“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.”…

“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.