At the end of one of the most bizarre奇异 weather years in American history, climate research stands at a crossroads. 徘徊之中

Scientists say they could, in theory, do a much better job of answering the question “Did global warming have anything to do with it?”全球变暖是否有关 after extreme weather events like the drought in Texas and the floods in New England.

But for many reasons, efforts to put out prompt及时 reports on the causes of extreme weather are essentially languishing困难. Chief among the difficulties that scientists face: the political environment for new climate-science initiatives项目 has turned hostile, and with the federal budget crisis, money is tight.

And so, as the weather becomes more erratic不规则，自由 by the year, the public is left to wonder what is going on.

When 2010 ended, it seemed as if people had lived through a startling year of weather extremes. But in the United States, if not elsewhere, 2011 has surpassed that. 有过之无不及

A typical year in this country features three or four weather disasters whose costs exceed $1 billion each. But this year, the National Oceanic and Atmospheric Administration has tallied记录 a dozen such events, including wildfires in the Southwest, floods in multiple regions of the country and a deadly spring tornado season. And the agency has not finished counting. The final costs are certain to exceed $50 billion.

“I’ve been a meteorologist 30 years and never seen a year that comes close to matching 2011 for the number of astounding, extreme weather events,” Jeffrey Masters, a co-founder of the popular Web site Weather Underground, said last month. “Looking back in the historical record, which goes back to the late 1800s, I can’t find anything that compares, either.”

Many of the individual events in 2011 do have precedents in the historical record. And the nation’s climate has featured other concentrated periods of extreme weather, including severe cold snaps in the early 20th century and devastating droughts and heat waves in the Dust Bowl era of the 1930s.

But it is unusual, if not unprecedented, for so many extremes to occur in such a short span. The calamities灾害 in 2011 included wildfires that scorched焦土化 millions of acres, extreme flooding in the Upper Midwest and the Mississippi River Valley and heat waves that shattered records in many parts of the country. Abroad, massive floods inundated泛滥 Australia, the Philippines and large parts of Southeast Asia.

A major question nowadays is whether the frequency of particular weather extremes is being affected by human-induced climate change.

【大气运动的动力来自冷热温差，气候变暖，导致大气温度差加大，导致空气动力加大。物体温度变化都会释放动力。】 

Climate science already offers some insight. Researchers have proved that the temperature of the earth’s surface is rising, and they are virtually certain that the human release of greenhouse gases, mainly from the burning of fossil fuels, is the major reason. For decades, they have predicted that this would lead to changes in the frequency of extreme weather events, and statistics show that has begun to happen.

【太阳能与风能的利用则是使大气变冷】

For instance, scientists have long expected that a warming atmosphere would result in fewer extremes of low temperature and more extremes of high temperature. In fact, research shows that about two record highs are being set in the United States for every record low, and similar trends can be detected in other parts of the world.

【气候变暖的总效应是气候波动加大。这倒是好消息，因为人类不会全部灭亡，只是有的地方不好过了。因为人类不怕雨雪，而是怕干旱。】

Likewise, a well-understood physical law suggests that a warming atmosphere should hold more moisture. Scientists have directly measured the moisture in the air and confirmed that it is rising, supplying the fuel for heavier rains, snowfalls and other types of storms.

“We are changing the large-scale properties of the atmosphere — we know that beyond a shadow of a doubt,” said Benjamin D. Santer, a leading climate scientist who works at the Lawrence Livermore National Laboratory in California. “You can’t engage in this vast planetary experiment — warming the surface, warming the atmosphere, moistening the atmosphere — and have no impact on the frequency and duration of extreme events.”

But if the human contribution to heat and precipitation is clear, scientists are on shakier ground analyzing many other events. Tornadoes, the deadliest weather disaster to hit the country this year, present a particularly thorny case.

On their face, weather statistics suggest that tornadoes are becoming more numerous as the climate warms. But tornadoes are small and hard to count, and scientists have little confidence in the accuracy of older data, which means they do not know whether to believe the apparent increase. Likewise, the computer programs they use to analyze and forecast the climate do not do a good job of representing events as small as tornadoes.

Some scientists have offered theories about how increasing heat and moisture may have made tornado outbreaks more likely, but these have not yet been tested in rigorous analyses. Many other types of extreme weather fall into this category, with scientists lacking a strong basis for attributing increases to human activity, or for discounting a human effect.

The question can sometimes be answered with focused studies of a specific weather event, but these are often finished years afterward. Lately, scientists have been discussing whether they can do a better job of analyzing events within days or weeks, not years.

“It’s clear we do have the scientific tools and the statistical wherewithal to begin answering these types of questions,” Dr. Santer said.

But doing this on a regular basis would probably require new personnel spread across several research teams, along with a strong push by the federal government, which tends to be the major source of financing and direction for climate and weather research. Yet Washington is essentially frozen on the subject of climate change.

This year, when the National Oceanic and Atmospheric Administration tried to push through a reorganization that would have provided better climate forecasts to businesses, citizens and local governments, Republicans in the House of Representatives blocked it. The idea had originated in the Bush administration, was strongly endorsed by an outside review panel and would have cost no extra money. But the House Republicans, many of whom reject the overwhelming scientific consensus about the causes of global warming, labeled the plan an attempt by the Obama administration to start a “propaganda” arm on climate.

In an interview, Jane Lubchenco, the director of NOAA, rejected that claim and said her agency had been deluged洪水 with information requests regarding future climate risks. “It’s truly unfortunate that we are not allowed to become more effective and efficient in delivering that information,” she said.

NOAA does finance research to understand the causes of weather extremes, as do the National Science Foundation and the Department of Energy. But with the strains on the federal budget, Dr. Lubchenco said, “it’s going to be more and more challenging to devote resources to many of our research programs.”

Some steps are being taken. Peter A. Stott, a leading climate scientist in Britain, has been pressing colleagues on both sides of the Atlantic to develop a robust capability to analyze weather extremes in real time. He is part of a group that expects to publish, next summer, the first complete analysis of a full year of extremes, focusing on 2011.

In an interview, Dr. Stott said the goal was to get to a point where “the methodologies are robust enough that you can do it in a kind of handle-turning way.”

But he added that it was important to start slowly and establish a solid scientific foundation for this type of work. That might mean that some of the early analyses would not be especially satisfying to the public.

“In some cases, we would say we have a confident result,” Dr. Stott said. “We may in some cases have to say, with the current state of the science, it’s not possible to make a reliable attribution statement at this point.”