From the Cliff Mass Weather Blog
Last Thursday, December 30th, powerful downslope winds resulted in a massive grass fire that rapidly moved into neighborhoods around Superior, Colorado–a town between Denver and Boulder.
Driven by winds exceeding 100 mph that rushed down the eastern slopes of the Colorado Front Range, a fire initiated by humans moved rapidly towards populated areas, with roughly 1000 homes lost, a number of businesses destroyed or damaged, and two people unaccounted for.
Large areas of dry grass surrounded the burning homes and businesses of Superior, CO and nearby Louisville.
Within hours of the event, several media outlets including the Washington Post, the Seattle Times, National Public Radio, NBC News, and Axios (to name only a few), were making broad claims that the fires were the result of global warming (or “climate change” in the modern vernacular) or that global warming played a major role.
Politicians, such as the Governor of Colorado, blamed climate change, as did a contingent of climate activists.
The truth is different and very clear. This event had little to do with climate change. And it is easy to show this.
In this blog, we shall examine why this terrible tragedy occurred and what steps must be taken to prevent it from happening again.
We will consider the necessary ingredients of this fire, one by one, and ask whether climate change could have contributed.
The Ignition Source: No Climate Change Connection.
The fire was human-caused, with no natural ignition origin (there was no lightning).Currently, the point of origin appears to be within the camp of a fundamentalist religious group, but investigations are ongoing.
Climate change had nothing to do with the ignition.
Huge increases in human population over the region during the past 50-years obviously made accidental ignition of a fire more probable.
The Potential Ignition Location
The Strong Winds: No Climate Change Connection
A key aspect of this event was the strong winds, which accelerated down the eastern slopes of the Front Range of the Rockies.
Such winds are connected with high amplitude mountain waves that can be produced under the right meteorological conditions, including strong flow from the west to northwest approaching the Rockies and a stable layer near or just above crest level. Such conditions occurred on December 30th and the strong downslope winds were forecast by high-resolution numerical prediction models (e.g., the NOAA/NWS HRRR model).
There is no reason to expect this downslope windstorm was the result of global warming, enhanced by global warming, or made more frequent by global warming.
In fact, the opposite is possible.
Although the winds reached 100-115 mph in a few locations, some historical front range windstorms have been stronger, such as the events in 1972 (144 mph) and 1982 (140 mph).
Downslope windstorms are not unusual along the Colorado Front Range and are most frequent during the cool season (November-January) as shown below. There appears to be a declining trend in the number of strong downslope events, which suggests that global warming does not encourage them.
In fact, some research, examining global climate models forced by increasing greenhouse gases, found that the conditions producing Front Range downslope windstorms will become less frequent and weaker under global warming (e.g., this reference).
The Fuel of the Fire: Dry Grasses. No Connection With Global Warming.
So if the ignition of the fire and the essential strong winds had nothing to do with global warming, the only possibility left is the fuels, in this case, the extensive grasslands of the region. But as I will show, it is implausible that global warming played any role in the fast-moving grass fire.
As shown in the picture below (courtesy of google maps), the region just to the west of Superior, CO was characterized by extensive grassland. These grasses grow and green up in the spring and naturally brown out and dry during the summer. Such grasses are known as one-hour dead fuels, which means that no matter how moist they are, they can dry enough to burn after ONE-HOUR of drying conditions.
And few environments are more drying than the combination of strong winds and low relative humidities that accompany downslope wind events (the relative humidity was around 23% the morning of the windstorm).
So whether the prior period was warm, wet, moist, or dry, IT DID NOT MATTER. The windstorm event itself ensured that the grasses were ready to burn.
So the claims by some activists that multi-month autumn drought set up the wildfire event are patently false.And the claims that global warming helped prepare the grass to burn are patently false.
Furthermore, measurements of 10-h dead fuel moisture (for plants slightly larger than grass) at the nearby USDA RAWS site (Sugarloaf Mountain) showed moisture levels of around 9% for the preceding days, which is near normal for this time of the year (9% for December). I should note that it had rained on December 25th.
10-h Dead Fuel Moisture % at theSugarloaf RAWS observing site.
But there is more.
The grass was particularly bountiful this year not because of drought, but because the region experienced a particularly wet spring and early summer. To show this, below is the observed cumulative precipitation for the past year at Boulder, Colorado, with the normal values shown as well.
Precipitation was normal to about March 1 but by June 1 precipitation was well ahead of normal…and that bountiful precipitation continued into the summer. The result was enhanced grass growth. And there is no reason to expect that global warming is INCREASING precipitation in spring–there is no climate model output to support that.
You will notice that the year as a whole came in near normal. The snowpack in the mountains above Boulder was above-normal last winter by the way.
The bottom line in all this: there is no apparent or plausible connection between the dry grass that produced this tragic fire and global warming.
Lack of snow: A Global Warming Connection?
There is another claimed global warming connection with the fires, the lack of snow this year from the dry, warm conditions during this fall. But that is without support as well.
First, having little or no snow on the ground is not unusual for the Boulder, Colorado area during late December. In fact, only about one-third of winter days have 1-inch or more of snow on the ground (one reference here), with an average snow depth of around 1.5 inches. And wildfires can occur in grasslands with a few inches of snow on the ground.
An interesting question is whether global warming is producing drier/warmer autumns along the Front Range (little evidence for that). And another is whether there is an alternative explanation for the dry/warm fall this year (there is).
If global warming is important for fall weather along the Front Range, one should find a significant trend over the past decades in autumn precipitation, drought indices, and temperature. Well, let’s take a look at this using the NOAA/NWS Climate Division Data for conditions from September through December for 1950-2020.
For precipitation (below), there is no apparent trend up or down:
So there does not appear to be a long-term global warming signal in this area that is contributing to drought and drying conditions. Or to a lack of snow
But there IS something that probably contributed to the warm, dry conditions and lack of snow this fall on the Colorado Front Range: La Nina.
We are now in a moderate La Nina year, with the tropical central and eastern Pacific experiencing below-normal sea surface temperatures. La Nina influences the circulation of the atmosphere over the entire planet and one La Nina “teleconnection” is dry, warm conditions over eastern Colorado.
To show this, I looked at the correlation between tropical sea surface temperatures and temperature/precipitation conditions over the U.S. using the wonderful NOAA ESRL site.
La Nina years are associated with drier than normal autumns over Colorado (orange/red colors)
So why blame global warming for the warm/dry conditions, when long-term trends don’t suggest a global warming signal and La Nina provides a ready explanation? Some media folks are not earning their keep!
Major Contributors to the Disaster
Multiple lines of evidence make it clear that global warming had little to do with the catastrophic Marshall fire in Colorado. Strong/dry downslope winds, bountiful grass for a wet spring, and human ignition explain the fire.
This was a disaster ready to happen and human actions and decisions contributed to the problem. Let me note a few of them.
Massive Population Increase in the Area
Between 1950 and today there has been explosive population growth in the area, which has not only increased the vulnerable population but increased the potential ignition sources and fuels (e.g, the homes). The town of Louisville, for example, saw population growth from approximately 2000 to 20,000 during the past 70 years.
Grasslands Next to Dense Population Areas
Ironically, for environmental reasons, vast tracks of “natural” grasslands have been set aside as part of the Boulder County Comprehensive Plan, with dense housing development next to wild areas (see map below showing protected Environmental Conservation Areas, with a red star where the homes were lost)
Thus, there are large areas of flammable grass adjacent to heavily populated areas, and worse than that, these grassy areas are generally upwind (west) of the developed areas. Thus, we have an extremely dangerous situation where the areas of strongest winds, just to the east of the Front Range, are dominated by grassland. Any ignition will result in fires that rush eastward into the populated regions. Flammable grassland upwind of large housing developments. It could hardly be worse.
Dense House Development
With so much land put aside for wildland areas, less remains for housing and development. As a result (and perhaps to enhance profit as well), many of the housing developments near Superior and Louisville, CO had very closely spaced homes (see imagery below).
Thus, once one house catches fire, neighboring homes are more likely to go up in flames. In many wildfire situations, homes provide massive amounts of fuel to help grow and propagate the fire, something documented for the Camp Fire in Paradise, CA, and clearly evident in this case.
Highly Flammable Invasive Grasses
During the past century, highly flammable invasive grasses (e.g., cheatgrass, oat grass) have moved into the region, greatly enhancing wildfire potential. Limited steps have been taken to deal with the problem.
Lack of Safe Zones
There has been little effort to create sufficiently wide grass-free safe zones around urbanized areas.
Historical Fires in the Region
Fires are frequent visitors to Boulder County, but most of the recent fires have been in tree-covered terrain, often with a grass understory (see map below). A few fires have been predominantly grass fires, but have not extended over heavily populated areas.
Global warming had very little to do with the destructive wildfire that occurred in Colorado on December 30th. Those pushing a global warming narrative for this event (e.g, some media, politicians, and activists) are misinforming the public.
But it is worse than that. Blaming global warming undermines efforts to clearly define the risks and to take coherent, effective actions to reduce the chances of such wildfire disasters happening again.