The story explaining the dramatic flooding in Houston during Hurricane Harvey has many chapters, ranging from climate history to the history of groundwater use and development zoning. The chapter on climate change has had a few pages filled, thanks to a study quickly published by an MIT hurricane scientist Kerry Emanuel. This week, two complementary studies flesh out the chapter more.
The first paper comes from a group of scientists who have worked to analyze a number of extreme weather events over the past few years, including flooding in Europe and Louisiana last year. The general process for this type of activity is not entirely different from tracking home run hitting by baseball players. You never really know if an individual’s home run will happen no pain no gain steroids, but that’s not the point. Instead, it works out whether the house is running as the one that just generally proves to be juicy.
In this case, the researchers were able to build on their analysis of the nearby Louisiana flood from 2016. As in that study, they analyzed the history of rainfall measurements in the region to work out how is unusual however the extraordinary rainfall totals from Harvey are- and perhaps the chances of an event like that have changed over time.
Even in today’s weather, it’s raining Harvey least 1,000-year event, meaning it only has a one-in-a-thousand chance of happening in a year. But researchers found evidence that the odds would have been even lower a century ago. They estimate that there has been (roughly) an 18-percent increase in the intensity of extreme rainfall events along the Gulf Coast since the late 1800s.
One of the simplest relationships in climate change is how the water-holding capacity of the atmosphere increases by about 6-8 percent for every degree Celsius of warming. The Houston area has warmed by about 1°C, so that’s about how much you can expect rainfall rates to change if nothing else is going on. One thing that can increase that simple relationship, though, is the fact that water vapor is added to the air along with release heat when it condenses into droplets. This causes the wind to continue until some water evaporates, which can mean hurricane events.
Rainfall records seem to support something like that scenario. Storms around Houston are producing more rain than ever.
To test the cause of this change in rainfall intensity, the researchers also simulated the region with several climate models. Model simulations like these will include the occurrence of “blocking high pressure systems” like the one that kept Hurricane Harvey pinned over Houston for so long. That is, after all, one way to get a high rainfall total.
They throw out a model because it can’t do a good job matching past weather patterns in a particular area. Interestingly, the two models that passed that test gave slightly different answers. One model shows a 17-percent increase in extreme rainfall between 1861 and 2017—similar to the trend seen in actual weather observations. However, another model predicts a smaller increase of 8 percent, which is close to a negligible effect of water vapor in the warm atmosphere.
In the end, the researchers went with an average of different estimates, concluding that our best understanding is that human-caused climate change has caused modern hurricanes—including Harvey—to drop by about 15 percent. more rain. Another way to describe that is to say that hurricanes of Harvey’s magnitude are about three times more likely now than they were in the late 1800s.
A second study, published by Mark Risser we had Michael Wehner at Lawrence Berkeley National Laboratory, performed a similar analysis of past weather station records. Their methods are slightly different: they focus on a seven-day rainfall average rather than a three-day average, for example, and check for the influence of El Niño/La Niña conditions. But their results are basically the same.
They estimate that Harvey’s rainfall is at least 19 percent higher due to global warming, which also means the probability of a Harvey-sized storm is now 3.5 times more likely.
The researchers used the previous record-high 7-day rainfall as an example. In 1950, that would be about a 200-year storm. But in 2017, it approached the 30-year storm.
Although a 1,000 year storm will occur somewhere in the normal world, it’s still “bad luck” when it happens to you. A number of natural events had to line up to make Hurricane Harvey the size it was. But on the list of things people did that made Harvey and its effects too bad than otherwise it would be, climate change cannot be observed.