New study closes in on geologic history of Earth’s deep interior

This schematic of Earth's crust and mantle shows the results of a new study that found that extreme pressures would have concentrated iron's heavier isotopes near the bottom of the mantle as it crystallized from an ocean of magma to its solid form 4.5 billion years ago. -  Louise Kellogg, modified by James Rustad & Qing-zhu Yin/UC Davis
This schematic of Earth’s crust and mantle shows the results of a new study that found that extreme pressures would have concentrated iron’s heavier isotopes near the bottom of the mantle as it crystallized from an ocean of magma to its solid form 4.5 billion years ago. – Louise Kellogg, modified by James Rustad & Qing-zhu Yin/UC Davis

By using a super-computer to virtually squeeze and heat iron-bearing minerals under conditions that would have existed when the Earth crystallized from an ocean of magma to its solid form 4.5 billion years ago, two UC Davis geochemists have produced the first picture of how different isotopes of iron were initially distributed in the solid Earth.

The discovery could usher in a wave of investigations into the evolution of Earth’s mantle, a layer of material about 1,800 miles deep that extends from just beneath the planet’s thin crust to its metallic core.

“Now that we have some idea of how these isotopes of iron were originally distributed on Earth,” said study senior author James Rustad, a Chancellor’s fellow and professor of geology, “we should be able to use the isotopes to trace the inner workings of Earth’s engine.”

A paper describing the study by Rustad and co-author Qing-zhu Yin, an associate professor of geology, was posted online by the journal Nature Geoscience on Sunday, June 14, in advance of print publication in July.

Sandwiched between Earth’s crust and core, the vast mantle accounts for about 85 percent of the planet’s volume. On a human time scale, this immense portion of our orb appears to be solid. But over millions of years, heat from the molten core and the mantle’s own radioactive decay cause it to slowly churn, like thick soup over a low flame. This circulation is the driving force behind the surface motion of tectonic plates, which builds mountains and causes earthquakes.

One source of information providing insight into the physics of this viscous mass are the four stable forms, or isotopes, of iron that can be found in rocks that have risen to Earth’s surface at mid-ocean ridges where seafloor spreading is occurring, and at hotspots like Hawaii’s volcanoes that poke up through the Earth’s crust. Geologists suspect that some of this material originates at the boundary between the mantle and the core some 1,800 miles beneath the surface.

“Geologists use isotopes to track physico-chemical processes in nature the way biologists use DNA to track the evolution of life,” Yin said.

Because the composition of iron isotopes in rocks will vary depending on the pressure and temperature conditions under which a rock was created, Yin said, in principle, geologists could use iron isotopes in rocks collected at hot spots around the world to track the mantle’s geologic history. But in order to do so, they would first need to know how the isotopes were originally distributed in Earth’s primordial magma ocean when it cooled down and hardened.

As a team, Yin and Rustad were the ideal partners to solve this riddle. Yin and his laboratory are leaders in the field of using advanced mass spectrometric analytical techniques to produce accurate measurements of the subtle variations in isotopic composition of minerals. Rustad is renowned for his expertise in using large computer clusters to run high-level quantum mechanical calculations to determine the properties of minerals.

The challenge the pair faced was to determine how the competing effects of extreme pressure and temperature deep in Earth’s interior would have affected the minerals in the lower mantle, the zone that stretches from about 400 miles beneath the planet’s crust to the core-mantle boundary. Temperatures up to 4,500 degrees Kelvin in the region reduce the isotopic differences between minerals to a miniscule level, while crushing pressures tend to alter the basic form of the iron atom itself, a phenomenon known as electronic spin transition.

Using Rustad’s powerful 144-processor computer, the two calculated the iron isotope composition of two minerals under a range of temperatures, pressures and different electronic spin states that are now known to occur in the lower mantle. The two minerals, ferroperovskite and ferropericlase, contain virtually all of the iron that occurs in this deep portion of the Earth.

These calculations were so complex that each series Rustad and Yin ran through the computer required a month to complete.

In the end, the calculations showed that extreme pressures would have concentrated iron’s heavier isotopes near the bottom of the crystallizing mantle.

It will be a eureka moment when these theoretical predictions are verified one day in geological samples that have been generated from the lower mantle, Yin said. But the logical next step for him and Rustad to take, he said, is to document the variation of iron isotopes in pure chemicals subjected to temperatures and pressures in the laboratory that are equivalent to those found at the core-mantle boundary. This can be achieved using lasers and a tool called a diamond anvil.

“Much more fun work lies ahead,” he said. “And that’s exciting.”

Sediment yields climate record for past half-million years

Researchers here have used sediment from the deep ocean bottom to reconstruct a record of ancient climate that dates back more than the last half-million years.

The record, trapped within the top 20 meters (65.6 feet) of a 400-meter (1,312-foot) sediment core drilled in 2005 in the North Atlantic Ocean by the Integrated Ocean Drilling Program, gives new information about the four glacial cycles that occurred during that period.

The new research was presented today at the Chapman Conference on Abrupt Climate Change at Ohio State University’s Byrd Polar Research Center. The meeting is jointly sponsored by the American Geophysical Union and the National Science Foundation.

Harunur Rashid, a post-doctoral fellow at the Byrd Center, explained that experts have been trying to capture a longer climate record for this part of the ocean for nearly a half-century. “We’ve now generated a climate record from this core that has a very high temporal resolution, one that is decipherable at increments of 100 to 300 years,” he said.

While climate records from ice cores can show resolutions with individual annual layers, ocean sediment cores are greatly compressed with resolutions sometimes no finer than millennia

“What we have is unprecedented among marine records.”

Dating methods such as carbon-14 are useless beyond 30,000 years or so, he said, so Rashid and his colleagues used the ratio of the isotopes oxygen-16 to oxygen-18 as a proxy for temperature in the records. The isotopes were stored in the remains of tiny sea creatures that fell to the ocean bottom over time.

When the researchers compared their record of past climate from the North Atlantic to a similar record taken from an ice core drilled from Dome C in Antarctica, they found it was remarkably similar.

“You can’t miss the similarity between the two records, one from the bottom of the North Atlantic Ocean and the other from Antarctica,” he said. “The record is virtually the same regardless of the location.”

Surprisingly, Rashid’s team was also able to score another first with their analysis of this sediment core – a record of the temperature at the sea surface in the North Atlantic.

They drew on knowledge readily known to chemists that the amount of magnesium trapped in calcite crystals can indicate the temperatures at which the crystals formed. The more magnesium present, the warmer the waters were when the tiny organisms were alive.

They applied this analysis to the remains of the benthic organisms in the cores and were able to develop a record of warming and cooling of the sea surface in the North Atlantic for the last half-million years.

Having this information will be useful as scientists try to understand how quickly the major ocean currents shifted as glacial cycles came and went, Rashid said.

The researchers were also able to gauge the extent of the ancient Laurentide Ice Sheet that covered much of North America during the last 130,000 years.

As that ice sheet calved off icebergs into the Atlantic, Rashid said that the “dirty underbelly” of those icebergs carried gravel out into the ocean. As the bergs melted, the debris fell to the bottom and of the ocean floor. The more debris present, the more icebergs had been released to carry it, meaning that the ice sheet itself had to have been larger.

“Based on this, we’ve determined that the Laurentide Ice Sheet was probably largest during the last glacial cycle than it was during any of the three previous cycles,” he said.

During the last glacial cycle, the Laurentide Ice Sheet was more than a kilometer (.6 miles) thick and extended to several miles north of Ohio State.

The Earth’s magnetic field remains a charged mystery

400 years of discussion and we’re still not sure what creates the Earth’s magnetic field, and thus the magnetosphere, despite the importance of the latter as the only buffer between us and deadly solar wind of charged particles (made up of electrons and protons). New research raises question marks about the forces behind the magnetic field and the structure of Earth itself.

The controversial new paper published in New Journal of Physics (co-owned by the Institute of Physics and the German Physical Society), ‘Secular variation of the Earth’s magnetic field: induced by the ocean flow?’, will deflect geophysicists’ attention from postulated motion of conducting fluids in the Earth’s core, the twentieth century’s answer to the mysteries of geomagnetism and magnetosphere.

Professor Gregory Ryskin from the McCormick School of Engineering and Applied Science at Northwestern University in Illinois, US, has defied the long-standing convention by applying equations from magnetohydrodynamics to our oceans’ salt water (which conducts electricity) and found that the long-term changes (the secular variation) in the Earth’s main magnetic field are possibly induced by our oceans’ circulation.

With calculations thus confirming Ryskin’s suspicions, there were also time and space correlations – specific indications of the integral relationship between the oceans and our magnetospheric buffer. For example, researchers had recorded changes in the intensity of current circulation in the North Atlantic; Ryskin shows that these appear strongly correlated with sharp changes in the rate of geomagnetic secular variation (“geomagnetic jerks”).

Tim Smith, senior publisher of the New Journal of Physics, said, “This article is controversial and will no doubt cause vigorous debate, and possibly strong opposition, from some parts of the geomagnetism community. As the author acknowledges, the results by no means constitute a proof but they do suggest the need for further research into the possibility of a direct connection between ocean flow and the secular variation of the geomagnetic field.”

In the early 1920s, Einstein highlighted the large challenge that understanding our Magnetosphere poses. It was later suggested that the Earth’s magnetic field could be a result of the flow of electrically-conducting fluid deep inside the Earth acting as a dynamo.

In the second half of the twentieth century, the dynamo theory, describing the process through which a rotating, convecting, and electrically conducting fluid acts to maintain a magnetic field, was used to explain how hot iron in the outer core of the Earth creates a magnetosphere.

The journal paper also raises questions about the structure of our Earth’s core.

Familiar text book images that illustrate a flow of hot and highly electrically-conducting fluid at the core of the Earth are based on conjecture and could now be rendered invalid. As the flow of fluids at the Earth’s core cannot be measured or observed, theories about changes in the magnetosphere have been used, inversely, to infer the existence of such flow at the core of the Earth.

While Ryskin’s research looks only at long-term changes in the Earth’s magnetic field, he points out that, “If secular variation is caused by the ocean flow, the entire concept of the dynamo operating in the Earth’s core is called into question: there exists no other evidence of hydrodynamic flow in the core.”

On a practical level, it means the next time you use a compass you might need to thank the seas and oceans for influencing the force necessary to guide the way.

Dr Raymond Shaw, professor of atmospheric physics at Michigan Technological University, said, “It should be kept in mind that the idea Professor Ryskin is proposing in his paper, if valid, has the potential to deem irrelevant the ruling paradigm of geomagnetism, so it will be no surprise to find individuals who are strongly opposed or critical.”

Greenland ice sheet larger contributor to sea-level rise

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

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.

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.”

Predicted ground motions for great earthquake in Pacific Northwest: Seattle, Victoria and Vancouver

A new study evaluates expected ground motion in Seattle, Victoria and Vancouver from earthquakes of magnitude 7.5 – 9.0, providing engineers and policymakers with a new tool to build or retrofit structures to withstand seismic waves from large “subduction” earthquakes off the continent’s west coast.

The Cascadia subduction zone in the Pacific Northwest has produced great earthquakes of magnitude 9.0 and larger, most recently in the 1700s. Now home to millions of people and a vast infrastructure of buildings and other man-made structures, scientists seek to determine the impact of large earthquakes on the region.

To simulate ground motions from a very large earthquake on the local region, this study combined detailed analysis of ground motions recorded from smaller earthquakes in the Pacific Northwest with recorded data from a severe subduction earthquake from another region – the M8.4 2003 Tokachi-Oki quake off the coast of Japan. The authors estimate ground motions for firm ground at the three sites and provide a model that engineers can adjust for local or site-specific soil conditions.

Co-author Gail Atkinson of the University of Western Ontario describes earthquakes in the Pacific Northwest as having rich energy content. “The expected ground motion may not be very large in amplitude – the peak accelerations are not that high – but the motion will go on for a very long time,” Atkinson explained. “The real hazard is that an earthquake here will affect a very large, very wide region – amplifying seismic motion and exciting vulnerable structures wherever there is an opportunity to do so.”

Surprise: Typhoons trigger slow earthquakes

This photo shows colleague Chiching Liu with the drill rig and the strainmeter on the ground front left. The small blue enclosure to the right houses the electronics. The picture was taken shortly before the installation of the strainmeter. -  Image courtesy Alan Linde
This photo shows colleague Chiching Liu with the drill rig and the strainmeter on the ground front left. The small blue enclosure to the right houses the electronics. The picture was taken shortly before the installation of the strainmeter. – Image courtesy Alan Linde

Scientists have made the surprising finding that typhoons trigger slow earthquakes, at least in eastern Taiwan. Slow earthquakes are non-violent fault slippage events that take hours or days instead of a few brutal seconds to minutes to release their potent energy. The researchers discuss their data in a study published the June 11, issue of Nature.

“From 2002 to 2007 we monitored deformation in eastern Taiwan using three highly sensitive borehole strainmeters installed 650 to 870 feet (200-270 meters) deep. These devices detect otherwise imperceptible movements and distortions of rock,” explained coauthor Selwyn Sacks of Carnegie’s Department of Terrestrial Magnetism. “We also measured atmospheric pressure changes, because they usually produce proportional changes in strain, which we can then remove.”

Taiwan has frequent typhoons in the second half of each year but is typhoon free during the first 4 months. During the five-year study period, the researchers, including lead author Chiching Liu (Academia Sinica, Taiwan), identified 20 slow earthquakes that each lasted from hours to more than a day. The scientists did not detect any slow events during the typhoon-free season. Eleven of the 20 slow earthquakes coincided with typhoons. Those 11 were also stronger and characterized by more complex waveforms than the other slow events.

“These data are unequivocal in identifying typhoons as triggers of these slow quakes. The probability that they coincide by chance is vanishingly small,” remarked coauthor Alan Linde, also of Carnegie.

How does the low pressure trigger the slow quakes? The typhoon reduces atmospheric pressure on land in this region, but does not affect conditions at the ocean bottom, because water moves into the area and equalizes pressure. The reduction in pressure above one side of an obliquely dipping fault tends to unclamp it. “This fault experiences more or less constant strain and stress buildup,” said Linde. “If it’s close to failure, the small perturbation due to the low pressure of the typhoon can push it over the failure limit; if there is no typhoon, stress will continue to accumulate until it fails without the need for a trigger.”

“It’s surprising that this area of the globe has had no great earthquakes and relatively few large earthquakes,” Linde remarked. “By comparison, the Nankai Trough in southwestern Japan, has a plate convergence rate about 4 centimeters per year, and this causes a magnitude 8 earthquake every 100 to 150 years. But the activity in southern Taiwan comes from the convergence of same two plates, and there the Philippine Sea Plate pushes against the Eurasian Plate at a rate twice that for Nankai.”

The researchers speculate that the reason devastating earthquakes are rare in eastern Taiwan is because the slow quakes act as valves, releasing the stress frequently along a small section of the fault, eliminating the situation where a long segment sustains continuous high stresses until it ruptures in a single great earthquake. The group is now expanding their instrumentation and monitoring for this research.

Cantabrian cornice has experienced 7 cooling and warming phases over past 41,000 years

In 1996, an international team of scientists led by the University of Zaragoza (UNIZAR) started to carry out a paleontological survey in the cave of El Mirón. Since then they have focused on analysing the fossil remains of the bones and teeth of small vertebrates that lived in the Cantabrian region over the past 41,000 years, at the end of the Quaternary. The richness, great diversity and good conservation status of the fossils have enabled the researchers to carry out a paleoclimatic study, which has been published recently in the Journal of Archaeological Science.

“We carried out every kind of statistical analysis over a six-month period at the University of New Mexico, analysing around 100,000 remains, of which 4,000 were specifically identified, and catalogued according to species and the number of individuals in each stratum”, Gloria Cuenca-Bescós, lead author of the study and a researcher in the Paleontology Department of the UNIZAR’s Institute for Scientific Research (IUCA), tells SINC.

The resulting study involves climatic inferences being drawn on the basis of the fossil associations of small mammals whose remains have been deposited in El Mirón over the past 41,000 years. The fossil associations of these mammals reveal the composition of fauna living around the cave at the time, and have made it possible to develop a paleoclimatological and paleoenvironmental reconstruction of the environment.

The research shows that there have been seven periods of cooling and warming in the Cantabrian cornice over the past 41,000 years. An analysis carried out by other authors on data relating to pollen, marine isotope stratigraphy, and materials deposited by glaciers backs this up this result.

The water rat was king of the Late Pleistocene

According to the study, there were four unstable cold periods, two more stable ones, and a temperate climatic period at the El Mirón cave. The scientists are unsure about dating the seventh and last period ended, as this “could correspond with the Bronze Age, the Ice Age, or the start of agricultural expansion by human beings, which certainly would have impacted on the wild animals living close to the caves.

However, the study shows that during earlier periods at the end of the Late Pleistocene, the species that predominated during cold periods were rodents and insectivores that were well-adapted to environments with only sparse vegetation. “When climatic conditions became more mild at the end of the last cold pulse of the Late Pleistocene, known as the Dryas III, forest-dwelling rodents and insectivores flourished and become more frequent in the associations”, explains Cuenca-Bescós. We now know that the water vole (Arvicola terrestris) dominated in this period.

According to the researcher, this domination by woodland species started to decline in the area only at the end of the Holocene, when human activities began to change the landscape, and when deforestation resulting from permanent settlements and agriculture can be observed “even though the climate continued to be favourable to these kinds of organisms”.

The study has also shown that the majority of the Pleistocene taxa became extinct around 10,000 years ago while “some cold-adapted species, which had managed to survive, moved to the north of Europe, leaving our warmer latitudes behind”, the scientist concludes.

Team starts first-ever Tennessee Valley earthquake survey

A new research project at the University of Tennessee, Knoxville will provide the first-ever record of seismic activity in the Tennessee Valley, providing new information not only on past quakes but insight into future activity, as well.

Led by Robert Hatcher, UT Knoxville distinguished scientist and professor of earth and planetary sciences, the research team will explore sites from just north of Knoxville, Tenn., through the Chattanooga area to just north of Rome, Ga.

The area, known as the East Tennessee Seismic Zone (ETSZ), is the second most active area for earthquake activity in the eastern U.S. The Nuclear Regulatory Commission is funding the study.

There are currently more than 20 applications for new nuclear plants in the Southeast, and Hatcher says the need for new information in determining their feasibility is behind the study.

“We have been working to get this study funded for more than 20 years,” said Hatcher. “An understanding of seismic history in the ETSZ will let us know what is possible here, and that’s vital in planning new large engineered structures.”

A lack of large historic earthquakes is the key factor in why the ETSZ receives less attention than an area such as the New Madrid seismic zone around Memphis. The team will scour the area, both on the ground and using satellite photos, for geologic features called “sandblows.”

Major earthquakes in the eastern U.S. generally occur on faults below the earth’s surface. Instead of leaving a gash in the earth, the activity can cause sand buried in stream deposits to liquefy and force its way to the surface forming sandblows. From the air, they look like mottled white spots on the ground.

Sandblows may act as a sign to pinpoint the sites of major earthquakes sometime in recent prehistory, said Hatcher. The government and private companies with plans to build major new facilities are most interested in quakes that have occurred in fairly recent times, at least in the geological scale, so team members are focusing on former streambeds that would show evidence of quakes that occurred in the past several thousand years.

“If we know that a major earthquake has occurred in an area,” said Hatcher, “then it greatly increases the likelihood that one will occur again.”

Knowing more about the possible strength and frequency of major earthquakes will allow engineers and architects to design new facilities, such as nuclear or conventional power plants, to stand up to the seismic rigors they may face over their lifetime.

For the research team, the information they’ll discover will also help fill in the blanks of how and possibly why earthquakes occur in areas like the Tennessee Valley or the New Madrid seismic zone.

Located far from the edges of tectonic plates, where the mechanics of earthquakes are better understood, quakes in these “intraplate” areas have included some of the most devastating recent events, such as the one in Gujarat, India, that killed more than 20,000.

Members of the research team include Hatcher, Stephen Obermeier of the U.S. Geological Survey, an authority on seismic features in the New Madrid seismic zone in West Tennessee and adjacent states; James Vaughn of the Missouri Geological Survey, an authority on formation of the present landscape of the Appalachians; and Hugh Mills of Tennessee Technological University, an authority on Appalachian landscape evolution.

Melting Greenland ice sheets may threaten Northeast United States, Canada

Melting of the Greenland ice sheet this century may drive more water than previously thought toward the already threatened coastlines of New York, Boston, Halifax, and other cities in the northeastern United States and in Canada, according to new research led by the National Center for Atmospheric Research (NCAR).

The study, which will be published Friday in Geophysical Research Letters, finds that if Greenland’s ice melts at moderate to high rates, ocean circulation by 2100 may shift and cause sea levels off the northeast coast of North America to rise by about 12 to 20 inches (about 30 to 50 centimeters) more than in other coastal areas. The research builds on recent reports that have found that sea level rise associated with global warming could adversely affect North America, and its findings suggest that the situation is more threatening than previously believed.

“If the Greenland melt continues to accelerate, we could see significant impacts this century on the northeast U.S. coast from the resulting sea level rise,” says NCAR scientist Aixue Hu, the lead author. “Major northeastern cities are directly in the path of the greatest rise.”

A study in Nature Geoscience in March warned that warmer water temperatures could shift ocean currents in a way that would raise sea levels off the Northeast by about 8 inches (20 cm) more than the average global sea level rise. But it did not include the additional impact of Greenland’s ice, which at moderate to high melt rates would further accelerate changes in ocean circulation and drive an additional 4 to 12 inches (about 10 to 30 cm) of water toward heavily populated areas in northeastern North America on top of average global sea level rise. More remote areas in extreme northeastern Canada and Greenland could see even higher sea level rise.

Scientists have been cautious about estimating average sea level rise this century in part because of complex processes within ice sheets. The 2007 assessment of the Intergovernmental Panel on Climate Change projected that sea levels worldwide could rise by an average of 7 to 23 inches (18 to 59 cm) this century, but many researchers believe the rise will be greater because of dynamic factors in ice sheets that appear to have accelerated the melting rate in recent years.

The new research was funded by the U.S. Department of Energy and by NCAR’s sponsor, the National Science Foundation. It was conducted by scientists at NCAR, the University of Colorado at Boulder, and Florida State University.

How much meltwater?

To assess the impact of Greenland ice melt on ocean circulation, Hu and his coauthors used the Community Climate System Model, an NCAR-based computer model that simulates global climate. They considered three scenarios: the melt rate continuing to increase by 7 percent per year, as has been the case in recent years, or the melt rate slowing down to an increase of either 1 or 3 percent per year.

If Greenland’s melt rate slows down to a 3 percent annual increase, the study team’s computer simulations indicate that the runoff from its ice sheet could alter ocean circulation in a way that would direct about a foot of water toward the northeast coast of North America by 2100. This would be on top of the average global sea level rise expected as a result of global warming. Although the study team did not try to estimate that mean global sea level rise, their simulations indicated that melt from Greenland alone under the 3 percent scenario could raise worldwide sea levels by an average of 21 inches (54 cm).

If the annual increase in the melt rate dropped to 1 percent, the runoff would not raise northeastern sea levels by more than the 8 inches (20 cm) found in the earlier study in Nature Geoscience. But if the melt rate continued at its present 7 percent increase per year through 2050 and then leveled off, the study suggests that the northeast coast could see as much as 20 inches (50 cm) of sea level rise above a global average that could be several feet. However, Hu cautioned that other modeling studies have indicated that the 7 percent scenario is unlikely.

In addition to sea level rise, Hu and his co-authors found that if the Greenland melt rate were to defy expectations and continue its 7 percent increase, this would drain enough fresh water into the North Atlantic to weaken the oceanic circulation that pumps warm water to the Arctic. Ironically, this weakening of the meridional overturning circulation would help the Arctic avoid some of the impacts of global warming and lead to at least the temporary recovery of Arctic sea ice by the end of the century.

Why the Northeast?

The northeast coast of North America is especially vulnerable to the effects of Greenland ice melt because of the way the meridional overturning circulation acts like a conveyer belt transporting water through the Atlantic Ocean. The circulation carries warm Atlantic water from the tropics to the north, where it cools and descends to create a dense layer of cold water. As a result, sea level is currently about 28 inches (71 cm) lower in the North Atlantic than the North Pacific, which lacks such a dense layer.

If the melting of the Greenland Ice Sheet were to increase by 3 percent or 7 percent yearly, the additional fresh water could partially disrupt the northward conveyor belt. This would reduce the accumulation of deep, dense water. Instead, the deep water would be slightly warmer, expanding and elevating the surface across portions of the North Atlantic.

Unlike water in a bathtub, water in the oceans does not spread out evenly. Sea level can vary by several feet from one region to another, depending on such factors as ocean circulation and the extent to which water at lower depths is compressed.

“The oceans will not rise uniformly as the world warms,” says NCAR scientist Gerald Meehl, a co-author of the paper. “Ocean dynamics will push water in certain directions, so some locations will experience sea level rise that is larger than the global average.”

Meteorite bombardment may have made Earth more habitable, says study

Large bombardments of meteorites approximately four billion years ago could have helped to make the early Earth and Mars more habitable for life by modifying their atmospheres, suggests the results of a paper published today in the journal Geochimica et Cosmochima Acta.

When a meteorite enters a planet’s atmosphere, extreme heat causes some of the minerals and organic matter on its outer crust to be released as water and carbon dioxide before it breaks up and hits the ground.

Researchers suggest the delivery of this water could have made Earth’s and Mars’ atmospheres wetter. The release of the greenhouse gas carbon dioxide could have trapped more energy from sunlight to make Earth and Mars warm enough to sustain liquid oceans.

In the new study, researchers from Imperial College London analysed the remaining mineral and organic content of fifteen fragments of ancient meteorites that had crashed around the world to see how much water vapour and carbon dioxide they would release when subjected to very high temperatures like those that they would experience upon entering the Earth’s atmosphere.

The researchers used a new technique called pyrolysis-FTIR, which uses electricity to rapidly heat the fragments at a rate of 20,000 degrees Celsius per second, and they then measured the gases released.

They found that on average, each meteorite was capable of releasing up to 12 percent of the meteorites’ mass as water vapour and 6 percent of the meteorites’ mass as carbon dioxide when entering an atmosphere. They concluded that contributions from individual meteorites were small and were unlikely to have a significant impact on the atmospheres of planets on their own.

The researchers then analysed data from an ancient meteorite shower called the Late Heavy Bombardment (LHB), which occurred 4 billion years ago, where millions of rocks crashed to Earth and Mars over a period of 20 million years.

Using published models of meteoritic impact rates during the LHB, the researchers calculated that 10 billion tonnes of carbon dioxide and 10 billion tonnes of water vapour could have been delivered to the atmospheres of Earth and Mars each year.

This suggests that the LHB could have delivered enough carbon dioxide and water vapour to turn the atmospheres of the two planets into warmer and wetter environments that were more habitable for life, say the researchers.

Professor Mark Sephton, from Imperial’s Department of Earth Science and Engineering believes the study provides important clues about Earth’s ancient past:

“For a long time, scientists have been trying to understand why Earth is so water rich compared to other planets in our solar system. The LHB may provide a clue. This may have been a pivotal moment in our early history where Earth’s gaseous envelope finally had enough of the right ingredients to nurture life on our planet.”

Lead- author of the study, Dr Richard Court from Imperial’s Department of Earth Science and Engineering, adds:

“Because of their chemistry, ancient meteorites have been suggested as a way of furnishing the early Earth with its liquid water. Now we have data that reveals just how much water and carbon dioxide was directly injected into the atmosphere by meteorites. These gases could have got to work immediately, boosting the water cycle and warming the planet.”

However, researchers say Mars’ good fortune did not last. Unlike Earth, Mars doesn’t have a magnetic field to act as a protective shield from the Sun’s solar wind. As a consequence, Mars was stripped of most of its atmosphere. A reduction in volcanic activity also cooled the planet. This caused its liquid oceans to retreat to the poles where they became ice.