Is climate change frequent hurricane and cyclones?

Hurricane Milton is approaching Florida, bringing “potentially catastrophic” winds, storm surges, and heavy rainfall, according to U.S. forecasters. The storm is expected to hit the densely populated Tampa Bay area, less than two weeks after the state was battered by Hurricane Helene.

Milton is the ninth hurricane of the 2024 Atlantic season, which lasts until the end of November. Hurricanes are powerful storms that form over warm tropical ocean waters. In other parts of the world, these storms are called cyclones or typhoons and are collectively known as tropical cyclones.

Tropical cyclones are defined by strong winds, intense rainfall, and storm surges, which often lead to extensive flooding and damage. Major hurricanes, classified as category three and above, have sustained winds of at least 111 mph (178 km/h).

Hurricanes, typhoons, and cyclones originate from atmospheric disturbances, such as tropical waves or low-pressure systems where thunderstorms and clouds develop. As warm, moist air rises from the ocean, the winds within the storm clouds start to rotate, a phenomenon influenced by Earth’s rotation near the equator.

For a hurricane to develop and sustain its spin, sea surface temperatures generally need to be at least 27°C, and the wind conditions should not vary significantly with altitude. When these factors align, a powerful hurricane can form, though the exact triggers of each storm are complex.

Globally, the frequency of tropical cyclones has not increased over the past century. In fact, the number may have decreased, though reliable long-term data is scarce in certain regions. However, according to the UN’s climate body, the IPCC, it is “likely” that a greater proportion of tropical cyclones globally now reach category three or higher.

The IPCC reports “medium confidence” that both average and peak rainfall rates associated with tropical cyclones have increased. In the Atlantic, “rapid intensification events,” where wind speeds surge quickly, have also likely become more frequent. These rapid changes can be particularly hazardous.

There is also evidence that tropical cyclones have slowed down, leading to prolonged rainfall over affected areas. For instance, in 2017, Hurricane Harvey stalled over Houston, releasing 100 cm of rain in just three days.

In some regions, such as the western North Pacific, tropical cyclones are reaching their peak intensity farther from the equator, which exposes new areas to storm risks. In the U.S., there is some evidence suggesting that the increased intensity of hurricanes is causing greater damage.

Determining the precise influence of climate change on individual storms is challenging, given their localized and short-lived nature, as well as their variability. Nonetheless, rising global temperatures are impacting these storms in measurable ways.

Warmer ocean waters, due to long-term greenhouse gas emissions, are allowing storms to gain more energy, leading to stronger winds. Record high sea surface temperatures were a key factor in the above-average Atlantic hurricane season forecast for 2024.

Additionally, a warmer atmosphere can hold more moisture, resulting in more intense rainfall. For example, the extreme rainfall from Hurricane Harvey in 2017 was estimated to be three times more likely due to climate change.

Sea-level rise, driven by melting glaciers and expanding warmer waters, further exacerbates coastal flooding during storm surges. Local factors can amplify these effects. It is estimated that the flooding from Hurricane Katrina in 2005 was 15-60% higher than it would have been under the climate conditions of 1900.

Overall, the IPCC has “high confidence” that human activity has contributed to increased precipitation linked to tropical cyclones and “medium confidence” that it has made storms more intense.

The IPCC suggests that while the total number of tropical cyclones may not increase, it is “very likely” that higher rainfall rates and more intense wind speeds will become more common as global temperatures rise. A greater proportion of storms are expected to reach the most severe categories, four and five.

If global temperatures rise by 1.5°C, the proportion of category four and five storms could increase by around 10%, going up to 13% at 2°C and 20% at 4°C, although these projections come with some uncertainty.