Antarctica’s melting speeds up | Science News for Students

Antarctica’s melting speeds up

The continent has lost about 3 trillion metric tons of ice since 1992, raising global sea levels
Jul 18, 2018 — 6:45 am EST
an image of an iceberg floating in the ocean near the Antarctic Peninsula

An iceberg floats near the Rothera Research Station on the Antarctic Peninsula. Antarctic ice is melting faster and faster, new research finds.

A. Shepherd/University of Leeds

Antarctica’s ice is melting faster and faster. In the past five years, the frozen continent has lost ice three times faster, on average, than it did over the previous 20 years.

An international team of scientists has made the most thorough analysis yet of Antarctica’s ice sheet mass. They combined data from two dozen satellite surveys for their study. The conclusion: The frozen continent lost an estimated 2,720 billion metric tons (3 trillion tons) of ice from 1992 to 2017. Most of that loss happened in recent years, particularly in West Antarctica.

Before 2012, the continent lost an average of 76 billion metric tons (84 billion tons) of ice each year. But from 2012 to 2017, the rate increased to 219 billion metric tons (241 billion tons) per year.

Combined, all that water raised the world’s sea level by an average of 7.6 millimeters (0.3 inch). Two-fifths of that rise happened in the last five years. That increase in melting is helping scientists understand how the Antarctic ice sheet is responding to climate change. The researchers report their results in the June 14 Nature.

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a graph showing the impact of Antarctic ice melt on sea level rise from 1992 through 2017
Melting ice in Antarctica has been fueling global sea level rise for a while. But that melting has sped up in the past five years. Most of the ice loss is happening in West Antarctica.
A. Shepherd et al/Nature 2018

Measuring melt

Before this study, Antarctica’s ice loss seemed to be on the low end of international predictions, says Andrew Shepherd. He’s an earth scientist at the University of Leeds in England and one of the study’s authors. “Now it’s tracking the upper end” of those estimates, he says.

Antarctica currently holds enough frozen water to raise the world’s oceans by 58 meters (190 feet). The sea is already rising, and melting ice from Antarctica is a major reason. That rising sea level is threatening coastal communities and ecosystems around the world with flooding. A good estimate of Antarctica’s ice loss will help climate scientists predict future sea level rise, Shepherd says, as the planet keeps warming.

But in a place as big as Antarctica, it’s not easy to tell how much ice there is or how it’s changing. Satellites can collect different kinds of data to help with these estimates. They can measure the mass of the ice sheet or the depth of the ice, or the speed at which glaciers flow into the ocean. But there are two kinds of changes happening in Antarctica. There are seasonal changes, such as snowfall that makes the sheet thicker each winter. And then there are more meaningful long-term changes, such as the overall loss of ice. It can be difficult for scientists to tease these two apart.

This group of researchers made their last big estimate of Antarctica’s shrinking ice sheet in 2012. They found that it had lost 1,320 billion metric tons (about 1.5 trillion tons) of ice from 1992 to 2011. The new analysis paints an even worse picture. “In 2012, we concluded that over the 20-year period before that, Antarctica had been losing ice at a steady state,” Shepherd says. But the new findings show that the rate of ice loss has tripled since then.

The new study combines data from three different ways of measuring ice with satellites. Researchers first found the average mass change in ice using each technique by itself. Then they put those data together. This let them account for specific kinds of errors in the different measurements.

A changing continent

West Antarctica is melting most, Shepherd and colleagues found. It was losing about 53 billion metric tons (58 billion tons) of ice each year, on average, in 1992. Now the rate has risen to about 159 billion metric tons (175 billion tons) per year.

That area probably loses ice more than other parts of the continent because it’s more sensitive to small temperature changes. The ice in West Antarctica sits fully underwater. Other parts of the continent’s ice sheet, such as East Antarctica, sit higher, exposed to air. Consider a small increase in ocean temperature (say, 0.5 degree Celsius, or 0.9 degree Fahrenheit). That will warm that ice much more than a similar increase in air temperature does, Shepherd says. And when warm water starts to eat away at an ice shelf from underneath, it thins the shelf and makes it melt even faster.

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a computer generated image the change in Antarctica's ice sheet thickness
Antarctica’s ice has thinned in some areas over the past 25 years, as shown in red. It’s grown slightly thicker in other places, shown in blue. But overall, the continent has far less ice today than it did in 1992.
planetaryvisions.com

East Antarctica seems to be more stable. It may even have slightly gained mass in recent years. But those measurements are more uncertain.

As scientists have learned more about Antarctica, they’ve discovered that it can change rapidly, says Steve Rintoul. He’s an oceanographer at the science agency CSIRO based in Hobart, Australia. Scientists had thought that such a large mass of ice was relatively stable. Now it seems more vulnerable. Rintoul was not involved in the new analysis. But he helped write articles on the future of Antarctica and the Southern Ocean in the same issue of Nature.

Ice cores can give clues about Antarctica’s climate going back thousands of years. But researchers have reliable satellite data on Antarctic ice going back only a few decades. To learn more about long-term trends for Antarctica — and for melting ice in Greenland and in glaciers around the world — they’ll need more data.

Power Words

(for more about Power Words, click here)

Antarctica     A continent mostly covered in ice, which sits in the southernmost part of the world.

average     (in science) A term for the arithmetic mean, which is the sum of a group of numbers that is then divided by the size of the group.

climate     The weather conditions that typically exist in one area, in general, or over a long period.

climate change     Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.

colleague     Someone who works with another; a co-worker or team member.

continent     (in geology) The huge land masses that sit upon tectonic plates. In modern times, there are six established geologic continents: North America, South America, Eurasia, Africa, Australia and Antarctica. In 2017, scientists also made the case for yet another: Zealandia.

core     Something — usually round-shaped — in the center of an object. (in geology) Earth’s innermost layer. Or, a long, tube-like sample drilled down into ice, soil or rock. Cores allow scientists to examine layers of sediment, dissolved chemicals, rock and fossils to see how the environment at one location changed through hundreds to thousands of years or more.

data     Facts and/or statistics collected together for analysis but not necessarily organized in a way that gives them meaning. For digital information (the type stored by computers), those data typically are numbers stored in a binary code, portrayed as strings of zeros and ones.

ecosystem     A group of interacting living organisms — including microorganisms, plants and animals — and their physical environment within a particular climate. Examples include tropical reefs, rainforests, alpine meadows and polar tundra. The term can also be applied to elements that make up some an artificial environment, such as a company, classroom or the internet.

error     (In statistics) The non-deterministic (random) part of the relationship between two or more variables.

glacier     A slow-moving river of ice hundreds or thousands of meters deep. Glaciers are found in mountain valleys and also form parts of ice sheets.

Greenland     The world’s largest island, Greenland sits between the Arctic Ocean and North Atlantic. Although it is technically part of North America (sitting just east of Northern Canada), Greenland has been linked more politically to Europe. Indeed, Vikings arrived in Greenland around the 10th century, and for a time the island was a colony of Denmark. In June 2009, Greenland became an independent nation. Ice covers roughly 80 percent of Greenland. Indeed, the Greenland ice sheet is the world’s largest. If its frozen water were to melt, it could raise sea levels around the world by 6 meters (about 20 feet). Although this is the 12th biggest nation (based on surface area), Greenland averages the fewest people per square kilometer of its surface area.

ice sheet     A broad blanket of ice, often kilometers deep. Ice sheets currently cover most of Antarctica. An ice sheet also blankets most of Greenland. During the last glaciation, ice sheets also covered much of North America and Europe.

mass     A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.

satellite     A moon orbiting a planet or a vehicle or other manufactured object that orbits some celestial body in space.

sea     An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty.

sea level     The overall level of the ocean over the entire globe when all tides and other short-term changes are averaged out.

survey     (v.) To ask questions that glean data on the opinions, practices (such as dining or sleeping habits), knowledge or skills of a broad range of people. Researchers select the number and types of people questioned in hopes that the answers these individuals give will be representative of others who are their age, belong to the same ethnic group or live in the same region. (n.) The list of questions that will be offered to glean those data.

trillion     A number representing a million million — or 1,000,000,000,000 — of something.

Citation

Journal: A. Shepherd et al. Mass balance of the Antarctic Ice Sheet from 1992 to 2017. Nature. Vol. 558, June 14, 2018, p. 219. doi:10.1038/s41586-018-0179.

Journal: S.R. Rintoul et al. Choosing the future of Antarctica. Nature. Vol. 558, June 14, 2018, p. 233. doi:10.1038/s41586-018-0173-4.

Journal: S.R. Rintoul. The global influence of localized dynamics in the Southern Ocean. Nature. Vol. 558, June 14, 2018, p. 209. doi:10.1038/s41586-018-0182-3.