爱上海同城419

Antarctica is losing ice at an increasingly rapid pace. In just the last five years, the frozen continent has shed ice nearly three times faster on average than it did over the previous 20 years.

An international team of scientists has combined data from two dozen satellite surveys in the most comprehensive assessment of Antarctica’s ice sheet mass yet. The conclusion: The frozen continent lost an estimated 2,720 billion metric tons of ice from 1992 to 2017, and much of that loss occurred in recent years, particularly in West Antarctica. Before 2012, the continent shed ice at a rate of 76 billion tons each year on average, but from 2012 to 2017, the rate increased to 219 billion tons annually.
Combined, all that water raised the global sea level by an average of 7.6 millimeters, the researchers report in the June 14 Nature. About two-fifths of that rise occurred in the last five years, an increase in severity that is helping scientists understand how the ice sheet is responding to climate change.

“When we place that against the [Intergovernmental Panel on Climate Change’s] sea level projections, prior to this Antarctica was tracking the low end of sea-level-rise projections,” says study coauthor Andrew Shepherd, an earth scientist at the University of Leeds in England. “Now it’s tracking the upper end.”

Antarctica currently contains enough frozen water to raise the oceans by 58 meters. Melting ice from the continent is a major driver of the sea level rise that’s threatening coastal communities and ecosystems around the world with flooding as the climate changes (SN: 12/27/14, p. 29). A good estimate of Antarctica’s ice loss will help climate scientists better predict future sea level rise, Shepherd says, as the planet continues to warm.
But in a place as big as Antarctica, it’s not easy to gauge the amount of ice and how it fluctuates. Satellites can collect different kinds of data to inform estimates, measuring the mass of the ice sheet or the depth of the ice or the speed at which glaciers flow into the ocean. But sorting out seasonal changes (such as the ice added annually from winter snowfall) from more meaningful long-term ones is hard.
This group of researchers made their last big estimate of Antarctica’s shrinking ice sheet in 2012 and found that it had lost 1,320 billion tons of ice from 1992 to 2011 (SN: 12/29/12, p. 10). The new analysis paints a more dire picture. “In 2012, we concluded that over the 20-year period before that, Antarctica had been losing ice at a steady rate,” Shepherd says. But the new findings indicate that the rate of ice loss has now tripled, compared with the average rate from 1992 to 2011.

The new study combines data from three different ways of measuring ice via satellite to pinpoint a composite number. Researchers first computed the average mass change in ice suggested by each technique. Then they integrated those data, accounting for specific kinds of errors introduced by the different measurements.

West Antarctica dominates the melting, Shepherd and colleagues found. It was losing about 53 billion tons of ice each year, on average, in 1992. Now the rate has risen to about 159 billion tons per year.

That region probably loses ice more than other parts of the continent because the ice in West Antarctica is more sensitive to small temperature fluctuations. It sits on the seabed submerged in water, while other parts of the continent’s ice sheet, such as East Antarctica, sit higher, exposed to air. Even a small increase in ocean temperature (say, 0.5 degrees Celsius) transfers a lot more heat to that ice than a comparable increase in air temperature, Shepherd says. And when warm water starts to eat away at an ice shelf from underneath, it thins the shelf and hastens melting even further.

East Antarctica seems to be more stable. It appears to have even slightly gained mass in recent years, but those measurements are more uncertain.
“I think it’s fair to say that the more work that gets done, the picture we get of Antarctica is that it’s more dynamic and more capable of rapid change than we used to think,” says Steve Rintoul, an oceanographer at CSIRO based in Hobart, Australia. Scientists had thought that such a large mass of ice was relatively resilient. Now its vulnerabilities are showing. Rintoul was not involved in the new analysis, but coauthored a perspective and a review on the future of Antarctica and the Southern Ocean in the same issue of Nature.

While ice cores can provide clues to Antarctica’s climate going back millennia, researchers have reliable satellite data on Antarctic ice going back only about 25 years. Getting a better handle on long-term trends for Antarctica — and for melting ice in Greenland and in glaciers around the world — requires more data. And that means keeping Earth-observing satellites, funded by governmental agencies such as NASA and the European Space Agency, in the air (SN Online: 2/9/18).

Narwhals are among the most elusive of whales. But for the first time, researchers have been able to eavesdrop on the creatures for days at a time as these unicorns of the sea dove, fed and socialized.

Biologist Susanna Blackwell and colleagues listened in on the clicks, buzzes and calls of the East Greenland narwhal (Monodon monoceros). The team’s findings, published June 13 in PLOS ONE, provide a peek into the daily behavior of the long-toothed whale. The research could help scientists determine how human-made noises may affect narwhals as the Arctic warms due to climate change and shipping lanes become more open.

Many whale sounds are recorded using hydrophones, underwater microphones that dangle in the water. But these acoustic devices have several drawbacks: They can’t sense the depth or direction from which noise comes, and they can’t detect which animal is making a sound.

Blackwell and colleagues skirted these issues by attaching an acoustic recording device to the narwhals themselves. “It is really like sitting on the back of a narwhal for a few days and experiencing the world,” Blackwell says.
With the help of native Greenland hunters, the researchers tagged six of the skittish creatures from 2013 to 2016. The devices were attached with suction cups, and held in place for several days by a nylon string threaded through a ridge of cartilage on the narwhals’ backs. After three to eight days in the water, magnesium links to the string degraded and released the device, which the researchers retrieved using GPS.
Tagging was stressful for the narwhals, says Blackwell, who works for Greeneridge Sciences, Inc., the Santa Barbara, Calif., company that manufactures the acoustic devices (SN Online: 12/7/17). But after a day of silence, the narwhals resumed their normal behavior.
Like other species of toothed whales, narwhals use echolocation to hunt in the dark arctic waters. “They’re like wet bats,” says Kate Stafford, an oceanographer at the University of Washington in Seattle who did not participate in the study. The researchers found that the narwhals clicked while diving to locate their prey, often arctic and polar cod or squid. When closing in on a meal, the clicking sounds turned into a rapid buzzing noise. At the surface, the narwhals used whistle and trumpetlike calls to communicate with one another.

“We were very surprised that they actually have a very specialized way of using sound,” says study coauthor Mads Peter Heide-Jørgensen, a biologist at the Greenland Institute of Natural Resources based in Copenhagen. The narwhals’ sounds varied not only with each activity the animals did, but also with their depth.
“Having some of this baseline data from an area that is relatively pristine is going to be really valuable going forward,” Stafford says.

Though the Scoresby Sound where the narwhal research took place is very remote, it won’t be for long. Human presence in the Arctic is increasing thanks to climate change, says Jens Koblitz, a bioacoustician at the University of Konstanz in Germany. Fishing boats, ships of oil prospectors and others are expected to spend more time in the Arctic as global warming reduces the extent of sea ice in the region (SN: 12/10/16, p. 15).

“This study is a great stepping-stone,” Koblitz says. If scientists discover that human sounds negatively affect the ways that the whales communicate, researchers might be able to protect some areas from human activity.