Climate science at a glance
- The 2017 Atlantic hurricane season produced 17 named storms, 10 hurricanes and six major hurricanes, including three Category 4 hurricanes that made landfall in the United States: Hurricanes Harvey, Irma, and Maria. It was the most active season since 1936.
- Human‐caused climate change is supercharging hurricanes and exacerbating the risk of major damage.
- Sea level rise has significantly extended the reach of storm surge and coastal flooding driven by hurricanes.
- Global warming is fueling extreme rainfall, increasing the threat of rain and flooding driven by hurricanes.
- Hurricanes have grown stronger over recent decades. There is a significant risk global warming may be driving that trend.
- Infrastructure built to withstand the storms of the past can collapse as new extremes overwhelm and push infrastructure past design thresholds.
Climate signal breakdown
Climate signal #1: Increased storm surge
The most important impact of tropical cyclones in coastal regions is storm surge. Increases in storm surge related to climate change can be due to rising seas, increasing size, and increasing storm wind speeds.
Climate change has already contributed about 8 inches (0.19 meters) to global sea level rise, and this has dramatically amplified the impact of cyclones by increasing baseline elevations for waves and storm surge. A small vertical increase in sea level can translate into a very large increase in horizontal reach by storm surge depending upon local topography. For example, sea level rise extended the reach of Hurricane Sandy by 27 square miles, affecting 83,000 additional individuals living in New Jersey and New York City and adding over $2 billion in storm damage. Aided by sea level rise, Hurricane Matthew set several storm tide records during its approach to the eastern sea board.
Observations consistent with climate signal #1
To be determined as hurricane season unfolds.
Climate signal #2: Increased extreme precipitation
Climate change is fueling extreme rainfall and dramatically increasing rainfall across many types of storms. Estimates of the impact have been calculated for individual hurricanes (Katrina and Ivan), and an increase in rainfall rates is one of the more confident predictions of the effects of climate change on tropical cyclones.
One of the clearest changes in weather globally is the increasing frequency of heavy rain. Over the past century the US has witnessed a 20 percent increase in the amount of precipitation falling in the heaviest downpours. Over the period from 1994-2008, extreme precipitation events linked to hurricanes accounted for more than 33 percent of the observed increase in heavy events across the US. The Southeast in particular saw a 40 percent increase in the amount of precipitation falling in the heaviest events, and 100 percent of this increase is linked to hurricane events.
The increase in extreme precipitation has increased the threat of flooding, dramatically illustrated by the 1,000-year rainfall dropped by Hurricane Matthew. Inland cities near large rivers in the US now experience more flooding, and this shift is consistent with climate change.
As the global average temperature increases, so too does the ability of the atmosphere to hold and dump more water when it rains. Atmospheric water vapor has been increasing. And the observed increases have been studied and formally attributed to global warming.
Storms reach out and gather water vapor over regions that are 10-25 times as large as the precipitation area, thus multiplying the effect of increased atmospheric moisture. As water vapor condenses to form clouds and rain, the conversion releases heat that adds buoyancy to the air and further fuels the storm. This increases the gathering of moisture into storm clouds and further intensifies precipitation.
Over the last three decades, extreme and record-breaking rainfall events have significantly increased globally, and the fingerprint of global warming has been firmly documented in this pattern. Climate change is now responsible for 17 percent of moderate extreme rainfall events, i.e. 1-in-3 year events. The more extreme the event, e.g, 1-in-30 year events, the more likely climate change is responsible, as climate change affects the frequency of the extreme events the most.
Observations consistent with climate signal #2
To be determined as hurricane season unfolds.
Climate signal #3: Increased sea surface temperatures
Tropical cyclones are fueled by available heat. Warming seas have increased the potential energy available to passing storms, effectively increasing the power ceiling or speed limit for these cyclones. In parallel there has been a global increase in the observed intensity of the strongest storms over recent decades.
According to the Fourth US National Climate Assessment (2017), "Both theory and numerical modeling simulations (in general) indicate an increase in TC intensity in a warmer world, and the models generally show an increase in the number of very intense TCs," And in looking at recent hurricane trends in the Atlantic, the Assessment further reports that "there is broad agreement that human factors have had an impact on the observed oceanic and atmospheric variability in the North Atlantic, and there is medium confidence that this has contributed to the observed increase in hurricane activity since the 1970s." 
There is an extremely wide range of natural variability in tropical cyclone activity, making definitive attribution of recent trends challenging. 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. In addition, other factors affected by climate change, such as wind shear and the pattern of regional sea surface temperatures, also play controlling and potentially contradictory roles. The balance of all these factors is not fully known.
Nevertheless, the fingerprint of global warming in the intensity of tropical cyclones has been identified in one ocean basin: the Northwest Pacific. In 2015, accumulated cyclone energy (ACE) in the western North Pacific was extreme, and human-caused climate change "largely increas[ed] the odds of the occurrence of this event," according to an assessment study conducted by a large group of scientists led by NOAA and published in the fifth edition of "Explaining Extreme Events from a Climate Perspective" by the Bulletin of the American Meteorological Society.
Virtually every study that has been done, dating to 1987, shows increasing potential intensity in most places as our climate continues to warm; the average trend is about 10 mph for every degree centigrade of tropical sea surface temperature increase, or roughly 20 mph for each doubling of atmospheric CO2 concentration.
- Kerry Emanuel, MIT, member National Academy of Sciences
Observations consistent with climate signal #3
To be determined as hurricane season unfolds.
North Atlantic trends
As seas warm and offer more heat energy, there has also been an increase in the observed intensity of the strongest storms in the North Atlantic over recent decades. The Third US National Climate Assessment (2014) reports that "the intensity, frequency, and duration of North Atlantic hurricanes, as well as the frequency of the strongest hurricanes, have all increased since the early 1980s."
Looking back further, a reconstruction of North Atlantic hurricanes dating back to the late 1800s adjusts the historical record in an attempt to account for hurricanes that went unrecorded in the early years, and after adjustment finds that there has been no century-scale trend. However, prior periods of increased hurricane activity due to natural variation do not rule out a role for global warming in the current period of increased activity since the early 1980s. North Atlantic hurricane power is strongly correlated with the temperature of the tropical Atlantic during hurricane season, and both have been increasing over the last several decades.
The close correlation between observed trends in recent sea surface temperatures and observed trends in the intensity of tropical cyclones may reflect a link between warming ocean temperatures and stronger storms. The increase is well-documented and is consistent with models projecting the future climate, but the correlation between these two trends is not fully understood and may be negated by other factors also driven by climate change. Scientists are still working to better understand the relation between these two trends.
Looking forward, there is a uniform projection by climate models that the frequency of the strongest cyclones will increase. Most models also project that the overall number of cyclones will decline even as the frequency of the most powerful storms increases. And there is some evidence this trend, driven by warming seas, may already be underway.
Unfortunately, these opposing trends do not balance out. The damage caused by hurricanes increases geometrically with wind speed, becoming disproportionately greater as wind speed increases. As a result, under the present climate, almost 93 percent of tropical cyclone damage is caused by the top 10 percent of storms. Moreover, some results point to an increase in frequency across all storms. There is a large uncertainty in these projections which represents considerable risk.
We expect to see more high-intensity events, Category 4 and 5 events, that are around 13 percent of total hurricanes but do a disproportionate amount of damage. The theory is robust and there are hints that we are already beginning to see it in nature.
- Kerry Emanuel, professor of atmospheric science, MIT
For a detailed, peer-reviewed consensus synthesis of the science published through 2014 on climate change and tropical cyclones see, "Tropical cyclones and climate change," by Walsh et al, 2015.
In addition, see the Global Warming and Hurricanes: An Overview of Current Research Results, a web resource published by NOAA's Geophysical Fluid Dynamics Laboratory.
Frequency of landfalling hurricanes
While there is a clear long-term trend in tropical cyclone activity over recent decades in the North Atlantic, the wide range of natural variability in tropical cyclone activity is particularly visible over shorter time periods. This is especially true for hurricanes making landfall in the United States, which are relatively scarce as the large majority of hurricanes in the Atlantic recurve away from the continent and travel eastward as they move north.
A frequency analysis published in the Bulletin of the American Meteorological Society established that the arbitrary choice of metrics is solely responsible for the illusion of a "drought" in land falling hurricanes in the U.S. prior to 2017. For example, these metrics exclude Hurricane Sandy, the strongest storm on record to make landfall north of Cape Hatteras.
Location and steering
Globally, tropical cyclone activity has migrated poleward (northward in the northern hemisphere) over the last several decades, and this movement as been tentatively linked to the expansion of the tropical zone driven by global warming.
Over the past 30 years, tropical activity (as measured by lifetime-maximum intensity) has not shifted northward for Atlantic Hurricanes. However, a recent analysis of Atlantic hurricanes extending back for 450 years reports a clear shift in the track of Cape Verde tropical cyclones north-eastward from the western Caribbean toward the North American east coast, driven since 1870 largely by anthropogenic greenhouse gas and sulfate aerosol emissions. Cape Verde hurricanes form in the low tropics, and are often largest, most intense, and longest-lived storms of the season.
The affect of climate change on the steering of hurricanes is extremely uncertain, and the research reports conflicting indications. Most Atlantic hurricanes recurve eastward away from the coast of the United States. However, recent science on blocking patterns formed by weather patterns driven by climate change indicates there is a possibility of new atmospheric weather conditions emerging that could steer hurricanes towards the eastern seaboard, as witnessed during the travel of Hurricane Sandy through the Atlantic. While the uncertainty is extremely high, this uncertainty translates as new risk.
Climate models project stronger but possibly fewer or the same number of storms in a warming climate. However the combination of fewer but stronger storms would likely lead to an increase in overall damages. Damage incurred by tropical cyclones is overwhelmingly and disproportionately incurred by the most powerful storms, indicating that overall damage will increase as the climate warms.
Cyclone wind damage increases geometrically with an increase in speed. A decrease in frequency would not be expected to offset the increase in damages incurred by increasing intensity. Moreover, damage escalates exponentially when storm strength crosses over the thresholds beyond which threatened infrastructure collapses.
It's the high end events that are the most destructive historically....More than half the damage that's been done in the United States by storms dating back to the middle of the 19th century has been done essentially by just eight events. So it really is the rare events, the Katrinas, the Sandys, that do the overwhelming amount of damage. Your average run-of-the-mill hurricane will do some damage and be memorable in the local place that if affects, but it doesn't really amount to a hill of beans compared to what the big storms do.
- Kerry Emanuel, Professor of Atmospheric Science, MIT