Fact Checking the Wildfire-Climate Link


Fact Checking the Wildfire-Climate Link


According to the C2ES website, the transformed successor of the PEW foundation’s Center on Global Climate Change, “Climate change has been a key factor in increasing the risk and extent of wildfires in the Western United States.” “Climate change enhances the drying of organic matter in forests and has doubled the number of large fires between 1984 and 2015 in the western United States.” NASA’s Global Climate Change webpage agrees stating, “hot and dry conditions in the atmosphere determine the likelihood of a fire starting, its intensity and the speed at which it spreads.


Thus every politician trying to excuse bad landscape policies, every environmental group and every scientist seeking funding, and every journalist promoting a crisis to attract readers repeatedly tells some version of the headline The Devastation of Human Life is in View: what a burning world tells is about climate change.  The good news is scientific facts totally refute such fearful narratives.


A Candle Can Light Your Way


The average temperature of a candle’s flame is a whopping ~1800°F (~1000°C) and readily demonstrates how heat is transferred. Hold your hand to the side of the flame. Despite the flame’s high temperature, you can bring your hand very close to the candle because the wick’s small mass produces relatively small amounts of radiation. The closer you bring your hand to the flame, the more heat you can feel. That heat reaches your hand via heat wave radiation. Just inches away however you can’t sense much heat at all because radiation spreads out as the distance from the heat source increases, making the heat that reaches your hand increasingly less dense, and thus less intense. Although a bon fire may only reach temperatures of ~1110°F (~600°C), its larger mass produces more energy forcing you to stay much further away where the heat intensity becomes adequately reduced.


Now place your hand over the top of the candle . There you can’t hold your hand as close to the flame because in addition to radiation, the candle heats the air and that hot air rises carrying heat upwards via convection. Convection is why a ground fire will burn overstory branches and perhaps evolve into a crown fire. When fire fighters estimate how fast a fire will spread, they must consider convection and  the slope of the terrain. Fires can spread more rapidly when convection carries more heat upslope.


The Wisdom of Firebreaks


Building firebreaks are fire fighters’ primary tactic.  Just as you can create enough space between you and a bon fire, a firebreak creates a safe distance between the searing heat of an approaching fire and potential fuels. Rivers and streams act as natural firebreaks. Fearless fire fighters armed with pulaskis and other hand tools, tirelessly clear swaths of land down to bare soil creating fire stopping intervening spaces. A small fire with limited fuels can be halted with a small fire break. Larger fires often require bulldozers to plow wider firebreaks, while the most intense fires also require airdrops of water and fire retardants.


Before our era of fire suppression, frequent wildfires naturally generated networks of firebreaks. After America’s era of fire suppression began in the early 1900s, not only did the supply of forest floor fuels accumulate, enabling bigger fires, but fewer natural firebreaks were created, enabling greater wildfire spread. To defend homes and towns, people must now maintain adequate “defensible spaces” by creating their own firebreaks.


Winds and Spot fires


Despite cooling down from peak summer temperatures, downed power lines and high winds ignited northern California’s Tubbs Fire in early October 2017. Called Diablo winds, these high winds arise every autumn as cooling inland deserts develop high pressure systems that drive dry winds across California towards lower pressure regions on a warmer Pacific Ocean. Large fires in southern California are driven similarly by the Santa Anna winds that peak during the inland’s coldest winter temperatures. These strong winds are not the result of global warming. In truth, climate models predict global warming should reduce these winds by warming inland deserts.


As the Diablo winds scattered burning embers, spot fires jumped firebreaks, and raining devastation on the town of Santa Rosa. Embers got trapped under eaves, entered attics through outside vents and ignited rooftops. As one house burnt, it generated enough radiant heat to ignite a neighboring home. Without burning a single neighborhood tree, house after house was reduced to ash. Such residential neighborhoods cannot create defensible spaces between established houses, so residents must install screens that prevent embers from igniting their homes and construct fire-proof roofing.


Tubbs Fire Santa Rosa Destruction


The 2018 Carr Fire was California’s 8th largest fire and started when a towed trailer blew a tire causing its wheel rim to scrape the asphalt. Resulting sparks ignited roadside grasses. Because sparks from power line failures or scraping wheel rims are carried by molten particles, extensive scientific studies have examined what size and temperature of these molten particles can ignite fires. When a molten particle lands on potential fuel, it transfers its heat via conduction. For fuels with 6% moisture content to reach ignition temperature, a small 6-millimeter (mm) particle must be over 1700°F (~950°C). Fuels with higher moisture content require more energy to first evaporate the water before combustion can begin. Thus fuel with 25% moisture content requires the same sized particle to have a temperature over 1800°F (~1000°C) to ignite the fuel. Because a larger particle (14 mm) can carry and transfer more energy, a lesser temperature of about 1300°F (~700°C) is needed to ignite fuel with a 25% moisture content.


Depending on moisture content, most fuels must reach ignition temperatures between 644°F (340°C) and 795°F (440°C) to start a fire. Stronger winds are more dangerous in part, because they transport larger embers. Small embers lack adequate energy to raise fuels from ambient temperatures of 70°F or 90°F to an ignition temperature of 644°F and higher. More so, the 2°F  increase in global air temperatures since the Little Ice Age, increases the fuel’s temperature insignificantly and thus highly unlikely to increase “the likelihood of a fire starting, or increasing the speed at which it spreads” as NASA claimed.



Seek and You Shall Find


Many of today’s climate scientists are eagerly funded to seek out any problems that climate change might rain down on society. However those seeking dire consequences of global warming, are blinded to the significance of critical dynamics like fire suppression, natural fire breaks, and the increase in human ignitions during colder months and so fail to account for their effects. Thus they obscure or misdiagnose the appropriate remedies. Instead, they insist that a 2°F increase in global temperature increases atmospheric aridity or increases water vapor pressure deficits, and dangerously dry out the accumulating fire fuels. They make their claims, not based on wildfire physics, but via simple short term statistical correlations between increasing drying trends and increasing burnt areas. They typically commit 2 scientific sins. First, they fail to control for how much other critical dynamics increased burnt areas. Second, they cherry picked 1970s or 1980s starting dates for their trends, dates which mark the reduction of  fire suppression policies that now allowed fires to burn for greater periods of time.



In contrast, carpenters and woodworkers long ago sought to determine how changing temperatures and relative humidity affect wood moisture because it affects the quality of their work. The average moisture content of newly logged “green” Douglas fir is 43%, the green heartwood of eastern white pine averages 50% and green heartwood of ponderosa pine averages 40%. The interior dryness of most homes dries the wood which finally equilibrates at roughly 8% moisture content. If high moisture green wood is installed, that wood shrinks and warps as it equilibrates with the interior dryness and undermines the integrity of their carpentry. So lumber yards dry green wood to the ~8% moisture content that carpenters demand. Because air drying may take 2 to 5 years to reach that moisture content, lumber yards speed up the drying process via kilns and other mechanisms. Furthermore, because changes in moisture content is an ongoing dynamic process, to minimize seasonal moisture fluctuations, homes are constructed with moisture barriers.


Because the precise moisture content of wood is economically important, tried and true estimates of wood moisture content have been developed. Calculations are driven mostly by changes in relative humidity. In the naturally hot dry Mediterranean climate of California, 3- to 8-inch diameter pieces of wood will absorb moisture during the rainy winter season, reaching ~30% moisture content by March. Moisture content then falls to between 10% and 5% in July and remains low through September until the rains return. From a global warming perspective, if relative humidity is kept constant during California’s rainless summers, for every 2 °F increase in temperature anomalies, calculations estimate that moisture content will only decreases by a rather insignificant 0.056% .


Rising CO2 Concentrations Don’t Correlate with Historical Wildfires


Historically bigger wildfires are indeed associated with drier years. In California, natural ocean oscillations cause decades long cycles of droughts followed by rainy periods. California is driest during La Nina events and La Nina events are more common during the negative phase of the Pacific Decadal Oscillation (PDO). Because an El Nino event shifts the location of greatest rainfall westward, every 3 to 7 years El Ninos produce wetter seasons for California, but simultaneously cause droughts in southeastern Asia. This dynamic was dramatically illustrated by the 1997-1998 El Nino that soaked California but concurrently caused severe drought and extensive wildfires throughout Indonesia.


Thus, some climate scientists have determined changes in precipitation and “century-long warming around the northeast Pacific margins, …can be primarily attributed to changes in atmospheric circulation” caused by the PDO. After 1999, the Pacific Ocean switched to a negative PDO phase, predicting the emergence stronger California droughts and wildfires for the coming decades. Similarly, in Colorado’s Rocky Mountain National Park between 1700 and 1975 AD,  70% of large fires burned during dry conditions created by La Nina events that coincided with a negative PDO, even though those phases co-occurred only 29% of the time. Scientific studies showing more western USA droughts and fires since 1970, have typically failed to account for the effects of La Nina and the Pacific Decadal Oscillation that naturally drive western USA’s current dryness.  


Swetnam SW fire


In addition to PDO and La Nina effects, dryness in the American southwest is modulated by the North American monsoon season. While California’s driest period occurs during July and August, Arizona and New Mexico’s dry season ends when the summer monsoons in July and August bring abundant moisture. Despite centuries of cooler Little Ice Age temperatures, wildfires were more frequent and burned more extensive areas then, than during the warmer 20th and 21st centuries. Before the era of fire suppression began in the early 1900s, Southwest lightning fires ignited in April could burn for months. Suddenly, with the advent of 20th century fire suppression policies, “very few or no fire scars were recorded on any of the trees represented after 1900”. When let-it-burn policies were re-instituted during  the 1970-1979 decade, the burnt area in several southwest forests increased by 40% compared to the previous decade.


Finally, based on changes in the amount of unique organic substances emitted from wildfires and transported to Greenland, ice cores have revealed maximum fire activity in boreal forests also occurred during the Little Ice Age between 1500–1700 AD. That higher fire frequency was attributed to multi-annual droughts caused by failed Asian monsoons. Colder temperatures had caused extensive droughts by pushing the Intertropical Convergence Zone’s rain belt southward, reducing Asian monsoon rains.


A Lit Household Match Can’t Ignite a Log


Still, natural droughts cannot fully explain many wildfire dynamics. A lit match can’t ignite a log, no matter how dry it is. Despite reaching temperatures of ~1100°F (~600°C), total combustion of the match’s small mass can’t provide enough energy to sufficiently raise the log’s temperature to the ignition point. Although lightning raises air temperatures to an astonishing 50,000°F, less than 4% of all lightning strikes start fires. Lightning’s extreme heat will boil the tree’s internal water, often causing the struck tree to explode. But lightning’s fleeting nature usually doesn’t sustain enough energy transfer to ignite the tree. More often, due to a much smaller combustible mass,` the duff and fine fuels at the base of the tree are more easily heated to ignition temperatures as lightning rapidly passes to the ground.


A lit match, small molten particles or lightning can easily ignite fine fuels, because their small mass only requires relatively small amounts of energy to reach ignition temperatures. Fine fuels are grasses and small diameter twigs with large surface-area to volume ratios that makes dead fine fuels very sensitive to changing humidity. Thus fire fighters also characterize fine fuels as 1-hour lag fuels, meaning on any typical dry summer day, dead grasses and small twigs lose 60% of their moisture in just one hour. Thus fine fuel flammability is a function of fire weather, regardless of how our climate has changed. However if fine fuels are sparse, then like a lit match, they burn out before providing enough heat required to ignite larger pieces of wood. Dense patches of burning fine fuels are needed to provide enough energy to ignite larger fires.


Abundant fine fuels act as small kindling, much like the crumpled newspaper we use to ignite larger kindling in our fireplaces. Fine fuels also act like fuses that rapidly carry a fire into more dense shrublands with larger twigs that, when ignited, can provide enough energy to burn tree branches. One theory attributes the lack of USA wildfires in the early 1900s, in part, to the beginning of overgrazing that removed much of the natural fine fuels. Now, as feedlots fattened cattle more efficiently, marginal pastures have been abandoned and have become overgrown, thickening with fire enabling fine fuels. Grazing also introduced Eurasian grasses that have further increased fine fuel densities, and now provide more kindling to start bigger fires.

grass fir


Because the complete combustion of grass or paper happens so rapidly, fast moving fine-fuel-fires have a very limited time frame during which they can ignite larger kindling, and not nearly enough time to ignite living trees. And this dynamic is greatly affected by the moisture content of larger kindling. If the moisture content of larger kindling is too high, longer periods of sustained heating are required to both evaporate the added internal water and then raise temperatures to the point of ignition. Thus during wet years, fine fuels are less capable of igniting larger fires. Conversely, dry years reduce the time needed to reach ignition temperatures, allowing fine fuels to more easily spread fire.


Once larger branches and pieces of wood ignite, combustion produces sustained temperatures  of 1110°F (~600°C) and higher. That combustion now provides enough heat to dry out and ignite any vegetation intercepting the approaching fire. And again, fire suppression dangerously allows the buildup of both fine fuels and larger kindling that then allows fires to reach sustaining ignition temperatures. Clearly the wisest fire policy requires better  management of the landscape’s fuels. From a climate change perspective,  at ~1110°F a fire emits dense radiant heat energy at the rate of ~31,700 watts of energy per square meter. (W/m2). In contrast, the amount of energy added to a wildfire by a doubling of CO2 is a mere 3.5 W/m2, which is a totally insignificant factor in the speed of wildfire spread.


Pants on Fire


pants on fire 

Fact checking the science of wildfires, NASA’s narrative that rising CO2 concentrations are increasing the “likelihood of a fire starting”, increasing “its intensity” and increasing “the speed at which it spreads” must get a rating of Pants on Fire.  Likewise claims that “climate change has doubled the number of large fires” gets a rating of Pants on Fire.  Wildfire physics simply does not support any such fear mongering narratives. Every politician, every environmental group and every scientist trying to scare up more funding by uncritically blaming wildfires on CO2 induced climate change are not only ignoring good published science, but they’re also pushing wrong remedies and downplaying the correct remedies needed to benefit society and our environment. Better managed landscapes that control fuel supplies, and the re-introduction of fires via prescribed burns, will create more effective firebreaks and more healthy open habitat that coincidentally also increases wildlife diversity. Those are treatments we all should support.


Other articles on causes of wildfires

1. Media Fearmongering Fires

2. Invasive Grasses and Larger Wildfires

3. Human Ignition

4. Wildfires: Separating Demagoguery from the Science

5. Highjacking Wildfire Tragedy: Science vs Politics

6. The Paradise Wildfire

May 24, 2021

Jim Steele is Director emeritus of San Francisco State University’s Sierra Nevada Field Campus, authored Landscapes and Cycles: An Environmentalist’s Journey to Climate Skepticism, and proud member of the CO2 Coalition

Contact: naturalclimatechange@earthlink.net