Such doubts have been a major factor in the American political debate over global warming, stalling efforts by Democrats and the Obama administration to pass legislation that would curb emissions of heat-trapping gases. Similar legislative efforts are likely to receive even less support in the new Congress, with many newly elected legislators openly skeptical about climate change.

A large majority of climate scientists argue that heat-trapping gases are almost certainly playing a role in what is happening to the world’s land ice. They add that the lack of policies to limit emissions is raising the risk that the ice will go into an irreversible decline before this century is out, a development that would eventually make a three-foot rise in the sea look trivial.

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 afoot that most climate scientists attribute to global warming.

Yet the rise of the sea could turn out to be the single most serious effect. While the United States is among the countries at greatest risk, neither it nor any other wealthy country has made tracking and understanding the changes in the ice a strategic national priority.

The consequence is that researchers lack elementary information. They have been unable even to measure the water temperature near some of the most important ice on the planet, much less to figure out if that water is warming over time. Vital satellites have not been replaced in a timely way, so that American scientists are losing some of their capability to watch the ice from space.

The missing information makes it impossible for scientists to be sure how serious the situation is.

“As a scientist, you have to stick to what you know and what the evidence suggests,” said Gordon Hamilton, one of the researchers in the helicopter. “But the things I’ve seen in Greenland in the last five years are alarming. We see these ice sheets changing literally overnight.”

Dodging Icebergs

In the brilliant sunshine of a late summer day in southeastern Greenland, the pilot at the controls of the red helicopter, Morgan Goransson, dropped low toward the water. He used the downdraft from his rotor to clear ice from the surface of Sermilik Fjord.