Aerial photo on the left of Denham Springs, Louisiana at the height of the flooding on 15 August 2016, collected by NOAA's National Geodetic Survey at sites identified by NOAA's National Weather Service and the Federal Emergency Management Agency. The photo on the right was taken three days later when flood waters had receded. Photo: NOAA

7 September 2016 (NOAA) – Human-caused climate warming increased the chances of the torrential rains that unleashed devastating floods in south Louisiana in mid August by at least 40 percent, according to a team of NOAA and partner scientists with World Weather Attribution (WWA) who conducted a rapid assessment of the role of climate on the historic heavy rain event.

“We found human-caused, heat-trapping greenhouse gases can play a measurable role in events such as the August rains that resulted in such devastating floods, affecting so many people,” said Karin van der Wiel, a research associate at NOAA’s Geophysical Fluid Dynamics Laboratory and the lead author. “While we concluded that 40 percent is the minimum increase in the chances of such rains, we found that the mostly likely impact of climate change is a near doubling of the odds of such a storm.”

For the assessment, scientists conducted a statistical analysis of rainfall observations and used two of NOAA’s high-resolution climate models to understand how the odds have changed for such three-day events between the early 20th century and the early 21st century. The results were consistent using observational data and climate models.

“This was by far the hardest fast attribution study we have done, given all the different small-scale weather types that cause precipitation in the region,” said Geert Jan van Oldenborgh of the Royal Netherlands Meteorological Institute, part of the international research network known as WWA that was formed to provide near real-time analysis of extreme weather events. “It was encouraging to find that our multi-model methods worked even for such a complicated case.”

The research focused on the central U.S. Gulf Coast, and investigated events as strong as that observed at the height of the storm (August 12-14) to provide a regional context and a broader assessment of risk. The climate model experiments involved altering the climate based on levels of greenhouse gases in the atmosphere, aerosols such as soot and dust, ozone and natural changes in the sun’s radiation and from volcanic eruptions for various periods of time to assess how extreme rainfall events respond to climate changes.

“Extreme event attribution analyses are meant to characterize the changing nature of weather risk,” said Heidi Cullen, chief scientist at Climate Central, an independent climate science and news organization that leads WWA. “The information is critical to insurers, policy makers, engineers and emergency managers as they work to make communities more resilient.”

The storm began when a low-pressure system carried massive levels of moisture from an unusually warm Gulf of Mexico over south Louisiana where the system stalled, leading to record breaking precipitation in the region around Baton Rouge. The rains were followed by inland flash flooding and river flooding that was slow to recede due to flooding downstream. As of Aug. 17, Louisiana officials reported that the flood had claimed 13 lives, more than 30,000 people had been rescued, more than 8,100 slept in shelters, and more than 60,000 homes had been damaged.

The rapid assessment was submitted to the open access journal Hydrology and Earth System Sciences Discussions, which has an open online peer review process. The rapid response research is two fold, providing useful information about a severe event nearly immediately to help scientists analyzing the event. However, the research also goes through a more lengthy open comment period and peer review. The robust findings from that process will advance our knowledge on the topic.

A science summary of the paper can be found at climatecentral.org.

A feature on this climate attribution research can be found at climate.gov.

Contact

Monica Allen, monica.allen@noaa.gov, 301-734-1123

Climate change increased chances of record rains in Louisiana by at least 40 percent


Models indicate that the return period for extreme rain events of the magnitude of the mid-August 2016 downpour in Louisiana has decreased from an average of 50 years to 30 years. A typical 30-year event in 1900 would have had 10 percent less rain than a similar event today, for example, 23 inches instead of 25. Graphic: NOAA

ABSTRACT: A stationary low pressure system and elevated levels of precipitable water provided a nearly continuous source of precipitation over Louisiana, United States (U.S.) starting around 10 August, 2016. Precipitation was heaviest in the region broadly encompassing the city of Baton Rouge, with a three-day maximum found at a station in Livingston, LA (east of Baton Rouge) from 12–14 August, 2016 (648.3 mm, 25.5 inches). The intense precipitation was followed by inland flash flooding and river flooding and in subsequent days produced additional backwater flooding. On 16 August, Louisiana officials reported that 30,000 people had been rescued, nearly 10,600 people had slept in shelters on the night of 14 August, and at least 60,600 homes had been impacted to varying degrees. As of 17 August, the floods were reported to have killed at least thirteen people. As the disaster was unfolding, the Red Cross called the flooding the worst natural disaster in the U.S. since Super Storm Sandy made landfall in New Jersey on 24 October, 2012. Before the floodwaters had receded, the media began questioning whether this extreme event was caused by anthropogenic climate change. To provide the necessary analysis to understand the potential role of anthropogenic climate change, a rapid attribution analysis was launched in real-time using the best readily available observational data and high-resolution global climate model simulations.

The objective of this study is to show the possibility of performing rapid attribution studies when both observational and model data, and analysis methods are readily available upon the start. It is the authors aspiration that the results be used to guide further studies of the devastating precipitation and flooding event. Here we present a first estimate of how anthropogenic climate change has affected the likelihood of a comparable extreme precipitation event in the Central U.S. Gulf Coast. While the flooding event of interest triggering this study occurred in south Louisiana, for the purposes of our analysis, we have defined an extreme precipitation event by taking the spatial maximum of annual 3-day inland maximum precipitation over the region: 29–31º N, 85–95º W, which we refer to as the Central U.S. Gulf Coast. Using observational data, we find that the observed local return time of the 12–14 August precipitation event in 2016 is about 550 years (95 % confidence interval (C.I.): 450–1450). The probability for an event like this to happen anywhere in the region is presently 1 in 30 years (C.I. 11–110). We estimate that these probabilities and the intensity of extreme precipitation events of this return time have increased since 1900. A Central U.S. Gulf Coast extreme precipitation event has effectively become more likely in 2016 than it was in 1900. The global climate models tell a similar story, with the regional probability of 3-day extreme precipitation increasing due to anthropogenic climate change by a factor of more than a factor 1.4 in the most accurate analyses. The magnitude of the shift in probabilities is greater in the 25 km (higher resolution) climate model than in the 50 km model. The evidence for a relation to El Niño half a year earlier is equivocal, with some analyses showing a positive connection and others none.

Rapid attribution of the August 2016 flood-inducing extreme precipitation in south Louisiana to climate change

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