December 9, 2013
As much of the United States shivers through a cold spell, readers may be hard pressed to remember the summer heat waves that have been coming in increasing frequency. The southwestern U.S. baked during this past summer. High heat in the Midwest and East Coast in summer 2012 killed 82 people, which followed a record summer in 2011. And that came after a 2010 summer that saw high heat across the Northern Hemisphere, from Asia to Europe to North America.
These events are not random and can be blamed on the disappearance of sea ice from the Arctic Ocean and, to a lesser extent, the melting of snow cover in the Arctic, say climate scientists from the Chinese Academy of Sciences in Beijing and Rutgers University. Their study was published December 7 in Nature Climate Change.
The ice that blankets the Arctic Ocean increases in winter and shrinks in extent in the summer. Likewise, Arctic lands become covered in snow in winter, and that snow melts in warmer months. This cycle is natural, but it’s been changing in recent years. The summer ice has been shrinking more, and the winter snow has been melting more. The region is warming more quickly than the rest of the world, and it’s having a variety of consequences, from alterations to the food web to a melting of permafrost to the opening up of shipping channels.
But climate scientists are also trying to figure out if the loss of snow and ice might be having larger effects on Earth’s weather patterns. Snow and ice act like mirrors, reflecting some of the Sun’s energy back out into space. When that mirror shrinks, the darker land and ocean can suck up more heat, which not only leads to more melting and a warmer Arctic but may also alter weather far away.
Arctic sea ice reaches its smallest extent in September, and that area has declined by about 8 percent every 10 years since the 1980s. Arctic snow cover, which reaches its minimum in June, has been shrinking even faster, declining about 18 percent every decade since 1979. In the new study, the researchers linked this data, as gathered from satellite observations, with atmospheric data and found that shrinking sea ice was associated with the jet stream moving northward. Snow cover also played a role but a smaller one, even though it is disappearing faster than the sea ice.
The jet stream is a ribbon of air that flows around the Northern Hemisphere from west to east and separates cold Arctic air from warmer air masses to the south. A jet stream stuck farther in the north helps to keep unbroken the warm weather patterns to the south, “increasing the probability of extreme weather events such as heat waves and droughts,” the researchers write, particularly in the eastern half of North America, eastern Europe and eastern Asia.
This study “provides further evidence linking snow and ice loss in the Arctic with summer extreme weather in mid-latitudes,” the researchers write. “As greenhouse gases continue to accumulate in the atmosphere and all forms of Arctic ice continue to disappear, we expect to see further increases in summer heat extremes in the major population centres across much of North America and Eurasia where billions of people will be affected.”
Though a heat wave may sound like a good thing right now, as many of us look out through frost-covered windows onto snowy streets, these are expensive, deadly events that kill more people than cold, cause droughts and contribute to devastating wildfires.
But the link between changes in the Arctic and heat waves in the populous mid-latitudes isn’t certain. The study showed an association, but climate scientists have yet to figure out the mechanism that might provide the link and most remain skeptical that such a link exists. “I would have more confidence in the linkage being ‘real’ if there was a well-understood and proven mechanism to support the correlations,” James Screen, a climate researcher at the University of Exeter in England, told Climate Central. And there is evidence that Arctic melting can also be associated with extremes in winter cold.
Though climate scientists have yet to understand exactly how the changes in the Arctic may be influencing weather elsewhere in the world, there is enough evidence to convince them that they should keep investigating, climate scientist James Overland of the NOAA/Pacific Marine Environmental Laboratory in Seattle, writes in an accompanying News & Views article. “The potential for an Arctic influence remains high given the outlook for further declines in summer sea-ice and snow cover over the next few decades and Arctic amplification of global temperatures.”
December 5, 2013
The magnitude 9.0 Tohoku-Oki earthquake that struck Japan on 11 March 2011, killing more than 15,000 people and setting off a devastating tsunami that the nation is still working to recover from, brought up a lot of troubling questions. For instance,
what made such a powerful earthquake possible, and could it happen again in Japan or somewhere else?
An international group of scientists that drilled miles beneath the Pacific Ocean and into the earthquake fault now have answers to these questions, and they report their findings in a trio of papers published today in Science.
The epicenter of the 2011 quake was in an unusual spot, about 130 kilometers east of Sendai, Japan, just off the northern coast of that nation
. In this area, a subduction zone, the Pacific plate is diving beneath the Eurasian plate. Strong earthquakes are possible here, but scientists hadn’t thought that there was enough energy to produce one larger than magnitude 7.5. They were wrong, and they’ve been interested in finding out more about what made the fault capable of producing such a large quake.
A little over a year after the earthquake, the deep sea drilling vessel Chikyu was tasked with the mission to drill into the fault off the Japanese coast and install a temperature observatory. By taking the temperature of a fault after an earthquake, scientists can measure how much energy was released in the quake and calculate a fault’s friction—how easily the rocks rub against each other.
“One way to look at the friction of these big blocks is to compare them to cross-country skis on snow,” Robert Harris, a study co-author and geophysicist at Oregon State University, said in a statement. “At rest, the skis stick to the snow and it takes a certain amount of force to make them slide. Once you do, the ski’s movement generates heat and it takes much less force to continue the movement…. The same thing happens with an earthquake.”
Getting that temperature measurement was tricky. The Chikyu team had to drill 850 meters into the seafloor, which itself was 6,900 meters below the ocean’s surface. They had to deal with bad weather, and the fault itself was still shifting, putting the instruments at risk.
The difficult work paid off, though, and it revealed residual heat from the earthquake, from which the scientists could calculate the fault’s friction, which was very low. Bottom line: “The Tohoku fault is more slippery than anyone expected,” Emily Brodsky, a study co-author and geophysicist at the University of California, Santa Cruz, said in another statement.
The slippery nature of the fault helps to explain some characteristics of the 2011 quake. The fault slipped an unprecedented 50 meters and the rupture, which began deep underground, reached the surface where it caused a sudden disturbance in the ocean and set off the tsunami.
The drilling and laboratory tests also revealed another characteristic of the fault that made it so dangerous. The low friction can be attributed to incredibly fine clay sediment within the fault. “It’s the slipperiest clay you can imagine,” Christie Rowe, a study co-author and geologist at McGill University, said in a statement. “If you rub it between your fingers, it feels like a lubricant.” Incidentally, the area between the Pacific and Eurasian plates that experiences slip is also very thin, less than five meters across, which would make it the thinnest known fault zone on the planet.
Measuring the earthquake’s thermal signal was a first for science. It “was a major accomplishment,” Harris said, “but there is still a lot we don’t yet know.” For example, researchers don’t yet know how generalizable these results are to other subduction zones across the world or what effect the thinness of fault zones has on earthquake hazards. Nonetheless, the drilling results “suggest that the shallow megathrust at the Japan Trench has special traits not seen in many other subduction zones,” Kelin Wang of Natural Resources Canada and Masataka Kinoshita of the Japan Agency for Marine-Earth Science and Technology—the agency that runs the Chikyu—wrote in an accompanying Perspectives article.
Similar conditions may be rare, but they do exist in some places of the north Pacific, such as the Kamchatka Peninsula in Russia and the Aleutian Islands in Alaska, notes Rowe.Deep sea drilling shows that these regions have that same usually slippery clay that lowered the friction in the Japan fault.
But the fact that the unusual circumstances of the Japan fault may be rare shouldn’t put scientists, or the public, at ease, Wang and Kinoshita say. Such huge, shallow slip isn’t necessary for a devastating tsunami to form, and it wasn’t what caused either the 2010 Chile tsunami that destroyed 370,000 homes or the 2004 Indian Ocean tsunami that killed nearly 230,000 people. “It’s hard to say how generalizable these results are until we look at other faults,” Brodsky added. “But this lays the foundation for a better understanding of earthquakes and, ultimately, a better ability to identify earthquake hazards.”
November 25, 2013
Official estimates of U.S. emissions of the greenhouse gas methane may be far too low, according to a report published today by the Proceedings of the National Academy of Sciences. Oil and gas production is contributing much more methane than either the U.S. Environmental Protection Agency (EPA) or the best global survey of the greenhouse gas assume.
Carbon dioxide tends to get the most attention in climate change discussions because it’s the greenhouse gas most responsible for the changes we’re now seeing on Earth. But methane (CH4) has similar heat-trapping effects, and pound for pound, it traps 70 times more heat than carbon dioxide (CO2). However, methane has a shorter atmospheric lifespan, sticking around only for about ten years, compared to a century for CO2.
Like carbon dioxide, methane has been on the rise. Atmospheric concentrations of CH4 have increased from around 680 to 715 parts per billion (ppb) before the Industrial Revolution to approximately 1,800 ppb today. Determining where all that extra methane is coming from is important for efforts to reduce greenhouse gas emissions and limit future climate change effects.
The EPA currently lists livestock production as the biggest methane contributor, followed by, in order, natural gas production, landfills and coal mining. Methane measurements made from aircraft, however, are calling that order, and the EPA’s methane estimates, into question. The EPA and the Emissions Database for Global Atmospheric Research (EDGAR) both use a “bottom up” method of estimating methane, which depends on taking samples and calculating how much methane comes from known emitters, such as livestock herds and petroleum fields, then adding it all up. The aircraft studies take a “top-down” approach instead, starting with measurements of methane in atmospheric samples.
In the new study, Scot M. Miller of Harvard University and colleagues used aircraft-based sampling and a National Oceanic and Atmospheric Administration/Department of Energy air-sampling network to tally 12,694 observations of methane from across the United States in 2007 and 2008. They then used those observations and a computer model to create estimates of monthly methane emissions. The analysis found large differences between their observations and the EPA and EDGAR estimates: The new figures were 1.5 times greater than those of the EPA and 1.7 times those from EDGAR.
Nearly a quarter of the nation’s methane emissions came from just three states—Texas, Oklahoma and Kansas. The estimates for CH4 emissions from these three states were 2.7 times higher than those of EDGAR. “Texas and Oklahoma were among the top five natural gas producing states in the country in 2007,” the researchers note in their paper. The team was able to trace the methane to oil and gas production not simply through coincidences of geography but also because of
their observations found propane in the atmosphere above certain areas in these states. Propane is not produced by methane sources such as livestock or landfills–rather, it is released during fossil fuel extraction. Thus, its presence indicates that some fraction of the methane over those those regions must come from fossil fuels.
“This is the first study to quantify methane emissions at regional scales within the continental United States with enough spatial resolution to significantly criticize the official inventories,” study co-author Marc L. Fischer, of the University of California Berkeley, said in a statement. “Even if we made emissions from livestock several times higher than inventory estimates would suggest for the southwest, you still don’t get enough to cover what’s actually being observed. That’s why it looks like oil and gas are likely responsible for a large part of the remainder…Cows don’t produce propane; oil and gas does.”
Cow farts aren’t getting off the hook here, and clearly the oil and gas industry is already known to be a big contributor to climate change. But one of the selling points of natural gas has been that it is more climate-friendly–or at least less climate-damaging–than other forms of fossil fuels, such as coal. If producing that natural gas results in more methane emissions than currently assumed, then it might not be such a good choice after all.
November 20, 2013
The world’s inland waterways move more than just water; they play a pivotal role in the global carbon cycle, soaking up carbon from the land and releasing it into the atmosphere as carbon dioxide. But are rivers or lakes bigger greenhouse gas contributors? A study published today in Nature finds that, cumulatively, rivers and streams release about five times more carbon dioxide than all the world’s lakes and reservoirs, even though the latter cover far more of the Earth’s surface.
Figuring out how much carbon dioxide these water bodies contribute to the carbon cycle is a complex task. Scientists have to determine the global surface area of the world’s lakes, streams, rivers and other water bodies. Then, they have to figure out how much carbon dioxide those bodies hold, and how quickly that carbon is transferred from water to atmosphere, a factor called the gas-transfer velocity. Uncertainties and a lack of data in all three areas have hamstrung efforts to determine exactly how much carbon inland waters are releasing.
To get better estimates, a team led by biogeochemist Peter Raymond of the Yale School of Forestry and Environmental Studies had to create more detailed data sets for all three parameters. They revised a census of lakes and reservoirs, and drew on data from sources as varied as space-shuttle missions and U.S. river monitors to determine the extents of global waterways. Inland waters are generally supersaturated with carbon dioxide, but how much carbon the waters hold differed by type. Gas-transfer velocities had been determined in earlier experiments; factors such as turbulence and lake size played a role in how quickly carbon dioxide moved through the system.
The researchers calculated that all the planet’s inland waters contribute about 2.1 gigatonnes of carbon to the atmosphere each year. Rivers and streams, which cover some 241,000 square miles (624,000 square kilometers) of the Earth, release about 1.8 gigatonnes of carbon each year. Another 0.32 gigatonnes come from lakes and reservoirs, which account for 1,200,000 square miles (3,000,000 square kilometers). These estimates were about twice as high as any done previously, the researchers note. However, the results are in line with detailed studies that have been done of places like the Amazon and temperate regions. To put this all in perspective, humans are expected to contribute about 36 gigatonnes of carbon to the atmosphere in 2013.
“Understanding the relative importance of these sources is crucial to the global carbon budget,” the researchers write. “A flux of 1.8 [gigatonnes of carbon per year] for streams and rivers is large considering their small surface area, reinforcing the concept that streams and rivers are hotspots for exchange.” In addition to giving researchers a better overall picture, the study highlights locations that are the biggest contributors of carbon dioxide
released through rivers, such as Southeast Asia, the Amazon, Europe and southeast Alaska.
There are still uncertainties in these calculations, however. The researchers left out the world’s wetlands because, with their vegetation, they function in a very different manner than open bodies of water–a wetland’s canopy can alter the movement of carbon dioxide into the atmosphere. There’s also a need for even better data than is currently available. “Because tropical regions are seriously under-represented in global data sets, additional studies of carbon concentrations in the predicted hotspot areas in the tropics are urgently needed,” Bernhard Wehrli, a biogeochemist at the Swiss Federal Institute of Technology in Zurich, writes in an accompanying News & Views article.
Plus, Wehrli notes, humans have been altering waterways for hundreds of years—damming them, draining them, channeling them. Some of these constructions, such as turbine releases associated with dams, along with natural features such as waterfalls, can be places of high gas emission. Others, such as human-made channels and drained wetlands, have produced such altered systems that they act very differently from the natural systems on which models of carbon budgets are based.
These uncertainties, however, give much food for thought. Do certain agricultural practices promote the transfer of carbon to rivers, which then escapes into the atmosphere as carbon dioxide? How much does the unnatural alteration of our waterways contribute to the amount of carbon dioxide released by rivers? Answering these questions will help scientists understand the degree to which human behavior is increasing greenhouse gas emission rates, giving us a fuller picture of the causes of human-induced climate change and where efforts to reduce carbon emissions might have the greatest effect.
November 19, 2013
The North Pole is losing about 30,000 square miles of sea ice per year. Over the past century, average global temperatures have climbed by 1.5 degrees Fahrenheit. And yet, over the past few years, the sea ice that surround the South Pole has steadily been growing.
This past September, at the end of the Southern Hemisphere’s winter, the extent of Antarctica’s sea ice reached 19.51 million square kilometers, breaking a 35-year record that dated back to the start of data being collected in 1978. (In comparison, from 1981 to 2010, the average extent on the same date was just 18.5 million square miles.)
Why are the Arctic and Antarctic such polar opposites? Climate change deniers have pounced upon the unexpected divergence to argue that the planet’s temperature isn’t actually rising. But new research suggests that a different mechanism—unrelated to climate change—is responsible for the ice growth. The real answer, says University of Washington oceanographer Jinlun Zhang, can be found blowing in the wind.
Specifically, according to a study he and colleagues published in the Journal of Climate, the vortex of winds that swirl around the South Pole has both strengthened and converged, a trend that can explain about 80 percent of the growth in ice extent that has been detected in recent years.
Atmospheric scientists had previously observed that these swirling winds had gradually strengthened since the 1970s. Using a computer model, Zhang’s team found that this mechanism drives ice growth—even in the face of rising temperatures—by pushing floating layers of sea ice together, compressing them into thick ridges that are slower to melt.
“Ice ridging increases the amount of open water and areas with thin ice, which are then exposed to cold air in winter, leading to enhanced ice growth,” Zhang says. “Meanwhile, the ridges, driven together by the wind, shrink less during the summer, because thicker ice tends to survive longer.” Based on this mechanism, the model accurately predicted ice growth in the same areas—the Weddell, Bellingshausen, Amundsen and Ross seas—that it’s been most distinctly observed.
Of course, the explanation brings to mind another question: Why is this vortex of swirling winds growing more powerful in the first place? Scientists are still unsure, but a few hypotheses have been put forth.
One possible culprit is the hole in the ozone layer, caused by lingering CFCs that were emitted before their use was phased out by the Montreal Protocol. Because ozone absorbs ultraviolet light from the Sun, missing ozone affects the local balance and transfer of energy, potentially leading to stronger winds. Another possibility is that the strengthened winds can simply be chalked up to natural variability.
Whatever the cause, the observed effect—a growth in Antarctic ice—has been relatively small, especially in comparison to the rapidly melting ice in the Arctic. For now, the winds are causing ice growth, but going forward, that trend is likely to be overwhelmed by a far more potent one: the continued rise in greenhouse gas emissions and the climate change they’re rapidly driving. “If the warming continues, at some point the trend will reverse,” Zhang says.