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 (weeklong), 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.
In that plan, Barsugli and his co-authors investigate the potential for heavy rain and flooding due to climate change. They note that models predict an increase in winter precipitation but that the spring and summer, which are when Boulder is prone to large rain events, are expected to have an overall decrease in precipitation.
Nonetheless, the report points out that "climate models clearly show that precipitation extremes can become larger even in areas experiencing unchanged or decreasing total precipitation. In that circumstance, rainy days would become less frequent, but if conditions are right for an extreme event, and more moisture is available in the atmosphere, then larger extreme events are possible."
In other words, even if it is drier overall, extreme events like the one Boulder just experienced are within the realm of possibility.
In the case of this recent rainfall, one of the reasons Boulder and surrounding areas experienced such severe flooding was basic geography, said Bruce Sullivan, a senior branch forecaster with NOAA's Weather Prediction Center.
"When you are over the mountains like that, the mountains kind of exacerbate the problem," Sullivan said.
More people and buildings in the floodplain
The Boulder area is prone to flash floods because it sits near the mouth of canyons in the Rocky Mountain foothills. Boulder's downtown was devastated in 1894 when a flood rushed down Boulder Creek out of the canyon and, similar to the events of last week, washed away bridges, buildings and roads.
More than a century later, there are more roads, more bridges and more buildings in the floodplain to be damaged when there is such an event.
Knowing it was only a matter of time until the next big flood, the city has been engaged in an ongoing discussion in recent years about a floodplain ordinance that would require buildings in its 100-year and 500-year floodplains to either be elevated or use flood-proofing measures.
That ordinance had not been put into effect by the time of the recent flood, however.
NOAA's Hoerling said that even if the climate change impact on the meteorology of the actual storm was not significant, there could also have been other climate-related factors that came into play. Soils could have been drier and hard-packed due to drought, leading to more runoff. Wildfire-charred landscapes hold less water, and the fire season has been increasing in length due to climate change.
"In some areas, the flood may have been made worse because of antecedent factors" like drought and wildfire, explained Hoerling.
He plans to rally a team to analyze the storm and see whether their models are even capable of producing an event like this.
"We want to be able to get our hands around it, describe it," he said. "Nature gives us an example: 'Hey, see if you can do this, modelers.' It put the gauntlet down."