by Joe Romm
Decadal index of two-day precipitation totals that are exceeded on average only once in a 5-year period. Changes are compared to the period 1901-1960. As data show, such once-in-five-year events have become much more common (via NCA.)
One of the most robust scientific findings is the direct connection between global warming and more extreme deluges. Scientists have observed a sharp jump in monster one- and two-day rainstorms in this country.
The 2014 National Climate Assessment (NCA), which is the definitive statement of current and future U.S. climate impacts, notes, “The mechanism driving these changes is well understood.” The Congressional-mandated report by 300 leading climate scientists and experts, which was reviewed by the National Academy of Sciences, explains:
Warmer air can contain more water vapor than cooler air. Global analyses show that the amount of water vapor in the atmosphere has in fact increased due to human-caused warming…. This extra moisture is available to storm systems, resulting in heavier rainfalls. Climate change also alters characteristics of the atmosphere that affect weather patterns and storms.
That final point from our leading scientists is very important. The worst deluges have jumped not merely because warmer air holds more moisture that in turn gets sucked into major storm systems. Increasingly, scientists have explained that climate change is altering the jet stream and weather patterns in ways that can cause storm systems to slow down or get stuck, thereby giving them more time to dump heavy precipitation.
The scientific literature on this is expanding rapidly. See this post: this on Arctic ice loss boosting the chance of extreme weather in the U.S., and this on how changes to the jet stream drives global warming-linked extreme weather. Similarly, a 2010 Journal of Climate study that found “global warming is the main cause of a significant intensification in the North Atlantic Subtropical High (NASH) that in recent decades has more than doubled the frequency of abnormally wet or dry summer weather in the southeastern United States.”
What does this all mean for the answer to these questions? “Did global warming cause Phoenix to have its wettest day ever recorded? Did global warming cause the Midwest to receive two months of rainfall in one week (in August)?
Dr. Kevin Trenberth, former head of the Climate Analysis Section at the National Center for Atmospheric Research and one of the world’s leading experts on extreme weather, wrote a must-read article in 2012 on how to “relate climate extremes to climate change” (PDF here, HTML here). As Trenberth explains:
The answer to the oft-asked question of whether an event is caused by climate change is that it is the wrong question. All weather events are affected by climate change because the environment in which they occur is warmer and moister than it used to be….
The air is on average warmer and moister than it was prior to about 1970 and in turn has likely led to a 5–10% effect on precipitation and storms that is greatly amplified in extremes. The warm moist air is readily adverted onto land and caught up in weather systems as part of the hydrological cycle, where it contributes to more intense precipitation events that are widely observed to be occurring.
Because global warming tends to make wet areas wetter and dry areas drier, this effect does not manifest itself the same way in every part of the country. Here is the NCA chart of “percent changes in the amount of precipitation falling in very heavy events (the heaviest 1%) from 1958 to 2012 for each region”:
Those of you in the Northeast who thought you’d noticed deluges becoming more intense were right. Thanks to climate change, when it rains, it pours, literally. As the NCA explained, “The heaviest rainfall events have become heavier and more frequent, and the amount of rain falling on the heaviest rain days has also increased.” Some 70 percent more precipitation falls in the heaviest rain events now than it did in 1958.
Ironically, what this means is that even for the regions that are expected to see a drop in total annual precipitation — such as the Southwest — more of the precipitation they do get will be in the form of deluges so intense they can create terrible flash floods. Sound familiar?
The bottom line is that scientists predicted that climate change would increase the intensity and frequency of the worst deluges — and we’ve observed that happening. Now scientists are telling us that things are going to get a lot worse. After all, the Earth has only warmed one degree Fahrenheit in the past half-century, while we are on track to warm as much as ten times that this century if we continue ignoring the warnings. Perhaps it’s time to listen and act…