Japan is being pummeled today by Tropical Storm Neoguri, which was briefly a super typhoon (meaning winds in excess of 150 miles per hour) before. Last week, the Atlantic hurricane season produced its first named storm, Arthur, which intensified to an unusually early Category 2 hurricane and struck North Carolina before weakening and moving to the northeast, dropping huge amounts of rain on parts of New England and Eastern Canada on the way. While both storms are atypical in some ways, neither has been catastrophic.
But with big storms so much in the news, this seems like a great time to address the role of climate change. Is global warming making hurricanes worse? That question is actually a whole swarm of questions, each of which is difficult to answer for its own particular reasons. Because are are, as yet, no simple soundbite answers, it is important to understand the science in order to be able to put the sometimes conflicting press releases we hear into perspective.
Today we’ll start by exploring just one of the swarm of questions; has climate change made hurricanes and related storms worse so far. We’ll save the other questions for subsequent posts.
A Note on Terminology
“Tropical cyclone” is the generic term that covers tropical storms, hurricanes, typhoons, and clones. All these storms have a distinct eye and draw their energy from the evaporation of water, rather than from temperature differences between adjacent air masses as extra-tropical cyclones do.
“Tropical storm” refers to a tropical cyclone with sustained winds of anywhere from 39 MPH to 74 MPH. Once a storm intensifies to 75 MPH or beyond, it is called a typhoon in the Northwest Pacific, a cyclone in the South Pacific or the Indian Ocean, and a hurricane everywhere else. I have not found any explanation for this diversity of names for the same kind of storm. Perhaps it is a relic from a time before we knew they were all the same.
Has Global Warming Make These Storms Worse?
The short answer is yes. Whether or not climate change has intensified the storms themselves, sea level rise is unquestionably making them more destructive.
Since the 1880’s, global sea level has gone up eight inches, so that storm surges, the dome of high water that tropical cyclones push ahead of them, are eight inches higher than they otherwise would have been.
Nine feet, the total height of Superstorm Sandy’s surge, might not seem that different from eight feet, four inches, which is what it would have been without sea level rise. For the people who got all nine feet, it probably isn’t that different. But for the people on the edge of the flood who got eight inches, the extra height mattered a lot. For Sandy, each extra inch meant roughly 6000 more people were inundated.
But Have Tropical Cyclones Changed?
This one is harder. You’d think that all a scientist would have to do would be to count up the number of tropical cyclones in each wind speed category per year, crunch the numbers, and see whether there is a trend.
Tropical cyclone records are being studied, but the problem is the data are “noisy.” That is, there are so many variations that are not related to the greenhouse effect that it’s hard to spot the variations that are.
A further complicating factor is that wind speed is only one aspect of a tropical cyclone. From a human perspective, it isn’t even the most important, because most people who die in these storms don’t die of wind. They die of water, either in storm surges along the coast or in freshwater floods from so much rain. The size of the storm surge, the amount of rainfall, the diameter of the storm, and how long it sits on top of any one area can all vary independently of wind speed and of each other. Wind speed category is only a rough approximation of how “bad” a storm is.
What’s All That Noise?
Some of the noise in tropical cyclone data is the natural variability in storminess from year to year. Normally scientists can tune out such noise by looking at a large enough dataset. The basic procedure is to let random variations cancel themselves out–years with a lot of hurricanes are balanced by years with very few, if you look at enough years. What variation doesn’t get cancelled out is actually the climate changing.
But with tropical cyclones that standard procedure doesn’t work very well because there are problems with the data:
- We don’t have good records of tropical cyclones before the Industrial Revolution. Scientists only started realizing that some large storms are spirals around 1820. Modern weather forecasting based on networks of weather stations didn’t begin until the 1860’s and most of the technology used to monitor hurricanes was only invented in the 20th century. It’s hard to do a before-and-after comparison if you have no “before” shot.
- The United States has been conducting aerial reconnaissance on hurricanes for decades, but since similar flights into typhoons have stopped, the data on storms in different parts of the world are not directly comparable. That makes it hard to really get a global picture.
- A lot of research on tropical cyclones is done by satellite, especially in the Pacific, but satellites are a relatively new technology so, again, we don’t have a good picture of how storms change over time.
- Which information we get about which storm is a little random. For example, getting a measurement of a storm’s highest winds at landfall depends on getting the right instrumentation into the right part of the storm at the right time. For obvious reasons, that doesn’t always happen.
- The conventions on how researchers analyze data and how they make estimates can change, subtly but definitely changing the numbers they record.
Scientists can and do work around these limitations, but they can’t make the limitations vanish. The bottom line is that just because scientists have trouble saying whether global warming has already changed tropical storm behavior doesn’t mean it hasn’t.
What We Do Know
At least one study has shown a trend towards more tropical cyclone intensity, at least for some ocean basins. That is, these storms are not getting more numerous, but they are getting stronger. A greater proportion of them reach Category 4 or 5. Another recent study suggests the storm tracks are changing, carrying more tropical cyclones into subtropical and temperate areas.
We know, as already mentioned, that the sea level is rising. We also know that the sea is getting warmer and that sea surface temperature and the total energy in tropical cyclones per year track each other pretty closely. Both have been trending up, at least in the Pacific.
Sea temperature at depth is also going up and that might matter even more.
When a large tropical cyclone passes across warm surface water, the storm grows, but it also stirs up the water, allowing cooler water from below to come up. This cooler water acts as a break, not only on the storm doing the churning, but also on any second storm that might be following behind. If the water at depth is also warm, though, there is no break.
Both Hurricane Katrina (which was a Category 5 storm before it weakened just before landfall) and Typhoon Haiyan intensified to catastrophic proportions right as they passed over very deep pools of unusually warm water. In fact, beneath Haiyan, possibly the most powerful storm ever to make landfall, the sea was hot enough to feed a typhoon—about 80 F.—even three hundred feet down.
No word yet on what the deep water is like under Neoguri.