Scientists said Wednesday that the Arctic has become a prime example of the built-in conservatism of their climate forecasts. As dire as their warnings about the long-term consequences of heat-trapping emissions have been, many of them fear they may still be underestimating the speed and severity of the impending changes.

In a panel discussion on Wednesday in New York sponsored by Greenpeace, the environmental group, James E. Hansen, a prominent NASA climate scientist, said the Arctic melting should serve as a warning to the public of the risks that society is running by failing to limit emissions.

“The scientific community realizes that we have a planetary emergency,” Dr. Hansen said. “It’s hard for the public to recognize this because they stick their head out the window and don’t see that much going on.”

A prime concern is the potential for a large rise in the level of the world’s oceans. The decline of Arctic sea ice does not contribute directly to that problem, since the ice is already floating and therefore displacing its weight in water.

But the disappearance of summer ice cover replaces a white, reflective surface with a much darker ocean surface, allowing the region to trap more of the sun’s heat, which in turn melts more ice. The extra heat in the ocean appears to be contributing to an accelerating melt of the nearby Greenland ice sheet, which does contribute to the rise in sea level.

At one point this summer, surface melt was occurring across 97 percent of the Greenland ice sheet, a development not seen before in the era of satellite measurements, although geological research suggests that it has happened in the past.