Notes on Heatwave Global Warming Attribution

Phillips and others (2021) have taken a rapid assessment run at global warming attribution of the 2021 Pacific Northwest heat event. A summary of key points is provided at the beginning of the paper. Two points stand out:

The observed temperatures were so extreme that they lie far outside the range of historically observed temperatures. This makes it hard to quantify with confidence how rare the event was. In the most realistic statistical analysis the event is estimated to be about a 1 in 1000 year event in today’s climate.

This simply emphasizes what most long-term Pacific Northwest residences felt - this event was very far from what anyone expects for northwest summers. For those that experienced the event, you have a story to tell. But this was more than a remarkable event for the Pacific Northwest; Christopher Burt author of Extreme Weather stated “This is the most anomalous regional extreme heat event to occur anywhere on Earth since temperature records began. Nothing can compare.”   So for those that went through the event (I missed it), you can say you went through a world historic weather event.

There are two possible sources of this extreme jump in peak temperatures. The first is that this is a very low probability event, even in the current climate which already includes about 1.2°C of global warming -- the statistical equivalent of really bad luck, albeit aggravated by climate change. The second option is that nonlinear interactions in the climate have substantially increased the probability of such extreme heat, much beyond the gradual increase in heat extremes that has been observed up to now. We need to investigate the second possibility further, although we note the climate models do not show it. All numbers below assume that the heatwave was a very low probability event that was not caused by new nonlinearities. 

Under the first possible source, that the event was a very low probability event, global warming additive attribution pushes the temperature upward some amount more. Phillips and others (2021) note that the observed annual maximum daily temperatures in the Pacific Northwest trend is approximately 2 times the global temperature trend. So regardless of the event being rare, our heatwave temperatures have been trending upward at a greater rate than the global temperature trend as well as our local temperature trend.

Figure 4 b) Annual maximum of the index series with a 10-yr running mean (green line)

Burt noted in 2017 that Seattle has had 122 record high temperatures since 2010 compared to 19 record low temperatures and Salem has had 98 record highs compared to only 11 record lows.  

Another potential added source of heat is the trend of 500hPa height associated with a warming expanding atmosphere (Christidis and Stott ,2015). 

Phillips and others (2021) also suggest a source of added heat may be from persistent drought in the west leading to higher temperatures.

Given the circulation associated with major heatwaves in the Pacific Northwest, drought and enhanced heating in the southwest will be a contributing factor as hot air from that region descends into the lower elevation areas of eastern Washington and across the Cascades to the west side. 

Phillips and others (2021) suggest that global warming pushed the temperatures about 2°C warmer than they otherwise would have been. 

The other question or cause would be the potential that the heatwave was caused by a 'nonlinear' factor related to global warming. Phillips and others (2021) do not attribute this event to a nonlinear event such as a wavy stalled jet stream, but do give it a brief discussion and suggest further analysis. 'Stuck' jet streams or omega blocks are a point of some debate in climate science. 

I would note that this attribution paper was a bit narrow. John Pollack (HERE) notes that this event was preceded by two western US heatwaves of similar pattern, but to the south in June that set records including tying the all time high temperature in Palm Springs, CA of 123 F. Those events likely provided some additional heat charge to the third high pressure event centered in south British Columbia with southeast flow of air coming up from the southwest U.S into the Pacific Northwest. And the pattern of high pressure heatwaves of long duration is continuing. From the National Weather Service: The current forecast for Las Vegas is just below the all-time record of 117.  This is the fourth western US major/record breaking heatwave in the past month.