Did Global Warming Play A Significant Role in the Recent Northwest Wildfires?

Reposted from the Cliff Mass Weather Blog

Monday, September 21, 2020

A number of groups and individuals are claiming that the recent major wildfires in the Pacific Northwest are predominantly or significantly the result of climate change produced by increasing greenhouse gases.
In fact, many have called these conflagrations “climate fires.”    Did global warming (a.k.a. climate change) have a significant impact on the Northwest wildfires of the past few weeks?

Consider the key fires in the Northwest U.S. this month: the huge, rapidly expanding fires on the western slopes of the Oregon Cascades. 

The fires that not only burned hundreds of thousands of acres, but produced most of the smoke that engulfed the region for over a week.  As I will demonstrate, the catastrophic Oregon Cascade fires of the past weeks were forced by strong easterly winds, and such winds may well weaken under global warming.  And I will show that the weather of the past summer was relatively normal.

Thus, although global warming will undoubtedly produce substantial changes in our climate in the future, the impacts of global warming on the recent Oregon fires were probably quite small.

Smoke reaches Portland.  Image by Tedder.I should note that BEFORE the recent fires I had been working on research and a paper on the meteorology of western Cascade fires and have a NSF grant to examine California wildfires.  Furthermore, I have worked closely with the Washington State Departments of Ecology and Natural Resources, as well as the USDA Forest Service, on Northwest wildfire meteorology and prediction. 

Fires on the Western Slopes of the Oregon Cascades and their Historical Context
A series of major fires exploded on September 7-8, 2020, ranging from the Big Hollow Fire northeast of Portland, the Riverside, Beach Creek and Lionshead Fires east of Salem, the Holiday Fire near Eugene, and the Archie Creek, South Obenchain and other fires to the south.  Several of the western Cascade Range fires, which have spread over more than 350,000 acres, had been initiated by lightning in mid-August, smoldering until strong easterly winds caused rapid expansion.

Fires along the western slopes of the Cascades are infrequent but regular, with research studies using proxy information (such as charcoal remnants in the soil and tree ring/scaring data) finding stand-killing fires occur roughly every 250 years (e.g. this reference).   Importantly, during the past century, few major fires has burned over western slopes of the Oregon Cascades, with the most prominent being the Yacolt Burn (1902, 500,000 acres) and the Eagle Creek Fire (2017, 50,000 acres), both near the Columbia Gorge east of Portland.  Interestingly, there has been far more fire activity over the coastal mountains of Oregon than along the western slopes of the Cascades during the past 120 years.  Thus, many of the recent fires along the western Cascades slopes were burning across terrain that had not experienced major fires in over a century.

Fires and their sizes since 1900 over Oregon. Image created by Lynne Palombo of the Oregonian.

Strong Easterly Winds Are Necessary for Major Wildfires on the Western Slopes of the Cascades
It has long been known that major wildfires over the western slopes of the Oregon Cascades have been limited to periods of sustained, strong easterly winds (from the east).   To quote a classic paper (FREQUENCY of DRY EAST WINDS OVER NORTHWEST OREGON and SOUTHWEST WASHINGTON, Cramer 1957):
The history of forest conflagrations in the Northwest is, for the most part, a history of the simultaneous occurrence of small fires and severe east winds.
Most of the biggest fires along the crest and western slopes of the Oregon Cascades started with small fires, either natural or human-initiated, which without winds would smolder or very slowly grow.  But add strong easterly winds and they can explode as seen on September 8th.
As part of my research, I determined every major fire on western side of the Oregon and Washington Cascades since 1900, and then examined the meteorology of each using observations, newspaper accounts, and the reanalysis grids (gridded analyses based on observation that go back to the 1860s).   EVERY such major fire, without exception, was associated with strong easterly winds.

The Tillamook Burn included a series of fires starting in 1933 that torched over 350,000 acres
Why are strong easterly winds required for the megafires on the Oregon western slopes?   

First, easterly winds tend to be very dry and usually warm, which helps desiccate surface fuels.  Westerly winds off the ocean are generally cool and moist, not only bringing high relative humidity but often moving fog and low clouds over the western Cascade slopes, both negatives for fires.  Easterly winds are from the dry, warm interior of the continent, and as the air sinks along the western slopes it is compressed and thus warmed as it moves to lower elevations (where pressure is higher).  Because of the warming, the relatively humidity plummets as the air sinks.   Very favorable for fires.

Furthermore, strong easterly winds not only facilitate warming and drying but also provides oxygen to fires, allowing them to expand rapidly.  In addition, strong winds push superheated gases ahead of fires (which helps then move quickly) and strong winds loft firebrands and embers that produce spot fires ahead of the main fire line.
The bottom line:  strong easterly winds are very important for starting fires on the western Cascade slopes, which are typically resistant to fire because of their relatively moist surfaces and extensive shading.

So the questions you must be asking at this point:   were the recent Oregon wildfires associated with strong easterly winds?   Is there a climate connection with such winds? Are easterly winds increasing as the earth has warmed the last 40 years?   And do climate models suggest that global warming will increase easterly winds over the western slopes of the Cascades?
The answer to the first question is an emphatic YES. The September event was associated with powerful and sustained easterly winds  over the Cascades, with some gusts reaching 50-70 mph (see map below of maximum gusts on September 8th in mph).  Enough to cause large numbers of power outages in Oregon.

Maximum winds on September 8 over western Oregon
The balloon-launched sounding at Salem, Oregon for 5 AM Tuesday, September 8th showed strong northeasterly and easterly winds though the entire lower atmosphere (shown up to roughly 10,000 ft in the figure).

Bottom line:  strong easterly winds occurred over the lower atmosphere of western Oregon from late on September 7, peaking on Sept 8th, and continuing in weakened form during the event.

Strong easterly winds are relatively unusual during the summer over western Oregon   So how unusual was this easterly flow event?   It turns out EXTREMELY unusual. 

To examine this, I searched gridded weather data (the NCAR-NCEP reanalysis) for a grid point on the western slopes of the Oregon Cascades.  Specifically, I examined the strength of the daily average zonal (east-west wind), looking for the days of strongest easterly wind.  For example, I checked surface wind (10-m above the surface) from 1950 to today for July through September–the relevant months– and found the top ten cases.

The grid point I used for this analysis
The results show that September 8 had the strongest easterly surface wind over the period examined (more negative means more easterly), substantially exceeding second place (Sept 17, 1971). And Sept. 9 was also on the list.  Only one other multi-day sequence was on the list (Sept. 16-18, 1971) and there were major fires that period as well.

Results of  one of my analyses
What produced this record-breaking easterly wind event starting Sept. 7th?  An extraordinarily area of COLD temperatures and associated high pressure area that moved southeastward to the east of our region, coupled with an unusual low pressure offshore.

Below is the pressure analysis at 5 AM on Sept. 8, with the solid lines indicating pressure (isobars) and the colors indicating differences from normal).  Strong high pressure was over Montana, while a trough of low pressure was along the coast.  Between these two features there was a large difference in pressure.  The colors indicate that the high pressure, centered in Montana was very unusual, with the difference from normal (the anomaly) being very unusual (4-5 standard deviations from normal for those of you knowledgeable about statistics)

Looking at winds at 925 hPa (about 2500 ft above sea level) at the same time, the strongest winds were over western Oregon, reaching over 6 standard deviations from normal.  Unprecedented conditions for this date.

Let me underline something I noted above: the anomalous high pressure was associated with very unusually cold air, air that would bring snow to Denver in the subsequent day.

Is there a trend of more easterly winds over Northwest Washington in the operational record?  
If one is interested in climate change, one MUST look at trends over time. Below is a plot of the top 10 cases of easterly wind at the grid point noted above–there is no evidence of an upward trend over time.    So with increasing temperatures as the planet has warmed, there is no apparent increase of easterly wind occurrence over the region.  This is a serious strike against the global warming/wildfire contention.

But let’s not stop there.  My group, in concert with Professor Eric Salathe of UW Bothell, are running a high-resolution climate model forced by increasing greenhouse gases–probably the most sophisticated local climate modeling in the country.  And we are doing this with an ensemble of many ultra-high resolution climate runs.  And we drove our regional climate model with global models forced with a very aggressive (and undoubtedly larger than expected) increase in CO2 (RCP 8.5). 

In these model how did the easterly flow near the crest of the Cascades change over time  (we picked a point near Washington’s Stampede Pass, but that is close enough)?

The answer is found below.  The figure shows the number of days per year during July through September that the winds exceed a certain speed (6.6 knots) from the east.  The simulations extend from 1970 through 2100 and the black line provides the mean of all the simulations. 

Wow.  The number of strong easterly events….the kind that start fires…DECLINES under global warming.  Let me say that again, it declines.

This makes a lot of physical sense and is consistent with results found by others in California.  As the planet warms, the interior of the continent warms more rapidly that the ocean.   Warms results in less dense air and pressure falls.  Thus, pressure falls more rapidly in the interior than on the coast, which increases westerly flow and decreases easterly flow.  Warming would also lessen the amplitude of the cold highs, like the one that occurred two weeks ago.
So we have observational data that shows that summer easterly flow over the Cascades did not increase during the last 70 years as the planet started to warm.  Furthermore, the gold standard in climate simulations shows late summer easterly flow declining under global warming.    So the absolutely key driver of major west side of Cascades wildfires–strong easterly winds– does not appear to be strengthened by global warming.  In fact, the OPPOSITE appears to be the case.  It appears to weaken.

These findings profoundly undermines the hypothesis that the Oregon fires are “climate fires” forced by increasing greenhouse gases.   As a popular TV series might say, this hypothesis is “busted.”

But let me take this one step further to completely address the “climate fire” claims. To put the proverbial “final nail” into the “climate fire” coffin.
How Unusual Were the Climate Conditions in the Months Before the Fire?
Were the weather conditions in the months leading the September fires highly unusual?  And has there been a significant observed trend towards considerably worse (dry/hot) conditions as would be expected if climate change was contributing to the Oregon fires?
To answer these question, let’s examine the precipitation over crest and western slopes of the northern Oregon Cascades—the region where many of the big fires originated and grew.  Below is a plot of the June to August precipitation over the region from the NOAA/NWS climate divisions data (Division 4 of Oregon) for 1900 to 2020.   The summer 2020 values is not exceptional at all (indicated by small arrow and the horizontal dashed line).  And there is little overall trend in the precipitation for that region.

Clearly,  precipitation in this region does not appear to be changing much with global warming.  Climate models suggests a small decline in summer precipitation (and an increase in overall precipitation) by the end of the century if we continue burning fossil fuels with abandon.
Temperature?  As shown below, the summer 2020 temperature for the western slopes of the Oregon Cascades was neither a record nor even exceptional.  One notes a modest upward trend during the past 30 years of approximately 1F.  That could be the global warming signal.   In any case, such a small warming hardly explains the catastrophic wildfires of this summer.

Finally, let me show you the Palmer Drought Severity Index (produced by NOAA) for September 12th.  The Palmer Index combines temperature and precipitation to evaluate whether drying/drought conditions are present.  This index indicates normal conditions over the western slopes of the north Cascades. 

The bottom line:  this summer was not one of significant drought or very unusual.  Until the September 7-8 fire initiation by the strong winds, the area encompassing fire was below normal in both Oregon and Washington.
Thus, considering observations and modeling, both strongly supporting each other, the major fire/smoke outbreak this month was the result of very, very unusually strong easterly winds, NOT global warming or climate change.

Those pushing the climate change narrative, including some politicians, activist groups and media are simply misinforming the public.

Such deceptive information undermines society’s ability to deal with wildfires in an effective way, such as improved forecast management (e.g., thinning, proscribed fires), more aggressive extinguishing of fires before dangerous situations like this month, and better warnings to move people out of harm’s way.
Short Comment on Washington Fires and Global Warming
Some individuals and groups have suggested that the explosive fires in Washington State that proceeded the Oregon fires, were the result of climate change.  This is also without any foundation.  Nearly all of the Washington State fires were grass fires (e.g., Cold Springs Fire–190,000 acres, Pearl Hills Fire–224, 000 acres) that were spread by extreme and highly unusual northerly winds.  Grass fires do NOT correlate well with climate, since grasses and small bushes inevitably dry out sufficiently to burn by early summer.  Even if the grass was not initially dry, it would dry out within hours under strong winds.  Such fires are controlled by the availability of ignition sources (often manmade) and strong winds and very powerful (50-70 mph) and unusual winds occurred on September 6th.

Grassland after fire in Eastern Washington this month.Courtesy WA State Department of Ecology

HT/Cam_S