As Tropical Storm Henri batters much of New England this weekend with damaging winds, heavy rainfall, storm surges and a large possibility of inland flooding, it may feel like one more item to add to the list of abnormal weather events we’ve seen this year.
And what a year it’s been — from wildfires out west and record heat here in New England, to deadly flooding in Germany and China — weather events in 2021 have led many to ask, “Is this climate change?”
To understand what the latest science tells us about the connection between climate change and hurricanes, we called Kerry Emanuel, professor of atmospheric science at MIT and an expert on how climate change affects extreme weather.
Our conversation has been edited for length and clarity.
Let’s start simple. What is a hurricane and how does it form?
"A hurricane is the local term for a generic phenomenon called a tropical cyclone that occurs worldwide — and the Atlantic, although it gets a lot of attention, only has about 11% of the world’s tropical cyclones.
"Hurricanes are rotating storms that can be very violent. They are powered by the transfer of heat from very warm ocean waters to the atmosphere, and they tend to develop exclusively over warm ocean waters. Most of them form over the tropics and they tend to move with the prevailing winds, which are east to west. If they begin to venture into higher latitudes, they can get caught up in the westerlies — west to east winds — and typically curve off to the east.
"We characterize hurricanes by their position, the way they're moving, how large they are in diameter, how strong they are, and, of course, how much rain they are producing."
Since the Industrial Revolution, humans have caused the planet to warm 1° C (1.8° F). How has this warming affected Atlantic hurricanes?
"We should begin with what we understand theoretically about this and then get to the observations. So theoretically, we've known for about 30 years that when you raise the temperature, especially if you do this with greenhouse gases, it's possible for tropical cyclones to become stronger. It doesn't mean they always will or necessarily will, but based on the laws of thermodynamics, per degree of warming, their wind speeds can increase by about 7%.
"The other thing that we're very confident of, because the physics is very simple, is that in a warmer climate, any strong storm like a hurricane will produce much more rain. That actually is the main source of concern because water is the big killer in hurricanes; wind is what we associate primarily with these storms, but water is the bigger problem. And not just freshwater from rain, but saltwater. Strong winds can create something called storm surge, which is physically the same phenomenon as a tsunami, except that it's produced by wind rather than the shaking sea floor."
Have we seen an increase in the number of Atlantic hurricanes because of climate change?
"Well, yes and no. There's no question there's been an increase in the number of Atlantic hurricanes since the 1980s. And while it’s climate change, it isn’t necessarily greenhouse gas-related. And again, it’s very specific to the Atlantic and not something we see anywhere else on the planet.
"There's been a lot of sleuthing on the part of scientists to figure out what's going on here, and we now think that there actually was a man-made hurricane 'drought' in the 1970s and '80s that was caused by a lot of particulate matter being released into the atmosphere from fossil fuel combustion. These particulates, particularly sulfur-based aerosols, produced hazes that somewhat cooled the tropical Atlantic in summertime, meaning fewer hurricanes.
"People who grew up in the East Coast in the '60s, '70s and '80s will remember that when it was hot and humid, it was always hazy. But since the passage of the Clean Air Act, we’ve been remarkably successful in cleaning up these particulates and the hurricanes have come roaring back.
"So the big increases in hurricanes that we've seen since the '80s are probably due to the decrease in man made aerosols — we would also call that climate change. But it's not to be confused with global warming, which is a very specific kind of climate change. [Editor’s Note: Many scientists also believe that natural climate variability may have played a role in this too.]
"That said, when we look at the number of tropical storms globally over the last few decades, it hasn’t changed much."
What about the intensity of hurricanes and the speed at which they move? Is climate change affecting those things?
"The two things we are confident of is that as you warm the climate, hurricanes can become more intense and we would expect to see more high category hurricanes. The other thing that we're very confident about is that it will rain more during these storms. What we're not confident about is the frequency of weak storms. They could go up, they could go down.
"As for the speed of storms, well, that's controversial in science. The models we use suggest that as the climate warms globally, we ought to see a diminution of the speed with which they move over the sea in the subtropics, which in the case of the U.S. includes places like Florida and the Gulf of Mexico. But this might not necessarily be the case further south in the Caribbean and further north at our latitudes.
"So while we really don't know, there have been some rather spectacular cases of slow moving storms producing huge rainfalls. For example, Hurricane Harvey in Texas in 2017 — it was a storm that stalled and caused really incredible, very rare kinds of flooding."
Is climate changing affecting the path of hurricanes?
"Climate change affects everything. So when we look at hurricanes, we have to look at everything. So, yes, it affects the path of hurricanes because hurricanes are embedded in airflow at various different levels in the atmosphere and climate change changes that. Climate change also changes where hurricanes form, and it changes how intense they get.
"However, as is true in almost any aspect of global climate change, it is different from one region to the next. There are places on the planet where we actually expect hurricane activity to go down, just as there are a few places on the planet where the temperature is actually falling. So I like to say all climate change is local, in that it affects different regions differently."
So let’s bring it local. It’s pretty rare, though obviously not unheard of, for hurricanes to make landfall in New England. How might climate change affect the likelihood of this happening in the future?
"We believe from calculations we've done that as the world continues to warm, we will see an increase in hurricane risk in New England. Almost all the models agree with that. Now, bear in mind, as you stated, hurricanes are rare in New England. The last time we had a real hurricane in New England was Bob in 1991, so 30 years ago.
"Hurricanes are somewhat unlikely in New England for a variety of reasons. They always form over warm tropical ocean water, and that is something we decidedly do not have around New England. They form much further south and tend to make landfall in the Caribbean, the Gulf of Mexico, or the southeastern U.S. Only a few manage to head toward New England and many that do end up curving out to sea as they encounter the west to east winds of our higher latitudes."
People around the world have experienced a lot of extreme weather this summer. In the past, we in the media have often used the caveat in our reporting that it’s hard to say that any specific weather event is the result of climate change, but in the last couple of years we’ve seen a rise in attribution science — the ability to calculate how much influence climate change likely had on the way any given weather event played out. Have scientists been able to “attribute” anything hurricane-related to climate change?
"Well, yes and no. It depends on the measure. So, for example, science was able to determine that a lot of the rain that flooded Houston in Hurricane Harvey could be attributed to climate change. But it’s not possible to do that in every storm.
"I think the strongest part of attribution science is when we can attach probabilities to events and determine how those probabilities are evolving. So if you ask a very specific question — like, what's the probability of experiencing 70 mile per hour winds on the south shore of New England during a hurricane, and how is that evolving over time? — we can begin to answer that."
The U.N. Intergovernmental Panel on Climate Change just came out with a new alarming report that says we’re pretty much locked into more global warming in the near future. So as we’re thinking about the next few decades, what can we expect to see during hurricane season?
"Given the IPCC report and all the science behind it — of which I've been involved in the hurricane-relevant parts — we can expect to see new records set for the intensity, or wind speeds, of hurricanes. Much more consequential than that in terms of human lives, suffering and destruction, is that we're going to see a lot of records broken for rainfall and rainfall-induced flooding from tropical cyclones. That’s mostly because storms are going to be wetter and because in some places they’re going to slow down and linger longer."
Any final thoughts?
"I think it's very important for people to be looking at climate change as a problem of assessing and managing risk, and not as a problem of signal detection. What I mean by that is that we shouldn't wait to deal with the problem until we can show with 95% certainty that the hurricanes, wildfires, the melting of the Arctic ice we’re seeing is because of climate change.
"We should be doing things like innovating carbon-free energy and possibly coming up with ways of taking carbon out of the atmosphere now because we can't afford to wait and because we'll never have complete certainty about anything."