FILE - In this May 24, 2011 file photo, a half-mile-wide tornado moves towards Piedmont, Okla. In 2011 the United States saw one of the busiest tornado seasons in generations: Nearly 1,

 

              FILE - In this May 24, 2011 file photo, a half-mile-wide tornado moves towards Piedmont, Okla. In 2011 the United States saw one of the busiest tornado seasons in generations: Nearly 1,
FILE - In this May 24, 2011 file photo, a half-mile-wide tornado moves towards Piedmont, Okla. In 2011 the United States saw one of the busiest tornado seasons in generations: Nearly 1,700 tornadoes that killed 553 people. With the planet heating up, many scientists seem fairly certain some weather elements like hurricanes and droughts will worsen. But as the traditional season nears, scientists are still trying to figure out if there be more or fewer tornadoes as global warming increases. (AP Photo/The Oklahoman, Paul Southerland, File)