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  • Ice, in its varied forms, covers as much as 16% of Earth’s surface, including 33% of

  • land areas at the height of the northern winter.

  • Glaciers, sea ice, permafrost, ice sheets and snow play an important role in Earth’s

  • climate.

  • They reflect energy back to space,

  • shape ocean currents,

  • and spawn weather patterns.

  • But there are signs that Earth’s great stores of ice are beginning to melt.

  • To find out where Earth might be headed, scientists are drilling down into the ice, and scouring

  • ancient sea beds, for evidence of past climate change.

  • What are they learning about the fate of our planet, a thousand years into the future and

  • even beyond?

  • 30,000 years ago, Earth began a relentless descent into winter,

  • Glaciers pushed into what were temperate zones.

  • Ice spread beyond polar seas.

  • New layers of ice accumulated on the vast frozen plateau of Greenland.

  • At three kilometers thick, Greenland’s ice sheet is a monumental formation built over

  • successive ice ages and millions of years. It’s so heavy that it has pushed much of

  • the island down below sea level.

  • And yet, today, scientists have begun to wonder how resilient this ice sheet really is.

  • Average global temperatures have risen about one degree Celsius since the industrial revolution.

  • They could go up another degree by the end of this century.

  • If Greenland’s ice sheet were to melt, sea levels would rise by over seven meters.

  • That would destroy or threaten the homes and livelihoods of up to a quarter of the world’s

  • population.

  • These elevation maps show some of the areas at risk. Black and red are less than 10 meters

  • above current sea level.

  • The Southeastern United States, including Florida,

  • And Louisiana.

  • Bangladesh.

  • The Persian Gulf.

  • Parts of Southeast Asia and China. That’s just the beginning.

  • With so much at stake, scientists are monitoring Earth’s frozen zones, with satellites, radar

  • flights, and expeditions to drill deep into ice sheets.

  • And they are reconstructing past climates, looking for clues to where Earth might now

  • be headed, not just centuries, but thousands of years in the future.

  • Periods of melting and freezing, it turns out, are central events in our planet’s

  • history. That’s been born out by evidence ranging

  • from geological traces of past sea levels, the distribution of fossils, chemical traces

  • that correspond to ocean temperatures, and more.

  • Going back over two billion years, earth has experienced five major glacial or ice ages.

  • The first, called the Huronian, has been linked to the rise of photosynthesis in primitive

  • organisms.

  • They began to take in carbon dioxide, an important greenhouse gas. That decreased the amount

  • of solar energy trapped by the atmosphere, sending Earth into a deep freeze.

  • The second major ice age began 580 million years ago. It was so severe, it’s often

  • referred to assnowball earth.”

  • The Andean-Saharan and the Karoo ice ages began 460 and 360 million years ago.

  • Finally, there’s the Quaternary, from 2.6 million years ago to the present.

  • Periods of cooling and warming have been spurred by a range of interlocking factors: volcanic

  • events, the evolution of plants and animals, patterns of ocean circulation, the movement

  • of continents.

  • The world as we know it was beginning to take shape in the period from 90 to 50 million

  • years ago. The continents were moving toward their present positions.

  • The Americas separated from Europe and Africa. India headed toward a merger with Asia.

  • The world was getting warmer. Temperatures spiked roughly 55 million years ago,

  • going up about 5 degrees Celsius in just a few thousand years. CO2 levels rose to about

  • 1000 parts per million, compared to 280 in pre-industrial times, and 390 today.

  • But the stage was set for a major cool down. The configuration of landmasses had cut the

  • Arctic off from the wider oceans.

  • That allowed a layer of fresh water to settle over it, and a sea plant called Azolla to

  • spread widely. In a year, it can soak up as much as 6 tons of CO2 per acre.

  • Plowing into Asia, the Indian subcontinent caused the mighty Himalayan Mountains to rise

  • up.

  • In a process called weathering, rainfall interacting with exposed rock began to draw more CO2 from

  • the atmosphere, washing it into the sea.

  • Temperatures steadily dropped.

  • By around 33 million years ago, South America had separated from Antarctica. Currents swirling

  • around the continent isolated it from warm waters to the north. An ice sheet formed.

  • In time, with temperatures and CO2 levels continuing to fall, the door was open for

  • a more subtle climate driver.

  • It was first described by the 19th century Serbian scientist, Milutin Milankovic.

  • He saw that periodic variations in Earth’s rotational motion altered the amount of solar

  • radiation striking the poles.

  • In combination, every 100,000 years or so, these variations have sent earth into a period

  • of cool temperatures and spreading ice. Each glacial period was followed by an interglacial

  • period in which temperatures rose and the ice retreated.

  • The Milankovic cycles are not strong enough by themselves to cause the shift. What they

  • do is get the ball rolling.

  • A decrease in solar energy hitting the Arctic allows sea ice to form in winter and remain

  • over summer, then to expand its reach the following year.

  • The ice reflects more solar energy back to space. A colder ocean stores more CO2, which

  • further dampens the greenhouse effect.

  • Conversely, when ocean temperatures rise, more CO2 escapes into the atmosphere, where

  • it traps more solar energy.

  • With so many factors at play, each swing of the climate pendulum has produced its own

  • unique conditions.

  • Take, for example, the last interglacial, known as the Eemian, from 130 to 115,000 years

  • ago.

  • This happened at a time when CO2 was at preindustrial levels, and global temperatures had risen

  • only modestly.

  • But with higher solar energy striking the north, temperatures rose dramatically in the

  • Arctic. The effect was amplified by the lower reflectivity of ice-free seas and spreading

  • northern forests.

  • There is still uncertainty about how much these changes affected sea levels. Estimates

  • range from a 5 to 9 meters, levels that would be catastrophic today.

  • That’s one reason scientists today are intensively monitoring Earth’s frozen zones, including

  • the ice sheet that covers Greenland.

  • Satellite radar shows the flow of ice from the interior of the island and into glaciers.

  • In the eastern part of the island, glaciers push slowly through complex coastal terrain.

  • In areas of higher snowfall in the northwest and west, the ice speeds up by a factor 10.

  • The landscape channels the ice into many small glaciers that flow straight down to the sea.

  • In the distant past, the center of the island may have been drained by a giant canyon, recently

  • discovered. Scientists found that it’s 550 kilometers long and up to 800 meters deep.

  • It leads from Greenland’s interior to one of today’s most volatile glaciers.

  • This is the Petermann Glacier in Northwest Greenland. Amid unusually warm summer temperatures

  • in 2012, satellites tracked a giant iceberg as it broke off and moved down the glacier's

  • outlet channel.

  • At about 31 square kilometers, this island of ice stayed together as it floated along.

  • After two months, it finally began to fragment.

  • The Jakobshavn glacier on Greenland’s west coast flows toward the sea at a rapid rate

  • of 20 to 40 meters per day.

  • At the ice front, where the glacier meets the sea, Jakobshavn has been retreating as

  • it dumps more and more ice into the ocean.

  • You can see it in this map. In 1851, the front was down here. Now it’s 50 kilometers up.

  • One reason, scientists say, is that water seeping down into its base is acting like

  • a lubricant. Another is that as the glacier thins, it’s more likely to break off, or

  • calve, when it interacts with warmer ocean waters.

  • Scientists are tracking the overall rate of ice loss with the Grace Satellite. They found

  • that from 2003 to 2009, Greenland lost about a trillion tons, mostly along its coastlines.

  • This number mirrors ice loss in the Arctic as a whole. By 2012, summer sea ice coverage

  • had fallen to a little more than half of what it was in the year 1980.

  • While the ice rebounded in 2013, the coverage was still well below the average of the last

  • three decades.

  • Analyzing global data from Grace, one study reports that Earth lost about 4,000 cubic

  • kilometers of ice in the decade leading up to 2012.

  • Sea levels around the world are now expected to rise about a meter by the end of the century.

  • What will happen beyond that?

  • To gauge the resilience of Greenland’s great ice sheet, scientists mounted one of the most

  • intensive glacial drilling projects to date, the North Greenland Eemian Ice Drilling Project,

  • or NEEM.

  • The ice samples they obtained from the height of Eemian warming told a surprising story.

  • If you were a visitor to Northern Greenland in those times, you would have stood on ice

  • over two kilometers thick.

  • Temperatures were warmer than today by about 8 degrees Celsius. And yet, the ice had receded

  • by only about 25%, a relatively modest amount.

  • That has shifted the focus to Earth’s other, much larger ice sheet, on the continent of

  • Antarctica.

  • Antarctica contains 90% of all the ice, and 70% of all the fresh water on the Earth.

  • Scientists are asking: how dynamic are its ice sheets? How sensitive are they to melting?

  • Data from Grace and other satellites shows that this frozen continent overall has lately

  • been losing as much ice as it gains.

  • The vast plateau of Antarctic ice is one of the driest deserts on Earth.

  • What little snow falls, remains, adding to the continent’s mass. You can see evidence

  • of this in the snow and ice that piles up at the South Pole research station.

  • This geodesic dome was built in the 1970s. By the time it was decommissioned in 2009,

  • the entrance was nearly buried.

  • With a thickness of up to 4 kilometers, the ice on which this outpost sits will not melt

  • easily.

  • That’s true in part because of the landmass below it, captured in an extraordinary radar

  • image.

  • The eastern part of the continent, the far side of the image, is a stable foundation

  • of continental crust.

  • In contrast, the western side dips as much as 2500 meters below present day sea level.

  • Along the Amundsen Sea Coast, the ice is disappearing at an accelerating rate.

  • Inland ice streams are moving toward the ocean at at least 100 meters per year. They end

  • up in floating ice shelves that extend hundreds of miles into

  • the ocean.

  • This region is the greatest source of uncertainty about global sea level projections.

  • When ice shelves like this grow, they become prone to fracturing. A giant crack, for example,

  • recently appeared in the Pine Island Glacier. Within two years, a 720 square kilometer iceberg

  • had broken off.

  • But the scientists are more concerned about what’s happening below the surface.

  • In recent times, the Southern ocean that swirls around the continent has been getting warmer,

  • at the rate of .2 degrees Celsius per decade.

  • That has affected ice shelves like Pine Island by melting them from below.

  • In a comprehensive survey of the continent, scientists concluded that this process was

  • responsible for 55 percent of the mass lost from ice shelves between 2003 and 2008.

  • It’s also been blamed for one of the more puzzling twists in the story of climate change,

  • the spread of sea ice all around Antarctica.

  • One possibility is that ramped up winds, circling the pole, are pushing the ice into thicker,

  • more resilient formations.

  • Another is that the melting of ice shelves has spread a layer of cold, fresh water over

  • coastal seas, which readily freezes.

  • A team of researchers has come to the Pine Island Glacier to try to monitor the melting

  • in real time. After five years of preparation, they drilled through 500 meters of ice to

  • begin measuring ice volume, temperature, salinity, and flow.

  • In some places, they found melt rates of about 6 centimeters per day, or about 22 meters

  • in a year.

  • Because ice shelves hold back inland glaciers, the melting could trigger larger changes.

  • That’s likely what happened to the Larsen ice shelf on the Antarctic Peninsula in the

  • year 2002. It’s thought to have been stable since the last interglacial.

  • Warmer ocean waters had been eating away at Larsen’s underside.

  • By early February of 2002, the shelf began to splinter into countless small icebergs.

  • By March 7th, when this picture was taken, it had completely collapsed, forming a vast

  • slush that drifted out to sea.

  • Without the shelf’s buttressing effect, a series of nearby glaciers picked up speed,

  • dumping an additional 27 cubic kilometers of ice into the ocean per year.

  • Evidence from the last interglacial, the Eemian, brings an ominous warning of what could lie

  • ahead.

  • It’s based on the height of ancient coral reefs, which grow to a depth relative to the

  • sea level above them.

  • Based on reefs along the Australian coast, a recent study published in the journal Nature

  • showed that sea levels remained stable for most of the Eemian, at 3-4 meters above those

  • of today.

  • But the authors found that in the last few thousand years of the period, starting around

  • 118,000 years ago, sea levels suddenly shot up to 9 meters above today.

  • The authors concluded, in their words, that “a critical ice sheet stability threshold

  • was crossed, resulting in the catastrophic collapse of polar ice sheets.”

  • Looking ahead, uncertainties about the future of our climate abound.

  • According to one study, the long cool down to the next glacial period is due to start

  • in the next 1500 years or so, based on the timing of Milankovic cycles.

  • But for this actually to happen, the study says, enough new ice would have to form to

  • get the ball rolling. CO2 would have to retreat to below pre-industrial levels.

  • Instead, it appears that a warming climate is becoming a fact of life.

  • The danger is that if the melting gains a momentum of its own, even reducing CO2 emissions

  • may not be enough to stop it.

  • The still unfolding story of Earth’s past tells us about the mechanisms that can shape

  • our climate. But it’s the unique conditions of our time that will determine sea levels,

  • ice coverage, and temperatures.

  • What’s at stake in the coming centuries is the world we know, the one that has nurtured

  • and sustained us.

  • The Earth itself will go on, ever changing on short and long time scales, a dynamic living

  • planet.

  • 1

Ice, in its varied forms, covers as much as 16% of Earth’s surface, including 33% of

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    Wonderful に公開 2021 年 01 月 14 日
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