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November 2008

Return of North Atlantic deep convection

The mixing of surface waters to depths greater than 1,000 metres in the northern North Atlantic Ocean has returned during the winter of 2007/2008, scientists report online this week in Nature Geoscience. This deep mixing, an essential part of the Atlantic Ocean circulation, has been absent for almost a decade, and is an important regulator of both carbon dioxide uptake by the oceans and heat transfer between the ocean and atmosphere. The lack of recent deep mixing in the Labrador Sea had been linked with climate warming, leading to concerns about possible future changes in ocean circulation.

Kjetil Våge, Robert Pickart and colleagues use data from a network of measuring floats to detect the deep mixing of surface waters. They also evaluate local observations, computer reconstructions of past climate and satellite data to understand the mechanisms that lead to deep convection. They find that a combination of air temperatures in the northern hemisphere, storm pathways, flow of freshwater to the Labrador Sea and the distribution of pack ice cooled the ocean surface, allowing convection to mix surface waters to greater depths.

The authors conclude that the convective system in the North Atlantic Ocean is too complex to allow straightforward predictions of future deep mixing events.

Surprising return of deep convection to the subpolar North Atlantic Ocean in winter 2007–2008

Kjetil Våge, Robert S. Pickart, Virginie Thierry, Gilles Reverdin, Craig M. Lee, Brian Petrie, Tom A. Agnew, Amy Wong and Mads H. Ribergaard

Published online: 30 November 2008 | doi 10.1038/ngeo382

Lessons from New Orleans

Densely populated coastal areas will become more vulnerable to the damaging effects of tropical cyclones, regardless of whether hurricane activity increases, suggests a Commentary online in Nature Geoscience this week. As sea levels rise, cities around the world could learn useful lessons from New Orleans following hurricane Katrina.

Torbjörn Törnqvist and Douglas Meffert suggest that preserving protective shorelines and wetlands, limiting the growth of urban centres on low-elevation coasts, concentrating the inhabitants of seaside cities in the least threatened areas and rebuilding destroyed housing in a more resilient fashion could all contribute to enhanced safety at the oceans' shores.

Sustaining coastal urban ecosystems

Torbjörn E. Törnqvist & Douglas J. Meffert

Published online: 30 November 2008 | doi 10.1038/ngeo365

Snowball Earth or open oceans?

According to the 'Snowball Earth' concept, the entire globe was frozen for extended intervals more than 600 million years ago. Not so, concludes a review article online in Nature Geoscience that examines earlier work on the topic.

Philip Allen and James Etienne evaluate earlier studies that investigated the Cryogenian period, approximately 840 to 635 millions of years ago, which was characterized by exceptionally severe periods of icehouse climate when low-altitude glaciers reached the low latitudes. The researchers conclude that, contrary to the Snowball Earth concept, the sedimentary evidence from this time points to a continuously active water cycle on Earth, which is inconsistent with the idea of oceans that were completely sealed by ice.

Determining whether substantial parts of the oceans remained ice-free has important consequences for our understanding of the survival and diversification of life and Earth's carbon cycle.

Sedimentary challenge to Snowball Earth

Philip A. Allen & James L. Etienne

Published online: 30 November 2008 | doi 10.1038/ngeo355

Wind-resistant ocean currents

The strength of Southern Ocean overturning circulation — a process that draws surface waters down to the ocean's abyss and deep waters up to the surface near Antarctica — has not been significantly affected by intensifying wind strength over recent decades, a study online in Nature Geoscience reports. A possible future intensification of the wind-driven circulation had been suggested as a mechanism that could inhibit the removal of carbon dioxide from the atmosphere into the deep ocean.

Claus Böning and colleagues analysed data from the Argo network of floating measurement instruments along with historical oceanographic data from the Southern Ocean. They found that the water in the Antarctic Circumpolar Current has become warmer and less salty over recent decades. From the resulting spatial distribution of water density, they conclude that neither the strength of the Antarctic Circumpolar Current nor of the overturning circulation were affected significantly by the recent strengthening of the westerly winds between 30 and 60° S.

The researchers conclude that the action of small-scale eddies, which are not resolved in most of the earlier ocean models, counteract any wind changes and stabilise the ocean circulation.

Response of the Antarctic Circumpolar Current to recent climate change

C. W. Böning, A. Dispert, M. Visbeck, S. Rintoul & F. U. Schwarzkopf

Published online: 23 November 2008 | doi 10.1038/ngeo362

Black carbon in soils affects terrestrial carbon dioxide release

The amount of carbon dioxide released from Australian savannah and grassland soils as temperatures rise may be lower than previously predicted, according to a study online in Nature Geoscience. Annual emissions of carbon dioxide from soil organic carbon are an order of magnitude greater than all human-made carbon dioxide emissions taken together, and are expected to increase as the Earth warms.

Johannes Lehmann and colleagues analysed soil samples stored in archives from hundreds of sites across Australia. According to their analyses, black carbon, which forms in wildfires, comprises a significant proportion of the total soil carbon. When these observation-based estimates of black carbon in soil were used in regional scale soil models, the amount of carbon dioxide predicted to be released from two Australian savannah regions under a 3°C warming scenario was 18.3% and 24.4% lower than previously calculated.

However, the team cautions that other responses to climate change, such as changing soil moisture and wildfire frequency, could affect the production and storage of black carbon in the soil.

Australian climate-carbon cycle feedback reduced by soil black carbon

Johannes Lehmann, Jan Skjemstad, Saran Sohi, John Carter, Michele Barson, Pete Falloon, Kevin Coleman, Peter Woodbury & Evelyn Krull

Published online: 16 November 2008 | doi 10.1038/ngeo358

Fast flow in Antarctic outlet glacier during drainage of subglacial lakes

The flow rate of a large outlet glacier in East Antarctica increased by about 10% in response to the flooding of two subglacial lakes, reports a paper online in Nature Geoscience. This acceleration is associated with an increase in ice loss from the continent into the ocean over the 14 months between December 2005 and February 2007.

Leigh Stearns and colleagues combined a 48-year record of ice velocities along Byrd Glacier, East Antarctica with satellite observations of ice surface elevation and found a marked increase in ice flow speed between December 2005 and February 2007. This coincides with rapid changes in ice surface elevation about 200 km upstream, which they interpret as the filling and draining of two subglacial lakes.

In an accompanying News & Views article, Helen Amanda Fricker writes that, "Their pivotal paper provides the piece in the water–iceflow puzzle that had been missing so far: direct evidence for glacier acceleration as a result of subglacial floods."

Increased flow speed on a large East Antarctic outlet glacier caused by subglacial floods

Leigh A. Stearns, Benjamin E. Smith & Gordon S. Hamilton

Published online: 16 November 2008 | doi 10.1038/ngeo356

News and Views: Glaciology: Water slide

Helen Amanda Fricker

Published online: 16 November 2008 | doi 10.1038/ngeo367

Himalaya rising to a monsoonal tune

The onset of the most intense phase of uplift in the Himalaya may have been triggered by a simultaneous strengthening of the Asian monsoons, suggests a study published online in Nature Geoscience.

Deformation and erosion of the Himalaya began around 50 million years ago, causing the mountain range to rise, but these processes accelerated much later. Peter Clift and colleagues integrated various types of data on the onset of intensification of both the Himalayan uplift and the Asian monsoons. Their results suggest that both these processes occurred at more or less at the same time, around 23 million years ago. Their results indicate that the increased erosion caused by the stronger monsoons may have contributed to the uplift of the Himalaya.

Correlation of Himalayan exhumation rates and Asian monsoon intensity

Peter D. Clift, Kip V. Hodges, David Heslop, Robyn Hannigan, Hoang Van Long & Gerome Calves

Published online: 09 November 2008 | doi 10.1038/ngeo351


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