Scientists warn
inaction may put
cities under water.

By JUSTIN GILLIS

TASIILAQ, Greenland AT GREAT RISK and with diminished support, scientists are racing to answer one of the most urgent - and most widely debated - questions facing humanity: How fast is the world’s ice going to melt? Scientists long believed that the collapse of the gigantic ice sheets in Greenland and Antarctica would take thousands of years, with sea level possibly rising as little as 18 centimeters in this century, about the same amount as in the 20th century.

The world’s glaciers and ice sheets show signs of melting faster than previously believed; some predict sea levels will rise one meter by 2100. Helheim Glacier, in Greenland.

Below, measuring ice is dangerous work. Fiammetta Straneo hangs off the side of a helicopter over Greenland to take a reading.

But researchers have recently been startled to see big changes unfold in both Greenland and Antarctica. As a result of recent calculations that take the changes into account, many scientists now say that sea level is likely to rise perhaps one meter by 2100 ? an increase that would pose a threat to coastal regions the world over.



And the calculations suggest that the rise could conceivably exceed 1.8 meters, which would put thousands of hectares of the American coastline under water and would probably displace tens of millions of people in Asia. The scientists say that a rise of even one meter would inundate low-lying lands in many countries, rendering some areas uninhabitable.

It would cause coastal flooding of the sort that now happens once or twice a century to occur every few years. It would cause much faster erosion of beaches, barrier islands and marshes. It would contaminate fresh water supplies with salt. Some of the world’s great cities ? London, Cairo, Bangkok, Venice and Shanghai among them ? would be critically endangered .

In the United States, parts of the East Coast and Gulf Coast would be hit hard. In New York, coastal flooding could become routine. Climate scientists readily admit that the one-meter estimate could be wrong. Their understanding of the changes is still primitive. But, they say, it could just as easily be an underestimate. “I think we need immediately to begin thinking about our coastal cities ? how are we going to protect them?” said John A.

Church, an Australian scientist who is a leading expert on sea level. “We can’t afford to protect everything. We will have to abandon some areas.” Sea-level rise has been a particularly contentious element in the global warming debate. One estimate suggested the threat was so dire that sea level could rise as much as 4.6 meters in this century.

Global warming skeptics contend that any changes occurring in the ice sheets are probably due to natural climate variability, not to greenhouse gases released by humans. A large majority of climate scientists argue that heat-trapping gases are almost certainly playing a role .

They add that the lack of policies to limit emissions is raising the risk that the ice will go into an irreversible declinebefore this century is out, a development that would eventually make a one-meter rise in the sea look trivial.

Besides the melting ice is by no means the only sign that the earth is warming, thermometers on land, in the sea and aboard satellites show warming. Heat waves, flash floods and other extreme weather events are increasing. Plants are blooming earlier, coral reefs are dying and many other changes are attributed to global warming by scientists.

Yet, while the rise of the sea could turn out to be the single most serious effect of global warming, no wealthy country has made understanding the changes in the ice a strategic national priority. The consequence is that researchers lack elementary information, and the missing information makes it impossible for scientists to be sure how serious the situation is. “The things I’ve seen in Greenland in the last five years are alarming,” said Gordon Hamilton, of the University of Maine.

“We see these ice sheets changing literally overnight.” Dodging Icebergs On a late summer day in southeastern Greenland, a helicopter pilot, Morgan Goransson, dropped low toward the water. Hanging out of the side of the aircraft, scientists sent a measuring device between the ice floes.

The frigid waters of Sermilik Fjord were only nine meters below, so any mechanical problem would have sent the chopper plunging into the sea. “It is so dangerous,” Mr. Goransson said. Taking the temperature of waters near the ice sheet is essential to understanding what is happening in Greenland.

But it is complex and risky. The scientists ? Fiammetta Straneo, of the Woods Hole Oceanographic Institution in Massachusetts, and Dr. Hamilton ? are part of a team that is financed by the National Science Foundation . A few weeks ago, they obtained a reading of 4 degrees Celsius near the bottom of the fjord, the highest they had seen there.

The reading fit a broader pattern . Warmer water that originated far to the south is flushing into Greenland’s fjords. Scientists suspect that as it melts the ice from beneath, the water is loosening the connection of the glaciers to the ground . This allows the glaciers to move faster and dump more ice into the ocean.

The overall loss of ice seems to be accelerating, an ominous sign given that the island contains enough ice to raise global sea levels by more than six meters. Strictly speaking, scientists have not proved that human-induced global warming is the cause of the changes.

They are mindful that the climate in the Arctic undergoes big natural variations. Yet scientists say that the recent changes in Greenland are occurring at the same time that air and ocean temperatures are warming, and ice melt is accelerating, in much of the world. Helheim Glacier, which terminates in Sermilik Fjord, is one of a group of glaciers in southeastern Greenland that have shown especially big changes.

S omething caused the glacier, one of Greenland’s largest, to speed up sharply in the middle of the last decade, and it spit so much ice into the ocean that it thinned by some 91 meters in a few years. The glacier has behaved erratically ever since, and that pattern is being repeated all over Greenland. A Rising Ocean To a majority of climate scientists, the question is not whether the earth’s land ice will melt, but whether it will happen too fast for society to adjust.

Recent research suggests that the volume of the ocean may have been stable for thousands of years. But it began to rise in the 19th century, around the same time that advanced countries began to burn large amounts of coal and oil. The sea has risen about 20 centimeters since then, on average. Such an increase is enough to cause substantial erosion.

In its last big report, in 2007, the United Nations group that assesses climate science, the Intergovernmental Panel on Climate Change, said that sea level would rise at least 18 more centimeters, and might rise as much as 61 centimeters, in the 21st century.

But the group warned that these estimates did not fully incorporate the possibility that the world’s big ice sheets would start spitting ice into the ocean at a much faster rate than it could melt on land. Scientific understanding of this prospect was so poor, the climate panel said, that no meaningful upper limit could be put on the potential rise of sea level.

Satellite evidence suggests that the rise of the sea accelerated late in the 20th century, so that the level is now increasing a little over 2.5 centimeters per decade, on average . Increased melting of land ice appears to be a major factor. Ano

ther is that most of the extra heat being trapped by human greenhouse emissions is warming the ocean, not the atmosphere, and as it warms, the water expands.

A developing consensus among climate scientists puts the best estimate of the pace of the rising sea at a little over one meter between now and 2100. Calculations suggest that that would cause shoreline erosion to accelerate markedly. In places that once flooded only in a large hurricane, a routine storm could have the same result.

The worst effects would probably occur in areas where land is sinking even as the sea rises. That applies to some of the world’s major cities. Defenses can be built to keep out the sea, of course, like the levees of New Orleans and the dikes of the Netherlands. But the expense is likely to soar as the ocean rises .

Storm surges battering the world’s coastlines every few years would almost certainly force people to flee inland. But it is hard to see where the displaced would go, especially in Asia, where huge cities ? and even countries, like Bangladesh ? are at risk.

And, scientists say, if their projections prove accurate, the sea will not stop rising in 2100. By that point, the ice sheets could be undergoing extensive melting. “Beyond a hundred years out, it starts to look really challenging,” said Richard B. Alley, a climate scientist at Pennsylvania State University.

“You start thinking about every coastal city on the planet hiding behind a wall, with storms coming.” No Plan, No Information Because no advanced country has made a priority of studying land ice, scientists lack elementary information that they need to make sense of what is happening.

They do not know the lay of the land beneath most of the world’s glaciers well enough to calculate how fast the ice might retreat. They have only haphazard readings of the depth and temperature of the ocean near Greenland, needed to figure out why so much warm water seems to be attacking the ice sheet. The information problems are even more severe in Antarctica.

Figuring out whether Antarctica is losing ice over all is essential, because that ice sheet contains enough water to raise global sea level by nearly 61 meters. Daniel Schrag, a geochemist and head of Harvard University’s Center for the Environment, praised the scientists who do difficult work studying ice, but added, “The scale of what they can do, given the resources available, is just completely out of whack with what is required.

” Climate scientists worry that extensive melting of land ice may become inevitable before political leaders find a way to limit heat-trapping gases, and before scientists even realize such a point of no return has been passed.

“The past clearly shows that sealevel rise is getting faster and faster the warmer it gets,” Dr. Rahmstorf said. “Why should that process stop? If it gets warmer, ice will melt faster.”