If it seems as if Houston is getting wetter, you’re not imagining things. The nation’s fourth-largest city found itself under multiple flash-flood warnings Friday night as an early winter storm doused Harris County with up to a half-foot of rain.
Houston’s George Bush Intercontinental Airport picked up 4.7 inches from the storm, 4.31 of which fell Friday alone. The neighboring suburb of El Lago saw 6.56 inches within 24 hours. That’s Houston’s single-greatest one-day rain total since Hurricane Harvey struck in August last year.
Friday was also the second-wettest December day on record at the airport, where bookkeeping dates to 1969. In one day, more rain came down than is normal for the entire month in Houston, where the average December total is 3.74 inches.
It’s all part of a startling trend that bears the fingerprint of climate change. Since 1970, Houston has warmed 3.5 degrees. The most dramatic warming has been in the late summer and fall.
Now consider that each degree Celsius the air heats up means that the atmosphere can contain about 7 percent more water. Scientists call that the Clausius-Clapeyron relationship.
By this metric, the roughly 1.9 degree Celsius warming that Houston has seen should correspond to a 10 to 15 percent increase in rainfall. That’s just theory, though. So what does the historical data show?
Between 1970 and 2017, Houston’s mean annual rainfall jumped from 47 inches to near 53. That’s a six-inch increase in 47 years or the equivalent of 12 percent more rain each year.
Harvey’s rainfall last year may have spiked the trend upward, so it’s important to compensate for that. By the same token, droughts like the one in 2011 — in which Houston fell two feet behind on rainfall — can have the opposite effect. For that reason, we can remove outliers in the data that obscure any long-term trends.
More than 85 percent of the years since 1969 in Houston have seen between 30 and 70 inches of rain. That’s a window of 20 inches on either side of the “normal” annual rainfall of about 50. Four years saw more; and three years, less. Axing the data from those years paints a more accurate picture of what’s happening.
The results are similar — showing a roughly 4.4-inch increase in average yearly rainfall since 1970. Houston is trending wetter, and the warming climate is very likely the leading cause.
It’s not just the day-to-day rainfall that’s increasing. High-end rainstorms are becoming markedly more common. Of the top 100 rainiest days since 1970 in Houston, 54 have occurred since 2000. Moreover, these top-tier rainy days are twice as common in the 2000-2017 period, compared with the 1970-1999 interval.
Houston has seen five rainstorms with more than 4 inches in the past year and a half. Assuming these trends do not slow or reverse, the city is likely to deal with even more severe challenges from heavy rainfall in the future.