The frequency and magnitude of tornado outbreaks with many, or clusters, of tornadoes has increased in the United States over the past 50 years, a new study reports. But whether this trend is driven by human-induced climate change, or other factors, remains unclear, making it difficult to predict whether it will continue. Tornado outbreaks are sequences of six or more tornadoes that occur in close succession; in the United States, these so-called clusters caused 79% of tornado-related fatalities between 1972 and 2010. In analyzing how tornado outbreaks have changed between 1965 and 2015, Michael K. Tippett and colleagues found that, over five-year periods, the estimated number of tornadoes in the most extreme outbreaks roughly doubled - from 40 in 1965 to nearly 80 in 2015. Even if the data is adjusted for differences in weather monitoring and recording, a significant increase in both frequency and intensity has occurred over recent decades. Intriguingly, the authors found that this increase did not correspond with factors that are associated with climate change, such as convective available potential energy. They propose that another factor, such as one that drives low-frequency climate variability, may be at play. One example of such a driver is the Atlantic Multidecadal Oscillation (AMO), an oscillating pattern of sea surface temperatures that's known to affect climate in North America. Indeed, changes in the AMO do correlate with changes in tornado outbreaks; however, more evidence is needed to truly pinpoint the underlying cause of the increase. A better understanding of the cause of these gustier, more frequent events is important for predicting whether this trend will continue in the future, the authors conclude.