California Wildfires, Climate Change, and the Hot-Dry-Windy Fire Weather Index

Reposted from Dr Roy Spencers Blog

California Wildfires, Climate Change, and the Hot-Dry-Windy Fire Weather Index

November 1st, 2019 by Roy W. Spencer, Ph. D.

Summer and early Fall are fire season in California. It has always been this way. Most summers experience virtually no precipitation over much of California, which means that the vegetation that grows during the cool, wet Winter becomes fuel for wildfires in Summer.

When you add the increasing population, risky forest management practices, and lack of maintenance of power lines, it should be little wonder that wildfire activity there has increased.

Few news reports of wildfires can avoid mentioning some nebulous connection of wildfires to human-caused climate change. This is a little odd from a meteorological perspective, however.

First of all, most of the historically significant wildfire events occur when COOL and DRY Canadian high pressure areas move south over the Great Basin region, causing strong downslope easterly winds (Santa Ana winds, Diablo winds). Global warming, in contrast, is supposed to result in WARMER and MOISTER air.

Secondly, the argument I’ve seen that excessive vegetation growth from a previous winter with abundant precipitation produces more fuel is opposite of the observation that fewer wildfires typically follow an unusually wet winter in California. They can’t have it both ways.

You might ask, why do SoCal temperatures sometimes rise so high before wildfire events if the source of the air is “cool” high pressure? It’s because the cooler high-altitude air over the Great Basin warms by compression as the air descends down the mountain slopes. Almost without exception (i.e., a super-adiabatic lapse rate), air at a higher altitude that is forced to descent to a low altitude will have a warmer temperature (and lower humidity) than the air it is displacing at low altitude. (While the warmth and dryness is widespread during these events, the high winds tend to be more localized to canyons and downslope areas.)

The dryness of this sinking air can be seen in this plot of the dewpoint temperature at LAX airport (Los Angeles) as dry air moved in from the east on December 4 with strong high pressure positioned over Nevada, and seven major wildfires developed and spread from the hot, dry, and locally windy conditions.

Hourly dewpoint temperatures at LAX airport from November 1 through December 31, 2017. Rapid drying is seen late on December 4, which is when the first of seven major wildfires (the Thomas fire) ignited.

But have such fire-enhancing weather events increased in, say, the last 50 years or more? And even if they have, was the cause due to greenhouse gas emissions from fossil fuels? While blaming some portion of recent global average warming on increasing CO2 is somewhat easier, blaming a change in regional or local weather patterns on it is much more difficult.

In the process of looking around for an answer to this question, I found some interesting recent work that would allow someone to analyze the appropriate meteorological station data, if it hasn’t already been done.

The Hot-Dry-Windy (HDW) Fire Weather Index

In 2018, a paper was published by a university research meteorologist and U.S. Forest Service (USFS) employees from three different USFS offices that describes a simple meteorological index related to wildfire risk. They call it the Hot-Dry-Windy (HDW) index, which is simply the product of (1) the surface wind speed times (2) the water vapor pressure deficit. The vapor pressure deficit uses the same information as relative humidity (temperature and dewpoint temperature), but it is a difference rather than a ratio, which better measures the potential of air to rapidly remove moisture from dead vegetation. For example a 10% relative humidity at 40 deg. F will have low drying potential, while 10% RH at 100 deg. F will have very high drying potential.

What is especially useful is that they used 30 years of weather forecast model (GFS) data to build a website that gives daily-updated forecasts of the HDW index across the United States. For example, here’s today’s forecast.

HDW-website-550x413

Importantly, the HDW index does not measure the actual fire danger, which must include how dry the vegetation currently is. It only shows whether the current weather will be conducive to the rapid spread of fire if a fire is started.

If you go to that website and click on a specific location, you get a time series plot of the HDW index values from 10 days ago up through the forecast for the coming days.

Unfortunately, the website does not provide any time series of the data over the last 30 years. But I can see the technique being applied to weather station data that goes back 50 years or more, for instance the formatted weather station data available here (which is where I got the Los Angeles airport data plotted above).

Until someone does this (if they haven’t already), I think it is a mistake to blame increased wildfire activity on “climate change”, when we don’t even know if there has been a change in the meteorological events most associated with major California wildfires: the intrusion of cool Canadian high pressure areas into the U.S. Southwest.

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42 thoughts on “California Wildfires, Climate Change, and the Hot-Dry-Windy Fire Weather Index

  1. “…Unfortunately, the website does not provide any time series of the data over the last 30 years. But I can see the technique being applied to weather station data that goes back 50 years or more…”

    Far more likely to see regionally-downscaled GCM results or reanalysis data looking back 30-50+ yrs and concluding high HDWs are trending upward.

  2. The words “climate change” roll freely off everybody’s lips these days. But there are few real examples of it happening. The Sahara was once green and now it’s a desert — that’s climate change. But a few tenths of a degree, or even a few degrees, is hardly noticeable. That does not change the climate map.
    https://en.wikipedia.org/wiki/Köppen_climate_classification

    You can determine the climate of a region by what grows there. In California, you have lots of grass, which leads to grass fires. Before that, there was lots of grass, and grass fires. The change is only in the number of houses and associated infrastructure.

  3. “Until someone does this (if they haven’t already), I think it is a mistake to blame increased wildfire activity on “climate change”, when we don’t even know if there has been a change in the meteorological events most associated with major California wildfires”

    There is a paper here from last July looking at historical climate changes linked to the wildfire increase in California over the last forty years. Fig 1 shows the increases in burnt area; Fig 2 shows the association with various climate variables, including vapor pressure deficit. Fig 5 shows historical variables relevant to summer fire like temperature (up), VPD (up), fuel moisture index (down), precipitation (mixed). Fig 6 has corresponding data for fall fires.

    They say
    “In this study we evaluated the various possible links between anthropogenic climate change and observed changes in California wildfire activity across seasons, regions, and land cover types since the early 1970s. The clearest link between California wildfire and anthropogenic climate change thus far has been via warming‐driven increases in atmospheric aridity, which works to dry fuels and promote summer forest fire, particularly in the North Coast and Sierra Nevada regions. Warming has been far less influential on summer wildfire in nonforest areas. In fall, the drivers of wildfire are particularly complex, but warming does appear to enhance the probability of large fall wildfires such as those in 2017 and 2018, and this effect is likely to grow in the coming decades.

    Importantly, the effects of anthropogenic warming on California wildfire thus far have arisen from what may someday be viewed as a relatively small amount of warming.”

    • I honestly don’t see how they can come to that conclusion…effects of anthropogenic..etc

      ..and not take into consideration all the fires that were arson, homeless camps, etc

      IIRC….some arsonist set many fires all at the same time…spreading their resources too thin and ability to contain them all

      • Same way they usually come to that conclusion…run a climate model and pretend it is reality. Interestingly, they run the “historical” scenario through 2005 even thought they were looking at wildfires into 2018. They ran the absurd RCP8.5 thereafter. GIGO.

        “…The simulated climate response to anthropogenic forcing was assessed using climate model simulations produced as part of the fifth phase of the Coupled Model Intercomparison Project (CMIP5; Taylor et al., 2012). The forcing scenarios considered were the Historical scenario for 1851–2005 and the 8.5 representative concentration pathway (RCP8.5) scenario for 2006–2100 (van Vuuren et al., 2011). The RCP8.5 represents a plausible upper‐end emissions scenario, but projections over the next few decades are similar to those projected for the RCP4.5 emissions scenario, which more strongly departs from RCP8.5 in the second half of this century due to reduced anthropogenic greenhouse gas emissions (Pierce et al., 2018)…”

        • I read somewhere that in 2017 over 8000 fires were,,,, arson….and thousands more were man made
          ….I guess that’s anthropogenic

          I mean really……over 8000 fires were arson

    • Stokes
      Two things to note about California. 1) Between 1848 and about the late-1930s, lumbering, to support the mining industry, rail lines,and homes for the growing population, significantly reduced the density of trees in the forests. Lumbering, as an industry, continued to be important in the National Forests until the 1970s, when environmentalists (aka Preservationists) exercised political power to curtail the industry, largely because of the aesthetics of ‘clear cutting.’ The trees have since been growing back unrestrained by aboriginal burning. 2) There has been an explosion in the population of people living in the urban/forest interface and the homes being built (or dragged in as “mobile homes”) appear to be more flammable than the trees typically surrounding them.

      The Oakland Hills firestorm of 1991 was the result of not only dense building construction on slopes to encourage the upward spread of flames, but a special gift from Australia, the highly flammable eucalyptus trees that grow quickly in the California climate, and an unwillingness of the owners to keep the properties cleaned of the bark, leaves, and seeds that are shed prolifically.

      In all, the land use influence is undoubtedly more important than any supposed anthropogenic control of temperatures in an area that already had fire-adapted vegetation because of climate and past practices of aboriginal burnings.

      • An additional factor in Southern California where I lived for much of my life is that prior to about 1960, most of the housing in SoCal consisted of relatively small houses on relatively small lots mostly on flat or flatish land. Dense housing mostly with limited, well tended, vegetation. And any fires that started were relatively easy to subdue. That situation didn’t (and still doesn’t?) support massive regional burns even with the annual onset of dry, hot, Easterly winds in the Autumn. Didn’t mean no brush fires. There were plenty of those. But they mostly stayed/were kept up in the hills and mostly didn’t interact with people. Since 1960, SoCal has largely run out of flat land and become more affluent. Result: More people living on larger lots in the rugged country around Santa Barbara, Ventura, Los Angeles, Orange County, and San Diego. Clearing the fire-prone native vegetation off those hills is somewhere between difficult and impossible. The terrain is rugged. The vegetation is hostile. And there are a lot of rattlesnakes. So it just isn’t done. That may fully explain SoCal wildfire history.

        I’m less familiar with Northern California although I’ve spent time there. I’m sure that human expansion into wild country is responsible for increased structure loss. But I’m at something of a loss to explain the large fires of recent years.

    • Gee Nick, did you just take Tamino’s post and make it your own? Or did he steal yours to make his? We know he’s got plagiarism issues, so I’ll try to give you the benefit of the doubt…

    • This study as does many others is using correlation mistakenly as causation.

      “Consistent with previous findings (e.g., Keeley & Syphard, 2017), the correlation between burned area and climate was relatively weak in Central and South Coast (Figure 2). This is likely partly because fire‐climate relationships in these regions are strongly manipulated by humans via ignitions, suppression, and land cover change (Balch et al., 2017; Sleeter et al., 2011; Syphard et al., 2017). In addition, aboveground biomass is generally lower in these regions due to warmer and drier conditions, causing fuel availability to often limit fire spread in grasslands and potentially shrublands with nonnative grasses (Keeley, 2004). Similar to relationships in traditionally fuel‐limited fire regimes, burned area in South and Central Coast was positively and significantly correlated with antecedent precipitation over the preceding 2 years, which promotes fine‐fuel accumulation (Bradstock, 2010; Batllori et al., 2013; Abatzoglou et al., 2018; Littell et al., 2018; Figures 2f, S4, S5, and S7). After removing the positive relationship with SPI via linear regression (identifying the range of months when SPI is most positively correlated with summer nonforest burned area in each region), burned area is secondarily promoted by current‐year moisture deficit, which promotes fuel drying (Figure S7). This highlights the likelihood that nonforest wildfire is promoted by large interannual swings in precipitation total, from wet conditions that drive accumulation of grasses to dry conditions that promote desiccation of fuels.”

      Instead of hanging their hat on climate, they should actually look at what their study found with regards to fuel and think deeper with more objectivity….and see that the increase in CO2 is actually responsible for much of this:

      https://www.nasa.gov/feature/goddard/2016/carbon-dioxide-fertilization-greening-earth

      The massive greening of the planet is clearly affecting this area, which has always and will always have a dry season that dries out the new vegetation. The dry season occurs too in the south………….but there’s not been the big increase in wildfires because of climate change. But the south does not have enough vegetation like the north and its the additional growth of vegetation because of the current climate optimum and greening planet that is the cause.

      The same beneficial CO2 and climate that are causing record crop yields and world food production. Rationalizing that we should cut back on CO2 because we don’t want plants to grow so well in California, ignores the positive global dynamic.

      The no brainer solution is to manage the forests better. There are a wide variety of ways to do this which would allow us to continue to reap trillions in agricultural productivity:

      https://www.drroyspencer.com/2013/10/the-social-benefit-of-carbon-3-5-trillion-in-agricultural-productivity/

      http://www.co2science.org/education/reports/co2benefits/MonetaryBenefitsofRisingCO2onGlobalFoodProduction.pdf

      At the same time address/manage this problem with adaptation which, if the humans that make those decisions in California, would stop blaming climate and just get to work cleaning the forest floors and doing the other things that are very effective at reducing the severity of wildfires……we could have our cake and eat it too.

      With regards to precipitation for this area. Many climate models have an increase.

      https://phys.org/news/2017-07-california-wetter-century.html

      Of course that precipitation mostly falls in the wet season, when there are no wildfires. This does the same thing as more CO2(and is considered good by most measures-charges up supplies for the inevitable dry periods). It also causes more plant growth and fuel for wildfires that occur during the dry season as stated in this paper. This paper spins this as a negative. C’mon, you have to be pretty biased to see additional precip in California and plants growing better as a negative.

      The no brainer solution is also the same.

      Is there more drying in the dry season which is a factor? Yes!

      So which of these options would you take to alleviate the problem if both were possible.

      1. Go back to the old climate with 1 deg. C cooling and 120 ppm less CO2……and have over 1 billion people starve because of the loss in food production which would result from those less favorable growing conditions(I estimate crop production based weather/CO2 for a living). This would also require cutting back severely on cheap, reliable, abundant and dense fossil fuels that are powering our economy and world.

      2. Manage the forest floors with adaptation using human technology that is being applied in a country like Finland and IS effective.

      https://wattsupwiththat.com/2018/12/12/trump-was-right-about-raking-finnish-forests/

    • Nick Stokes<=== This paper inserts the term "anthropogenic" before "climate change" gratuitously. The "anthropogenic" part is at great issue – why bring it up when the argument is over whether climate change per se is linked to wildfires? What’s the point?

      That, of course, is rhetorical. The point is to continually make “climate change” a package deal, which inextricably intertwines anthropogenicicity with climate change and bad, bad, awful, horrible things. Even though individually, not a single one of these things has been established scientifically.

  4. Methinks that there is a Montreal Canadiens fan involved in that website, the logo at the bottom left is suspiciously similar to that of Les Habitants

    • Apparently it is the logo of Saint Cloud State (MN). Nice for the double-meaning of “ST” but a horrible rip-off of the Habs.

    • Loydo,

      You must be kidding. Did you read the Roy Spencer’s above article?

      California has dry summers and the autumn winds that cause the fires are a natural phenomena.

      Wet winters increase the amount of vegetation that can burn.

    • It’s funny how some people pretend to be so scientifically authoritative, yet completely miss an obviously crucial factor: how have fuel load characteristics and quantities varied over time?

    • Intriguing link, the ‘Kids lives Matter’ line in the page title ? One would have to draw the conclusion that those who cast a critical eye at CC have a Herod like disregard towards Kids Lives ?

      The use of kids to add stoutness to an argument usually means the argument has problems 🙂

      • ‘It’s for the Children” is the first call to action for non existent problems that call for government solutions that will raise your cost of living and lower the quality of your life.

    • Nice use of truncated graphing by Tamino. If he put the high and low plots on the same chart we would see a more useful representation of how much temperatures have increased.

      • Reminds me of the way he’s been caught truncating posts from dissenters to omit things like being proven wrong.

        • But you don’t provide any evidence? Just a a slur. You guys should have read the link a little more closely, the conclusion is that a warming climate and fires are linked is a no-brainer.

          • An increase of one degree or there about is not going to have any dramatic effect on fire behaviour. An increase in fuel loads on the forest floor due to miss management however does.

          • It’s a correlation, that’s all. A slight increase in temp does not cause an increase in out-of-control wildfires. The hot Diablo and Santa Anna winds dessicate fuel loads much more than a small increase in temperature could. The two key factors driving out-of-control wildfires are high speed winds and large fuel loads. Both of those make fires very hot and difficult to control. The result is more damage and larger acreages burned. Decrease the fuel loads and they are much easier to contain.

          • aussiecol
            Most (>50%) of the global warming is occurring at night and in the Arctic. The actual change in daytime warming in California over the last 50 years is in the noise level! Purported AGW becomes a Whipping Boy for landuse mismanagement and political meddling. Neither the past or present governors of California, nor the members of the legislature, have any competence in the areas of fire safety, wild lands management, or climatology. The caricature of the “pointy haired boss” in the cartoon series Dilbert is a good analogy for those playing God in Sacramento.

          • “…But you don’t provide any evidence? Just a a slur…”

            Evidence? This wasn’t a criminal case. This was over a decade ago. I didn’t keep an archive. But here is some nice selective editing he made relevant to WUWT https://wattsupwiththat.com/2010/11/02/the-full-tamino/

            “…You guys should have read the link a little more closely…”

            The link? So the actual paper too hard for you to comprehend? You needed Tamino to spoon-feed it to you? How precious.

          • “The actual change in daytime warming in California over the last 50 years is in the noise level!”

            Important point! And that applies to the entire Earth, too.

            The Alarmists assume the Earth is currently experiencing the warmest temperatures in human history, thus: “the warming” they talk about. But that’s not really true, because it was just as warm in the 1930’s, all over the world, as it is today, according to unmodified surface temperature charts.

            Alarmists are assuming things not in evidence. The bogus, bastardized temperature record (Hockey Stick) is not evidence of unprecedented warming, it is evidence of unprecedented fraud. Alarmists and Lukewarmers have no evidence to show that what they claim is true. Massive assumptions based on not very much. Sloppy thinking, considering the evidence available that refutes, or at the very least, casts into doubt, the human-caused climate change narrative.

            A REAL scientist would assume Mother Nature is causing the Earth’s climate to do what it does, as She has done on the Earth since the beginning of time, until proven otherwise. It hasn’t been proven otherwise. It has been speculated otherwise. Not good enough for a REAL scientist.

            Repeat after me: Mother Nature is in charge of the Earth’s climate until proven otherwise.

      • Really, what’s the problem with reading the graphs? Shows an increase in the High T of ~3ºF and in the Low T of ~2ºF

  5. One thing that I have not seen discussed is the increase in CO2 leading to increased plant growth which the leads to more fuel available for the fires.

    Other than that, the problems caused by fires seem to be more of vegetation control than changes in weather. An example is Thousand Oaks, which has had numerous nearby fires the last couple of years, but has had a weed abatement law on the books since the early 1960’s.

  6. Let’s see here! Commifornia stopped harvesting timber so the forests could become overcrowded with many diseased or dead trees, vast areas were taking out of ranching for parks and open space so the grasses could grow wild and no controlled burns were allowed to keep the brush from growing to the maximum allowed by annual precipitation. Then you throw in millions of new residents, forced to move out of the urban areas due to high housing costs from over-regulation of the construction industry, many of whom build cabins and homes from LOGS (dead trees) and other flammable materials. Then you want to blame these over-destructive fires on climate change? Maybe you should try blaming them on witches or aliens.

    In the state where I live now, after fleeing the insanity growing in Commifornia, residents are encouraged to harvest dead and diseased trees for firewood, controlled burns take place every year to keep overgrowth from building up and most houses built near fire areas have metal roofing and cleared spaces around them for better insurance rates. But I sure the only reason we have fewer and less destructive fires is that we are not experiencing climate change! We still have a fire season but it is usually from late winter into spring; summertime brings the monsoon rains that turn everything green and end fire season in spite of numerous lightning strikes! It is sad watching the once great state of California sliding into the tragedy of socialism; it’s even worse watching morons making excuses for governmental failure!

  7. I’ve noticed an interesting relationship between the wet bulb temperature and the total heat content (enthalpy) of a parcel of air. That is, you can know the enthalpy of the air by measuring the wet bulb temperature alone, at a particular pressure at least. So if a particular heat record is beat, but the wet bulb temperature is the same, the total heat content of the air has not increased.

    That leads me to wonder, are wet bulb temperature records increasing? If not, then our atmosphere may be getting hotter, but it isn’t gaining energy. And it should be if global warming theory is true.

    This dry, hot air, supposedly caused by global warming, contains very little heat energy compared to a cool, moist, California day in June- when fires rarely start.

    • when humans fail and OVERmanage the forests……

      There, fixed it for you. In other words, let lots of small spot fires burn themselves out and use up the fuel so a catastrophe is less likely.

      • I think we are trying to make the same point. Proper forest management includes cleaning out the fuel, harvesting mature trees, etc. that the misguided eco-freaks have prevented.

  8. Dear Dr. Spencer,

    You ask “have such fire-enhancing weather events increased in, say, the last 50 years or more?” This question may be expressed as a model: Y = delta f(x), where Y is fire starts, size, or intensity, and delta f(x) is some time-based or change-over-time function of weather.

    This model presupposes or assumes that no other explanatory factors are involved, or significant, or else that such factors have been constant over time (50 years or more).

    Let us examine that assumption. First, let us note that fires are the rapid oxidation of fuel. Second, let us note that the fuel is plant biomass: cellulose and other vegetative materials.

    Is it logical, rational, or sufficient to presume/assume that plant biomass does not change over time? Do plants not grow? Is the biomass loading in California forest or chaparral exactly the same as it was 50 years ago?

    Of course not. What you have presented here is an inadequate model, one that does not include all the possible explanatory factors, and one that does not account for the change over time in those missing factors.

    California hillsides are quite capable of accumulating 1 to 5 tons per acre per year of plant biomass (net growth). That means after 50 years an acre which was barren in year zero could have as much as 250 tons per acre of fuel in year 50. This fuel aggregation over time most definitely affects the burn-ability or fire hazard thereupon.

    Any time series fire model which ignores the delta (fuel factor) — the change in fuels over time — is inadequate, insufficient, and lacking. By narrowly focusing on weather only, you miss the key explanatory factor. By focusing on climate change, you seem to be ignorant or unaware of the biomass fuel change.

    That problem plagues many analyses of this issue. Scientists with Ph.Ds in one particular limited field, when venturing beyond their area of expertise, are similar to blind men groping an elephant. That kind of poor science galls me. Does it not gall you?

    • As a very long term California resident, the key worry in fire season is just how long has it been since the area last burned. What Dr Spencer was concerned about was what, if any, affect the changes in climate had on the likelihood of weather conducive to fire (e.g. Santa Ana winds) as opposed to the severity of the fire due to accumulation of fuel. Quite a few weather stations “in the back country” of southern California have reports for “fuel moisture”, which is a gauge of the fire hazard.

      • Let’s get our terms straight. Fire “intensity” is how hot the fire is at the fire front. Fire “severity” is how much damage the fire does. Fire “likelihood” is the probability that a fire will ignite.

        All these depend on the quantity, quality, and continuity of the fuels. The weather is a minor factor. Fires can and do ignite, burn, and destroy in a variety of weather conditions. Santa Ana winds are not required. Look at the fire history. Many fires occur when winds are slight and temperatures moderate.

        The climate is not a factor at all, of course, let alone “climate change”. Forest fires occur from the Equator (recall the Amazon fires this year) to boreal forests near the Arctic Circle (Canada, Siberia) across every major climatic zone. Fuel is necessary, climate is unimportant.

        Logic dictates that we exclude climate from our fire model since it has no influence on fire intensity or likelihood, whether the climate “changes” or not.

        Under-specified models or models with extraneous factors are of no use. Combine the two — a model with only extraneous factors — and you get scientific “garbage”.

        Garbage will not prevent fires. Garbage will not save your home, community, or state. The fire question is not an academic parlor debate; it is about life and death in the real world.

  9. I am a 4th generation Californian who grew up “running the hills” of San Diego County in 1960+-, fires in late summer and fall were common. California has always burned, and burned more in the past than it does now. Much of it is desert with too many d&mn people for the supporting resources. Where I grew up almost all of the houses were on flat land with a few in the hills surrounded by well maintained groves. There were some houses built by idiots who somehow thought it was a good idea to live on the top of a ridgeline and ravine “chimney” Read the below for a view of the past when natural fires and Indian set fires were MUCH more common with more acreage burned per year.
    https://www.waterboards.ca.gov/water_issues/programs/tmdl/records/region_1/2003/ref1039.pdf
    Published SUMMARY of the study
    Estimates for the prehistoric areas of California burned per year are several times the areas burned today by wild and prescribed fires. The exclusion of fires from wildlands has led to extensive fuel buildup and large, damaging wildfires. Area and number of structures lost to wildfires each year has increased since the 1960’s, and the nature of fire regimes has changed. For most of the area burned in wildfires, fire regimes have changed, with a resultant change in the biota.

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