A new study of Antarctica’s past climate reveals that temperatures during the warm periods between ice ages (interglacials) may have been higher than previously thought. The latest analysis of ice core records suggests that Antarctic temperatures may have been up to 6°C warmer than the present day. The findings, reported this week by scientists from the British Antarctic Survey (BAS), the Open University and University of Bristol in the journal Nature could help us understand more about rapid Antarctic climate changes.

Slice of ice core from Berkner Island, depth 120m. Trapped air bubbles (an archive of the past atmosphere) are visible in the ice.

Previous analysis of ice cores has shown that the climate consists of ice ages and warmer interglacial periods roughly every 100,000 years. This new investigation shows temperature ‘spikes’ within some of the interglacial periods over the last 340,000 years. This suggests Antarctic temperature shows a high level of sensitivity to greenhouse gases at levels similar to those found today.

Lead author Louise Sime of British Antarctic Survey said, “We didn’t expect to see such warm temperatures, and we don’t yet know in detail what caused them. But they indicate that Antarctica’s climate may have undergone rapid shifts during past periods of high CO₂.”

During the last warm period, about 125,000 years ago, sea level was around 5 metres higher than today. Ice core scientist Eric Wolff of British Antarctic Survey is a world-leading expert on past climate. He said, “If we can pin down how much warmer temperatures were in Antarctica and Greenland at this time, then we can test predictions of how melting of the large ice sheets may contribute to sea level rise.”

The Paper: Evidence for warmer interglacials in East Antarctic ice cores by Louise C. Sime, Eric W. Wolff, Kevin I. C. Oliver and Julia C. Tindall is published online this week in the journal Nature.

By the British Antarctic Survey (BAS) Press Office

As Many as 138,000 Persons Killed in Disaster.

Atlanta —Tropical Cyclone Nargis made landfall in the Asian nation of Myanmar on May 2, 2008, causing the worst natural disaster in the country’s recorded history—with a death toll that may have exceeded 138,000. In the July 2009 issue of the journal Nature Geoscience, researchers report on a field survey done three months after the disaster to document the extent of the flooding and resulting damage.

Cyclone Nargis caused significant coastal erosion and land loss, shown here at Aya near the Ayeyarwady estuary in Myanmar. At top left is the golden Buddhist stupa, which was originally built on dry land. (Photo: Hermann Fritz)

The information—which may be the first reliable measurements of cyclone damage in the area—could lead to development of computer models for predicting how future storms may impact the geologically complex Ayeyarwady River delta. Those models could be the basis for planning, construction and education that would dramatically reduce future loss of life.

Among the findings of the study: the cyclone created a storm surge as much as five meters high—topped by two-meter storm waves—that together inundated areas as much as 50 kilometers inland. Fatality rates reached 80 percent in the hardest-hit villages, and an estimated 2.5 million people in the area lived in flood-prone homes less than 10 feet above sea level.

“The recorded high water marks serve as benchmarking for numerical models for the complex hydraulic response of the giant Ayeyarwady delta,” noted Hermann M. Fritz, an associate professor in the School of Civil and Environmental Engineering at the Georgia Institute of Technology. “Ongoing numerical simulations will allow us to determine flood zones and vulnerabilities for future cyclone scenarios. Based on those, evacuation scenarios and evaluation plans will be derived in collaboration with international partners and the Myanmar government.”

Already, a local non-governmental organization in the nation has developed a cyclone education program to raise awareness among residents, said Fritz, who was the only international scientist leading a team that surveyed 150 kilometers of the country’s coastline during a two-week period August 9-23, 2008.

“The aim of our project was to document the extent of the flooding and associated damage in the delta,” Fritz explained. “Field surveys in the immediate aftermath of major disasters focus on perishable data, which would otherwise be lost forever—such as infrastructure damage prior to repair and reconstruction.”

In the flood zone, for instance, the researchers searched for evidence of water marks on buildings, scars on trees and rafted debris as indicators of the maximum water height.

“Nargis washed away entire settlements, often without leaving a single structure standing, which forced us to focus on evidence left on large trees,” added Fritz, who has studied other natural disasters in Asia, Africa and the United States. “High water marks were photographed and located using global positioning system instruments. Transects from the nearest beach or waterway to the high water marks were recorded with a laser range finder.”

The survey team documented soil erosion of as much as one meter vertically and more than 100 meters horizontally. Highlighting the loss of land was a golden Buddhist stupa—originally constructed on dry land—that was left 150 meters offshore following the storm. Cyclone Nargis also scoured several drinking water wells, leaving them in the beach surf zone—and depriving survivors of safe water supplies.

While the storm surge and waves weren’t unusually high, the impact may have been worsened by the lack of nearby high ground for evacuation and loss of coastal mangrove forests that could have slowed the storm waves, Fritz said. Structures in the area were not built to survive cyclones, and there was no evacuation plan for the area—where people had no previous experience with such storms.

Rafted debris in trees showed how high the flood waters reached during Cyclone Nargis in Myanmar. (Photo: Hermann Fritz)

Those finding point to recommendations, including implementation of a cyclone education program, development of flood and vulnerability maps, construction of cyclone-safe buildings to serve as shelters, implementation of an improved warning system, and planning for evacuation, Fritz said. Partial reconstruction of the mangroves that had been removed for agriculture and fuel could also help protect the coastline.

The expedition’s itinerary was planned based on unofficial damage reports, physical storm and cyclone track data, satellite imagery, numerical model benchmark requirements and experience gained in surveying other disasters. The group traveled to the country by cargo boat and did most surveying from the vessel.

The research was in part supported by the Pyoe Pin Programme of the Department for International Development in the United Kingdom. The program is also sponsoring detailed modeling and a follow up study being done at Georgia Tech by Fritz and Christopher Blount, one of his doctoral students.

A Category 4 storm, Nargis was the eighth deadliest cyclone recorded worldwide. It is one of seven tropical cyclones generated in the Bay of Bengal that had death tolls in excess of 100,000. With damage estimated at more than $10 billion, the storm is the most destructive ever recorded in the Indian Ocean.

Fritz hopes the work done by the survey team—which also included Swe Thwin of the Myanmar Coastal Conservation Society and Moe Kyaw and Nyein Chan of the Mingalar Myanmar NGO—will ultimately help reduce the human cost of major cyclones.

“In the 21st century with modern communication and all that has been learned about cyclones in the Bay of Bengal, there is no need for 138,000 people to be killed by a storm like this,” Fritz said. “With adequate planning, education and shelters, it should be possible to reduce fatality rates from future cyclones by at least one order of magnitude.”

Related Links

• School of Civil and Environmental Engineering
• Hermann Fritz

By John Toon- Georgia Institute of Technology – Georgia Tech

Already 40,000 years ago people fed themselves to a large degree on fish.

The isotopic analysis of a bone from one of the earliest modern humans in Asia, the 40,000 year old skeleton from Tianyuan Cave in the Zhoukoudian region of China (near Beijing), by an international team of researchers from the Max Planck Institute for Evolutionary Anthropology in Leipzig, the Graduate University of Chinese Academy of Sciences and the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, the University of British Columbia in Vancouver and Washington University in Saint Louis has shown that this individual was a regular fish consumer (PNAS, 07.07.2009).

Fig.: Lower mandible of the 40 000 year old human skeleton, found in the Tianyuan Cave near Beijing. Analyses of collagen extracted from this bone prove that this individual was a regular consumer of fish. Image: Hong Shang / Chinese Academy of Sciences, Beijing

Freshwater fish are a major part of the diet of many peoples around the world, but it has been unclear when fish became a significant part of the year-round diet for early humans. Chemical analysis of the protein collagen, using ratios of the isotopes of nitrogen and sulphur in particular, can show whether such fish consumption was an occasional treat or part of the staple diet.

The isotopic analysis of the diet of one of the earliest modern humans in Asia, the 40,000 year old skeleton from Tianyuan Cave near Beijing, has shown that at least this individual was a regular fish consumer. Michael Richards of the Max Planck Institute for Evolutionary Anthropology explains “Carbon and nitrogen isotope analysis of the human and associated faunal remains indicate a diet high in animal protein, and the high nitrogen isotope values suggest the consumption of freshwater fish.” To confirm this inference the researchers measured the sulphur isotope values of terrestrial and freshwater animals around the Zhoukoudian area and of the Tianyuan human.

Since fish appeared on the menu of modern humans before consistent evidence for effective fishing gear appeared, fishing at this level must have involved considerable effort. This shift to more fish in the diet likely reflects greater pressure from an expanding population at the time of modern human emergence across Eurasia. “This analysis provides the first direct evidence for the consumption of aquatic resources by early modern humans in China and has implications for early modern human subsistence and demography”, says Richards.

Original work:

Yaowu Hu, Hong Shang, Haowen Tong, Olaf Nehlich, Wu Liu, Chaohong Zhao, Jincheng Yu, Changsui Wang, Erik Trinkaus, Michael P. Richards
Stable Isotope Dietary Analysis of the Tianyuan 1 Early Modern Human
PNAS. July 7, 2009. Vol. 106, No. 27

Max Planck Society

The mysterious 1908 Tunguska explosion that leveled 830 square miles of Siberian forest was almost certainly caused by a comet entering Earth’s atmosphere, says new Cornell research. The conclusion is supported by an unlikely source: the exhaust plume from the NASA space shuttle launched a century later.

Noctilucent clouds observed from Donnelley Dome near Fairbanks, Alaska, resulting from a post-space shuttle plume in August 2007. M.J. Taylor and C.D. Burton / Utah State University

The research, accepted for publication (June 24) by the journal Geophysical Research Letters, published by the American Geophysical Union, connects the two events by what followed each about a day later: brilliant, night-visible clouds, or noctilucent clouds, that are made up of ice particles and only form at very high altitudes and in extremely cold temperatures.

“It’s almost like putting together a 100-year-old murder mystery,” said Michael Kelley, the James A. Friend Family Distinguished Professor of Engineering at Cornell, who led the research team. “The evidence is pretty strong that the Earth was hit by a comet in 1908.” Previous speculation had ranged from comets to meteors to black holes.

The researchers contend that the massive amount of water vapor spewed into the atmosphere by the comet’s icy nucleus was caught up in swirling eddies with tremendous energy by a process called two-dimensional turbulence, which explains why the noctilucent clouds formed a day later many thousands of miles away.

Noctilucent clouds are the Earth’s highest clouds, forming naturally in the mesosphere at about 55 miles over the polar regions during the summer months when the mesosphere is around minus 180 degrees Fahrenheit (minus 117 degrees Celsius).

The space shuttle exhaust plume, the researchers say, resembled the water vapor from the comet. A single space shuttle flight injects 300 metric tons of water vapor into the Earth’s thermosphere, and the water particles have been found to travel to the Arctic and Antarctic regions, where they form the clouds after settling into the mesosphere. The thermosphere is the layer of the atmosphere above the mesosphere.

Kelley and collaborators saw the noctilucent cloud phenomenon days after the space shuttle Endeavour (STS-118) launched on Aug. 8, 2007. Similar cloud formations had been observed following launches in 1997 and 2003.

Following the 1908 explosion, known as the Tunguska Event, the night skies shone brightly for several days across Europe, particularly Great Britain — more than 3,000 miles away. Kelley said he became intrigued by the historical eyewitness accounts of the aftermath, and concluded that the bright skies must have been the result of noctilucent clouds. The comet would have started to break up at about the same altitude as the release of the exhaust plume from the space shuttle following launch. In both cases, water vapor was injected into the atmosphere.

The scientists have attempted to answer how this water vapor traveled so far without scattering and diffusing, as conventional physics would predict.

“There is a mean transport of this material for tens of thousands of kilometers in a very short time, and there is no model that predicts that,” Kelley said. “It’s totally new and unexpected physics.”

This “new” physics, the researchers contend, is tied up in counter-rotating eddies with extreme energy. Once the water vapor got caught up in these eddies, the water traveled very quickly — close to 300 feet per second.

Scientists have long tried to study the wind structure in these upper regions of the atmosphere, which is difficult to do by such traditional means as sounding rockets, balloon launches and satellites, explained Charles Seyler, Cornell professor of electrical engineering and paper co-author.

“Our observations show that current understanding of the mesosphere-lower thermosphere region is quite poor,” Seyler said.

The paper is also co-authored by Clemson University physicist Miguel Larsen, Ph.D. ‘79, a former student of Kelley. The work performed at Cornell was funded by the Atmospheric Science Section of the National Science Foundation.

On July 1, Kelley will give a lecture, “Two-dimensional Turbulence, Space Shuttle Plume Transport in the Thermosphere, and a Possible Relation to the Great Siberian Impact Event,” at a plenary session of the annual meeting of Coupling, Energetics and Dynamics of Atmospheric Regions in Santa Fe, N.M.

The paper is available at: http://www.agu.org/journals/gl/papersinpress.shtml.

By Anne Ju – Cornell University - Cornell Chronicle Online

Scientists at ETH Zurich have developed a new method of calculating ice volume and used it to recalculate the size of the Swiss glaciers. In 1999, the total ice volume of approximately 1500 glaciers in Switzerland was estimated at 74 cubic kilometres. Since then, however, some 12 percent has already melted away.

The huge Aletsch Glacier, as seen from Eggishorn, with a view up to the Jungfraujoch and the Mid-Aletsch Glacier (Image: F. Funk-Salamí)

The earth’s glaciers are one of the “uncertain” factors in climate forecasts. It is difficult to precisely calculate the ice volume, making its effects – such as a rising sea level – unpredictable. However, melting glaciers do not only contribute to a rising sea level. They cause a reduction in freshwater supplies and change the landscape and ecosystems forever. Switzerland is also affected: Swiss glaciers have reduced dramatically in size over the past twenty years, particularly during the past ten years, which has been the warmest decade of the past 150 years.

A new method of calculation can now more accurately determine just how much ice the Swiss glaciers have lost. The process was developed by Martin Funk, professor and head of the Department for Glaciology at the Laboratory for Hydraulics, Hydrology and Glaciology (Versuchsanstalt für Wasserbau, Hydrologie und Glaziologie – VAW) and his team at ETH Zurich, in order to estimate the total ice volume of the Swiss glaciers. Knowing the exact volume is important to be able to put the observed change in the right context. Previously, however, the volume of only very few glaciers was known reliably. For these glaciers, it was possible to determine the ice thickness and the approximate volume through drillings or echo-sounding examinations. The researchers were able to calibrate their new process on these glaciers.

Estimation based on mass conservation

Up until now, glacier volumes were calculated based on empirical relationships between glacier area and volume, according to Daniel Farinotti, postgraduate student studying under Funk. The scientists developed their method of calculation based on the law of mass conservation for glaciers. This states that the surface mass budget must be balanced by the flow of ice and the changes in the thickness of the glacier’s ice. The ice volume is ultimately calculated using the glacier’s topography and the estimated area spread of the surface mass budget. The ice flow is first determined using the basic laws of glacial mechanics, including the geometry of the glacier surface, which in turn is used to calculate the thickness of the ice. Unlike previous estimation methods, the newly developed method does not just give indications on the ice volume of the glacier, but also the spread of the ice’s thickness. This also allows the topography of the glacial bed to be determined.

Large glaciers are decisive

The scientists have now applied their method to 59 Swiss glaciers, which were larger than three square kilometres in 1999, and three smaller glaciers for which thickness measurements were already available. For the approximately 1400 remaining glaciers, the scientists deduced the ice volume using an empirical volume-area approach. Their calculations showed that the ice volume amounted to 74 cubic kilometres in 1999, with a possible deviance of 9 cubic kilometres. All of the Swiss glacial ice masses would therefore easily fit into Lake Geneva – which has a water volume of 89 cubic kilometres.

The researchers further calculated an average ice thickness of seventy metres. They were also able to show that 88 percent of the ice mass is contained in the 59 largest glaciers, of which 24 percent are in the Aletsch region alone. “The area of the Great Aletsch Glacier is roughly the same as the total area of all Swiss glaciers less than one square kilometre in size. Their total ice volume, however, is twenty times smaller than the Great Aletsch Glacier. Scientists maintain that in order to provide a more accurate estimation of the regional ice volume, it is vital to carry out further thickness measurements on the largest glaciers in Switzerland, as these are obviously particularly consequential when calculating the volume.

Scientists calculated the development of the size of the glaciers over the past ten years by using an average mass timeline. “The timeline took an average from around 30 glaciers where we have data on changes in volume”, states Farinotti. This showed that the heat wave in 2003 was responsible for 2.6 of the total of 9 cubic kilometres of ice lost.

Glaciers have been shrinking since 1850

The development of the Swiss glaciers has been observed and documented since 1880. The current calculations are based on data recorded in 1999 at the time when the survey of Swiss glaciers was last updated with satellite images combined with a geographic information system and a digital contour model. The last time this was done was in 1973. Based on these data, calculations on the volume of Swiss glacial ice were between 74 and 67 cubic kilometres. “Although these figures are similar to ours, they are based on much less accurate data. The fact that a considerable amount of ice has melted since then suggests that the ice volume for 1973 was somewhat underestimated”, states Farinotti.

Since the last minor ice age, which ended around 1850, the area covered by glaciers has been shrinking. In 1999, around 1063 square kilometres remained.

References:

Farinotti D, Huss M, Bauder A, Funk M & Truffer M.: A method to estimate ice volume and ice thickness distribution of alpine glaciers. Journal of Glaciology (2009), 55, 422-430.

Farinotti D, Huss M, Bauder A & Funk M: An estimate of the glacier ice volume in the Swiss Alps. Global and Planetary Change (2009).

By Simone Ulmer – ETH Zürich

Banning or restricting the use of certain types of fishing gear could help the world’s coral reefs and their fish populations survive the onslaughts of climate change.

Credit: ARC Centre of Excellence

An international team of scientists led by Dr Josh Cinner of the ARC Centre of Excellence for Coral Reef Studies at James Cook University has proposed that bans on fishing gear – like spear guns, fish traps, beach seine nets, and gill nets – could aid in the recovery of reefs and fish populations hard hit by coral bleaching events.

Around the world corals have been dying at alarming rates, due to unusually warm water events resulting from global warming.

Research carried out in Kenya and Papua New Guinea has shown that certain types of gear are more damaging to corals, to coral-dependent fish and to the key species of fish that are needed to help reefs recover from bleaching or storm damage.

“This is creating a double jeopardy for both the corals and certain types of reef fish. They are already on the edge because of the overfishing– and the additional impact caused by a bleaching even can push them over” Dr Cinner explains. The result can be an accelerated decline of the reef, its fish populations – and their ability to sustain local people.

“From an ecological perspective, the best response to bleaching is to close reefs to fishing entirely.  But that is not feasible everywhere and is a particularly hard sell among the impoverished fishers in developing countries” says co-author Dr. Tim McClanahan of the Wildlife Conservation Society. “In areas where fishery closures are impractical, managers don’t have many options and haven’t been able to do much but watch the reef die and often not recover.”

“Selective gear restrictions offer reef managers and fishers alike some middle ground, reducing pressure on the reef and its fish while it is in the recovery phase, while also providing fishers with some options for their livelihood” Dr Cinner says.  This middle way is also more likely to be taken up by fishers.  “In other research we’ve found that fishers themselves prefer gear restrictions to total closures, because most fishers use several types of gear so they can still earn a living when the use of one sort of gear is banned. They are more likely to comply.”

The team investigated the effects of five main types of gear on different types of fish: spear guns, traps, hook and line, beach seine nets and gill nets.

They found that spear guns were the most damaging of all – to corals themselves, to susceptible fish species and to the fish needed to help reefs recover, such as parrot, surgeon and trigger fish, which keep seaweeds and urchins in check while the coral re-grows.

“Spear guns target a high proportion of species that help maintain the resilience of coral reefs, but also can result in a surprising amount of damage to the corals themselves.  When a fish is shot with a spear gun, it often hides in the reef, so some fishermen break the corals in their attempts to get it.” Dr Cinner says.

But in developing countries, spear guns can be the fishing tool most used by the poorest fishers because they are cheap to make and the yield can be high, so they are an important source of income for poor fishers.

Credit: ARC Centre of Excellence

“You can’t simply impose an arbitrary ban on their use – you need to consider issues like compensation, other fishing options, or alternative livelihoods for the affected fishers,” says co-author Dr. Shaun Wilson of the Western Australian Department of Environment and Conservation. “One key issue may be educating fishers about the importance of reef habitat and the species that help to maintain reef quality – and the need to be selective in what they shoot. This would mean fishers could still use this cheap and effective fishing tool without necessarily damaging habitat and reef resilience.”

Fish traps also targeted both the most susceptible reef fish and the ones most involved in reef recovery. Beach seine nets didn’t target as many key fish species as gill nets, traps, or spear guns, but were damaging both to corals directly and took large amounts of juvenile fish.

“Where people really depend on reef resources, it may not be possible to permanently ban all of these types of gear.   By creating temporary bans for specific types of gear following a coral bleaching event, reef managers could ease pressure on the reef and its fish population for a time when corals ecosystems are most sensitive without causing undue hardship to the human populations that depend on it.” Dr Cinner says

“Of course, where the conditions are right, managers and fishers don’t have to wait for a bleaching event- preventative gear bans are a good idea, particularly in areas that are highly susceptible to the impacts of bleaching,” says co-author Dr Nick Graham. “And our new research provides managers with some ideas about the trade-offs involved in banning certain gear.”

Dr Cinner says that temporary bans or imposing permanent restrictions on the use of various types of gear can apply to virtually any coral reef management – whether in the developing world or in developed countries such as on Australia’s Great Barrier Reef.

“In principle, it can be used anywhere. It offers both communities and reef managers much greater flexibility.  Around the world, communities are increasingly making their own decisions about how to protect their reefs and they could impose voluntary bans on certain gears.

The article Gear-based fisheries management as a potential adaptive response to climate change and coral mortality, by Cinner J et al. appears in the latest issue of the Journal of Applied Ecology.

ARC Centre of Excellence for Coral Reef Studies
James Cook University

The Greenland ice sheet is melting faster than expected according to a new study led by a University of Alaska Fairbanks researcher and published in the journal Hydrological Processes.

Photo by Sebastian Mernild. Melting water from a glacier in Greenland runs into the ocean.

Study results indicate that the ice sheet may be responsible for nearly 25 percent of global sea rise in the past 13 years. The study also shows that seas now are rising by more than 3 millimeters a year–more than 50 percent faster than the average for the 20th century.

UAF researcher Sebastian H. Mernild and colleagues from the United States, United Kingdom and Denmark discovered that from 1995 to 2007, overall precipitation on the ice sheet decreased while surface ablation–the combination of evaporation, melting and calving of the ice sheet–increased. According to Mernild’s new data, since 1995 the ice sheet lost an average of 265 cubic kilometers per year, which has contributed to about 0.7 millimeters per year in global sea level rise. These figures do not include thermal expansion–the expansion of the ice volume in response to heat–so the contribution could be up to twice that.

The Greenland ice sheet has been of considerable interest to researchers over the last few years as one of the major indicators of climate change. In late 2000/early 2001 and in 2007, major glacier calving events sent up to 44 square miles of ice into the sea at a time. Researchers are studying these major events as well as the less dramatic ongoing melting of the ice sheet through runoff and surface processes.

Ice melt from a warming Arctic has two major effects on the ocean. First, increased water contributes to global sea-level rise, which in turn affects coastlines across the globe. Second, fresh water from melting ice changes the salinity of the world’s oceans, which can affect ocean ecosystems and deep water mixing.

“Increasing sea level rise will be a problem in the future for people living in coastal regions around the globe,” said Mernild. “Even a small sea level rise can be a problem for these communities. It is our hope that this research can provide people with accurate information needed to plan for protecting people and communities.”

On the Web: http://www3.interscience.wiley.com/journal/89013836/issue

University of Alaska Fairbanks

New research shows abrupt climate change over 14,000 years ago associated with a shift in monsoon patterns and a decline vegetation growth.

Oxygen gas in ice cores has shown evidence for abrupt climate change.Credit: Zina Deretsky, National Science Foundation

When the climate warmed relatively quickly about 14,700 years ago, seasonal monsoons moved southward, dropping more rain on the Earth’s oceans at the expense of tropical areas, according to climate researchers. If the same pattern occurs in the coming decades as the Earth’s temperatures rises due to climate change, the highly-populated regions of the world that depend on monsoons could face more wildfires, water shortages and lower agricultural production.

In an article to be published in the June 12 issue of the journal Science, researchers from the Desert Research Institute in Nevada, the Scripps Institution of Oceanography and Oregon State University present their findings after comparing oxygen isotopes in air that was captured in ice cores and previously published data from ancient stalagmites found in caves. The research was supported by the National Science Foundation.

The ice cores, gathered from different locations in Antarctica and Greenland, contain air bubbles that were trapped as the ice formed over tens of thousands of years. By measuring the amount of certain oxygen isotopes in those air bubbles, the researchers were able to determine patterns in vegetation growth worldwide over that same span of time.

The researchers found that beginning about 14,700 years ago, the mixture of oxygen isotopes began to change in a way that suggests less vegetation growth, and this process continued for at least 200 years. The researchers then compared these findings with data from an earlier study that determined the amount of rainfall that fell in China over many millennia by examining stalagmites in caves. They discovered that this period of low vegetation growth corresponded with a time of reduced monsoon rainfall.

By climate standards, the researchers say, this shift happened abruptly over a few decades. They also caution that observations of past climate events may not be able predict future conditions. Given the vital roll that monsoons play in sustaining billions of people, however, this connection between climate change and monsoon patterns may be an ominous sign of what climate change in the 21^st century may bring.

The National Science Foundation

For decades, geologists have been puzzled by the mechanisms that give rise to the kind of volcanoes that form the so-called “ring of fire” around the Pacific Ocean. These arc volcanoes, which account for about 10 to 25 percent of all volcanoes, are produced when one of the plates that make up Earth’s crust plunges beneath another plate, a process called subduction.

Photo courtesy / Christy B. Till A picture of Mt. Lassen, an arc volcano in northern California

What was unclear was what factors controlled when, how and at what depth fluids and molten rock from these subducting plates are released, giving rise to the molten magma in the Earth’s mantle that would then come spewing to the surface in the form of a volcanic eruption. This process produces many of the world’s major deposits of important metals, so understanding how it works could help in locating these sources.

The mystery has now been solved, thanks to fieldwork, experiments and computer modeling carried out by Professor of Geology Timothy Grove of the Department of Earth, Atmospheric and Planetary Sciences (EAPS), graduate student Christy Till, and three colleagues. The results were published in the June 4 issue of Nature.

The new findings will force a rewriting of textbooks and encyclopedias, Grove says. The conventional understanding has been that the depth to these descending slabs under arc volcanoes is always 100 kilometers, but recent analysis shows that in fact the depth can vary considerably, from around 60 km to more than 170 km, depending on a number of factors.

Grove says the discovery of this variability in depths led his team to question why that is. One key variable turned out to be the characteristics of a particular mineral called chlorite that is found in the oceanic crust material. Chlorite contains a large amount of water, and this water is released when the chlorite breaks down at specific combinations of temperature and pressure. Chlorite breakdown occurs at particular depths in the Earth’s mantle determined by the exact angle of the slab as it plunges downward. “The stability of this mineral is the key factor in our paper,” Till says, because that’s what limits the melting process to such a narrow range of conditions. The speed at which the two plates are converging, the team found, has relatively little effect on the melting depth.

Photo courtesy / Christy B. Till A group picture atop Mt. Lassen. From left: Etienne Medard, Jay Barr, Christy Till, Mike Krawczynski and Timothy Grove. Grove, Till and Medard are co-authors of the new report.

“By knowing that process, we can independently come up with a model for the thermal structure below these volcanoes, and why arc magmas come from these certain depths,” Till says. Until this research, she says, “we were still missing that link in how arc volcanoes form.”

Understanding the process that produces arc volcanoes is important because, among other things, most of the world’s major deposits of such metals as silver, copper and molybdenum occur in these formations. Knowing exactly how they form could eventually lead to a better ability to locate such deposits, Grove says.

In addition to Grove and Till, the research was carried out by EAPS graduate student Einat Lev, Nilanjan Chatterjee, a principal research scientist in EAPS, and Etienne Medard, a former EAPS researcher who is now a professor at Clermont-Ferrand in France. The research was supported by a grant from the NSF.

By David Chandler, MIT News Office – Massachusetts Institute of Technology