The “Feast and Famine” perspective on precipitation and climate change

Two separate things caught my attention this week and really put in context the challenges that climate change presents when it comes to precipitation. As an expert in atmospheric science, I have been in rooms where high level policy makers want me to tell them that climate change is going to bring more floods or more droughts? Frankly, the answer is both. It is not an “one or the other” thing. Using two timely examples, I will present the societal challenges associated with both sides of the precipitation spectrum.

Tanja Fransen is the meteorologist in charge of the National Weather Service office in Glasgow, Montana. She shared with friends the following statement: “If you don’t worry about the lack of snow yet this year, you should be.” The graph below shows the deviation of snowfall from recent averages for the month of November (2021). The key message is that there isn’t as much snowfall this time of year in the western states as you might expect. Why is it a problem? Well, it sure could have an impact on the skies and the tourism industry, but I’m also worried about the snowpack and its more vital impact – the water supply.

The snow that falls to the ground in the fall and winter is an essential water source for millions of people in the West. The current U.S. Drought Monitor (below) shows the extent of the current drought. What I see does not bode well for the seasonal snowpack. Snowmelt in the spring and summer provides runoff and meltwater, which serves as a source of drinking water, agricultural irrigation, and hydroelectricity.

Unfortunately, the snowpack in the western United States has shrunk over the past few decades due to climate change. According to the United States Environmental Protection Agency website, “From 1955 to 2020, the April snowpack decreased to 86% of the sites measured (see figure below). The average change across all sites equates to a decrease of about 19%. This is the ‘famine side of precipitation’ when it comes to climate change. And the “feast” side?

Even thinking about the term “party,” I’m more inclined to call it climate-induced “overindulgence.” The US National Climate Assessment (2018) report stated that “the frequency and intensity of heavy precipitation events across the United States have increased more than average precipitation and are expected to continue to increase over the next century, with stronger trends under a compared to a lower scenario. If the history of precipitation rate in the graph above does not suit you, consider this fact. New York’s National Weather Service confirmed that during the remnants of Hurricane Ida (2021), Central Park received more than 3 inches of rain in an hour. Such intensity of precipitation falling on large paved areas and in stormwater management systems designed for the rainstorms of past centuries highlights our flooding problem.

Policy makers seem increasingly aware of this facet of the problem. Earlier in 2021, a bipartisan bill was introduced that directs the National Oceanic and Atmospheric Administration (NOAA) to:

• Update the precipitation frequency estimates for the United States.
• Evaluate, through the National Academies of Science, Engineering and Medicine, where we are on precipitation estimation capabilities.
• Determine ways to update the development and release of the Likely Maximum Precipitation (PMP) estimates.

What does all this mean? Precipitation frequency and PMP estimates are used to assist engineers or other stakeholders in planning, design, and changes related to infrastructure. Let’s explore some practical examples. According to LeConstructeur.org, The PMP is “the greatest or the most extreme precipitation for a given duration which is physically possible on a pluviometric station or a basin”. One application of PMP is in the design of PMP is structures like spillways in large dams. Precipitation frequency estimates are also used in a number of engineering design applications related to roads, stormwater disposal, etc.

This PRECIP bill is still making its way through the legislative sausage mill, but it will be an important step forward. Coupled with emerging concepts for estimating precipitation on the ground and from space, this bill offers hope to advance the needle. To be clear, it will be irresponsible and negligent to design current and future infrastructure systems without considering climate impacts on the “wet” and “dry” sides of the precipitation spectrum. Remember that it is “and” and not “or”.