How Do We Know There Is A Link Between Climate Change and Wildfires?

How We Know There Is A Link Between Climate Change and Wildfires
A firefighter runs after trying to save a home in Lakeport, California, suffering its biggest fires ever.
AP Photo/Noah Berger

Once again, the summer and fall 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 climate change plays a major role by greatly increasing the risk of wildfire.

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 attribution 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.

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.

The flows of energy through the climate system (How we know there is a link between climate change and wildfires)

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. Trenberth et al 2009

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.

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.

Global ocean heat content for the top 2000 meters of the ocean (How we know there is a link between climate change and wildfires)
Global ocean heat content for the top 2000 meters of the ocean, with uncertainty estimates by the pink region.
ScienceAdvances, CC BY-NC

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.

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 global warming. 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.The Conversation

About The Author

Kevin Trenberth, Distinguished Senior Scientist, National Center for Atmospheric Research

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Books by this Author

List Price: $125.00
Sale Price: $125.00 $90.83 You save: $34.17


List Price: $1.95
Price: $0.95


List Price: $199.99
Sale Price: $199.99 $79.36 You save: $120.63


enafarzh-CNzh-TWnltlfrdehiiditjakomsfaptruesswsvthtrurvi

LATEST VIDEOS

Blue Ocean Event : Game Over?
by Just Have a Think
A Blue Ocean Event, or Ice-Free Arctic, is the source of almost fever pitch speculation in the climate science world.…
Climate Change - The Facts by Sir David Attenborough
by David Attenborough, BBC
After one of the hottest years on record, Sir David Attenborough looks at the science of climate change and potential…
Why it’s time to think about human extinction
by Kerwin Rae
After listening to this ep with Dr David Suzuki, you’ll never be the same again. The environmentalist, activist,…
Record Temperatures 20-25C Above Norm in far North
by Paul Beckwith
The Northwest Territories of Canada had March temperatures above 20C for the first time (hit 21.6C or 71F); breaking…
Why New CO₂ Capture Technology Is Not The Magic Bullet Against Climate Change
Why New CO₂ Capture Technology Is Not The Magic Bullet Against Climate Change
by Chris Hawes
According to a recent major UN report, if we are to limit temperature rise to 1.5 °C and prevent the most catastrophic…
Why Climate Change Will Dull Autumn Leaf Displays
Why Climate Change Will Dull Autumn Leaf Displays
by Matthew Brookhouse
Every autumn we are treated to one of nature’s finest seasonal annual transitions: leaf colour change and fall.
Climate denial isn’t stopping climate action.
by David Wallace-Wells
Climate change denial draws headlines. But is it actually an obstacle to climate action? A great majority of Americans…
Energy Storage: How to store renewable energy?
by Total
Under your bed, in the attic even on your mobile phone, it seems there's never enough storage. It turns out it's also…

LATEST ARTICLES

Crops at risk from changing climate
Crops at risk from changing climate
by Tim Radford
Global warming could bring yet more challenges to a hungry world. New studies have identified precise ways in which a…
Seeing The Planet Break Down In Climate Crisis Is Depressing – How To Turn Your Pain Into Action
Seeing The Planet In Climate Crisis Is Depressing – Turn Your Pain Into Action
by Cameron Brick
Environmentalism can feel like a drag. People trying to reduce their environmental impact often feel stressed and…
Global Inequality Is 25% Higher Than It Would Have Been In A Climate-stable World
by Nicholas Beuret
Those least responsible for global warming will suffer the most. Poorer countries – those that have contributed far…
Jason Kenney's Victory Means We'll All Pay The Price For Fossil Fuel Emissions
Jason Kenney's Victory In Alberta Means We'll All Pay The Price For Fossil Fuel Emissions
by D.T. Cochrane
Jason Kenney has led the United Conservative Party to victory in Alberta. There were manyobjectionablecomponents to the…
How Can Trees Really Cool Our Cities Down?
How Can Trees Really Cool Our Cities Down?
by Roland Ennos, University of Hull
In cities around the world, trees are often planted to help control temperatures and mitigate the effects of the “urban…
Beto O’Rourke Releases $5 Trillion Climate Change Proposal
Beto O’Rourke Releases $5 Trillion Climate Change Proposal
by Global Warming & Climate Change
But Varshini Prakash, executive director of the Sunrise Movement, a youth-led activist group that has advocated for the…
Should The Sahara Desert Be Turned Into A Huge Solar Farm?
Should The Sahara Desert Be Turned Into A Huge Solar Farm?
by Amin Al-Habaibeh
Whenever I visit the Sahara I am struck by how sunny and hot it is and how clear the sky can be.
How Retreating From The Sea Level Rise Will Affect Our Health?
How Retreating From The Sea Level Rise Will Affect Our Health?
by Jackson Holtz
Managed retreat in the face of sea level rise will be a mixed bag, researchers predict.