Explaining Extreme Events of 2013 from a Climate Perspective — American Meteorological Society

Click here to download the report (Herring, S. C., M. P. Hoerling, T. C. Peterson, and P. A. Stott, Eds., 2014: Explaining Extreme Events of 2013 from a Climate Perspective. Bull. Amer. Meteor. Soc., 95 (9), S1–S96.) Here’s the abstract:

Attribution of extreme events is a challenging science and one that is currently undergoing considerable evolu- tion. In this paper, 20 different research groups explored the causes of 16 different events that occurred in 2013. The findings indicate that human-caused climate change greatly increased the risk for the extreme heat waves assessed in this report. How human influence affected other types of events such as droughts, heavy rain events, and storms was less clear, indicating that natural variability likely played a much larger role in these extremes. Multiple groups chose to look at both the Australian heat waves and the California drought, providing an opportunity to compare and contrast the strengths and weak- nesses of various methodologies. There was considerable agreement about the role anthropogenic climate change played in the events between the different assessments. This year three analyses were of severe storms and none found an anthropogenic signal. However, attribution assessments of these types of events pose unique challenges due to the often limited observational record. When human-influence for an event is not identified with the scientific tools available to us today, this means that if there is a human contribution, it cannot be distinguished from natural climate variability…


Introduction. Welcome rains over northeast Colo- rado starting on 9 September 2013 turned into a deluge during 11 September and continued through 15 September. Boulder, an epicenter of this regional event (http://www.crh.noaa.gov /bou/?n=stormtotals_092013), almost doubled its daily rainfall record (from 12.2 cm in July 1919 to 23.1 cm on 12 September 2013), with 43.6 cm for the week. Widespread flooding took 10 lives and caused at least $2 billion in property damage, second only to the June 1965 floods of eastern Colorado (http://www.reuters.com/article/2013/09/19/us-usa-colorado-flooding-idUSBRE98H1BA20130919).

Events of similar magnitude are not unprecedented during summer in the Colorado Front Range (Hansen et al. 1978; McKee and Doesken 1997). Some reach that size in a few hours and are more localized (e.g., Big Thompson in late July 1976), while others take longer and have larger footprints as in June 1965 and September 1938. Interestingly, attributes of the 2013 event, including its late-summer occurrence, regional scale, long duration, and slowly changing atmospheric circulation (see Gochis et al. 2014, manuscript submitted to Bull. Amer. Meteor. Soc.) that transported extreme moisture into the Front Range, also characterized the 1938 event.

Does the recent occurrence of this extreme event indicate that its likelihood has increased due to global warming? Globally, the atmosphere has become warmer and moister, with the observed rate of increase since the 1970s broadly consistent with that expected from the Clausius–Clapeyron relation (~7% per °C; Hartmann et al. 2014). Heavy precipitation events have increased over much of the United States since 1901, however, with no sig- nificant long-term trends over the northern Great Plains or Southwest (Kunkel et al. 2013). Further, the relationship between heavy precipitation and atmospheric water vapor varies seasonally, with moisture availability rather than moisture-holding capacity being a more dominant factor in summer than winter (Berg et al. 2009). Thus, the answer to our question cannot be readily gleaned from globally and annually averaged statistics but requires careful consideration of place and time…

Conclusion. Our analysis of the GEOS-5 simulations leads to a diagnosis that the occurrence of extreme five-day rainfall over northeast Colorado during September 2013 was not made more likely, or more intense, by the effects of climate change. From an observational perspective, analogous events have occurred before in the Front Range, perhaps most strikingly similar in September 1938, long before appreciable climate change.

Although our model results suggest that the occurrence of this recent extreme has become less probable over northeast Colorado due to climate change, model projections do show an increase in the intensity of maximum five-day precipitation over the globe and for annual averages as a whole by the end of the 21st century (Sillman et al. 2013). Yet, a slight decline in intensity of the maximum five-day precipitation over the central Great Plains during summer is also projected (Sillman et al. 2013), emphasizing that global and annual perspectives of climate change may not always pertain to events at a specific place and time.

A strength of our study is the availability of an ensemble of long-term climate simulations spanning 1871–2013, conducted at 1° spatial resolution, that permits an analysis of statistical properties of the change in extreme events. For the purpose of study- ing regional five-day rainfall events over northeast Colorado, the GEOS-5 model has the attribute of re- alistically characterizing the tails of the distribution. A weakness of our study is that results are based on a single model and thus require confirmation using additional models. Also, the physical reasons for the decline in simulated frequency of extreme five-day rainfall over northeast Colorado during September are not addressed. Better understanding of the deliv- ery mechanisms for atmospheric moisture that pro- duce heavy rain events and how those mechanisms respond to global warming will be critical.

From Climate Central (Bobby Magill):

…climate change may have had little to do with those extreme rains, and global warming could reduce the likelihood that they’ll happen again, according to a new study, which is disputed by one of the nation’s most prominent climatologists.

The new study, published Monday in a supplement to the Bulletin of the American Meteorological Society (BAMS) analyzing climate change’s role in extreme weather events across the globe in 2013, uses a single computer model to reach its conclusion and says the results have yet to be confirmed by other models.

Using a computer model called GEOS-5, the study concludes climate change made last September’s rains neither more likely nor more intense. Similar events have occurred in the past before human-caused climate change was a factor in any region’s weather, the study says…

He said the local geography, where the Great Plains end abruptly at the point where the Rocky Mountains rear skyward out of the prairie near Boulder, lends itself to greater precipitation due to the orographic effect. The orographic effect is the way landscape topography, mountains in Colorado’s case, affects local weather.

Hoerling cautioned that more research needs to be done to confirm his team’s findings, and the study should not be viewed as the final word on climate change’s role in the Colorado floods.

“We don’t have a lot of answers,” he said. “It raises more questions.”

One critic of the study, which was announced with a news release under the headline, “Climate Change Not to Blame for 2013 Colorado Floods,” didn’t mince words after it was published.

“There is no justification for the headline of the news release at all, and the study has little relevance to the flood in Colorado in September 2013,” said National Center for Atmospheric Research senior scientist Kevin Trenberth, who is also based in Boulder. “They ask the wrong questions, do the wrong analysis with inadequate tools and come up with the wrong answer.”

In any extreme rainfall event, the weather situation is always the main player in how such a storm develops, but says nothing about the role of climate change, Trenberth said in a written statement.

“They do note the importance of having abundant moisture in the region in order to produce high enough rainfall amounts,” he said. “But they fail to analyze where the moisture comes from.”

Water vapor above Denver hit a record during the rains at the same time as sea surface temperatures south of Baja California, Mexico, were briefly 30°C, about 1°C warmer than normal, making it the hottest spot for the ocean in the Western hemisphere, Trenberth said.

“An incredible 75 mm of moisture was recorded in the atmosphere in that region by NASA satellites,” Trenberth said in his statement.

High sea surface temperatures (SSTs) led to the large-scale convergence of moisture into the region, and it was siphoned north by a very unusual pattern in the atmosphere creating a large river of moisture flowing straight toward Colorado, he said.

“I think there is no doubt that those extremely high SSTs and record water vapor amounts likely would not have occurred without climate change,” Trenberth said. “This study is largely irrelevant; it misses the big picture and gets the wrong answer.”

Hoerling said the study’s model accounted for high sea surface tempertures in the Pacific, the river of moisture flowing toward the state and other factors. He said he is perplexed as to why Trenberth was unaware of that.

“I’m confident that other experiments will repeat the experiments we published in this BAMS report, and those will help reveal if the indications from the first such effort are robust or not,” Hoerling said.

Colorado State Climatologist Nolan Doesken told Climate Central that the study’s conclusions do not surprise him.

“Maybe it will be more obvious in 35 years, but from a 2014 vantage point here in our part of the country, changes in temperatures on a seasonally and annually averaged time frame do show a trend; precipitation probably does not, and the extreme tail of the precipitation distribution — the really big ones — are just too few and far between here to yet get much of a handle on,” Doesken said.

Giant floods and extreme rain happened in the same region at least twice in the 20th century, so it’s hard to peg climate change on something that has ample precedent in the area, and even harder to associate local extreme events to an increase in global greenhouse gas emissions, he said.

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