Climate change and wildfires–how do we know if there is a link?

August 10, 2018 by Kevin Trenberth, The Conversation
This year is proving to be another active wildfire season. Credit: Climate Central, CC BY-NC

Once again, the summer of 2018 in the Northern Hemisphere has brought us an epidemic of major wildfires.

These burn forests, houses and other structures, displace thousands of people and animals, and cause major disruptions in people's lives. The huge burden of simply firefighting has become a year-round task costing billions of dollars, let alone the cost of the destruction. The smoke veil can extend hundreds or even thousands of miles, affecting air quality and visibility. To many people, it has become very clear that human-induced plays a major role by greatly increasing the risk of .

Yet it seems the role of climate change is seldom mentioned in many or even most news stories about the multitude of fires and heat waves. In part this is because the issue of attribution is not usually clear. The argument is that there have always been wildfires, and how can we attribute any particular wildfire to climate change?

As a climate scientist, I can say this is the wrong framing of the problem. Global warming does not cause wildfires. The proximate cause is often human carelessness (cigarette butts, camp fires not extinguished properly, etc.), or natural, from "dry lightning" whereby a thunderstorm produces lightning but little rain. Rather, global warming exacerbates the conditions and raises the risk of wildfire.

Even so, there is huge complexity and variability from one fire to the next, and hence the can become complex. Instead, the way to think about this is from the standpoint of basic science – in this case, physics.

Global warming is happening

To understand the interplay between global warming and wildfires, consider what's happening to our planet.

Climate change and wildfires – how do we know if there is a link?
The flows of energy through the climate system are schematically illustrated with numbers on the top-of-atmosphere values and net energy imbalance at the surface. Credit: Trenberth et al 2009

The composition of the atmosphere is changing from human activities: There has been over a 40 percent increase in carbon dioxide, mainly from fossil fuel burning since the 1800s, and over half of the increase is since 1985. Other heat-trapping gases (methane, nitrous oxide, etc.) are also increasing in concentration in the atmosphere from human activities. The rates are accelerating, not declining (as hoped for with the Paris agreement).

This leads to an energy imbalance for the planet.

Heat-trapping gases in the atmosphere act as a blanket and inhibit the infrared radiation – that is, heat from the Earth – from escaping back into space to offset the continual radiation coming from the sun. As these gases build up, more of this energy, mostly in the form of heat, remains in our atmosphere. The energy raises the temperature of the land, oceans and atmosphere, melts ice, thaws permafrost, and fuels the water cycle through evaporation.

Moreover, we can estimate Earth's energy imbalance quite well: It amounts to about 1 watt per square meter, or about 500 terawatts globally.

While this factor is small compared with the natural flow of energy through the system, which is 240 watts per square meter, it is large compared with all other direct effects of human activities. For instance, the electrical power generation in the U.S. last year averaged 0.46 terawatts.

The extra heat is always the same sign and it is spread across the globe. Accordingly, where this energy accumulates matters.

Global ocean heat content for the top 2000 meters of the ocean, with uncertainty estimates by the pink region. Credit: ScienceAdvances, CC BY-NC

Tracking the Earth's energy imbalance

The heat mostly accumulates ultimately in the ocean – over 90 percent. This added heat means the ocean expands and sea level rises.

Heat also accumulates in melting ice, causing melting Arctic sea ice and glacier losses in Greenland and Antarctica. This adds water to the ocean, and so the sea level rises from this as well, rising at a rate of over 3 milimeters year, or over a foot per century.

On land, the effects of the energy imbalance are complicated by water. If water is present, the heat mainly goes into evaporation and drying, and that feeds moisture into storms, which produce heavier rain. But the effects do not accumulate provided that it rains on and off.

However, in a dry spell or drought, the heat accumulates. Firstly, it dries things out, and then secondly it raises temperatures. Of course, "it never rains in southern California" according to the 1970s pop song, at least in the summer half year.

So water acts as the air conditioner of the planet. In the absence of water, the excess heat effects accumulate on land both by drying everything out and wilting plants, and by raising temperatures. In turn, this leads to heat waves and increased risk of wildfire. These factors apply in regions in the western U.S. and in regions with Mediterranean climates. Indeed many of the recent wildfires have occurred not only in the West in the United States, but also in Portugal, Spain, Greece, and other parts of the Mediterranean.

The conditions can also develop in other parts of the world when strong high pressure weather domes (anticyclones) stagnate, as can happen in part by chance, or with increased odds in some weather patterns such as those established by either La Niña or El Niño events (in different places). It is expected that these dry spots move around from year to year, but that their abundance increases over time, as is clearly happening.

A satellite image of the Carr Fire in California. Drought conditions, in addition to a lot of dead trees and vegetation, are contributing to another year of severe wildfires. Credit: NASA

How big is the energy imbalance effect over land? Well, 1 Watt per square meter over a month, if accumulated, is equivalent to 720 Watts per square meter over one hour. 720 Watts is equivalent to full power in a small microwave oven. One square meter is about 10 square feet. Hence, after one month this is equivalent to: one microwave oven at full power every square foot for six minutes. No wonder things catch on fire!

Attribution science

Coming back to the original question of wildfires and global warming, this explains the argument: there is extra heat available from climate change and the above indicates just how large it is.

In reality there is moisture in the soil, and plants have root systems that tap soil moisture and delay the effects before they begin to wilt, so that it typically takes over two months for the effects to be large enough to fully set the stage for wildfires. On a day to day basis, the effect is small enough to be lost in the normal weather variability. But after a dry spell of over a month, the risk is noticeably higher. And of course the global mean surface temperature is also going up.

"We can't attribute a single event to climate change" has been a mantra of climate scientists for a long time. It has recently changed, however.

As in the wildfires example, there has been a realization that climate scientists may be able to make useful statements by assuming that the weather events themselves are relatively unaffected by climate change. This is a good assumption.

Also, climate scientists cannot say that extreme events are due to global warming, because that is a poorly posed question. However, we can say it is highly likely that they would not have had such extreme impacts without . Indeed, all weather events are affected by climate change because the environment in which they occur is warmer and moister than it used to be.

In particular, by focusing on Earth's Energy Imbalance, new research is expected to advance the understanding of what is happening, and why, and what it implies for the future.

Explore further: Science Says: Record heat, fires worsened by climate change

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dirk_bruere
1 / 5 (5) Aug 10, 2018
"Well, 1 Watt per square meter over a month, if accumulated, is equivalent to 720 Watts per square meter over one hour. 720 Watts is equivalent to full power in a small microwave oven. "

Are you a scientific illiterate? That statement makes no sense. Watts do not "accumulate"

Scroofinator
2 / 5 (4) Aug 10, 2018
The question for me is how fast would co2 levels rise had we not been fighting forest fires for a hundred years? By prohibiting nature from doing what it does, who knows what all the consequences are.
antigoracle
1.5 / 5 (8) Aug 10, 2018
Climate change and wildfires–how do we know it is just PATHOLOGICAL LIES from the AGW Cult and their PATHOLOGICAL "science"

https://www.fs.fe...ds33.JPG
humy
5 / 5 (8) Aug 10, 2018
"Well, 1 Watt per square meter over a month, if accumulated, is equivalent to 720 Watts per square meter over one hour. 720 Watts is equivalent to full power in a small microwave oven. "

Are you a scientific illiterate? That statement makes no sense. Watts do not "accumulate"

I think it is obvious what he meant.
The heat energy generated from 1 Watt per square meter over a month is roughly about the same as from 720 Watts per square meter over one hour.

Phyllis Harmonic
4.4 / 5 (7) Aug 10, 2018
"That statement makes no sense."

Maybe this helps: (24hrs x 30 days) x 1w = 720w
rgw
1 / 5 (7) Aug 10, 2018
At least one of the major Western US wildfires was started deliberately. Does this mean that Global Warming causes arsonists?
howhot3
5 / 5 (6) Aug 10, 2018
Good article. I'm no expert on wildfires, but I've met one and he's convinced that the linkage to Anthropogenic global warming is causing the increase in not only the frequency of their occurrence, but the severity and scale in addition.
Ken_Fabian
5 / 5 (7) Aug 10, 2018
Excellent article. This fits with my Australian experience of wildfires.

At least one of the major Western US wildfires was started deliberately. Does this mean that Global Warming causes arsonists?
-

rgw, you could actually read the article for an answer -

Global warming does not cause wildfires. The proximate cause is often human carelessness (cigarette butts, camp fires not extinguished properly, etc.), or natural, from "dry lightning" whereby a thunderstorm produces lightning but little rain. Rather, global warming exacerbates the conditions and raises the risk of wildfire.


Whilst not specifically mentioning arsonists, they count as one of the human "proximate causes"- which the author does not blame on climate change.
Surveillance_Egg_Unit
1.5 / 5 (8) Aug 11, 2018
Good article. I'm no expert on wildfires, but I've met one and he's convinced that the linkage to Anthropogenic global warming is causing the increase in not only the frequency of their occurrence, but the severity and scale in addition.
says howhot3

As there are many causes of wildfires - lightning is one cause that occurs naturally - the tendency for "climatologists", politicians and the media to claim that global warming is most exclusively a product of human behaviour, is sorely in need of revision, as for the most part, observation of most or all individuals cannot be accomplished to provide unequivocal evidence that such is the case. Positive evidence for such claims must be done on an individual case by case basis.

Placing blame on the whole human race for global warming is tantamount to the Nazi claim that all Jews were inferior and were to be exterminated for being a Jew.
The term "anthropogenic" is a wide reference to ALL humans and is discriminatory.
humy
5 / 5 (9) Aug 11, 2018

Placing blame on the whole human race for global warming is tantamount to the Nazi claim that all Jews were inferior and were to be exterminated for being a Jew.
Surveillance_Egg_Unit

1, No, it isn't. It doesn't involve gassing or killing people nor claiming all humans are inferior (to what?).
2, No scientist hatefully blames ALL humans for global warming including bushmen and babies.
3, man made global warming is now scientific fact. Nobody is a Nazi for accepting facts.
Surveillance_Egg_Unit
1.6 / 5 (7) Aug 11, 2018
Well then, humy, perhaps it is time to start praying since the planet Earth is about to become as hot as an oven set to 350 degrees F.

1, You are projecting. I did not say that all of the human race is gassing and killing Jews, or anybody, although some have done it. For anyone to claim that humans are to blame for all of the environmental meltdowns is zany and foolish. And yes, it is anti-human to blame humanity for all or most of the ills in the world.
2. Have you inquired of every scientist whether they blame all humans or not? It is possible that some of them do blame all humans for global warming. I doubt that you can read minds.
3. When I see fish swimming in the streets of downtown Chicago, Miami, or NYC, I will then admit that it is scientific fact. I will then take my fishing pole, bait and tackle box to do some fishing in the flooded streets. Until such time that I can see severe flooding caused by global warming, I will continue to be sceptical.
Surveillance_Egg_Unit
2.7 / 5 (6) Aug 11, 2018
Personally, I feel sad for all the innocent animals who have burnt to death in the wildfires. Houses and other structures can be rebuilt, but animals are the ones who are really taking a beating in these wildfires through no fault of their own.
itfitzme
5 / 5 (6) Aug 11, 2018
"Well, 1 Watt per square meter over a month, if accumulated, is equivalent to 720 Watts per square meter over one hour. 720 Watts is equivalent to full power in a small microwave oven. "
Watts do not "accumulate"

dirk,

Watts is a measure of joules (energy) per second. So 10 watts for 5 seconds is 50 joules which I can store in a battery. Another 10 watts for an additional 5 second is another 50 joules stored in that battery. The total then is 100 joules. So, effectively, the energy does accumulate.

Consider a 600 watt light bulb. Hold your hand next to it and your hand gets warm. Keep your hand next to it and your hand gets even warmer until it becomes so hot you are forced to move your hand away.

Now back to that battery. So I have accumulated 100 joules in that battery over 10 seconds. I can now discharge that battery,in 1 second and the output will be 100 watts. So, yes, the statement is correct, assuming the numbers work out.
antigoracle
1.5 / 5 (8) Aug 11, 2018
Good article. I'm no expert on wildfires, but I've met one and he's convinced that the linkage to Anthropogenic global warming is causing the increase in not only the frequency of their occurrence, but the severity and scale in addition.

howhot3...AKA...howShat Da Turd, brays yet again.

Take a gander at actual facts -- https://www.fs.fe...ds33.JPG
Or, you can be an obedient Chicken Little Jackass and do as your Cult commands and keep staring at the sky, wishing for it to fall.
humy
5 / 5 (7) Aug 11, 2018
For anyone to claim that humans are to blame for all of the environmental meltdowns is zany and foolish.
Surveillance_Egg_Unit

Straw man; nobody I know of is claiming this.
Have you inquired of every scientist whether they blame all humans or not?
No, I haven't inquired to find out if there is at least one complete moron amongst them. So what? At least 99.9% of them wouldn't hatefully blame ALL humans, including babies, for climate change.
And, no, accepting the fact of man made warming is NOT nazism nor racist
...Until such time that I can see severe flooding caused by global warming, I will continue to be sceptical.
Straw man: Nobody who understand the theory would CLAIM that a particular weather event is entirely attributed to global warming but rather global warming would merely increase the average frequency and probabilities of certain kinds of severe weather events. Weather isn't climate.

greenonions1
5 / 5 (6) Aug 11, 2018
Surveillance
When I see fish swimming in the streets of downtown Chicago, Miami, or NYC, I will then admit that it is scientific fact
Round and around we go. What you admit is a scientific fact - is of relevance to you - and no one else. If you want to believe the earth is flat, or vaccines kill babies, or what ever you want is up to you. Unfortunately the rest of us have to live with homo stupidus - but what is new? We hope that there are not too many of you - so on balance we can make better decisions. That hope is not working out too well in many regards. My cell phone works well, so I guess science is making slow progress - but it would be a lot faster without denier goons. Here is a graph - that is a series of facts - https://ane4bf-da..._eZ.ufnQ

You will wait until there are fish in NY, or some other silly metric.
greenonions1
5 / 5 (5) Aug 11, 2018
goracle says
Take a gander at actual facts
And then presents of graph of areas burned by wild fires in the U.S. - with of course no context. Here is an interesting discussion of wild fires in the u.s. - with a lot more context. https://fivethirt...-bigger/
Graph 2 and 3 seem very pertinent to me.

Again - here is a graph that is a series of facts - https://ane4bf-da..._eZ.ufnQ

Home stupidus ignores it at his own peril....
zz5555
5 / 5 (4) Aug 11, 2018
How incompetent do you have to be to wait for Chicago (> 500 ft above sea level) to flood? SEU's whole philosophy seems to be to ignore what fundamental physics says and to ignore what happened in the past on earth. He's exactly like the guy that jumps off the Empire State building and, as he passes the 40th floor, claims, "Everything's fine so far." ;)

Why aren't there any intelligent, competent "skeptics"?
Anonym590659
1 / 5 (1) Aug 11, 2018
This looks interesting:

https://phys.org/...ric.html
Whart1984
Aug 11, 2018
This comment has been removed by a moderator.
Whart1984
Aug 11, 2018
This comment has been removed by a moderator.
antigoracle
1.7 / 5 (6) Aug 11, 2018
"That statement makes no sense."

Maybe this helps: (24hrs x 30 days) x 1w = 720w

Only the ignorant Chicken Little Jackasses would fall for that most obvious piece of bullshit.
Please tell us, what forest on the planet gets sunshine 24 hrs a day, year round?
Is it any wonder, these jackasses can only bray the word "science", since they are incapable of an independent thought.
SteveS
5 / 5 (6) Aug 11, 2018
The composition of the atmosphere is changing from human activities
It changes at least five-times faster than the human activity could release with fossil fuels burning. The carbon dioxide rise also ignores all trends in fossil fuel consumptions, like the economical crisis, which impeded their consumption a lot. Former Environmental Protection Agency (EPA) Administrator Scott Pruitt made a claim last year that humans are not the central cause of climate change, according to internal documents.


You are aware that none of your links support your arguments aren't you?
greenonions1
5 / 5 (7) Aug 11, 2018
goracle
what forest on the planet gets sunshine 24 hrs a day
No one made the assertion that there is sunshine 24 hours a day. If you read the article carefully - you see that it talks about an energy balance. Energy in - vs energy out. So the 1 watt per square meter - is talking about a net energy gain - in terms of heat accumulation.
these jackasses can only bray the word "science"
I think the word science is often invoked - because it describes a process that we feel comfortable - in terms of providing a framework for knowing. Deniers on the other hand seem more aligned with 'q'anon. In other words - believing in shit without needing any evidence to support said belief. Just like Surveillance above.
antigoracle
1 / 5 (4) Aug 11, 2018
goracle
what forest on the planet gets sunshine 24 hrs a day
No one made the assertion that there is sunshine 24 hours a day. If you read the article carefully - you see that it talks about an energy balance.....HAWW...HEE...HAWW....HEE.

The Onion Jackass brays again.
Please, find someone you trust, who actually got a brain, to read and explain this bullshit of how he arrived at that 720 value.
How big is the energy imbalance effect over land? Well, 1 Watt per square meter over a month, if accumulated, is equivalent to 720 Watts per square meter over one hour.

Surveillance_Egg_Unit
2.3 / 5 (3) Aug 12, 2018
@green onions
Perhaps you don't remember several months ago I told you that I recycle steel, aluminium, plastics, and cardboard plus eggshells for the garden. IF there is any flooding in downtown city streets, it certainly won't be MY fault as I am doing what I can to help alleviate the problems.
However, millionaires such as former VicePresident Al Gore talks a big talk but still flies around in a big gas-guzzling aeroplane and gets driven about in a gas guzzling limousine.
If rich climate-change-ologists were truly sincere, they would not have that "do as I tell you, not as I do" behaviour just because they are wealthy.
And Red China is THE biggest polluter of all; but American and European politicians are too cowed by Communists to accuse the Chinese of not doing their share to stop polluting the planet.
greenonions1
4.2 / 5 (5) Aug 12, 2018
surveillance
Perhaps you don't remember several months ago I told you that I recycle

Which of course has nothing to do with you declaring
When I see fish swimming in the streets of downtown Chicago, Miami, or NYC, I will then admit that it is scientific fact
So you set some random metric - which becomes your determinant of what is and is not a scientific fact. So you don't understand how science works.

And Al Gore is not a scientist. How many times do you denier goons have to raise the same - Alex Jones level bullshit - and it gets rebuffed - and around and around we go. Mean time - homo stupidus is stuck in the age of witches and scientology....
greenonions1
5 / 5 (4) Aug 12, 2018
Goracle demonstrates the usual level of rudeness and ignorance with another gem
to read and explain this bullshit of how he arrived at that 720 value.
No where did he assert that the sun shines 24 hours a day. That is Goracle's inability to understand a topic - and incessant need to announce to the world - his/her lack of comprehension skills - and of course childishness. Again - when talking about an energy imbalance - no one is declaring that the sun shines 24/7. The calculation is simply regarding an energy imbalance. The earth radiates energy at night.
Surveillance_Egg_Unit
2 / 5 (4) Aug 12, 2018
surveillance
Perhaps you don't remember several months ago I told you that I recycle

Which of course has nothing to do with you declaring
When I see fish swimming in the streets of downtown Chicago, Miami, or NYC, I will then admit that it is scientific fact
So you set some random metric - which becomes your determinant of what is and is not a scientific fact. So you don't understand how science works.

And Al Gore is not a scientist. How many times do you denier goons have to raise the same - Alex Jones level bullshit - and it gets rebuffed - and around and around we go. Mean time - homo stupidus is stuck in the age of witches and scientology....
says green

For one thing, when the time comes that I see fish swimming in the big downtown city streets, it will have been too late to stop it. Which would mean that you and many millions of others are STILL flying in gas guzzling airplanes and driving gas guzzling cars.
greenonions1
4.8 / 5 (4) Aug 13, 2018
For one thing, when the time comes that I see fish swimming in the big downtown city streets, it will have been too late to stop it. Which would mean that you and many millions of others are STILL flying in gas guzzling airplanes and driving gas guzzling cars
Very probably true. What does that have to do with your declaring that you will not believe something to be a scientific fact - until some random occurrence? The point is that science is not about belief. What you choose to believe - has no effect on what is. I am just pointing out that you don't understand science - which is reinforced by your throwing out some rubbish about Al Gore.

It does not matter what Tom Cruise believes - there is no EVIDENCE that humans were seeded by Xenu - millions of years ago.
howhot3
5 / 5 (2) Aug 17, 2018
Science is science and facts are facts. Goofballs like @AlGoreacle and @small_eggs just don't get it, and that's why you find them in the reject section. Basically climate denier goon squad types that pledge loyalty to trump and the far right. Goons man. Just plain goons. The want BREXIT to flop type goons.

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