How Unusual Were the Recent Floods? Did Global Warming Play a Significant Role?

There is a lot of incorrect information and false claims regarding the recent heavy rain and flooding in the Pacific Northwest.

Claims that the recent heavy rain events were unprecedented or that global warming (climate change from human emissions of greenhouse gases) was a major contributor.

The truth is that there is a long history of similar and larger events.

The truth, supported by extensive evidence,  is that global warming played a very minor role, if any.

False

False

Major Flooding Events Often Occur in the Northwest

Heavy rain and massive flooding are frequent visitors to our region, and THERE IS NO EVIDENCE THAT MAJOR EVENTS ARE INCREASING in intensity or frequency.

For example, in February 1996, major flooding in Oregon and Washington resulted in over a billion dollars of loss.

The Willamette Valley flooded in 1996

The 2006 flood event destroyed many roads and bridges in Mount Rainier Park, with massive damage along the Skagit and Cowletz rivers. 

Mt. Rainier Park, 2006

If global warming were a significant contributor to Northwest flooding, then flooding events would become more frequent or intense.

This is not happening.

Precipitation is not increasing

Most of the arguments for a global warming origin of heavy rain depend on arguments about the atmosphere "holding" more water as temperatures increase.

The problem with this claim is that many other factors modulate the location, intensity, and longevity of atmospheric rivers and their ability to produce heavy precipitation.

Let's evaluate these claims by looking at the ACTUAL changes in precipitation over our region.

Below is the wet season (November through February) annual precipitation over Washington State for 1895-2024 (below).

There is NO UPWARD trend, even as the planet warms.

Is the number of days with heavy rain increasing in our region?  

The data says no.   

To illustrate, this figure shows the number of days with heavy (2 inches or more) precipitation at Stampede Pass in the central WA Cascades.    NO UPWARD trend in apparent.  If anything, it is going down.

I have looked at many other stations...same story...no increase.

Not convinced yet?    

Let me show you something only my research group has at this time: high-resolution regional climate simulations in which greenhouse gases are increasing substantially (the RCP4.5 scenario).

This figure shows the change in the annual highest 5 days of precipitation between 2070-2100 and 1970-2000.

Very little change over WA State, but drier in CA.

The bottom line of all this is that both historical data and state-of-science climate simulations do NOT suggest an uptick in heavy precipitation from global warming forced by mankind.

Finally, there is ANOTHER major error regarding global warming and flooding in the claims noted above....in this case, dealing with snow.

Their argument is that warming causes less snow, and snow "soaks up" the precipitation, thus lessening flooding.

Wrong.    As noted by distinguished regional hydrometeorological expert Professor Dennis Lettenmaier, less snow meant LESS SNOWMELT, which REDUCES the amount of water for flooding.

In short, there are a lot of false claims about the relationship between global warming and local flooding.

The truth:

Northwest flooding is not increasing in frequency.

Northwest flooding is not getting worse.

There is little change in precipitation amounts or intensity during the past decades over the region as the Earth has slowly warmed.

Truth does matter.