After the cold winter of 2012-13, the percentage of Americans who believe global warming is occurring has dropped 7 points since last fall, a poll indicates. Researchers at Yale University and George Mason University said the drop was likely influenced by the relatively cold winter and an unusually cold March just before their survey, Climate Change in the American Mind, was conducted. \"Some people think weather and climate are the same thing,\" Yale research scientist Anthony Leiserowitz said. \"When they experience cold weather or a big snowstorm, they think that perhaps global warming isn\'t happening. \"When they experience a heat wave, they are more likely to say it is happening. Recent weather experiences can thus influence the opinions of some members of the public.\" In the survey of a random sample of 1,045 adults aged 18 and up interviewed April 8-15, 63 percent said they believe global warming is happening. Forty-nine percent of survey respondents said they believe global warming is caused mostly by human activities, a decrease of 5 points since fall 2012 but similar to levels stretching back several years, the researchers said.