Is Climate Change to Blame?: Although it is still relatively young, practitioners in the field of attribution science are looking at the relationship between climate change and extreme weather events such as the recent Colorado flooding. Image: Nurpu/Flickr
After a weather event as extreme and record-breaking as the recent rainfall and flooding in northern Colorado, the question often arises: Was climate change to blame?
Answering questions like this is part of an effort to place the consequences of climate change in terms that people understand. Two degrees of warming worldwide seems abstract, but bridge-collapsing, home-destroying, killer floods are the sorts of weather events that can bring the impact of climate change home.
The science of linking extreme events to climate change is relatively young. Practitioners in this field, called attribution science, work to understand whether any part of an event like a flood, drought or heat wave can be attributed to climate change.
Martin Hoerling, a research meteorologist at the National Oceanic and Atmospheric Administration's Earth System Research Laboratory in Boulder, has been one of the leaders in the field of attribution research.
Asked how the Boulder flood could be put into a climate change context, Hoerling first listed some statistics highlighting the extreme nature of the event.
He noted that the amount of rain for the week at Boulder's weather observation site had hit 17.6 inches. This quantity of rain has made 2013 the single wettest year on record in Boulder, with three more months left to go.
Hoerling also pointed out that the record-setting rains were not limited to Boulder but occurred all the way from the Colorado Springs area up to the Wyoming border. And unlike the normal monsoonal storms that have led to past record flooding events, like the Big Thompson flood of 1976, which tend to come in shorter, more intense bursts, this storm affected a wide geographic area and lasted for many days.
An extremely rare event
"In sum, this Front Range flood event was rare for its duration (week long), its large spatial extent (spanning approximately 155 miles of the Front Range), and its cumulative intensity (breaking single-day, three-day, weekly, seasonal and annual rainfall records)," said Hoerling.
According to both Hoerling and meteorologists with the National Weather Service, the conditions that led to this widespread, long-lasting rainfall stemmed from a moist tropical air mass from the Gulf of Mexico that was displaced into the region by air coming from the south.
When the air hits the mountains, it is moved upward rapidly and cools, causing precipitation. An upper-level high-pressure system in the area to the west pinned this weather in for about a week, so the rain kept falling.
What role does climate change play in any of this? Hoerling said in this storm, the amount of precipitable water measured in the atmosphere was record-high. Global warming is known to increase the amount of moisture in the atmosphere, although the effect is not large -- perhaps a 3 to 5 percent increase in the precipitable water would be a "reasonable estimate," he said.
But what really caused the rain to fall the way it did, for the amount of time it did, was the unusual atmospheric circulation. One way to examine how climate change might affect such circulation patterns in the future is to look at what is predicted in the most recent suite of climate models, known as the Coupled Model Intercomparison Project Phase 5, or CMIP5.
These models actually indicate a slight decline in summertime precipitation in the Front Range in the 2001-2020 period with further drying occurring further out in the future, said Hoerling. And for most of this decade, this is what Colorado and other parts of the Southwest have looked like: dry.
In fact, when the rains hit Colorado last week, the majority of the state was in some type of drought, according to the U.S. Drought Monitor.
Geography and drought contributed
Joe Barsugli, a research scientist at the Cooperative Institute for Research in Environmental Sciences (CIRES) and the Western Water Assessment, was a co-author of the Boulder County Climate Preparedness Plan, which was completed in May 2012.