li: Geophysical Institute, University of Bergen, 5007 Bergen, Norway and Bjerknes Centre for Climate Research, 5007 Bergen, Norway;

David S. Battisti: Department of Atmospheric Sciences, University of Washington, Seattle WA 98195, USA and Bjerknes Centre for Climate Research, 5007 Bergen, Norway;

Catherine Kissel: Laboratoire des Sciences du Climat et de l'Envionnement, CEA/CNRS/UVSQ, 91198 Gif-sur-Yvette Cedex, France.

6. Reconstructing ancient ocean temperature from deep sea shells

Measuring isotopes preserved in the shells of ancient single-celled foraminifera, tracking two sets of isotope ratios—carbon-13 to carbon-12, and oxygen-18 to oxygen-16—is one of the main tools used by paleoceanographers to reconstruct the temperature of the ancient ocean and global carbon cycling. Foraminifera build their shells from the carbon and oxygen in the seawater, and the relative uptakes of these isotopes change with temperature. Unfortunately, foraminifera shells can undergo a process called "diagenetic recrystallization"—the shells can dissolve away and recrystallize long after the creature is dead, resetting the original isotopic ratios. Diagenetic recrystallization is known to be a big problem for those working with foraminifera that live in the surface ocean, but researchers have long assumed that recrystallization is largely irrelevant if the foraminifera lived on the seafloor. Yet, little research has been done to confirm this assumption.

Using foraminifera samples that were drilled from the eastern Pacific ocean, Edgar et al. shore up the utility of deep water foraminiferal temperature reconstructions, finding that though diagenetic recrystallization can occur, it doesn't have a large effect on preserved isotope ratios there. Most recrystallization takes place within the first 10 million years after the shell was buried, they find, suggesting that deep sea foraminifera typically recrystallize under conditions similar to when they first built their shell.

Source: Paleoceanography, doi:10.1002/palo.20045, 2013 http://onlinelibrary.wiley.com/doi/10.1002/palo.20045/abstract

Title: Testing the impact of diagenesis on the δ18 O and δ13 C of benthic foraminiferal calcite from a sediment burial depth transect in the equatorial Pacific

Authors: Kirsty M. Edgar: School of Earth and Ocean Sciences, Cardiff University, Cardiff, United Kingdom;

Heiko Pälike: MARUM – Center for Marine Environmental Sciences, University of Bremen, Bremen, Germany;

Paul A. Wilson: School of Ocean and Earth Science, National Oceanography Centre, University of Southampton, Southampton, United Kingdom.

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