The following highlights summarize research papers that have been recently published in Geochemistry, Geophysics, Geosystems (G-Cubed), Geophysical Research Letters (GRL), the Journal of Geophysical Research-Atmospheres (JGR-D), the Journal of Geophysical Research-Biogeosciences (JGR-G), the Journal of Geophysical Research-Space Physics (JGR-A), Paleoceanography (PA), and Water Resources Research (WRR).
1. Analyzing subsurface oil and gas intrusions from Gulf spill
After the Deepwater Horizon blowout in 2010, hydrocarbons were released into the Gulf of Mexico. These hydrocarbons were found to have formed large subsurface horizontal intrusions. Socolofsky et al. study the dynamics of the formation of these intrusions. They looked at observations from the Deepwater Horizon blowout and adapted relationships developed from lab experiments to describe the mechanisms underlying the formation of the hydrocarbon intrusions. The authors find that the intrusions form from density stratification of multiphase plumes containing dissolved gas and oil as well as small liquid drops. They develop a method for estimating intrusion elevation and find that their estimates agree well with observations that the intrusions were primarily between 800 and 1200 meters (2,600 and 3,900 feet) in depth. The models could help researchers studying the fate of subsurface oil and gas.
Source:
Geophysical Research Letters, doi:10.1029/2011GL047174, 2011
http://dx.doi.org/10.1029/2011GL047174
Title: Formation dynamics of subsurface hydrocarbon intrusions following the Deepwater Horizon blowout
Authors: Scott A. Socolofsky: Zachry Department of Civil Engineering, Texas A&M University, College Station, Texas, USA;
E. Eric Adams: Ralph M. Parsons Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA;
Christopher R. Sherwood: U.S. Geological Survey, Woods Hole, Massachusetts, USA.
2. Cold snaps still a threat despite global warming
Long stretches of extreme cold weather can cause serious damage to agriculture as well as transportation, water, and energy infrastructure. Cold snaps have the potential to kill, with deaths attributed to cold weather often outpacing those caused by extreme heat. With climate projections anticipating at least 2 degrees Celsius (3.6 degrees Fahrenheit) increases in global average temperature by the end of the century, some regional planners may be taking solace in the idea that the threat of cold weather extremes could fade as the world warms. Research by Kodra et al., however, suggests that on a global scale the intensity and duration of extreme cold weather events will persist and in some regions will possibly even increase by the end of the 21st century.
The researchers use climate projections from nine global circulation models running a moderate emissions scenario to compare the occurrence of extreme cold weather for the period 2091 to 2100 against the events of 1991 to 2000. The authors count three main measures of cold weather: the average maximum temperature of each year's three coldest consecutive days (intensity), the number of days in a row in which the minimum temperature dropped below 0 degrees C (32 degrees F) (duration), and the total number of days in each year with a minimum temperature below freezing (frequency). The authors find that while the patterns hold globally, increases in the intensity and duration of cold weather extremes are seen most strongly for midlatitudes, predominantly affecting South America, the Middle East, and the western United States. The researchers caution that despite the anticipated increases in global average temperature throughout the next century, infrastructure intended to combat extreme cold weather must be maintained.
Source:
Geophysical Research Letters, doi:10.1029/2011GL047103, 2011
http://dx.doi.org/10.1029/2011GL047103
Title: Persisting cold extremes under 21st century warming scenarios
Authors: Evan Kodra: Computational Science and Engineering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA; and Department of Statistics, Operations, and Management Science, University of Tennessee at Knoxville, Knoxville, Tennessee, USA;
Karsten Steinhaeuser: Computational Science and Engineering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA; and Department of Computer Science and Engineering, University of Notre Dame, Notre Dame, Indiana, USA;
Auroop R. Ganguly: Computational Science and Engineering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA; and Department of Civil and Environmental Engineering, University of Tennessee at Knoxville, Knoxville, Tennessee, USA.
3. Part of Gulf of Mexico became greener after oil spill
Biological changes have been observed in the northeastern Gulf of Mexico since the April to July 2010 Deepwater Horizon oil spill, the largest offshore spill event in U.S. history. To overcome the difficulty of lack of sufficient field sampling data for a region-wide impact assessment, Hu et al. use multiyear satellite measurements of ocean phytoplankton's "glow" under sunlight to document the surface ocean's biological changes. They observe that during August 2010, several weeks after the leaking oil well was capped, an area stretching more than 11,000 square km (4,200 square mi) in the northeastern Gulf of Mexico was especially green, caused by increased phytoplankton biomass. The area was greener than it had been during any previous August since 2002. The researchers' analyses of ocean circulation and other oceanographic and environmental data indicate that the increased greenness was probably caused by the oil spill. Their findings highlight the unique value of satellite technology in observing changes in the ocean as well as the need for an integrated and sustained ocean observing system.
Source:
Geophysical Research Letters, doi:10.1029/2011GL047184, 2011
http://dx.doi.org/10.1029/2011GL047184
Title: Did the northeastern Gulf of Mexico become greener after the Deepwater Horizon oil spill?
Authors: Chuanmin Hu, Robert H. Weisberg, Yonggang Liu, Lianyuan Zheng, Kendra L. Daly, David C. English, Jun Zhao, and Gabriel A. Vargo: College of Marine Science, University of South Florida, St. Petersburg, Florida, USA.
4. Tundra sites show no carbon uptake rise from early Arctic melt
Snow has been melting earlier in the spring in the Arctic in recent years, and this earlier melting is predicted to continue as the Arctic continues to warm. Some studies have suggested that earlier snowmelt, which can lead to longer growing seasons and increased photosynthetic activity, could help Arctic ecosystems sequester more carbon dioxide (CO2), thus helping to slow global warming.
To determine whether this is indeed the case, Humphreys and Lafleur measured CO2 exchange between the tundra and the atmosphere at two sites in central Canada from 2004 to 2010. One site was a wet sedge meadow, and the other was mixed upland tundra.
They find that both sites absorbed more CO2 than they emitted to the atmosphere in all years, but there was large interannual variability. The researchers did not find a statistically significant correlation between snowmelt date and seasonal accumulated net ecosystem production, a measure of photosynthetic capacity. Factors other than snowmelt date were more influential in controlling the interannual variation in CO2 uptake by tundra. Therefore, the earlier snowmelt date probably does not result in greater CO2 uptake and reduced climate warming, the authors conclude.
Source:
Geophysical Research Letters, doi:10.1029/2011GL047339, 2011
http://dx.doi.org/10.1029/2011GL047339
Title: Does earlier snowmelt lead to greater CO2 sequestration in two low Arctic tundra ecosystems?
Authors: Elyn R. Humphreys: Department of Geography and Environmental Studies, Carleton University, Ottawa, Ontario, Canada;
Peter M. Lafleur: Department of Geography, Trent University, Peterborough, Ontario, Canada.
5. Tropical air flows, rain in flux - global warming likely factor
As Earth's climate has warmed over the past several decades, atmospheric and hydrological cycle changes are being observed globally and regionally. For instance, Zhou et al. analyze trends in the hydrological cycle in the tropics over the past 20 to 30 years using precipitation, cloud, and radiation data.
In particular, they look at Hadley and Walker cell atmospheric circulation patterns. Hadley circulation is a major circulation pattern in the tropical atmosphere in which air masses, warmed by the Sun, rise near the equator, then travel poleward, sink back toward Earth's surface in the subtropics, and return to the equator. Similarly, in the Walker circulation, air rises over the warmer parts of the oceans near the equator, travels zonally toward the colder parts, then sinks back to the surface. There are indications that both of these circulation patterns are changing with global warming and could be altering precipitation and cloud radiation distributions in the tropical regions.
The researchers find that tropical precipitation has increased in regions where air rises in Walker and Hadley circulation and has decreased in regions where air masses sink. They also find a poleward shift of subtropical dry zones and a broadening of the Hadley circulation. In general, they find that wet and dry extremes are intensifying; wetter areas are becoming wetter and dry areas have become drier. These results support other studies that have found that the strengthening tropical hydrological cycle and expanding Hadley cell trends are likely related to global warming.
Source:
Journal of Geophysical Research-Atmospheres, doi: 10.1029/2010JD015197, 2011
http://dx.doi.org/10.1029/2010JD015197
Title: Recent trends of the tropical hydrological cycle inferred from Global Precipitation Climatology Project and International Satellite Cloud Climatology Project data
Authors: Y. P. Zhou: Goddard Earth Sciences and Technology Center, University of Maryland, Baltimore County, Catonsville, Maryland, USA; Laboratory for Atmospheres, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA;
Kuan-Man Xu: Climate Science Branch, Science Directorate, NASA Langley Research Center, Hampton, Virginia, USA;
Y. C. Sud: Laboratory for Atmospheres, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA;
A. K. Betts: Atmospheric Research, Pittsford, Vermont, USA.
6. Ice sheet collapse affects ocean circulation
As Earth's climate warms and ice melts, freshwater input to oceans could weaken the large-scale Atlantic meridional overturning circulation, which acts as an important conveyor of heat and has significant effects on climate.
Green et al. use an intermediate complexity climate model to study how freshwater input to oceans can affect the meridional overturning circulation. They apply their model to the collapse of the Barents ice sheet about 140,000 years ago—the first study of this kind for the time period—which resulted in a huge influx of freshwater to the North Atlantic Ocean as large icebergs calved off the ice sheet.
The researchers find that compared to a large influx of freshwater from a river or melting glacier, the collapse of an ice sheet into icebergs has a smaller initial effect on the ocean circulation, but its effects last longer. They explain that icebergs breaking off from a collapsing ice sheet move large distances as they melt over time and thereby distribute freshwater over a larger area. The authors conclude that future studies of freshwater effects on ocean circulation should take into account whether the freshwater is in the form of meltwater or icebergs.
The study also shows that the ocean 140,000 years ago responded differently to ice sheet collapses than the more studied ice age ocean 20,000 years ago. During the more recent ice age the overturning circulation was more sensitive to ice sheet collapses than during the earlier period. The reason for the different response has yet to be explained.
Source:
Paleoceanography, doi:10.1029/2010PA002088, 2011
http://dx.doi.org/10.1029/2010PA002088
Title: Simulating the impact of freshwater inputs and deep-draft icebergs formed during a MIS 6 Barents Ice Sheet collapse
Authors: Clare L. Green: Department of Geography, University of Sheffield, Sheffield, UK;
J. A. Mattias: Green School of Ocean Sciences, College of Natural Sciences, Bangor University, Menai Bridge, UK;
Grant R. Bigg: Department of Geography, University of Sheffield, Sheffield, UK.
7. Mapping mayhem where Sun's magnetic influence wanes
When Voyager 1 passed into the heliosheath in 2004, it became the first man-made object to explore the remote edge of the Sun's magnetic influence. The heliosheath, from 1.5 to 15 billion kilometers thick (930 million to 9.3 billion miles) and starting roughly 14 billion km (8.7 billion mi) from the Sun, is where the outgoing flows of solar wind start to be pushed back by interstellar particles and magnetic fields that are heading toward the solar system. While passing through the heliosheath, Voyager 1 experienced many sudden and drastic changes in the surrounding magnetic field driven by structures called current sheets.
Using Voyager 1's ongoing measurements of the magnetic field, Burlaga and Ness identify three distinct types of current sheets. The structures, appearing as proton boundary layers (PBLs), magnetic holes or humps, or sector boundaries, were identified by characteristic fluctuations in either magnetic field strength or direction as the spacecraft crossed nearly 500 million km (310 million mi) of heliosheath in 2009. PBLs are defined by a rapid jump in magnetic field strength, with one observed event resulting in a doubling of the field strength in just half an hour. Passing through a sector boundary led to a sudden change in direction of the magnetic field. Magnetic holes saw the field strength drop to near zero before returning to the original background strength. Magnetic humps consisted of a sudden spike in strength and then a return to initial levels. The firsthand detections made by Voyager 1 are likely to be extremely important for researchers trying to decide between current leading theories for the source and structure of current sheets.
Source:
Journal of Geophysical Research-Space Physics, doi:10.1029/2010JA016309, 2011
http://dx.doi.org/10.1029/2010JA016309
Title: Current sheets in the heliosheath: Voyager 1, 2009
Authors: L. F. Burlaga: Geospace Physics Laboratory, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA;
N. F. Ness: Institute for Astrophysics and Computational Sciences, Catholic University of America, Washington, D. C., USA.
8. Study examines trading of 'virtual water' in food
While some countries have substantial supplies of freshwater, others need to import water to sustain their populations. Because food products contain significant amounts of water, global trade in food effectively moves water from one country to another in a "virtual water trade."
Konar et al. consider the global virtual water trade as a weighted complex network. The nations that participate in international food trade correspond to the nodes, and the links represent the flow of virtual water; weights are assigned to the links based on the volume of virtual water traded.
They find that the number of trade connections follows an exponential distribution. There is a global hierarchy in which nations that trade large volumes of water are more likely to link to other nations that trade large volumes of water. Several nations play a critical role in this network. For instance, the United States is the dominant exporter of virtual water, and Japan is the dominant importer.
Furthermore, trade volume follows a power law relationship with the number of trade partners of each nation—the more trading partners a country has, the more water it trades. The study could help in global water resource management; for instance, water-scarce nations could consider increasing their access to water resources by increasing the number of nations with which they trade food.
Source:
Water Resources Research, doi:10.1029/2010WR010307, 2011
http://dx.doi.org/10.1029/2010WR010307
Title: Water for food: The global virtual water trade network
Authors: M. Konar and C. Dalin: Department of Civil and Environmental Engineering, Princeton University, Princeton, New Jersey, USA;
S. Suweis: Department of Civil and Environmental Engineering, Princeton University, Princeton, New Jersey, USA; Laboratory of Ecohydrology, ECHO/IEE/ENAC, École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland;
N. Hanasaki: National Institute for Environmental Studies, Tsukuba, Japan;
A. Rinaldo: Laboratory of Ecohydrology, ECHO/IEE/ENAC, École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland; Department IMAGE and International Centre for Hydrology "Dino Tonini," Università di Padova, Padua, Italy;
I. Rodriguez-Iturbe: Department of Civil and Environmental Engineering, Princeton University, Princeton, New Jersey, USA.
9. Modeling monthlong slow-slip earthquakes
The slow crawl of the Earth's tectonic plates is periodically punctuated by the release of decades or centuries of accumulated stress. Standard earthquakes, lasting from fractions of a second to minutes in duration, are known to range from imperceptible to those that can flatten a city, but recent geodetic measurements have led to the discovery of a previously unrecognized style of fault slip – episodic slow earthquakes. These slow earthquakes recur roughly annually, involve a weeklong or monthlong speeding up of a plate's motion by a factor of 10 to 100, and are associated with low-level seismic tremors. First detected about 10 years ago in Japan and western North America, they occur along the same faults that produce devastating magnitude 9 earthquakes but at greater depths. How and why slow earthquakes differ from regular earthquakes, and the extent to which they might foreshadow the shallower truly great earthquakes, are currently topics of wide interest.
The most detailed view of how slow earthquakes unfold comes from the locations of the seismic tremors that accompany them. One of their more remarkable properties is that behind the main slow front, which propagates horizontally at about 10 km (6.2 mi) per day, there appear to be secondary fronts that travel perpendicular to this, up and down the face of the fault, up to 100 times faster. Rather than trying to model these observations with an assumed friction law, Rubin borrows some basic ideas from elasticity theory to describe the classes of laws that are required to generate this behavior. Slow slips are fickle in that the friction law governing slip must let the fault accelerate, while still preventing it from reaching the meter-per-second speeds of regular earthquakes. The author's two-dimensional fault model relies on the knowledge that the ratio between the propagation speed of the front and the fault slip speed, both of which can be estimated from observations, depends upon the frictional behavior of the propagating front. While abstract, the model produces stress drops, slip speeds, and a hierarchy of propagation speeds within the ranges observed in the Japanese and North American slow slip events.
Source:
Geochemistry, Geophysics, Geosystems, doi: 10.1029/2010GC003386, 2011
http://dx.doi.org/10.1029/2010GC003386
Title: Designer friction laws for bimodal slow slip propagation speeds
Author: Allan M. Rubin: Department of Geosciences, Princeton University, Princeton, New Jersey, USA.
10. Improving model estimates of plants' carbon use
Balancing the global carbon budget is a daunting task complicated by the fact that even the most essential values elude direct observation. Gross primary production, the amount of carbon used by terrestrial vegetation to fuel its annual growth, is often estimated by extrapolation from observations using climate models, including the Community Land Model (CLM). Terrestrial ecosystems are an extremely important sink for atmospheric carbon, so any misrepresentations of gross primary production can have strong effects on the understanding of the global carbon budget and potentially affect lawmakers' ability to plan around it. The widely used model, which is now in its fourth major version and seeks to represent the interplay between vegetation and climate, is known to overestimate global gross primary production by around 35 billion metric tonnes (38.6 billion short tons (U.S.))per year when compared to estimates derived from observations and other models, a surplus equivalent to around 6 times the annual carbon emissions of the United States.
To bring the aberrant estimates back into line, Bonan et al. use a novel data set of gross primary production derived from observations of a global network of microclimate detectors to reevaluate the model's representations of a number of key processes. The researchers updated the CLM's calculations for the temperature dependency of leaf behavior, the effects and abundance of direct versus diffuse sunlight, and the rate of photosynthesis for different plant types. The authors also adjusted how the activity of an individual leaf is interpreted on the scale of the whole canopy. On the basis of their suggested modifications, the authors are able to bring the estimates of gross primary production made by CLM to well within the range of estimates derived from the global observational network.
Source:
Journal of Geophysical Research-Biogeosciences, doi:10.1029/2010JG001593, 2011
http://dx.doi.org/10.1029/2010JG001593
Title: Improving canopy processes in the Community Land Model version 4 (CLM4) using global flux fields empirically inferred from FLUXNET data
Authors: Gordon B. Bonan, Peter J. Lawrence, Keith W. Oleson, Samuel Levis, David M. Lawrence and Sean C. Swenson: National Center for Atmospheric Research, Boulder, Colorado, USA;
Martin Jung and Markus Reichstein: Max Planck Institute for Biogeochemistry, Jena, Germany.
11. New ice core record of atmospheric methane
Atmospheric concentrations of methane, an important greenhouse gas, have varied in the past on time scales ranging from seasons to hundreds of thousands of years. Understanding past variations is important to interpreting current natural and anthropogenic changes. Mitchell et al. present a new high-precision, high-resolution atmospheric methane record covering 1000 to 1800 C.E. from an ice core from the West Antarctic Ice Sheet divide that has confirmed the existence of multidecadal-scale variability during this time period. The new record, which complements other existing ice core methane records, shows that multidecadal-scale methane variability is only weakly correlated or uncorrelated with reconstructed temperature and precipitation variations. The authors also find that time periods when war or plague resulted in population declines are coincident with global atmospheric methane decreases.
Source:
Journal of Geophysical Research – Biogeosciences , doi:10.1029/20102010JG001441, 2011
http://dx.doi.org/10.1029/2010JG001441
Title: Multidecadal variability of atmospheric methane, 1000-1800 C.E.
Authors: Logan E. Mitchell and Edward J. Brook: Department of Geosciences, Oregon State University,Corvallis, Oregon, USA;
Todd Sowers: Earth and Environmental Systems Institute, Pennsylvania State University, University Park, Pennsylvania, USA;
J. R. McConnell and Kendrick Taylor: Desert Research Institute, Nevada System of Higher Education, Reno, Nevada, USA.
12. Understanding regional Arctic temperature patterns
The oceans and atmosphere act as a giant heat mixer. However, they do not spread energy evenly across the planet—the overall effect is a net poleward transfer of energy. While it is known that this energy is predominantly moved by traveling air packets and heat exchange between different media, what is less well understood is how these mechanisms interact over small scales to produce regional patterns of temperature. Understanding these interactions is particularly pressing in the Arctic, where an amplified response to greenhouse gas–induced global warming could potentially skew the relationship between these mechanisms, thereby triggering serious changes in climate dynamics.
To sort out the primary forces dictating regional Arctic temperature anomalies, and to determine if they are changing with global warming, Serreze et al. compiled records of sea surface temperature, sea ice extent, wind speed and direction, atmospheric temperature, and snow cover stretching back to 1979. The researchers determine that wind direction is overall one of the most powerful dictators of local weather, being associated with temperature swings of up to 10 degrees C (18 degrees F) for some areas. The authors also find that if wind passes over regions affected by spikes in sea surface temperature or dips in sea ice cover, these anomalies can spread to surrounding areas. Although the researchers do not identify any particular changes in the balance of regional energy transfer mechanisms, they find that the past 30 years have seen a dramatic positive temperature anomaly for all areas and wind directions, which they attribute to background global warming.
Source:
Journal of Geophysical Research-Atmospheres, doi:10.1029/2010JD015127, 2011
http://dx.doi.org/10.1029/2010JD015127
Title: Circulation and surface controls on the lower tropospheric air temperature field of the Arctic
Authors: Mark C. Serreze, Andrew P. Barrett, and John J. Cassano: Cooperative Institute for Research in Environmental Sciences, University of Colorado at Boulder, Boulder, Colorado, USA.
13. Investigating agricultural chemical transport in a watershed
Understanding how agricultural chemicals filter through a catchment is important for managing water quality. Using a concept of the catchment as a physicochemical filter, Guan et al. examine nitrate, phosphate, and atrazine loads in the Little Vermillion River watershed in Illinois. They analyze a 10-year data set using mathematical signal processing to investigate spatial and temporal patterns in chemical concentrations and discharge rate. They find that export of these chemicals had a linear relationship with streamflow at annual scales—the higher the streamflow, the more these chemicals were exported from the watershed. The researchers' approach helps identify the roles of different hydrological flow paths in controlling chemical export at different spatial and temporal scales and reveals that chemical inputs overwhelm normal biogeochemical processing in these agricultural systems, leading to high long-term average rates of export.
Source:
Water Resources Research, doi:10.1029/2010WR009997, 2011
http://dx.doi.org/10.1029/2010WR009997
Title: Spatiotemporal scaling of hydrological and agrochemical export dynamics in a tile-drained Midwestern watershed
Authors: K. Guan: Department of Civil and Environmental Engineering, Princeton University, Princeton, New Jersey, USA;
S. E. Thompson: School of Civil and Environmental Engineering, Purdue University, West Lafayette, Indiana, USA; Now at Nicholas School of the Environment, Duke University, Durham, North Carolina, USA;
C. J. Harman: Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, USA;
N. B. Basu: Department of Civil Engineering, University of Iowa, Iowa City, Iowa, USA;
P. S. C. Rao: School of Civil and Environmental Engineering, Purdue University, West Lafayette, Indiana, USA;
M. Sivapalan: Department of Civil and Environmental Engineering and Department of Geography, University of Illinois at Urbana-Champaign, Urbana, Illinois, USA; Department of Water Management, Delft University of Technology, Delft, Netherlands;
A. I. Packman: Department of Civil Engineering, Northwestern University, Evanston, Illinois, USA;
P. K. Kalita: Department of Agriculture and Biological Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, USA.
14. Calculating specific catchment area
Specific catchment area, defined as the area of land upslope of a width of contour, divided by the contour width, is a commonly used quantity in hydrology to describe complex terrain for analyzing water flow on hill slopes; it can be a surrogate for water discharge per unit flow width. Though specific catchment area is important in hydrological, ecological, and geological studies, it can be difficult to estimate.
Gallant and Hutchinson provide a simple differential equation that describes the rate of change of specific catchment area along a flow path. The equation can be directly integrated to calculate specific catchment area at any point on a digital elevation model. The method avoids use of catchment area and width estimates, which have errors.
Although the method is more computationally intense than most methods for calculating specific catchment area, it can be used as a reference against which other methods can be tested.
Source:
Water Resources Research, doi:10.1029/2009WR008540, 2011
http://dx.doi.org/10.1029/2009WR008540
Title: A differential equation for specific catchment area
Authors: John C. Gallant: CSIRO Land and Water, GPO Box 1666, Canberra ACT 2601, Australia;
Michael F. Hutchinson: Fenner School of Environment and Society, Australian National University, Canberra ACT 0200, Australia.
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