The big melt: Earth’s ice sheets are under attack | Science News for Students

The big melt: Earth’s ice sheets are under attack

Massive zones are losing ice six times as fast as during the 1980s
Jan 31, 2019 — 6:45 am EST
a photo of Crane Glacier, Antarctic Peninsula from the air

Crane Glacier lost the protective floating ice shelf at its lower edge in 2002. Since then, the glacier has more than tripled its original speed. As the glacier sped up, it also splayed out. This caused its ice to thin in some places by 100 meters (325 feet).

Erin Pettit/Oregon State University

This is the sixth in a 10-part series about the ongoing global impacts of climate change. These stories will look at the current effects of a changing planet, what the emerging science suggests is behind those changes and what we all can do to adapt to them.

December 10, 2012 was an unpleasant day for the people camping on Pine Island Glacier. This slow-moving river of ice sits on a remote coastline of West Antarctica. The ice is up to 3.2 kilometers (two miles) thick. Only a few rocky hills poke above it. Or they look like hills. In fact, they are the tops of huge mountains buried up to their noses in ice.

Despite the ice, this December day was unusually warm. A steady rain pattered onto the wet snow. That made life hard for the scientists.

They were hauling their camp from one place to another. Their snowmobiles swerved back and forth in the sloppy slush. The engines strained and whined, giving off a smell of burnt rubber. And the sleds of gear that being towed behind constantly bogged down, forcing people to hop off and push.

“It was really creepy,” says Dale Pomraning. “Really unexpected.” He is an ice driller from the University of Alaska in Fairbanks.

The rain might seem like a harbinger of global warming. And it is. But Pomraning and the others knew that something even more serious was already underway. Large amounts of ice are melting off this glacier. It’s happening in a hidden place, where humans can’t easily see it.

These 14 scientists had come here to drill several holes, 500 meters (1,600 feet) down through the ice, into the ocean water below. They would lower scientific instruments down through the hole, to study the weak underbelly of Antarctica.

an aerial photo of a field camp on the Pine Island Glacier
A field camp on the upper reaches of Pine Island Glacier in West Antarctic. In December 2012, scientists used this camp as a jumping-off point to study the glacier’s rapidly-melting ice shelf.
August Allen/NSF

All told, Antarctica has lost three trillion tons of ice since 1992. The rate of ice loss has tripled since 1992. And since 1979, the rate of ice loss has actually increased six-fold — from roughly 40 billion tons per year in the 1980s, to around 250 billion tons per year now.

But things could get much worse.

Scientists fear that by 2100, the rate of ice loss could increase to more than 10 times what it is today. This rapid melt, spread over all of the world’s ice, could raise sea levels an extra 0.6 to 1.8 meters (2 to 6 feet). And if things play out this way, it will be partly because of the oceans.

Deep, warm ocean currents are already melting the edges of Antarctica’s ice from below. The scientists who visited Pine Island Glacier in 2012 were trying to understand how quickly it’s happening — and if it can still be stopped.

Mighty snowflakes

Our planet has three major ice sheets. One, in the Arctic, covers Greenland. Two others, in the south, cover the east and west parts of Antarctica. Collectively, they cover an area almost as large as the United States and Australia combined.

Greenland and Antarctic ice sheets
These side-by-side maps show where the majority of the world’s ice sheets reside. The big white portion in left global view is Greenland. Antarctica is the white area at bottom of right global view.
NASA

These massive slabs of ice formed from tiny snowflakes that fell over many thousands of years. As this fluffy snow piled up on land, it squished under its own weight, creating solid ice.

The ice is more than 4,000 meters (13,000 feet) thick in some places. Because it is under so much pressure, it bends and oozes like silly putty. So even as snow continues to sprinkle down on Antarctica and Greenland, forming new ice all the time, the old ice beneath it is always spreading outward toward the ocean.

Most of this ice moves only a few meters (yards) per year. But near the coastlines, it speeds up. Huge rivers of ice, called glaciers, flow hundreds — or even thousands — of meters per year. They follow deep canyons in the bedrock beneath them.

As Antarctic glaciers reach the coast, their ice begins to float on the ocean. Most of Antarctica’s coastline is ringed with these floating “ice shelves.” They are hundreds of meters thick. And they do an important job. They hold back the glaciers behind them, slowing their flow into the ocean.

a diagram showing how snow falls, forms ice sheets and then moves towards the oceans in Greenland and Antarctica
The ice sheets of Antarctica and Greenland are formed by snow that falls, gathers and then slowly compresses into ice. Like silly putty, that ice oozes toward the oceans, where it breaks off (a process known as calving) to form icebergs.
Hugo Ahlenius/UNEP/GRID-Arendal; adapted by L. Steenblik Hwang

But scientists are worried. Some ice shelves are showing signs of weakness. Deep, warm ocean currents are melting them from below.

Today, Pine Island Glacier’s ice shelf is, on average, about 400 meters (1,300 feet) thick. That’s about equal to the height of New York City’s 102-story Empire State Building. But since 1992, the ice shelf has thinned by 70 meters (roughly 230 feet). And as it has weakened, the glacier has sped up — by 70 percent, just since 1980.

Upside-down canyons

Antarctic ice sheet movement
Antarctica is covered by a sheet of thick slowly oozing ice. Near the coastline, this ice merges into faster-moving rivers known as glaciers. They flow into the ocean. In this map (made using satellite radar data), faster-moving ice appears dark blue, purple and red.
Eric Rignot/NSIDC/University of Colorado, Boulder

Once Pomraning and the others set up their new camp, they shoveled 3,500 kilograms (7,700 pounds) of snow into a metal tank. They melted it, then ran it through a machine known as a hot-water drill. As its name implies, it uses hot water to melt a narrow hole down through the ice. Once the hole was in place, the researchers lowered scientific instruments through to the ocean below.

Ice was melting off the bottom of the shelf a rate of 6 centimeters (2.4 inches) per day, they found. As warm water flows along the underside of the ice shelf, it melts deep channels into the ice. These upside-down “canyons” cut as far as 180 meters (600 feet) up into the ice shelf. The ice over these canyons no longer supports its own weight. So it sags. The entire ice shelf, 60 kilometers (37 miles) wide and 90 kilometers (56 miles) long, is constantly bending, buckling and breaking as the grooves melt ever deeper into its underside.

Pomraning and the others heard pops and cracks as they slept on the ice shelf at night. They would wake the next morning to find some new crack, a centimeter (four-tenths of an inch) wide — and seemingly bottomless — running between their tents. Some of the cracks on Pine Island Ice Shelf can eventually grow wide enough to swallow a house.

The warm ocean currents assaulting Antarctica are one of the least-recognized aspects of climate change.

Unstoppable heat

As the planet warms, 95 percent of that extra heat is going into a place where we don’t directly feel it. It is seeping into the deep oceans.

Since 1920, the world’s oceans have absorbed 450 zettajoules (ZET-uh-jewls) of heat. That’s enough to melt 1.5 million cubic kilometers (360,000 cubic miles) of ice. That’s enough meltwater to raise global sea levels by 4.3 meters (14 feet). And more heat enters the oceans every day, notes Douglas Martinson. He’s a polar oceanographer at the Lamont-Doherty Earth Observatory in Palisades, N.Y.

Antarctic glacial bed
Much of Antarctica’s ice sits on a rocky bed that dips below sea level, exposing it to warm, deep ocean currents. This map was built using ice-penetrating-radar data and other remote-sensing techniques. Areas below sea level are shown in blue.
P. Fretwell et al/The Cryosphere, 2013. (CC BY 3.0)

The amount of extra heat in the world’s oceans actually doubled between 1997 and 2015. Ocean currents are only now starting to deliver this heat to Antarctica.

Even if we stop producing carbon dioxide tomorrow, we will “still have all this heat in the ocean,” says Martinson. “It’s hard to imagine how we’re going to shut this off.”

People like Robert DeConto are trying to figure out how the world’s ice sheets will respond to this heat. He is a glaciologist at the University of Massachusetts in Amherst. And he has created computer models. These simulations show how Antarctica’s ice will likely change as the world gradually warms.

The world has so far warmed about 1 degree Celsius (1.8 degrees Fahrenheit) since 1850. As warming increases to 1.5 and then 2 degrees C (2.7 and then 3.6 degrees F) over the coming decades, Antarctica will gradually lose more ice. At first it would not be dramatic, says DeConto. But his simulations show that when the warming hits 3 degrees C (5.4 degrees F), “all hell breaks loose.”

His computer models show that at this point, West Antarctica’s ice shelves will be under attack from two directions at once. They will continue to melt from the ocean below. And they will also start to feel serious effects from the warming air above.

Shattering ice

Warm summers will melt snow on top the ice shelves, forming vast blue ponds. “A lot of liquid water on the ice shelves is not a good idea,” observes DeConto.

The water from those lakes drains into deep cracks in the ice, called crevasses (Kreh-VOSS-sez). The weight of the water drives the cracks ever deeper — until they break through bottom of the ice. If this happens with many crevasses at once, an entire ice shelf can shatter in a period of days. Three thousand square kilometers (1,160 square miles) of ice shelf can disintegrate into a swarm of icebergs that drift away into the ocean.

This has already happened in a warmer part of the continent, called the Antarctic Peninsula. This finger of land stretches up toward the bottom tip of South America. And since 1988, four of its ice shelves have disintegrated. They have fallen like dominoes, from north to south, as warm summers reach farther down the peninsula.

an aerial photo of Leppard Glacier, Antarctica
Shown here is Leppard Glacier, on the Antarctic Peninsula. The fast-moving glaciers around edges of Antarctica are gateways that allow ice, from the vast interior of the continent, to pour out into the ocean.
Erin Pettit/Oregon State University

When the Antarctic Peninsula’s ice shelves collapsed, the glaciers that they had been holding back rampaged out of control. Those glaciers were then flowing two to nine times their original speed. This dumped ice into the ocean faster than ever.

The Antarctic Peninsula does not hold enough ice to raise sea levels more than a few centimeters. “But it showed us what could happen” farther south in West Antarctica, says Ted Scambos. He’s a glaciologist at the National Snow and Ice Data Center in Boulder, Colo.

Greenland’s fastest glacier

Scambos believes that if large glaciers in West Antarctica suddenly lose their entire ice shelf, it could be even worse. Right now, these glaciers end in cliffs of ice that rise about 30 meters (100 feet) above the water. That’s about as high as a seven-story building. But if the floating ice shelf breaks off the front of the glacier, it exposes a taller cliff of ice. And if the cliff gets too tall, the ice will no longer be strong enough to support its own weight. When that happens, Scambos warns, “the ice fails almost instantly.”  

Scientists learned this from watching a glacier in Greenland. Called Jakobshavn, it lost the floating ice off its front between 1998 and 2002. The ice cliff at the head of the glacier now towers 100 meters (about 325 feet) above the water — as high as a 20-story building. Every few days, it collapses with a deafening roar. Icebergs as big as apartment buildings pile up in the water in front of it. They briefly clog up an ocean bay, called a fjord in front of the glacier. That fjord is 13 kilometers (8 miles) across. Within a few days, however, those blocks drift away. This gives the ice cliff space to collapse again.

Jakobshavn is now losing 13 kilometers of ice off its front face every year. That’s faster than any other glacier in the world. But if glaciers in West Antarctica succumb to summer melting, and lose their floating ice shelves, it could be even worse.

West Antarctica has several dozen large coastal glaciers. And researchers have become especially worried about one. Called Thwaites Glacier, it sits 100 kilometers (roughly 60 miles) west of Pine Island.

An aerial image of the Thwaites glacier from October 2012, showing where it meets seawater.
This aerial image of the Thwaites glacier from October 2012 shows where it meets seawater.
James Yungel/NASA IceBridge

Deep and dangerous

The front of Thwaites, where it touches the ocean, sits in water 600 meters (2,000 feet) deep. But as you go farther inland, the bed of the glacier slants downward. This deepening groove connects all the way to the center of West Antarctica, 500 kilometers (310 miles) inland. There, the ice is up to 3,400 meters (11,000 feet) thick. And it sits in a bowl that plunges as far as 2,500 meters (8,200 feet) below sea level.

a representation of thinning ice on Greenland and Antarctica
Satellite altimetry (height measurements) shows that ice is rapidly thinning in some parts of Greenland (left) and Antarctica (right). In these maps, areas of thinning are shown in yellow, orange and red. The red spot on the left-hand side of Antarctica marks the location of Pine Island, Thwaites and several other major glaciers
Hamish Pritchard/British Antarctic Survey

This geography sets up a bad situation. As the ice cliffs of Thwaites begin to crumble some years from now, each collapse will expose an even taller cliff behind it. And these progressively higher cliffs could collapse more and more quickly.

As the glacier front retreats into deeper water, it will also receive more and more heat from deep ocean currents. And here’s another concern: Thwaites Glacier is 120 kilometers (75 miles) across — eight times wider than Jakobshavn. And unlike Jakobshavn, it isn’t walled in by mountains on either side. So the ice blocks that tumble down might not pile up and protect the new cliff face, even for a short while.

Scambos imagines what a full-blown collapse of Thwaites Glacier might look like, 50 or 100 years from now. ”You’d see this ripple effect of crumpling glacier,” he says. The wave of collapse would expand a few kilometers further into Antarctica each year, as ice tumbled in from all sides. At this point, he says, ”you’re really ripping out the icy heart of the continent.”

Some regions of West Antarctica are thought to be stable right now. They aren’t exposed to as much deep warm water as the Amundsen Coast, where Pine Island and Thwaites sit. But the collapse of Thwaites could rip open the back doors on those glaciers, too.

As the wave of collapse eats into their rear ends, those supposedly stable glaciers could actually change direction, and flow backward. They could spill their ice into the widening hole at the center of the continent. Scambos says it’s “just like digging out the center of a big sand pile.”

Changing predictions

Scientists used to think that Antarctica and Greenland would resist the effects of global warming, at least for the next 100 years or so. They believed that warmer oceans would cause more water to evaporate. This would make more clouds — and drop more snow onto the ice sheets. Many scientists predicted that more snowfall would offset the effects of faster melting. A major 2001 report by the Intergovernmental Panel on Climate Change (IPCC) still voiced this opinion. But since then, things have changed quickly.

Antarctica’s rate of ice loss has increased by six-fold since 1979. And between just 2000 and 2004, Greenland’s rate of ice loss also nearly tripled.

Antarctica is now losing some 250 billion tons of ice per year. In places like Canada, Alaska and the high mountains of Asia and South America, the world’s smaller glaciers are collectively losing 215 billion tons per year. And Greenland is losing about 280 billion tons of ice per year.

But among these masses of ice, Antarctica is the sleeping giant. It holds 85 percent of the world’s ice. That’s seven times as much as Greenland and all of the world’s other glaciers combined. And compared to Greenland, much more of Antarctica’s ice sits in deep basins, vulnerable to ocean heat. So as the world continues to warm, the growing heat will attack Antarctica from both the air and the sea. And within a few years, Antarctica will become the leading contributor to sea level rise.

New worries

West Antarctica holds enough ice to raise sea level by 3 meters (10 feet). The continent’s other half, East Antarctica holds four times as much ice as the west. Scientists had thought that East Antarctica’s ice sat on high ground, protecting it from warm ocean currents.

But scientists have since realized that this is not true. They have spent a decade using a method, called ice-penetrating radar, to map the mountains and valleys under East Antarctica’s ice. Those studies have shown that vast swaths of this ice sheet sit in ocean basins that dip 1,000 to 2,500 meters (3,300 to 8,200 feet) below sea level. That’s just as deep as West Antarctica.

Just two of these glaciers in East Antarctica — called Totten and Denman — hold enough low-lying ice to raise sea levels by 5.5 meters (18 feet). In other words, these two glaciers by themselves could raise sea levels by more than all of West Antarctica combined.

If these glaciers become unstable, “then you’re really in trouble,” says Helen Fricker. She’s a glaciologist at the Scripps Institution of Oceanography in San Diego, Calif. “These are the places that keep me up at night.”

On September 15, 2018, NASA launched a satellite into orbit that will help Fricker monitor the situation. The craft is called the IceSat-2. (That’s short for Ice, Cloud and land Elevation Satellite 2.) It will orbit 15 times per day. As it passes overhead, it will send a laser beam down to the surface to measure the of glaciers and ice shelves below. This laser altimeter can show Fricker how quickly ice shelves like Totten and Thwaites are thinning. (At the end of January, Fricker and other researchers were still testing IceSat-2, to make sure that it’s taking accurate measurements. Within a few months, the satellite should begin providing an updated view of the ice sheets.)

And starting last November, the United States and the United Kingdom began a four-year research project at Thwaites Glacier. They are putting teams of scientists on the glacier and its ice shelf to better understand how it is responding to the slow, feverish rise in Earth’s surface temperatures.

Already melting

Old satellite measurements have shown that already in the mid-2000s, the ice shelves in front of Thwaites and other nearby glaciers were thinning, each year, by up to another 6 meters (20 feet). One ice shelf in front of nearby Smith Glacier actually thinned by 490 meters (1,600 feet) between 2002 and 2009. The thinning of all of these ice shelves was due to ocean melting on their undersides.

But people studying Thwaites want to know how soon the other line of attack will begin: the heavy summer melts that form ponds on top the ice shelves. Those ponds will set the stage for the shelves to shatter into thousands of pieces and drift away. That’s what happened on the Antarctic Peninsula.

The scientists who visited Pine Island in 2013 could already see the earliest signs of this.

When they arrived on the ice shelf, their first job was to dig out equipment that had been dropped off by airplanes the year before. The snowmobiles and barrels of fuel were buried in snow — visible only from flags that marked their positions.

But as Pomraning and his friends began to dig, they made a surprise discovery. Beneath the surface of that snow were thick rinds of ice. They took two days to chip their snowmobiles out of it.

That solid ice had formed the summer before, as warm air melted the snow. It left pools of water that trickled down and re-froze. No one was there to see it happen, but the assault from the air had already begun.

Power Words

(more about Power Words)

altimeter     An instrument used to determine altitude, or height above a surface.

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

Arctic     A region that falls within the Arctic Circle. The edge of that circle is defined as the northernmost point at which the sun is visible on the northern winter solstice and the southernmost point at which the midnight sun can be seen on the northern summer solstice. The high Arctic is that most northerly third of this region. It’s a region dominated by snow cover much of the year.

atmosphere     The envelope of gases surrounding Earth or another planet.

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.

basin     (in geology) A low-lying area, often below sea level. It collects water, which then deposits fine silt and other sediment on its bottom. Because it collects these materials, it’s sometimes referred to as a catchment or a drainage basin.

bedrock     The thick, solid rock layer than underlies the soil and other broken, rocky materials on Earth’s surface.

carbon dioxide (or CO2)    A colorless, odorless gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. Carbon dioxide also is released when organic matter burns (including fossil fuels like oil or gas). Carbon dioxide acts as a greenhouse gas, trapping heat in Earth’s atmosphere. Plants convert carbon dioxide into oxygen during photosynthesis, the process they use to make their own food.

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.

computer model     A program that runs on a computer that creates a model, or simulation, of a real-world feature, phenomenon or event.

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.

crevasse     (in Earth sciences) A large, deep crack in a glacier, whose surface may or may not be masked by a thin layer of snow.

current     A fluid — such as of water or air — that moves in a recognizable direction. (in electricity) The flow of electricity or the amount of charge moving through some material over a particular period of time.

fjord     A long, narrow inlet with steep sides, created in a valley carved by glacial activity.

geography     The study of Earth’s features and how the living and nonliving parts of the planet affect one another. Scientists who work in this field are known as geographers.

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.

global warming     The gradual increase in the overall temperature of Earth’s atmosphere due to the greenhouse effect. This effect is caused by increased levels of carbon dioxide, chlorofluorocarbons and other gases in the air, many of them released by human activity.

Greenland     The world’s largest island, Greenland sits between the Arctic Ocean and North Atlantic. Ice covers roughly 80 percent of Greenland. Indeed, this island’s 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).

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.

Intergovernmental Panel on Climate Change, or IPCC.     This international group keeps tabs on the newest published research on climate and on how ecosystems are responding to it. The United Nations Environment Programme and the World Meteorological Organization jointly created the IPCC in 1988. Their aim was to provide the world with a clear scientific view on the current state of knowledge in climate change and its potential environmental and social impacts.

laser     A device that generates an intense beam of coherent light of a single color. Lasers are used in drilling and cutting, alignment and guidance, in data storage and in surgery.

meltwater     The water that comes from melting ice. The quantities can be large and show up quickly when it comes from melting glaciers, ice sheets and snow-capped mountains.

monitor     To test, sample or watch something, especially on a regular or ongoing basis.

NASA     Short for the National Aeronautics and Space Administration. Created in 1958, this U.S. agency has become a leader in space research and in stimulating public interest in space exploration. It was through NASA that the United States sent people into orbit and ultimately to the moon. It also has sent research craft to study planets and other celestial objects in our solar system.

observatory     (in astronomy) The building or structure (such as a satellite) that houses one or more telescopes.

oceanography     (adj. oceanographic ) The branch of science that deals with the physical and biological properties and phenomena of the oceans. People who work in this field are known as oceanographers.

orbit     The curved path of a celestial object or spacecraft around a star, planet or moon. One complete circuit around a celestial body.

peninsula     A parcel of land that is that is attached to the mainland but surrounded by water on three sides.

planet     A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and has cleared other objects out of the way in its orbital neighborhood.

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.

simulation     (v. simulate) An analysis, often made using a computer, of some conditions, functions or appearance of a physical system. A computer program would do this by using mathematical operations that can describe the system and how it might change over time or in response to different anticipated situations.

swarm     A large number of animals that have amassed and now move together. People sometimes use the term to refer to huge numbers of other moving things.

theory     (in science) A description of some aspect of the natural world based on extensive observations, tests and reason. A theory can also be a way of organizing a broad body of knowledge that applies in a broad range of circumstances to explain what will happen. Scientists who use mathematics and/or existing data to project what might happen in new situations are known as theorists.

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

wake     An area of disturbed air or water left behind an object (such as a boat or animal) moving through it.

wave     A disturbance or variation that travels through space and matter in a regular, oscillating fashion.

Citation

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