Colorado's 2013 Floods Not Due to Climate Change

Soon after the floods in the Boulder, Colorado area during September 2013, climate science commentators and journalists were quick to assign blame for the floods on anthropogenic climate change.

---

Bryan Walsh's piece at Time quoted Andrew Freedman of Climate Central with the following: "But as Andrew Freedman of Climate Central noted, it wasn't just weather that was playing a role in the biblical Colorado floods: 'An increase in the frequency and intensity of extreme precipitation events is expected to take place even though annual precipitation amounts are projected to decrease in the Southwest.'" So it is more than just weather? Climate change is increasing the frequency and intensity of extreme precipitation events in this region? We'll come back to that claim. Over at Salon.com, Steve Hendricks was unequivocal in his linkage that "Colorado flooding: Yes, it's climate change." Errors abound in Hendricks' piece, such as this one:

"Boulder's last hundred-year flood, the great storm of 1894; and others of the genre. But this year's deluge, you will have noticed, occurred in September, which ranks in the bottom half of our months for rain. We normally get just a bit over an inch and a half in September, and our previous record was a hair over 3 inches, set back in 1901. We've received nearly six times that, at 17.17 inches so far."

---

---

Wrong. Prior to the record year of 2013 for September precipitation in Boulder, which ended up with 18.16 inches of rain, the previous record was in 1940 -- with 5.50 inches of rain, not "a hair over 3 inches, set back in 1901." In fact, September 1901 was one of the driest ever months of September on record for Boulder, with only 0.16 inches of rain the entire month. Who knows what Hendricks is talking about, but it doesn't seem to match up with the NOAA National Weather Service historical database for Boulder. Hendricks went on to claim that "the flood was compounded by other climatic devastation, chiefly years of drought. The problem with drought is that when soils are overcooked, they become hard. When they become hard, they don't soak up water as quickly. They send it downhill -- into creeks, into your basement, wherever." The average annual precipitation at Boulder since complete yearly records began in 1898 is 18.80 inches. During 2009, 2010, and 2011, Boulder received significantly more than the long-term average annual precipitation, and 2012 was drier than normal by only 17 percent -- not severe by any means. The first eight months of 2013 prior to the "biblical floods" of September were only slightly drier (7 percent) than the historical average. The last few years leading up to the September 2013 rainfall event in Boulder weren't even particularly warm. Sure, 2012 was warmer than the long-term average (but nowhere near a record), but 2008 and 2009 were cooler than normal, and 2010 and 2011 were normal. The first eight months of 2013 was also cooler than normal. Years of drought in Boulder leading up to the September 2013 deluge? I think not. Chris Mooney also wrote an article at Mother Jones in which he linked climate change to the Colorado floods. Mooney shows a graph of "Contiguous U.S. Extremes in 1-Day Precipitation (Step 4*) Annual (January-December) 1910-2012" from the NOAA-NCDC climate database, and then states that "since the 1970s, there has been an uptick in one-day extreme precipitation events ... In this sense, the Colorado Floods are consistent with the general picture of what we've been seeing, and what we would expect to see, under climate change." The flaw with Mooney's discussion is that the storm in Boulder took place in September, and if we look at the more relevant seasonal dataset for "Contiguous U.S. Extremes in 1-Day Precipitation (Step 4*) Annual (September-November) 1910-2013," we find absolutely no hint of a statistically significant trend towards more extreme precipitation events during this part of the year. Thus, Mooney's rationalization linking climate change to the extreme rainfall event in Boulder gets dismissed fairly readily. Faulty reasoning to link the precipitation event to anthropogenic climate change was also employed by Climate Central's Andrew Freedman in his Huffington Post article: "It will take climate scientists many months to complete studies into whether manmade global warming made the Boulder flood more likely to occur, but the amount by which this event has exceeded past events suggests that manmade warming may have played some role by making the event worse than it would have otherwise been." Incorrect logic. Basic reasoning dictates that if "manmade global warming" played any role in the extreme rainfall event during September 2013, we would expect to see a trend towards more extreme rainfall events in this region -- and during this timeframe -- as atmospheric greenhouse gas concentrations have increased over the past century, and especially since 1970 (this, as always, is the time-frame over which the National Climate Assessment tells us the anthropogenic climate change signature should be most evident). Never forget that causation always -- repeat, always -- requires correlation. For Colorado as a whole, there is no evidence that precipitation during September is changing or becoming more extreme. September 2013 sticks out clearly in the historical record as an anomaly with no suggestion that it is part of a post-1970 trend (and certainly there is no sign of a trend since records began in 1895). The historical record for either monthly precipitation during September, or the maximum daily precipitation during this month, shows no sign of a trend towards increasingly extreme rainfall at Boulder. Neither since 1970, nor since records began in 1897, up to 2012 was there a significant trend in monthly or daily maximum precipitation at Boulder. And since 1897, there is effectively a perfect non-correlation for the maximum annual one-day precipitation over time. Thus, there is no sign of increasingly extreme precipitation events in this area of Colorado, which allows us to rule out any reasonable linkages to anthropogenic climate change. With the new field of climate change "attribution science" apparently upon us, I suspect we'll see much more written about these floods and climate change -- but, in the absence of more of these events in the Boulder region over the coming decades, any causal attributions towards anthropogenic climate change will not be correct.

---