AGU journal highlights -- Feb. 9, 2011

1. No tipping point for Arctic Ocean ice, study says
2. Using microearthquakes to evaluate potential carbon sequestration sites
3. Observing flares from Jupiter's aurora
4. Change in atmospheric patterns behind Arctic sea ice summer 2010 low
5. Antarctic ice sheet melting would affect sea ice margin, marine food chain
6. Simulating ocean carbon storage during the Last Glacial Maximum

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://www.agu.org/pubs/search_options.shtml and inserting into the search engine the full doi (digital object identifier), e.g. 10.1029/2010GL045698. 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. Instructions for members of the news media, PIOs, and the public for downloading or ordering the full text of any research paper summarized below are available at http://www.agu.org/news/press/papers.shtml.

1. No tipping point for Arctic Ocean ice, study says

Declines in the summer sea ice extent have led to concerns within the scientific community that the Arctic Ocean may be nearing a tipping point, beyond which the sea ice cap could not recover. In such a scenario, greenhouse gases in the atmosphere trap outgoing radiation, and as the Sun beats down 24 hours a day during the Arctic summer, temperatures rise and melt what remains of the polar sea ice cap. The Arctic Ocean, now less reflective, would absorb more of the Sun's warmth, a feedback loop that would keep the ocean ice free.

However, new research by Tietsche et al. suggests that even if the Arctic Ocean sees an ice-free summer, it would not lead to catastrophic runaway ice melt. The researchers, using a general circulation model of the global ocean and the atmosphere, find that Arctic sea ice recovers within 2 years of an imposed ice-free summer to the conditions dictated by general climate conditions during that time. Furthermore, they find that this quick recovery occurs whether the ice-free summer is triggered in 2000 or in 2060, when global temperatures are predicted to be 2 degrees Celsius (3.6 degrees Fahrenheit) warmer.

During the long polar winter the lack of an insulating ice sheet allows heat absorbed by the ocean during the summer to be released into the lower atmosphere. The authors find that increased atmospheric temperatures lead to more energy loss from the top of the atmosphere as well as a decrease in heat transport into the Arctic from lower latitudes. So the absence of summer sea ice, while leading to an increase in summer surface temperatures through the ice-albedo feedback loop, is also responsible for increased winter cooling. The result is a swift recovery of the Arctic summer sea ice cover from the imposed ice-free state.

Title: Recovery mechanisms of Arctic summer sea ice

Authors: S. Tietsche, D. Notz, J. H. Jungclaus, and J. Marotzke: Max Planck Institute for Meteorology, Hamburg, Germany.

Source: Geophysical Research Letters, doi: 10.1029/2010GL045698, 2011 http://dx.doi.org/10.1029/2010GL045698

2. Using microearthquakes to evaluate potential carbon sequestration sites

With the world turning on to concerns about global climate change, strategies are being weighed to combat rising atmospheric carbon dioxide levels. One proposed solution is geologic carbon sequestration—storing liquid carbon dioxide deep underground. But for long-term underground storage of carbon dioxide, stability of the underground reservoirs is a major concern. Selecting the best storage locations requires a detailed understanding of the rock's internal structure.

Pytharouli et al. suggest using the detection of microseismic events to map, and determine the ability of fluids to move within, fractures in the rock of any potential geologic storage site. Their technique would allow the detection of these fractures as deep as 2.5 kilometers (1.5 miles). As a test bed, the authors turned to the Açu reservoir in northeastern Brazil. Seasonal filling of the reservoir changes the pressure exerted on the surrounding rock. As the water enters subterranean fractures, the change in pressure sets off a swarm of low-frequency ground movements. Using a web of eight seismometers, the authors watched the pressure wave move down individual fractures. The cascading microseisms allowed them to pinpoint the fracture's location and give an assessment of its permeability.

The authors find an array of previously unknown small fractures, each at least 100 meters (328 feet) long. They think similar detections should be made at any proposed long-term storage site, because the last thing wanted is carbon dioxide thought to be locked up for thousands of years escaping through an unknown fracture.

Title: Microseismicity illuminates open fractures in the shallow crust

Authors: Stella I. Pytharouli and Rebecca J. Lunn: Department of Civil Engineering, University of Strathclyde, Glasgow, UK;

Zoe K. Shipton: Department of Civil Engineering, University of Strathclyde, Glasgow, UK; and Department of Geographical and Earth Sciences, University of Glasgow, Glasgow, UK;

James D. Kirkpatrick: Earth and Marine Sciences Department, University of California, Santa Cruz, California, USA;

Aderson F. do Nascimento: Departamento de Geofısica, Universidade Federal do Rio Grande do Norte, Natal, Brazil.

Source: Geophysical Research Letters, doi:10.1029/2010GL045875, 2011 http://dx.doi.org/10.1029/2010GL045875

3. Observing flares from Jupiter's aurora

Jupiter's aurora often emits dramatic flares of ultraviolet light lasting several tens of seconds. Bonfond et al. capture high-time-resolution image sequences of the flares using the Space Telescope Image Spectrograph on board the Hubble Space Telescope. The authors find that these flares occur quasi-periodically, with a time scale of about 2 to 3 minutes. They also identify the magnetospheric region that corresponds to these emissions, and by analogy with similar flares on Earth, they determine that the flares are probably related to pulsed reconnections of the magnetic field at the planet's dayside magnetopause (boundary where the planet's magnetic field meets the solar wind of particles flowing from the sun).

Title: Quasi-periodic polar flares at Jupiter: A signature of pulsed dayside reconnections?

Authors: B. Bonfond: Laboratoire de Physique Atmosphérique et Planétaire, Université de Liège, Liège, Belgium; and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California, USA;

M. F. Vogt: Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California, USA; and Department of Earth and Space Sciences, University of California, Los Angeles, California, USA;

J.-C. Gérard, D. Grodent, A. Radioti, and V. Coumans: Laboratoire de Physique Atmosphérique et Planétaire, Université de Liège, Liège, Belgium.

Source: Geophysical Research Letters, doi:10.1029/2010GL045981, 2011 http://dx.doi.org/10.1029/2010GL045981

4. Change in atmospheric patterns behind Arctic sea ice summer 2010 low

Arctic sea ice extent has been declining in recent years, although ice extent varies with changes in atmospheric circulation, especially the phase of the Arctic Oscillation. The negative phase of the Arctic Oscillation has generally favored survival of sea ice, but while an extreme negative phase of the Arctic Oscillation took place in winter 2009 to 2010, the September 2010 Arctic sea ice extent was the third lowest in the satellite record.

Stroeve et al. investigate the causes of the low ice extent in 2010. The authors analyze sea-ice concentrations as well as sea-level pressure and air temperature data from 1979 through 2010 and find that atmospheric circulation during winter of 2009 to 2010 was different from previous negative Arctic Oscillation events. In typical negative Arctic Oscillation events, winds drive thick multiyear ice in the Beaufort Sea northward to areas where the ice thickens and survives the summer melt season, but during the 2009 to 2010 winter, winds drove older ice across the Beaufort Sea into the warmer southern areas of the Beaufort and Chukchi seas.

Furthermore, overall ice volume in the Arctic Sea was low at the start of the 2010 melt season. They also note that in recent years, thick multiyear ice in the Arctic has been disappearing and is being replaced by thinner first-year ice that is less likely to survive the summer melt season.

Title: Sea ice response to an extreme negative phase of the Arctic Oscillation during winter 2009/2010

Authors: Julienne C. Stroeve, Mark C. Serreze, and Walter Meier: National Snow and Ice Data Center, Cooperative Institute for Research in Environmental Sciences, University of Colorado at Boulder, Boulder, Colorado, USA;

James Maslanik, Charles Fowler: Colorado Center for Astrodynamics Research, University of Colorado at Boulder, Boulder, Colorado, USA;

Ignatius Rigor: Polar Science Center, Applied Physics Laboratory, University of Washington, Seattle, Washington, USA.

Source: Geophysical Research Letters, doi:10.1029/2010GL045662, 2011 http://dx.doi.org/10.1029/2010GL045662

5. Antarctic ice sheet melting would affect sea ice margin, marine food chain

The West Antarctic Ice Sheet (WAIS) could collapse in the future as the rising sea level and warming climate destabilize the sheet. Previous studies have shown that melting of the WAIS could contribute to several meters of global sea level rise over the next few centuries. Some studies have pointed to evidence that the WAIS has collapsed in the past during previous interglacial periods, which could give clues to what might happen in the future.

To gain additional insight into how the Earth's climate would respond to a collapse of the WAIS, Menviel et al. simulate the effects on global climate and the carbon cycle of a massive meltwater discharge from the collapse of the ice sheet. The authors find that a large meltwater discharge into the Southern Ocean would lead to a substantial cooling of the Southern Ocean, causing a northward expansion of the sea ice margin. Southern Hemisphere westerly winds would intensify. Furthermore, the formation of cold Antarctic Bottom Water would be suppressed, and subsurface warming would take place in areas where Antarctic Bottom Water is formed under present-day conditions. This subsurface warming would lead to positive feedback that would accelerate the melting of the ice sheet.

In addition, the new model suggests that WAIS melting would lead to decreased marine productivity in the Southern Ocean but would not lead to significant changes in global atmospheric carbon dioxide levels. The simulations of the WAIS collapse are consistent with paleoproxy records from the last interglacial period, about 123,000 years ago, suggesting that the WAIS may have collapsed during that period.

Title: Climate and biogeochemical response to a rapid melting of the West Antarctic Ice Sheet during interglacials and implications for future climate

Authors: L. Menviel: Climate and Environmental Physics, Oeschger Centre for Climate Change Research, University of Bern, Bern, Switzerland;

A. Timmermann and O. Elison Timm: IPRC, SOEST, University of Hawai'i, Honolulu, Hawaii, USA;

A. Mouchet: Département AGO, Université de Liège, Liège, Belgium.

Source: Paleoceanography, doi:10.1029/2009PA001892, 2010 http://dx.doi.org/10.1029/2009PA001892

6. Simulating ocean carbon storage during the Last Glacial Maximum

During the cold period of the Last Glacial Maximum, about 21,000 years ago, atmospheric carbon dioxide concentration was about 190 parts per million, compared to 280 ppm in the preindustrial era and about 385 ppm today. While less carbon was stored in the atmosphere during the Last Glacial Maximum, the oceans probably held more carbon. Atmospheric carbon isotope ratios were similar to today's values, but the oceans had a steeper surface-to-deep gradient in atmospheric carbon isotope ratios. Previous simulations had trouble simulating simultaneously atmospheric carbon dioxide levels and oceanic atmospheric carbon isotope ratios. To reconcile both data, Bouttes et al. run simulations including a new combination of three mechanisms: brine-induced ocean stratification, stratification-dependent diffusion, and iron fertilization. Including these effects makes it possible to account for the recorded glacial carbon cycle changes, reconciling the Last Glacial Maximum atmospheric carbon isotope ratios values and atmospheric carbon dioxide levels. The Last Glacial Maximum has been considered a case study for climate models used for climate change projections. The new study could help improve understanding of the mechanisms involved in the global carbon cycle and the link between carbon and climate.

Title: Last Glacial Maximum carbon dioxide and atmospheric carbon isotope ratios successfully reconciled

Authors: N. Bouttes and D. Paillard: Laboratoire des Sciences du Climat et de l'Environnement, IPSL/CEA-CNRS-UVSQ, Gif-sur-Yvette, France;

D. M. Roche and L. Bopp: Laboratoire des Sciences du Climat et de l'Environnement, IPSL/CEA-CNRS-UVSQ, Gif-sur-Yvette, France; and Section Climate Change and Landscape Dynamics, Faculty of Earth and Life Sciences, Vrije Universiteit Amsterdam, Amsterdam, Netherlands;

V. Brovkin: The Land in the Earth System, Max-Planck-Institute for Meteorology, Hamburg, Germany.

Source: Geophysical Research Letters, doi:10.1029/2010GL044499, 2011 http://dx.doi.org/10.1029/2010GL044499

Source: American Geophysical Union