Many scientists also say that the cool spell in no way undermines the enormous body of evidence pointing to a warming world with disrupted weather patterns, less ice and rising seas should heat-trapping greenhouse gases from burning fossil fuels and forests continue to accumulate in the air.  “The current downturn is not very unusual,” said Carl Mears, a scientist at Remote Sensing Systems, a private research group in Santa Rosa, Calif., that has been using satellite data to track global temperature and whose findings have been held out as reliable by a variety of climate experts. He pointed to similar drops in 1988, 1991-92, and 1998, but with a long-term warming trend clear nonetheless.  “Temperatures are very likely to recover after the La Niña event is over,” he said.  Mr. Morano, in an e-mail message, was undaunted, saying turnabout is fair play: “Fair is fair. Noting (not hyping) an unusually harsh global winter is merely pointing out the obvious. Dissenters of a man-made ‘climate crisis’ are using the reality of this record-breaking winter to expose the silly warming alarmism that the news media and some scientists have been ceaselessly promoting for decades.”  More clucking about the cold is likely over the next several days. The Heartland Institute, a public policy research group in Chicago opposed to regulatory approaches to environmental problems, is holding a conference in Times Square on Monday and Tuesday aimed at exploring questions about the cause and dangers of climate change.  The event will convene an array of scientists, economists, statisticians and libertarian commentators holding a dizzying range of views on the changing climate — from those who see a human influence but think it is not dangerous, to others who say global warming is a hoax, the sun’s fault or beneficial. Many attendees say it is the dawn of a new paradigm. But many climate scientists and environmental campaigners say it is the skeptics’ last stand.  Michael E. Schlesinger, an atmospheric scientist at the University of Illinois, Urbana-Champaign, said that any focus on the last few months or years as evidence undermining the established theory that accumulating greenhouse gases are making the world warmer was, at best, a waste of time and, at worst, a harmful distraction.  Discerning a human influence on climate, he said, “involves finding a signal in a noisy background.” He added, “The only way to do this within our noisy climate system is to average over a sufficient number of years that the noise is greatly diminished, thereby revealing the signal. This means that one cannot look at any single year and know whether what one is seeing is the signal or the noise or both the signal and the noise.”  The shifts in the extent and thickness of sea ice in the Arctic (where ice has retreated significantly in recent summers) and Antarctic (where the area of floating sea ice has grown lately) are similarly hard to attribute to particular influences.  Interviews and e-mail exchanges with half a dozen polar climate and ice experts last week produced a rough consensus: Even with the extensive refreezing of Arctic waters in the deep chill of the sunless boreal winter, the fresh-formed ice remains far thinner than the yards-thick, years-old ice that dominated the region until the 1990s.  That means the odds of having vast stretches of open water next summer remain high, many Arctic experts said.  “Climate skeptics typically take a few small pieces of the puzzle to debunk global warming, and ignore the whole picture that the larger science community sees by looking at all the pieces,” said Ignatius G. Rigor, a climate scientist at the Polar Science Center of the University of Washington in Seattle.  He said the argument for a growing human influence on climate laid out in last year’s reports from the Intergovernmental Panel on Climate Change, or I.P.C.C., was supported by evidence from many fields.  “I will admit that we do not have all the pieces,” Dr. Rigor said, “but as the I.P.C.C. reports, the preponderance of evidence suggests that global warming is real.” As for the Arctic, he said, “Yes, this year’s winter ice extent is higher than last year’s, but it is still lower than the long-term mean.”  Dr. Rigor said next summer’s ice retreat, despite the regrowth of thin fresh-formed ice now, could still surpass last year’s, when nearly all of the Arctic Ocean between Alaska and Siberia was open water.  Some scientists who strongly disagree with each other on the extent of warming coming in this century, and on what to do about it, agreed that it was important not to be tempted to overinterpret short-term swings in climate, either hot or cold.  Patrick J. Michaels, a climatologist and commentator with the libertarian Cato Institute in Washington, has long chided environmentalists and the media for overstating connections between extreme weather and human-caused warming. (He is on the program at the skeptics’ conference.)  But Dr. Michaels said that those now trumpeting global cooling should beware of doing the same thing, saying that the “predictable distortion” of extreme weather “goes in both directions.”  Gavin A. Schmidt, a climatologist at NASA’s Goddard Institute for Space Studies in Manhattan who has spoken out about the need to reduce greenhouse gases, disagrees with Dr. Michaels on many issues, but concurred on this point.  “When I get called by CNN to comment on a big summer storm or a drought or something, I give the same answer I give a guy who asks about a blizzard,” Dr. Schmidt said. “It’s all in the long-term trends. Weather isn’t going to go away because of climate change. There is this desire to explain everything that we see in terms of something you think you understand, whether that’s the next ice age coming or global warming.”  A version of this article appears in print on , on Page A18 of the New York edition with the headline: Skeptics on Human Climate Impact Seize on Cold Spell. Order Reprints| Today's Paper|Subscribe
Extraordinary winter conditions in many parts of the world have been seized on by those who challenge warnings about dangerous human-caused global warming.