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Yellowstone: Statistics Versus Climate Change

The Times They are a-Changin

Published in the EarthSphere Blog. Feature Image: Let It Flow by CF Lovelace & WM House (©Archean Enterprises, LLC; ArcheanArt)

My grandfather liked to quote, “There are two types of liars; statistical liars and damn liars.” As an aside, DJ Trump doesn’t appear to have much mathematical aptitude, so we can understand where he falls on the spectrum.

The recent dangerous and devastating flooding in Yellowstone National Park is reported by the US Geological Survey (USGS) as representing a 1-in-500-year event. I have a great deal of respect for the USGS, and I have no reason to doubt the accuracy of their assessment. But their analysis is, by definition, based on historical records. Numbers and data are required to establish flooding patterns over time, and past events become the data for gauging the statistical probabilities of future flooding.

My first experience with major flooding was in my hometown of Richmond, Virginia, when in 1969, hurricane Camille caused major flooding on the James River. This dramatic event was described to me as a 1-in-100-year flood. The river crested at 25 feet. Homes beside the river, near where I lived, were completely underwater and ultimately destroyed. My young mind was uninitiated in the art of statistics, and I took the assessment at face value. My flooded neighbors must have also taken the 1-in-100-year assessment at face value because they rebuilt their riverside homes.

Flash forward to 1972, when hurricane Agnes pounded the East Coast with torrential rains. You would be correct in assuming Richmond experienced another 1-in-100-year flood. This time the river crested above 28 feet. My neighbors’ new homes did not survive this ordeal either. Two 100-year floods in three years made an impression on me — these weather guys didn’t know what they were talking about. But several years later, while studying environmental sciences at the University of Virginia, I came to understand the seemingly deceptive nature of statistics. The probability of flooding is the same in any given year, telling you nothing about the temporal spacing between large floods.

Statistics are Only as Good as the Data

The Yellowstone River could have a 1-in-500-year flood next year, or maybe we will have to wait the full 500 years for the next deluge. But I personally think we will see an increasing frequency of major flooding. Of course, if we get enough major floods, the statistics will change, and perhaps occurrences like the recent Yellowstone River flood will be downgraded to 1-in-100-year floods. Our current predictions are based on past observations from a colder planet. Global warming and climate change introduce new variables into the flooding equation. Variables we can’t factor into a statistical approach to flood probabilities.

As always with science, statistical analyses provide useful information, But we shouldn’t conflate the term “useful” with accuracy. As was famously said by Mr. Dylan, “The times they are a-changin.” Perhaps statical analysis integrated into detailed hydrological models with a Monte Carlo simulation engine thrown in can improve our accuracy.

Our observations about recent Yellowstone flooding apply to a wider range of natural events. Climate change is altering our reality. It is only in retrospect we understand that the dry American West was settled during a long period of relatively “wet” conditions. Now, 20 years into a megadrought, our rivers and reservoirs are drying up, threatening a way of life for over 40 million people. Our lived experience for over 100 years is proving to be an inaccurate indicator of the future, a false flag.

Back to Yellowstone

The Greater Yellowstone Climate Assessment has several noteworthy quotes.”

“We know from the assessment, for example, that temperatures in the GYA have increased by 0.35°F/decade since 1950 and are projected to increase at a higher rate in the future. Warmer temperatures have already led to decreased snowpack at elevations ranging from 5000 to 7000 ft.”

“Through the 21st century more winter precipitation will fall as rain instead of snow and the amount of water stored annually in snowpack will decline (Figure 9–3). Snowmelt and runoff will occur earlier in spring, and higher evapotranspiration and reduced runoff will create water shortages in summer.”

In short, we should expect to see more spring rain and less winter snow. More high-intensity flooding is a likely result of this combination. Precipitation in the form of snow is advantageous in mitigating flooding since the water is stored in snow packs and gradually released as the weather warms. Rain, on the other hand, demands a place to go, so it rapidly fills the streams and rivers, leading to flooding. This Climate Assessment points toward increased spring flooding and more severe summer droughts for Yellowstone National Park. The times they are a-changin.



Yellowstone flooding: Why is it happening now? (Source: National Geographic) 

Greater Yellowstone Climate Assessment

Yellowstone River flooding is a 1 in 500-year event, US Geological Survey says (Source: CNN) 

William House
William is an earth scientist and writer with an interest in providing the science "backstory" for breaking environmental, earth science, and climate change news.