Tropical Storm Imelda was named just after 2 p.m. Tuesday, the rainstorm drenching Houston and surrounding areas with rainfall totals reaching the double digits in spots. It’s another puzzle piece that fits into an alarming trend — a trend that bears the fingerprint of climate change. Let’s look at the numbers.
We’ll start with temperature. Reliable records at Houston’s Hobby Airport date back to 1948; at Bush Intercontinental, the city’s other airport, to 1970. Below, we compare the Hobby and Intercontinental data from 1970 onward when data was available at both locations. The result? Similar warming rates.
(At Hobby, from the 1940s into the 1970s, there was little trend in temperature before warming later took off. The lack of warming in those decades is consistent with a global pause in warming around that time, possibly connected to an increase in reflective sulfate aerosols.)
Our analysis suggests Houston has warmed between 2.9 and 3.5 degrees Fahrenheit since 1970 (1.6 to 1.9 degrees Celsius).
That alone is impressive. 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 12 percent increase in rainfall. That’s just theory, though. So what does the historical data show?
Between 1970 and 2018, Houston’s mean annual rainfall jumped from 49 inches to near 57 at Hobby Airport. That’s a nearly eight-inch increase in 48 years or the equivalent of 16 percent more rain each year.
In other words, it’s as if Houston is seeing 58 days’ worth of extra rainfall every year, compared with the 1970 benchmark.
Do note above that Hurricane Harvey, in 2017, is not an outlier at Hobby. We explored removing the effects of Harvey and found that the result made about a 20 percent difference in the observed increased annual rainfall.
Just to prove this, let’s look at Intercontinental. Here’s the plot with Harvey, since 1970:
Here, Harvey’s supersoaked 2017 is an outlier. We’ll examine compensating 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 at Intercontinental.
More than 85 percent of the years since 1969 at Intercontinental 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. No matter how you slice it, Houston is trending dramatically wetter. And the warming climate is very likely the leading cause.
We can also look at the time of year in which Houston is seeing its “extra” rainfall. It’s interesting to note that, while an increase in average daily precipitation has been noted in most months of the year, the winter months exhibit the opposite. In fact, Houston is seeing less rain in the months of February and November, with essentially no change in December and January.
There was little change observed during the dog days of summer. July, August and September saw increases but not dramatic ones. The most impressive increases came on either side of summer, including in May, June and October. That lends support to the hypothesis that an earlier warming in the spring and a later cooling in the fall, thanks to climate change, is bringing moisture-laden rain systems characteristic of summertime during an expanding annual window.
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 six days with more than 4 inches of rain since 2017. 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.