Was Global Warming The Cause of the Great Northwest Heatwave? Science Says No.

Synopsis

Society needs accurate information in order to make crucial environmental decisions. Unfortunately, there has been a substantial amount of miscommunication and unscientific handwaving about the recent Northwest heatwave, and this blog post uses rigorous science to set the record straight. First, the specific ingredients that led to the heatwave are discussed, including a high-amplitude ridge of high pressure and an approaching low-pressure area that “supercharged” the warming. Second, it is shown that global warming only contributed a small about (1-2F) of the 30-40F heatwave and that proposed global warming amplification mechanisms (e.g., droughts, enhanced ridging/high pressure) cannot explain the severe heat event. It is shown that high-resolution climate models do not produce more extreme high temperatures under the modest global warming of the past several decades and that global warming may even work against extreme warming in our region. Importantly, this blog demonstrates that there is no trend towards more high-temperature records. Finally, the communication of exaggerated and unfounded claims by the media, some politicians, and several activists are discussed.
__________________________________________________

During the past week, the Pacific Northwest experienced the most severe heat event of the past century.  

All-time high-temperature records were broken throughout the region, often by large margins. 

Many in the media, several local and national politicians, and some activist environmental scientists have claimed that this event was "driven by" or predominantly forced by human-inspired global warming (usually referred to as "climate change").

But such global warming claims are not supported by the facts and our best scientific understanding.  

Truth and Rigorous Science About Climate Change is Necessary for Wise Decisions

In this blog, I will use observations, modeling, climatological data, and the peer-reviewed scientific literature to demonstrate that human-caused global warming played a very small role in the extreme heat event that we just experienced here in the Pacific Northwest.

I will describe the origins of a meteorological black swan event and how the atmosphere is capable of attaining extreme, unusual conditions without any aid from our species.

As you read this, consider that I have actively pursued research on Northwest heatwaves, published several papers in the peer-reviewed literature on this specific topic, and have run both weather prediction and climate models that simulate such events.  This subject is in my wheelhouse.

I also discuss the seriousness of misinformation.   You and others can not make wise decisions when the information provided to you is not based on truth and science.

>published in the New York Times is that global warming produces a weakened, "lazy" jet stream that forces more wave-like undulations and thus more ridging.    This hypothesis has been thoroughly debunked in a number of papers in the peer-reviewed literature (see this >my blog).  But I need not cite others, since I and my students have investigated this very issue in research sponsored by the National Science Foundation and published the research in the peer-reviewed literature.

For example, Matt Brewer and I looked at global climate models driven by rapidly increasing greenhouse gases and found that high-pressure areas like that seen last week will WEAKEN under global warming.  Just the opposite of what Professor Mann was suggesting.

And last week I examined the frequency of high-amplitude ridging over our region from 1948 to today using NOAA reanalysis gridded data.  I found no trend in the extreme highs aloft.

As further confirmation, Dr. Joe Zagrodnik of Washington State University examined the radiosonde sounding data (balloon-launched weather stations) at Quillaytate on the Washington coast and found no upward trend in the maximum heights (that is big ridges of high pressure) at 500 hPa (around 18,000 ft) at that location.

Dr. Zagrodnk also did not find any trend in extreme high temperatures in the lower troposphere (around 5000 ft).

Bottom line: none of the claims that global warming increases ridging (high pressure)  over time are supported by data or modeling.  Unfortunately, Dr. Mann and others continue to push this unfactual narrative.

Perhaps, The Most Compelling Evidence of All

If global warming was producing extreme heatwaves in our region, such as the event last week, there would be a long-term trend towards more extreme high temperatures.  A single event does not reflect climate, only a trend or changes in long-term average do.

Well-known climatologist (and the official Alabama State climatologist) Dr. John Christy has run the numbers to determine if there are trends in record-breaking extreme temperatures in Washington State or Oregon.  

As shown below, there IS NO INCREASING TREND for more record high temperatures over our region during the past century.  In fact, the past decade (2011-2020) had no all-time records.  I suspect many of you are surprised at this, but it is true.

Average number of days with temperatures above 99F in our region? Also no trend.

These results are consistent with what others have found.  For example, the U.S. National Climate Assessment found the warmest day of the year over the Northwest actually COOLED between a historic (1901-1960) and a contemporary period (1986-2016).

I had a conversation about these findings with Dr. Nick Bond, Washington State Climatologist.  He told me he was not surprised, and that he and Associate State Climatologist Karin Bumbaco found similar results, published in a peer-reviewed paper.

You may ask why there is little or no upward trend in record high temperatures even though the region and planet are warming.   

There are a number of possibilities.  For example, natural variability may be so dominant that a small amount of global warming is hard to notice.  Or there may be climate feedbacks that work against extremes in a warming world....at least in our area.  I have been researching one of these feedbacks:  the tendency of heat-wave producing easterly flow to weaken under global warming and describe it below. And I can think of several more.

Still not convinced?  Read on.

High Resolution, State-of-Science Regional Climate Models Do Not Suggest A Global Warming Origin of the Extreme Heat

One of my major research projects (with Professor Eric Salathe of UW Bothel and supported by Amazon) has been to run high-resolution climate simulations driven by the most realistic global climate models.  And the results are very relevant for this discussion.

Below is a graphic from this work showing the extreme high temperature each year at Seattle Tacoma Airport by an ensemble of 12 regional climate simulations (12-km grid spacing---VERY high resolution for climate work) driven by a highly aggressive (and unrealistic) increase of greenhouse gas emissions (RCP 8.5).  These simulations started in 1970 and each forecast is shown by a different color. The green line is the mean of the regional forecasts and the observed highs are shown by black dots.  I have indicated SeaTac's high temperatures this year (108F, 42C) by the blue line.

You will notice two important features of these regional climate projections.  

First, and of great importance, there is little upward trend in the extreme high temperatures.  

Second, the most extreme temperatures of the ensembles over the entire period climbed to about  42C (108F).  To do so, required rare, but possible, combinations of several factors.   Improbable, but possible.   You might also notice that the climate model was typically about 1-2C to warm.  I believe that reflects the inability of even my very high-resolution simulations to properly simulate the effects of a narrow, improperly resolved cold Puget Sound. 

Could Global Warming REDUCE Heat Waves West of the Cascade Crest?

Most people assume that global warming can only increase our high temperatures, but it can do the opposite as well.  The Northwest's mountains and nearby cold water paradoxically might reduce heatwaves over the populated areas west of the Cascade crest under global warming.  

Global warming preferentially warms the interior of western North America compared to the Pacific coast (land warms up much more quickly than the Pacific Ocean).  The heating results in enhanced pressure falls over the interior (warm air is less dense than cold air), which strengthens cool, onshore flow and lessens warm easterly (offshore) flow.   Bad for heatwaves!

All the great heatwaves west of the Cascade crest occurred under strong easterly flow. 

Both global and regional climate models have shown that global warming lessens heatwave-producing easterly flow.    To illustrate this, consider the figure produced by the research done by my group with Dr. Eric Salathe of UW Bothell.  This plot shows the maximum easterly flow each year simulated by an ensemble (many forecasts) of high-resolution regional climate models.  Importantly, the mean strength of the strong easterly flow (black line) goes DOWN under global warming, working against the kind of heatwave we had last week.

The Evidence is Overwhelming

As I described above, the natural origins of the "black swan" heatwave we experienced last week can be demonstrated in a number of independent ways, from examining the origins and frequency of the ingredients (such as the high-amplitude ridge of high pressure), considering the statistics of warm temperatures  (the lack of trend in extreme high-temperature days), and through high-resolution climate modeling.

The evidence for a predominantly natural origin of the high temperatures records of last week is compelling, with global warming marginally increasing the peak temperatures by perhaps a few degrees.  Without global warming, we still would have experienced the most severe heatwave of the past century.

Politicization and Miscommunication of Science

The inaccurate information being distributed about the origins of this heatwave is very disturbing.

Some of this is being done out of ignorance or laziness, but a few individuals are deceiving the public deliberately.   Science journalism is only a shadow of what it was decades past, and a number of scientists now see social activism as more important than the determination and communication of truth.

Our nation has made costly mistakes when the truth was twisted for political reasons, such as for the Iraq war, when our nation spent trillions of dollars and initiated a war that killed hundreds of thousands of people based on misinformation about non-existent weapons of mass destruction.

We are now making similar mistakes with global warming, with politically inspired misinformation slowing realistic and life-saving steps, such as thinning our forests and restoring natural fire, or proceeding rapidly with nuclear energy.  Hyping global warming puts unrealistic and unnecessary fear into the hearts of our fellow citizens.  Unconscionable.  Global warming is an issue we can deal with, but only if truthful, factual, and science-based information is provided to decision-makers and the nation's citizens.

I have spent my life trying to understand the weather and climate of our region and it is so frustrating that the media (e.g., KNKX public radio, the Seattle Times, the Seattle Stranger) and local politicians (such as our governor) have placed such a low priority on providing accurate information regarding climate change and other environmental challenges.  

They have put political agendas ahead of truth and we are all the worse for it.