Climate science at a glance

• Hurricanes have grown stronger over recent decades. There is a significant risk global warming may be driving that trend.
• Increasing surface land and ocean temperatures are increasing the potential energy available to passing storms.[1]
• Irma intensified over sea surfaces that were 0.9°F to 2.25°F (0.5°C to 1.25°C) above average, relative to a 1961-1990 baseline.[2] (Note: This does not account for warming that occurred before the baseline period.)
• The record strength of Irma is consistent with observed trend of increasing hurricane intensity.
• Irma’s rapid intensification is also consistent with the observed trend toward rapidly intensifying tropical cyclones, particularly in the North Atlantic and Caribbean.[3][4]
• Irma’s track is consistent with the observed shift in the track of Cape Verde tropical cyclones, driven by human-caused climate change.

The warmer the waters, the more energy available to passing storms

Atlantic hurricanes are getting stronger. Studies have found a 30-year trend showing an increase in Atlantic tropical cyclones strength alongside an increase in ocean temperatures over the Atlantic Ocean and elsewhere.[5][4] So far, the fingerprint of global warming in the intensity of tropical cyclones has been definitively identified in one ocean basin: the Northwest Pacific.[6]

“[Hurricanes] extract heat energy from the ocean to convert it to the power of wind, and the warmer the ocean is, the stronger a hurricane can get . . . So, scientists are confident that as we continue to heat up the oceans, we’re going to see more of these high-end perfect storms.”[7] — Jeff Masters, meteorologist and former hurricane hunter

Climate change also affects other factors that shape and control hurricane development, such as wind shear. In addition, hurricanes are powered by the difference between ocean temperature and the temperature at various levels in the atmosphere. It is the difference that matters, not the absolute value of the sea surface temperature. The balance of all these factors is not fully known. However, hurricanes have grown stronger over recent decades. And there is a significant risk global warming may be driving that trend.

“Irma certainly fits the pattern of increasingly strong hurricanes [which is] precisely what studies have predicted we would see as a result of human-caused warming.”[7]

— Michael Mann, a leading climatologist

Irma rapidly intensified over unusually warm water

In just over 24 hours, from September 4 to 5, Irma intensified from a Category 2 hurricane with observed wind speed of 106 mph to a Category 5 hurricane with a maximum observed wind speed of 160 mph.[8] Irma continued to intensify on September 5 over unusually warm water, reaching wind speeds up to 185 mph. Irma’s wind speeds are so high, they have challenged the region’s maximum estimated storm intensity,[9] an estimate based on current water temperature and other conditions.[10]

Irma grew in size amid nearly ideal circumstances for intensification[10], with low wind sheer and sea surface temperatures in the range of 0.9°F to 2.25°F (0.5°C to 1.25°C) above average, relative to a 1961-1990 baseline.[2] Sea surface temperatures (SSTs) were a very warm 29.5°C (85°F), and the total heat content of the ocean was a high, giving the storm plenty of heat energy to fuel intensification.[11]

Sea surface temperatures have been consistently higher during the past three decades than at any other time since reliable observations began in 1880.[12]

Warming seas are increasing the potential energy available to passing storms, effectively increasing the power ceiling or speed limit for these cyclones.[13] This trend is strongest in the Atlantic, where rising ocean temperatures correlate closely to an increase in Atlantic tropical cyclone strength.[5]

However, other factors, such as wind shear and the global pattern of regional sea surface temperatures, also play controlling roles. The balance of these factors is not fully known.[14]

Climate change driven hurricane migration

Globally, tropical cyclone activity has migrated poleward (northward in the northern hemisphere) over the last several decades, and this movement has been tentatively linked to the expansion of the tropical zone driven by global warming.[15]

In the Atlantic, a recent analysis extending back 450 years reports a clear shift in the track of Cape Verde tropical cyclones like Hurricane Irma.[16] Cape Verde hurricanes form in the low tropics, and are often the largest, most intense, and longest-lived storms of the season. The study found a north-eastward shift from the western Caribbean toward the North American east coast, driven largely by human-caused climate change since 1870.[16]

Irma’s track is consistent with this shift.

Major storms are spinning up faster

In the North Atlantic, storms have shaved almost a day (20 hours) off their spin-up to Category 3. Storms are intensifying at a much more rapid pace than they used to 25 years back,” reports the author of a 2012 study. They are getting stronger more quickly and also [to a] higher category. The intensity as well as the rate of intensity is increasing.” The vast majority (79 percent) of major storms are rapid intensification storms, and the most intense storms are those that undergo rapid intensification according to a 2016 study.[17] These findings have been confirmed by a 2016 study which in turn linked the trend to warming seas.[18]

This trend toward more rapid intensification makes it much harder to predict and plan for the most intense storms.