Tornadoes are inflicting more damage and destruction across North America more than ever, a new study has revealed. Instead of scattered incidents, they continue to come in clusters.

Since the 1950s, the average of number of tornado clusters, which are often four or more tornadoes that last for one to three days, is skyrocketing.

What's more, tornado outbreaks are turning deadlier, leaving hundreds of casualties behind.

Climate scientist and study lead author Michael Tippett said they do not know what causes the surge in tornado outbreaks.

"It could be global warming," said Tippett, who is a weather researcher at Columbia University. "But our usual tools -- the observational record and computer models -- are not up to the task of answering this question yet."

Additionally, their research discovered that the chances of extreme tornado outbreaks in North America have increased four-fold. Extreme outbreaks occur when hundreds of tornadoes spawn in storms.

The largest tornado outbreak ever was recorded in April 2011, which spawned 363 tornadoes in the United States and Canada combined, left 350 people dead, and lasted for about three days. Tippett said outbreaks are responsible for 79 percent of tornado-related deaths.

Scientists predict the frequency of atmospheric conditions that trigger tornadoes increase in warmer climates. However, the "right" conditions do not guarantee that a tornado will spawn.

Tippett and his colleagues examined tornado records from the National Oceanic and Atmospheric Administration (NOAA) from 1954 to 2014, spanning for 60 years. They calculated the average number of tornadoes per outbreak, as well as the variability or the chances of extreme outbreaks.

The team's findings are expected to help insurance companies better understand the dangers posed by tornado outbreaks. Over the last decade, tornado outbreaks have cost the industry an average of $12.5 billion in insured losses annually, said global reinsurance advisor Willis Re.

Meanwhile, Tippett said extreme outbreaks have become more common because of two factors: first, the average number of tornadoes per outbreak has increased. Second, the high variability means numbers above the average are more common.

One limitation of the study, however, is that the findings could be artifacts of tornado observational data, which are comprised of eyewitness accounts and have been known to have issues in consistency and accuracy.

In order to get around this, Tippett re-ran calculations after replacing historical tornado data with environmental proxies for tornado occurrence and the number of tornadoes per occurrence. It was not flawless, but Tippett said it provided an independent measure of tornado activity. The results were very closely identical.

The team's findings are featured in the journal Nature Communications.

Photo : NOAA Photo Library | Flickr