Hurricanes have definitely been in the news this year. Storms named Harvey, Irma, and Maria did billions of dollars of damage. We still haven't counted up the full cost. Four weeks after Hurricane Maria tore up Puerto Rico, the vast majority of the island is still without power.
Without question, these were terrible storms. Many quickly proclaimed that these storms must have been the result of global climate change, and the public should get prepared for many more. But is that an accurate assessment? The funny thing is that the same thing was said after Hurricane Katrina pounded the Gulf Coast in 2005, causing incredible destruction, including an unprecedented flood of New Orleans.
And then another funny thing happened. Not a single hurricane struck Florida, and not a single major hurricane (Category 3+) hit anywhere in the USA, for a decade. Of course, what about Hurricane Ike and Superstorm Sandy? Both were terrible storms, but Ike was Category 2 when it hit, and Sandy wasn't even officially a hurricane.
So what's happening? Greenhouse gas emissions are causing changes in our climate. I don't question that, but what that means seems to be uncertain. That likely means even more uncertainty in the future, unless and until scientists gain a better understanding of the relationship between climate change and hurricane frequency and intensity.
As a starting point for this discussion, consider how and why hurricanes form. The most basic reason is because of a combination of warm ocean water and certain types of intense thunderstorm activity. The rule of thumb is that the water temperature must be at least 79.5 degrees Fahrenheit to form and/or sustain a hurricane. The absence of warm water seems to prevent hurricanes from forming and from sustaining. That's why hurricanes break up when the encounter land.
Water temperature in the Atlantic Basin didn't dramatically decrease during the years between Katrina and the 2017 hurricanes, so why did hurricane intensity go down? Most likely because hurricane formation, strength and durability depend upon multiple factors, not just warm water and thunderstorms. So let's take a look at what those appear to be. I think you'll see that this is indeed very complicated, and thus we should be careful about our predictions of hurricane intensity due to global climate change. With that in mind, let's take a look at what else may at play.
One very key factor is what is called wind shear. It has to do with differential wind speeds between the surface and up to the troposphere, around 40,000 feet above sea level. Wind shear is a factor in both hurricanes as well as storms on land. The interesting thing is that on land, wind shear can make storms more dangerous, especially with what are called supercells, but wind shear is probably the worst enemy a hurricane can have. Wind shear, if it comes from a certain direction, tears hurricanes apart. Interestingly, if the wind shear is from a north/south direction, it doesn't seem to affect the hurricane, but if it is from the east/west direction, it can be deadly.
A 2007 study done by researchers at the University of Miami said that wind shear would likely reduce the frequency and intensity of hurricanes in the Atlantic Basin and the Eastern Pacific. The data for the ten years up to 2017 seems to bear that out, at least for the Atlantic Basin. At the same time, the Miami researchers said wind shear would likely have little effect on the frequency and intensity of hurricanes in the Western Pacific and Indian Oceans.
Why the difference? The Miami researchers hypothesized that this difference is due to what's called the "Walker circulation". This phenomenon has to do with the interplay between a high pressure system that tends to reside over the Eastern Pacific and a low pressure one that tends to reside around Indonesia. These two systems create a pressure gradient, and the interplay between the two is dynamic. From time to time, the pressure gradient weakens, or reverses, and causes a phenomenon increasingly described in weather reports: El Nino. Conversely, the Walker pressure gradient periodically strengthens, causing the opposite phenomenon: La Nina. El Nino tends to warm the waters of the Eastern Pacific. In the USA, the downside of El Nino is that there tend to be more tornadoes and other bad weather in the Southeast in the wintertime. But the nice side benefit of El Nino is that hurricane activity in the Atlantic Basin goes down. Conversely, in a La Nina, it tends to intensify.
The Miami researchers hypothesized that Walker was weakening, suggesting more El Nino events. That may have been happening over the past few years, and might explain the respite Florida had from hurricanes for ten years. However, another study, called the Twentieth Century Re-Analysis Project, calls that into question. The Twentieth Century Project has been accumulating world wide weather data for the period 1851-2014. Researchers involved in the project say that they do not see any long term weakening or strengthening of the Walker circulation.
In studying wind shear, other scientists have made another interesting observation. It appears that where hurricanes are gaining their greatest intensity may be shifting. Historically, hurricanes have been strongest in the lower latitudes, both in the Northern and Southern Hemispheres. That makes intuitive sense because water temperatures closer to the Equator are likely to be more intense. That's probably still the case, but peak hurricane intensity seems to be shifting away from the Equator, both in the North and South. The cause of the move: wind shear. The researchers found that closer to the Equator, increasing wind shear is weakening hurricanes, but farther away from the Equator, wind shear may be weakening, at least in a relative sense. That would suggest that the latitude where hurricanes are most intense would be increasing over time. In fact, the researchers note, that's what seems to be happening. The bad news about that is that population centers in higher latitudes can expect more intense hurricanes over time, other things being equal. Once again, however, the key variable appears to be wind shear.
Why were Hurricanes Irma, and Maria so strong? The evidence suggests that wind shear was pretty weak this year in the Atlantic Basin.
Strong wind shear also seems to explain why hurricanes don't form in places like the South Atlantic. Dr. Bill Gray, the noted Colorado State meteorologist who has long been an authority on hurricanes, said this is clearly the case for the South Atlantic. Only one known hurricane, Hurricane Catarina, has formed in the South Atlantic, that being in 2004, even though water temperatures there are very warm.
When it comes to hurricanes, wind shear may be our best friend, or at least the best weapon against hurricanes. The good news is the hypothesis that climate change may be increasing wind shear. In that case, higher water temperatures would be offset by increased wind shear. That could explain why we had ten years of relative calm in the Atlantic Basin, bookended by several years of bad hurricane activity.
But wind shear isn't the only variable to consider besides water temperature. Another factor is what is called the Atlantic Multidecadal Oscillation. Researchers such as Rong Zhang and Thomas Delworth have studied not only Atlantic Basin hurricanes but also rainfall in the Sahel region of Africa and the Indian Monsoon. Incidentally, Gilbert Walker of the "Walker circulation", described above, became famous because of his identification of patterns in the Indian monsoon at the beginning of the 20th century. Zhang and Delworth, as well as others, have noted the relationship between these seemingly disparate weather events. Thus, the frequency and intensity of Atlantic Basin hurricanes, as well as Sahel rainfall and Indian monsoons, may depend upon the Atlantic Multidecadal Oscillation (AMO). What impact is climate change having on the AMO? Unclear.
But an even more intriguing, and recent, study suggests even another variable in determining hurricane intensity. Earlier this month, Michael Toomey published findings online in Geology that about 12,000 years ago, Florida was ravaged by severe Category 5 hurricanes. The USA mainland was struck by Category 5 hurricanes only about four times in the past 100 years, but Toomey suggests it may have been worse back then. Here's the amazing thing: Toomey says that the water temperature was likely lower back then.
How could that be? Toomey believes that the hurricane suppressing effects of cooler water were outweighed by the side effects of slower ocean circulation. So the water might have been cooler even than now, but ocean currents were such that extremely powerful hurricanes ravaged Florida.
That was 12,000 years ago. What evidence does Toomey have? He studied sediment cores of what are called Turbidites from the Dry Tortugas, a series of islands near Key West, Florida. Turbidite is a rock that forms when sediments are disturbed and flow down across the ocean floor. Turbidites are often the result of earthquakes, but there is no evidence of earthquakes in the Dry Tortugas, so there must be a different explanation. Toomey believes the Turbidites were formed as the result of intense hurricanes.
Toomey was able to measure the size of the Turbidites. Those from about 12,000 years ago averaged 23 microns in diameter whereas more recent ones average 19 microns in diameter. Micron size thus served as a proxy for hurricane intensity.
What are we to conclude from all of this? I think the key takeaway is that the effects of climate change are clearly complicated, maybe more complicated than we ever thought. Once again, I am not denying that greenhouse gases are causing climate change. However, what exactly is the relationship between climate change and hurricane frequency and intensity is up in the air. Warmer waters certainly would suggest greater frequency and intensity, but as noted here, wind shear, ocean circulation, rain patterns on other continents, and the Atlantic Multidecadal Oscillation, also seem to play important roles.
Given the reality of climate change, perhaps the best we can hope for is that climate change will affect wind shear, ocean currents and the oscillation in ways that will tend to reduce hurricane intensity and frequency. In the meantime, when disasters like Hurricanes Harvey, Irma, and Maria occur, let's focus on helping victims. We know what difference that will make.
But even if hurricanes are becoming more frequent, and stronger, there are things we can and should be doing. However, as I pointed out in a recent blog post, what we should be focusing on actually doesn't have anything to do with climate change. That's not because climate change is unimportant (it is), just that we can and should focus on things we can more immediately control. Those include strengthening building codes, controlling construction in flood zones, and stopping the artificial subsidization of flood insurance. Doing those things will help us to control, or reduce, the cost hurricane damage. We're not going to eliminate hurricanes, even if we completely eliminate climate change, but we can significantly reduce the cost of hurricane damage if we pursue some of these policies.