Tropical Storm Barry formed in the Gulf of Mexico on July 11. It strengthened, and churned ashore as a Category 1 hurricane in western Louisiana on July 13. It weakened, and moved northward, causing rain in Arkansas and here in Missouri.
As Figure 1 shows, land-falling hurricanes in July are uncommon, though not unknown: there have been 26 in the 167 years that records have been kept. That’s one every 6.4 years. It seems like a good time to ask whether climate change has been affecting tropical cyclones?
Why might we expect climate change to affect tropical cyclones? To answer that question, you have to understand the “engine” that drives a tropical cyclone (see Figure 2). Tropical cyclones get their energy from warm, humid air on the surface of the ocean. Convection causes the warm, humid air to rise, and as it does so, it enters cooler regions of the atmosphere. This causes the humid air to condense into clouds and rain (often thunderstorms). Condensation is an exothermic process – that means that the water gives off heat as it condenses. The heat keeps the humid air rising, condensing more water, and giving off more heat. This process continues. As the air rises, it leaves an empty place where it used to be, so more air rushes in from the sides to take its place. If this air is also warm and humid, then it will rise, too, and condense into rain. If this process strengthens, then the air rises faster and faster, and the air moving in to take its place moves faster and faster. The air begins to rotate, and presto, you have your tropical cyclone.
The energy that drives all of this is the warm humid air on the surface of the ocean. Thus, it is easy to understand that anything that causes the air on the surface of the ocean to be warmer and more humid can provide more energy to a storm that might form.
What if, over the decades, the water in the oceans got warmer? Well, it would make the air above it warmer. It would also evaporate into the air more effectively, as we all know that warm water evaporates more quickly than does cold water. So warming oceans would seem to be a perfect recipe for providing more energy to tropical cyclones, making them more intense. Climate change is projected to cause the oceans to warm, and this brings us to the first article I wanted to report.
Multiple studies have reported that the heat content of the oceans has been rising. The IPCC 5th Assessment Report put the rate at 0.20-0.32 watts per square meter. However, there were many uncertainties. A recent article by Cheng, Abraham, Hausfather, and Trenberth (2019) reports that since the 5th Assessment Report, scientists have made progress in identifying and resolving the uncertainties. They review 3 studies that incorporated the advances, and find that the rate has actually been 0.36-0.39 watts per square meter. Compared to the IPCC estimate, that represents an increase of somewhere between 0.04-0.19 watts per meter.
Doesn’t sound like much, does it? The oceans are huge, however, 361,900,000 square kilometers, which translates to 361,900,000,000,000 square meters. So, the increase represents an increase of 14,476,000 – 68,761,000 megawatts. The Callaway Nuclear Generating Station in Missouri is rated at 1,190 megawatts, so the increase is equal to 12,164 – 57,782 Callaway Nuclear Generating Stations. That’s a lot of heat!
So, have tropical cyclones become more severe? Well, that is really two questions. One involves wind speed, the other involves rainfall amounts. Too many other factors affect wind speed and rainfall amounts to permit a simple comparison across storms. There is no scientific consensus regarding how climate change has affected tropical cyclones, or how it may do so in the future.
Patricola and Wehner (2019) recently published a study where they modeled the wind speed and rainfall in a suite of 15 tropical cyclones from around the world under different climates. Thus, this study doesn’t really prove anything. Rather, it clarifies what kind of effects our current theories might predict. From coolest to warmest they simulated pre-industrial climate, historical climate, RCP 4.5, RCP 6.0, and RCP 8.5. (The RCPs are standardized emission scenarios used to project the effects of climate change. The terms 4.5, 6.0, and 8.5 represent the level of radiative forcing caused by climate change. All are projected to be warmer than current climate.) They then made comparisons between the models.
Table 1 presents the results for peak wind speed measured for at least 10 minutes. The 1st column lists the name of the storm. The 2nd column gives the difference between the result of the historical and pre-industrial models. The 3rd column gives the difference between the result of the RCP 4.5 and historical models. The 4th column gives the difference between the result of the RCP 6.0 and historical models. The 5th column gives the difference between the result of the RCP 8.5 and historical models. The 6th column gives the wind speed projected by the historical model. The 7th column gives the wind speed as it was actually observed in the real storm.
Remember that the goal here is not to actually predict wind speed, but to understand the kind of effects our climate models project. The average difference in wind speed projected for RCP 4.5 vs. historical climate was 6.7 knots. The average difference in wind speed projected for RCP 6.0 vs. historical climate was 7.8 knots. The average difference in wind speed projected for RCP 8.5 vs. historical climate was 13.0 knots. Thus, for those comparisons, the hotter the climate scenario, the higher the wind speed. The outlier was for pre-industrial climate. Being cooler, the pre-industrial climate scenario should have resulted in lower wind speeds than the historical climate, yet the projection resulted in higher wind speed. Why, I’m not sure.
Table 2 presents the results for rainfall. The average difference in rainfall projected for RCP 4.5 vs. historical climate was 10.9 inches. The average difference in rainfall projected for RCP 6.0 vs. historical climate was 13.5 inches. The average difference in rainfall projected for RCP 8.5 vs. historical climate was 18.4 inches. Here, pre-industrial climate was not an outlier: the average rainfall projected for pre-industrial climate was 5.8 inches less than for historical climate.
Thus, the study shows that modeling projects that climate change will result, on average, in more rainfall per storm. The trend was linear and consistent, and no storm bucked the trend. For wind speed, results were less consistent.
Hurricane Harvey was a great example of what is projected for rain. The storm parked itself over Houston and Eastern Texas, dropping 40 inches of rain in some areas, causing extensive flooding.
But what about Hurricane Barry? It’s storm path initially projected that it would come very close to New Orleans, and it would dump 15-20 inches of rainfall. Would there be another Katrina-like disaster?
The reality turned out to be much less dire. The storm came ashore west of New Orleans, and though there were some spots that received heavy rain, most areas received much less. According to the official weather service reports from New Orleans, Baton Rouge, and Shreveport, between July 1 and July 15 they received 3.66 inches, 4.41 inches, and 0.42(!) inches. It sounds like most of the rain came from scattered thunderstorms associated with Barry, not from a widespread downpour.
Cheng, Lijing, John Abraham, Zeke Hausfather, and Keven E. Trenberth. 2019. “How fast are the ocearns warming?” Science. Vol 363 (6423), pp. 128-129. Downloaded 1/20/2019 from http://science.sciencemagazine.org.
Landsea, Chris. “Frequently Asked Questions: How many hurricanes have there been in each month?” Atlantic Oceanographic & Meteorological Laboratory. Data downloaded 7/16/2019 from https://www.aoml.noaa.gov/hrd/tcfaq/E17.html.
National Centers for Environmental Information. “Volume of the World’s Oceans from ETOPO1.” Viewed online 7/15/2019 at https://ngdc.noaa.gov/mgg/global/etopo1_ocean_volumes.html.
National Weather Service Forecast Office, New Orleans/Baton Rouge, LA. Daily Climate Report.Viewed online 7/16/2019 at https://w2.weather.gov/climate/index.php?wfo=lix.
National Weather Service Forecast Office, Shreveport, LA. Daily Climate Report. Viewed online 7/16/2019 at https://w2.weather.gov/climate/index.php?wfo=shv.
Patricola, Christina M. and Michael F. Wehner. 2018. “Anthropogenic Influences on Major Tropical Cyclone Events.” Nature. 563, 11/15/18., pp. 339-346.
Weather.gov. “Hurricane Facts”. Downloaded 7/16/2019 from https://www.weather.gov/source/zhu/ZHU_Training_Page/tropical_stuff/hurricane_anatomy/hurricane_anatomy.html.
Wikipedia contributors, “Hurricane Barry (2019),” Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/w/index.php?title=Hurricane_Barry_(2019)&oldid=906405193 (accessed July 15, 2019).
Wikipedia contributors, “Tropical cyclone,” Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/w/index.php?title=Tropical_cyclone&oldid=906385211 (accessed July 15, 2019).