Storms and Confidence

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While trends in extreme heat, cold episodes, and heavy rainfall have shown some global variation, scientists are confident that global warming significantly influences these patterns. However, determining how global warming impacts smaller-scale or short-lived storms—like hurricanes (tropical cyclones), mid-latitude low-pressure systems (extratropical cyclones), and severe thunderstorms (convective storms)—is more complex.

The figure below provides a sense of the certainty levels regarding links between global warming and different types of extreme weather events. Those toward the upper right of the graph represent trends with higher confidence in their connection to global warming, while those in the bottom left indicate areas with less certainty. Notably, the graph includes "extreme rainfall" but not "flooding" as a separate category. Trends in flooding are highly localized, influenced by factors such as urbanization, which affects water absorption and drainage into nearby streams and rivers. Poor urban planning, for instance, can lead to increased flooding regardless of trends in heavy rainfall. This means that while global warming may influence extreme rainfall, local land-use changes often play a major role in determining flooding outcomes, making it challenging to link flooding trends directly to climate-driven changes in rainfall.

Scientists are most confident in linking changes in episodes of extreme heat and extreme cold to global warming, followed by droughts and extreme rainfall. Confidence in attributing changes to smaller, shorter-lived atmospheric phenomena like tropical cyclones (such as hurricanes), extratropical cyclones (mid-latitude low-pressure systems), and severe convective storms (thunderstorms) is lower.

Credit: National Academy of Sciences

Interest in the links between global warming and extreme storms like hurricanes, severe thunderstorms, or tornadoes is high. However, there’s less certainty in understanding these connections. Let’s focus on tornadoes and tropical cyclones (hurricanes), since they’re often in the spotlight. Part of the challenge with linking these storms to global warming is the relatively short period of quality observations we have of them.

For instance, while the number of strong tornadoes in the U.S. hasn’t shifted much since 1950, the frequency of weak tornadoes has risen significantly (see the graph below). Is this increase due to global warming? Probably not. Instead, the rise is largely due to improved storm detection with the introduction of NEXRAD Doppler radar in the early 1990s, which allows for more accurate tracking of storms that might produce tornadoes. Prior to advanced radar, only tornadoes easily observed and reported from the ground were included in the records—meaning in parts of the central U.S. where much of the landscape is dominated by acres and acres of crops instead of humans, weaker tornadoes likely went undetected and/or reported!

The number of tornados observed per year over the United States, broken down by Fujita scale intensity.

Credit: Ian Livingston / ustornadoes.com

A similar story applies to tropical cyclones. From 1980 to 2017, hurricanes dominated the list of costliest U.S. weather disasters (credit: NCEI), underscoring their substantial societal impact. But is this impact due to human-induced warming making hurricanes more frequent or intense? It’s complicated. Reliable observations only go back to the 1970s when global satellite coverage began; prior to that, storms that didn’t make landfall or encounter ships often went unrecorded. Adjusting for this, the data suggest the overall number of tropical cyclones worldwide has changed very little with warming, although a higher percentage of hurricanes are reaching extreme strength (sustained winds over 110 mph). Still, the rise in hurricane damage appears to be driven more by increased coastal development—more people and infrastructure to be affected—than by warming itself.

Yet, some trends consistent with global warming are evident. For instance, rising sea levels amplify coastal flooding during tropical cyclones. Also, the areas where tropical cyclones develop are expanding, and these storms are reaching peak intensities farther from the equator than a few decades ago.