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Forest fires seem to be more numerous and larger, and the forest fire season is getting longer. Some of these fires are caused by arson. For example, from USA Today:

The man suspected of igniting the Holy Fire in Southern California was charged on Thursday with three counts of felony arson, among other offenses.

The Carr Fire, another large 2018 fire, was caused accidentally by a spark from a motor vehicle. According to CNN:

... on one road near Redding, California, when a tire failed last month on a trailer and its rim scraped the asphalt, the result was catastrophic for an entire region.

The sparks that shot out July 23 from that minor incident, California fire officials said, ignited what is now the sixth-most destructive wildfire in state history.

Improperly attended campfires have caused major forest fires, although my limited search did not turn up a campfire as the cause of a major forest fire in 2018.

In the past few years, what have been the immediate causes of major forest fires in the US and Canada? Suggested breakout: arson, vehicles, campfires, other human causes, lightning, other natural causes (if any). Canada and the US should probably be listed separately.

"Several years" is intentionally vague; it depends on what data is available. Let's say several years means two to ten. "Major forest fires" is also vague. I suggest Class F and Class G fires, or perhaps just Class G fires, as defined by The National Wildfire Coordinating Group:

◦Class E - 300 acres or more, but less than 1,000 acres;

◦Class F - 1,000 acres or more, but less than 5,000 acres;

◦Class G - 5,000 acres or more**

If someone has a better definition for a major fire, please use it.

Note that I am not asking about what may be the ultimate causes such as climate change or fire management policies.

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One of the significant causes of fires being major or large is fire suppression. The initial cause of the fire is incidental. They are major because the amount of fuel available is high.

Forest fires are natural events, forest evolution grew to depend on them happening at intervals and being allowed to burn without interference.

To partially address the forest service will often have controlled burns, but it is not always sufficient, and larger fires will occur

Wilderness managers need a way to quantify and monitor the effects of suppressing lightning-caused wildfires, which can alter natural fire regimes, vegetation, and habitat. Using computerized models of fire spread, weather, and fuels, it is now possible to quantify many of the hidden consequences of fire suppression. Case study watersheds in Yosemite and Sequoia–Kings Canyon National Parks were used to simulate where fires might have spread if they had not been suppressed, and what effects those fires would have had on fuels. Source

Managing forest fires is a complex task with many variables, if or when to suppress them is outside of the scope of this answer. But if you have a lot of fuel in the forest it will be a bigger fire when it happens.

P.S. I realize this might not be the answer you are looking for "Note that I am not asking about what may be the ultimate causes such as climate change or fire management policies."

  • I suggest you ask the followup question that I was going to ask but can't get to this week: Why have forest fires become larger and more destructive in recent years? Then you can give this answer to that question. I will upvote both Q and A. – ab2 Sep 5 '18 at 12:27
  • @ab2 if I get time. Been busy as well. – James Jenkins Sep 5 '18 at 12:38
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This report from the National Academy of Sciences of the United States of America says that in the years between 1992 to 2012 humans were the cause of 84% of wildfires and 44% of the total area burned. Additionally,

Human-started wildfires were dominant (>80% of ignitions) in over 5.1 million km2, the vast majority of the United States, whereas lightning-started fires were dominant in only 0.7 million km2, primarily in sparsely populated areas of the mountainous western United States.

From Natural Resources Canada, humans cause slightly less than half of the wildfires while lightning accounts for nearly 67% of the total land burned. The fires caused by lightning are usually more remote than those caused by humans and are therefore more likely to be left to burn longer.

If one were to want to do a more exhaustive study of recent wildfires and causes this wikipedia web site would be a good starting point.

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Drought.

The BC Wildfire Service has an interactive map that shows every reported fire in BC and it's suspected cause. Most of them are ignited by lightning, but it's the unusually dry season, high temperatures and wind that are the cause for the spread of the wildfire.

Back home on Kootenay Lake we used to sit out on the deck and watch lighting hit the mountains accross the lake. You'd occasionally see a tree flare up, then burn out as the rains came and extinguished it. One of the problems with the past couple fire seasons is that the smoke plumes coming off of the big fires have been generating their own lightning, and there hasn't been enough relative humidity or rain to control the spread of the fire.

I was backcountry camping with my family a couple weeks ago when the Boundary Creek fire in Waterton/Glacier National Park flared up. We were camped in a bay eating our supper when the dark plume of smoke went overhead, it was shortly after we noticed it that park crews swept through the camp and told us we had to evacuate because they had just discovered a new fire 5km away from us. It had grown from 20 - 700 hectares in size in just a few hours. They suspect lightning to be the cause, but it was the winds that caused the fire to spread so quickly that day. Fortunately, we had rain that weekend, and 20mm of rain fell only couple days after the fire flared up, which helped bring the fire under control.

The fires aren't necessarily more numerous in the sense of more fires are being ignited. The problem is that the conditions the past couple years have been favourable for fires, so once one is ignited, it burns out of control instead of flaring up and burning out like they do when there is more humidity and rain.

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