“By the standards of the average climate during 1981-2000,” MIT’s Kerry Emanuel writes in Proceedings of the National Academy of Sciences, “Harvey’s rainfall in Houston was ‘biblical’ in the sense that it likely occurred around once since the Old Testament was written.”
Emanuel is a professor of atmospheric science with the Lorenz Center, Massachusetts Institute of Technology, Cambridge, MA. and is an officer of WindRiskTech, LLC, a firm that provides hurricane risk assessments to clients worldwide. The firm also provides datasets free of charge to scientific researchers.
Emanuel has attempted to answer a number of questions that will be critical to recovery and rebuilding as we begin to reassess the risks associated with extreme weather events brought on by a warming climate. He writes: “Should buildings, homes, roads, and associated infrastructure be built in the same place again? Are building codes, levees and sea walls tough enough for the future? As the world warms, every community will have to grapple with these questions on their own.
A Physics-based study of risk assessment
With the U.S. being hit with Hurricanes Harvey, Irma, and Maria, one after the other, the need for quantitative estimates of risk assessment are needed in looking toward the future. The only problem is the lack of enough recorded historical events and the fact that some of the records are of poor quality. They also lack any datasets showing changes in the climate.
Emanuel’s study takes into account a recently developed physics-based risk assessment method that can be applied to assessing the probabilities of extreme hurricane rainfall, allowing for quantitative assessment of hurricane flooding risks in all locations affected by such storms, regardless of the presence or quality of historical hurricane records.
Keep in mind that global warming did not cause Hurricane Harvey, Irma or Maria. However, global warming did influence the intensity, duration, and amounts of rainfall dumped by hurricanes. This means the likelihood of heavier rains, flooding, and destruction of property and infrastructure will increase.
What’s behind global warming’s impact on weather events?
“When you’re planning for the future of cities like Houston, it would be unwise to assume that the climate of the future is pretty much similar to what it’s been for the last 100 years or so,” Emanuel said. “Even if the climate wasn’t changing, records of rainfall are too short, and the quality of them is too low, to really get a handle on flooding risks.”
This is where a physics-based methodology is helpful. By combining observations with well-understood meteorological physics and with powerful computer models that can simulate past trends and “run” thousands of future storms, and by incorporating three climate reanalyses and six climate models, Emanuel was able to forecast that in the state of Texas, rainfall amounts will increase about 6 percent. Before this century’s out, that probability may rise to 18 percent.
Global warming will worsen tropical cyclones, also called hurricanes, for two reasons. Actually, storms of all types, not just hurricanes, are retaining more moisture because of the warmer climate. “It’s one of the risks that we’re most confident of,” Emanuel told the Atlantic. “The physics are so elegantly simple: Warm the air and it can hold more water.”
Secondly, storms will move slower because of global warming. Those of you who followed the hurricane tracking of Harvey. Irma and Maria will remember, they did not move very fast. As a matter of fact, Harvey came ashore and ended up rotating in the same place for three days.
Harvey became an atmospheric conveyor belt, picking up waters from the Gulf of Mexico and dumping them over the Houston metropolitan area. These days of rain caused the city’s devastating inland floods. Keep in mind that while tropical cyclones can have some of the fiercest winds on Earth, their movement, or storm track is determined by the slower high-altitude winds summoned by distant high- and low-pressure systems.
Emanuel explains this “planetary engine” is slowing down as climate change pushes ocean and land temperatures closer together. “If [general circulation] slows down, then places near the coast will get more rain,” Emanuel said. “But the main reason our technique shows increasing rainfall is that there’s more water in the air.”
