Insured severe weather losses have already topped $15.5 billion in the United States this year, three times the full-year average of the last 20 years, Reuters reported.

Almost all of that comes from the outbreak of tornadoes and thunderstorms that struck the country in April and May, according to a report by insurance brokerage Aon Corp.

Over the past 20 years, the United States has averaged just more than $5 billion in annual severe weather losses, the company said, and a potentially devastating hurricane season could make losses soar further this year.

Eight storm outbreaks drove most of the costs. The May 22 tornado in Joplin, Mo., and the April 27 tornado in Tuscaloosa, Ala., now rank as the most expensive tornadoes in history, even adjusting for inflation.

Aon estimated total economic losses at $21.65 billion. Combined with earthquakes in Japan and New Zealand, flooding in Australia and unrest in the Middle East, insurers and reinsurers worldwide are estimated to have already lost more than $60 billion this year in claims.