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Transcript

Before the Spark · The Climate Translation

• 0:00
  A few weeks ago, I was on vacation in Northern California when a wildfire started burning nearby.
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  The fire itself never threatened where I was staying, but the smoke did.
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  For about 24 hours, the air quality became so bad that I changed my plans and stayed
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  indoors.
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  It was a reminder that you don't have to be anywhere near the flames for a wildfire
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  to affect your life.
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  And this summer, that experience is being repeated across large parts of North America.
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  Today, we're going to look at how the changing climate has led to a greater fire danger.
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  Why determining what caused wildfires is more complicated than it sounds and how smoke
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  from distant fires can eventually find its way to you.
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  Welcome to the podcast, I'm Dr. Mac, and this is the Climate Translation.
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  When we talk about wildfires, we tend to focus on the moment they begin.
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  What started the fire?
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  Was it lightning, a campfire, a down power line?
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  Maybe a piece of equipment?
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  Those are important questions, especially when investigators are trying to determine
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  responsibility.
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  But from a meteorological standpoint, I'm more interested in a different question.
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  Once that first flame appears, what happens next?
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  To understand, let's go back to something many of us may have learned in science class,
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  the fire triangle.
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  Every fire needs three basic ingredients: heat, fuel, and oxygen.
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  If you take away any one of them, the fire can't continue.
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  That's why when you pour water on a campfire to remove the heat, it goes out.
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  It's why a fire extinguisher can smother a flame by separating it from oxygen.
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  But a wildfire operates on a much larger scale.
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  Oxygen is not the problem.
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  There's plenty of that in the atmosphere, and once something provides enough heat for
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  ignition, that part of the triangle is satisfied as well.
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  So that leaves the fuel, and in a forest, grassland, or shrubland, fuel is everywhere.
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  There are trees, grass, pine needles, fallen branches, leaves, and dead shrubs.
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  Even the layer of organic materials sitting on top of the soil can be a fuel source.
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  But not all fuel burns the same way.
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  If you've ever tried to start a campfire with damp wood, you probably know this already.
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  You can hold a match underneath it, you can add paper, you can even blow on the flame.
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  But much of the energy from that fire first has to evaporate the water inside the wood.
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  Now compare that to a pile of dry twigs and leaves.
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  One spark can be just enough.
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  The difference isn't the amount of fuel, it's the amount of moisture that's inside it.
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  The fuel source can respond especially quickly to changes in atmosphere.
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  Dead grasses, leaves, and small twigs have a lot of surface area compared to their size.
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  They can gain or lose moisture relatively quickly as the atmosphere humidity changes.
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  A large log responds more slowly, and a living tree is different as well.
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  Its moisture depends not only on today's weather, but what's been happening over weeks,
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  months, even years.
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  Has there been enough rain?
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  Is the soil dry?
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  Has the vegetation been stressed by drought?
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  How much water is the plant losing that leaves?
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  So, when meteorologists and fire managers look at wildfire danger, they're not just looking
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  at today's temperature.
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  They're looking at the climate, the condition of the entire landscape.
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  And then you add in wind.
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  Wind can transform a manageable fire into something very different.
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  This is what I saw in Northern California.
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  Wind supplies fresh oxygen to the flames, but more importantly it bends those flames towards
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  unburned vegetation.
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  That preheats the fuel ahead of the fire.
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  Wind can also carry burning embers away from the main fire, and those embers can land
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  hundreds of feet, sometimes even farther, away from the flames and start new fires.
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  Firefighters call that spotting.
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  Now instead of fighting one continuous fire line, crews have many fires starting ahead
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  of them.
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  And the terrain matters too.
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  Fire generally moves uphill fast.
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  This flames burn vegetation on a slope below, heat rises, and warms the fuel above the
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  fire before the flames actually reach it.
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  The steeper the slope, the more efficiently that can happen.
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  So fire behavior becomes an interaction between three things:
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  the fuel, the weather, and the terrain.
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  Change any one of those, and the same ignition can produce a very different outcome.
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  And as our climate has warmed, it has changed the background conditions in which that ignition
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  occurs.
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  A longer period without rain, dries vegetation.
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  An intense heat wave increases moisture loss.
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  A warm spring can melt mountain snow earlier and give landscapes more time to dry before
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  the hottest part of the summer arrives.
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  And when those conditions occur together, the landscape becomes much more receptive to
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  fire.
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  Think about two identical matches dropped into two different places.
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  One lands in damp grass after several days of rain.
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  The other lands on vegetation that is spent weeks drying under the hot, sunny weather
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  with low humidity.
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  The ignition is identical, but the outcome is going to be completely different.
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  And that's why a wildfire caused by lightning, a power line, or a person doesn't
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  really tell us everything we need to know about why that fire became so large or so difficult
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  to control.
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  The spark tells us how the fire started.
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  But the condition of the landscape tells us what happens afterwards.
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  And there's a way meteorologists can measure one more important part of that condition.
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  It tells us something about how strongly the atmosphere is demanding moisture from plants
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  and fuels at any given time.
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  It's called vapor pressure deficit.
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  And as our atmosphere warms through climate change, that number is becoming increasingly
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  important to understanding wildfire risk.
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  So what is vapor pressure deficit?
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  The name sounds complicated, but the idea, it really isn't.
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  Think about a warm summer afternoon.
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  There's an amount of water in the air, but there's also an amount that's needed for that
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  air to be at get saturation.
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  The vapor pressure deficit, or the VPD, is the difference between the two.
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  The larger the difference, the greater the atmosphere's demand for moisture.
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  You can think of it as an atmospheric thirst.
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  When VPD is low or small, the atmosphere is very moist and it's not demanding any additional
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  moisture, or very little.
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  When VPD is high, or a large number, water can evaporate more quickly from soils and
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  dead vegetation and plants lose water more quickly through their leaves.
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  And temperature plays an important role in this.
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  As air gets warmer, the amount of moisture or water vapor it can hold increases.
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  And thus, the amount of water vapor or moisture needed to reach saturation increases.
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  That means if the temperature rises, but the amount of moisture in the atmosphere doesn't
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  increase, VPD goes up, and the atmosphere becomes thirstier.
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  That's why relative humidity doesn't always tell us everything we need to know about
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  how quickly a landscape can dry.
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  Imagine two afternoons with the same amount of moisture in the air.
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  One is relatively cool, the other is extremely hot.
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  Even though the amount of moisture is the same, the hotter atmosphere has a greater or
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  larger vapor pressure deficit.
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  It has a greater need to suck in water to fill up that gap.
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  It creates a strong demand for water from the landscape below.
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  And plants respond to that demand.
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  During photosynthesis, plants have tiny openings on their leaves called stomata.
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  These openings allow carbon dioxide to enter.
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  But there is a trade-off.
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  When they're open, water can escape.
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  On a day with a high vapor pressure deficit, a very thirsty atmosphere, plants can lose
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  water quickly.
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  Some respond by closing those openings to conserve moisture, and it's a useful defense,
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  but it also limits the carbon dioxide they can pull in, thus slowing photosynthesis and
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  putting plants under stress.
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  If the conditions continue for days or weeks, vegetation can become increasingly dry.
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  And dead vegetation responds even faster.
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  Grass, leaves, pine needles, and small branches, they exchange moisture directly with the surrounding
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  air.
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  Under hot, dry conditions, they can become more flammable in a very short period of time.
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  Now put that all together with what I discussed earlier.
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  A spark occurs, but if the spark lands where the VPD is high and fuels have been drying
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  for an extended period, the fire is entering a landscape that is much more prepared to burn.
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  This is one reason researchers have become very interested in the vapor pressure deficit.
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  Across the Western United States, studies have found a strong relationship between VPD
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  and the amount of land burned by wildfire.
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  And as temperatures have risen due to climate change, the atmosphere's ability to create
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  those high VPD conditions has increased as well.
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  It doesn't mean that every hot day is going to produce a wildfire, a wet winter or spring
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  can provide important moisture, but it can encourage vegetation growth that may become fuel
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  later in the season.
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  Wind, terrain, vegetation type, the ignition still matters, but VPD gives us another way
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  to understand why two landscapes can look similar, but behave very differently when fire
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  arrives.
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  And perhaps more importantly, it explains something about climate change that can be
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  easy to miss.
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  A region doesn't necessarily have to receive dramatically less rainfall for wildfire conditions
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  to become more favorable.
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  Sometimes the atmosphere is simply better at sucking that moisture away.
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  So when a major wildfire occurs, one of the questions we ask is what caused it?
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  And as you can tell, the question is more complicated than it sounds.
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  Let's say investigators determine a fire started when lightning struck a tree.
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  Well you know what caused the ignition, but have you explained why it burned 100,000 acres?
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  Not really.
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  To understand that, you have to ask several questions.
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  In addition to what started the fire, you have to ask what allowed it to spread?
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  What conditions were present in the landscape when the fire began?
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  What was the VPD?
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  These are not necessarily going to have the same answer.
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  In the United States, people caused the majority of wildfires.
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  Campfires, equipment, vehicles, power lines, debris burning, fireworks, even arson.
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  They all can provide the initial spark.
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  Lightning is a major source, particularly across remote parts of the West and the vast
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  boreal forest of Canada.
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  But once that ignition occurs, the atmosphere really doesn't care where the spark came
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  from.
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  A fire started by lightning or one start by person is going to respond to the same
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  wind, the same humidity, and the same fuels.
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  And then we have the condition of the forest itself, and this is where forest management
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  enters the conversation.
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  For much of the 20th century, the goal in North America forests was to suppress fire as
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  quickly as possible.
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  The goal was to protect communities and valuable timber.
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  But fire is a natural part of the forest ecosystem.
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  Small fires, removed grasses and shrubs, fallen branches and other vegetation.
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  When you suppress the fires, the fuel can accumulate.
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  In some forests that can give a future fire material to burn and potentially make it
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  more difficult to control.
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  That's why prescribed burning and forest thinning can be useful tools.
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  But here's where the conversation goes off track, forest management and climate change
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  are often presented as competing explanations.
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  Was the fire caused by poor management?
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  Was it caused by climate change?
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  Scientifically, those are the wrong questions, because they can influence fire risk at the
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  same time.
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  A forest can contain high amounts of fuel because of its management history, and that same forest
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  can experience weeks of extreme heat, high vapor pressure deficits, and very little rainfall.
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  Add in some strong winds, providie ignition, and you've got the factors to contribute
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  to a major event.
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  And the importance isn't the same everywhere.
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  A dense forest in California is different from a grassland in Washington.
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  A remote, boreal forest in northern Canada is different from either of them.
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  Different ecosystems have different fire histories, different fuels, and different relationships
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  with people.
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  That's why scientists are careful when attributing changes to wildfire activity to any single
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  cause.
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  Climate change does not explain every fire.
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  Forest management does not explain every fire, and neither does lightning or human carelessness.
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  Instead, we have to look at how each factor changes the probability of a particular outcome.
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  Science isn't always trying to identify one culprit.
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  Sometimes it is trying to understand how several conditions come together at the same time.
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  So when you hear investigators have determined what caused a wildfire, remember that they've
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  only answered one part of the question.
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  They've told you what lit the match.
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  But understanding why the landscape burned the way it did requires looking at a lot of
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  other variables that were already waiting when the match was struck.
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  So far we've talked about what happens around a wildfire, but fire doesn't just change
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  the landscape where it burns, it also changes the atmosphere.
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  When vegetation burns, it releases a complicated mixture of gases and tiny particulates in
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  the air.
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  I know in the fire that I experienced, one thing they talked about was PM2.5.
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  It's an important element in human health.
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  The name refers to particularly matter smaller than about 2.5 micrometers.
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  That's very small.
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  We're talking about something that is 20 to 30 times smaller than a human hair.
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  Because these particles are so tiny, they can remain suspended in the atmosphere and travel
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  with the wind.
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  And they're small enough to penetrate deep into your lungs when you breathe them.
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  That's why you can be hundreds of miles away from a wildfire.
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  See no flames at all, and still receive an air quality alert on your phone.
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  But where that smoke goes depends on the atmosphere.
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  The heat from a wildfire causes air above it to rise, and that carries smoke with it.
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  How high that plume rises depends on the intensity of the fire and the structure of the atmosphere
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  above it.
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  If the atmosphere is stable, smoke usually remains trapped close to the surface.
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  That can cause terrible air quality near the fire.
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  But if the atmosphere allows the smoke to rise higher, the winds above the surface can
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  carry it farther away.
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  And those winds don't all blow in the same direction or at the same speed.
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  A smoke plume a few thousand feet above the ground may travel very differently in the
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  air than we've experienced at the surface.
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  That's how smoke crosses state lines, national borders, even entire continents.
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  And we've seen remarkable examples of that with the Canadian wildfires.
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  Smoke produced in remote forest can travel south into the United States, affecting air quality
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  across the Midwest, the Northeast, and sometimes even farther away.
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  Under extreme conditions, a wildfire can even create its own thunderstorm.
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  They're called pyrocumulonimbus clouds.
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  The intense heat creates a powerful updraft that can carry smoke high into the atmosphere,
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  occasionally even reaching the lower stratosphere.
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  But smoke doesn't need to travel that high to become someone else's problem.
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  Ordinary winds in the lower atmosphere are more than capable of carrying wildfire smoke
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  hundreds or thousands of miles.
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  So that brings me back to where we started today.
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  A couple of weeks ago in Northern California, I wasn't threatened by the wildfire itself.
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  I didn't have to evacuate.
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  I wasn't watching flames approach where I was staying.
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  But I still lost a day of my vacation because the atmosphere carried the fire to me.
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  Not its flames, but as PM2.5 smoke.
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  And millions of people across the North America are increasingly familiar with that experience.
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  That's an important change in how we should think about wildfire risk.
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  The people living beside a burning forest face dangers.
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  Those of us hundreds of miles away may never experience.
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  But the effects don't stop at the fire line.
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  Once smoke enters the atmosphere, the weather can take over.
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  Winds determine where it travels.
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  And stability helps to determine whether it stays near the ground or goes higher.
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  Eventually, changing weather conditions determine when cleaner air finally returns.
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  A wildfire can begin as an event on a landscape, but once the smoke rises, it becomes a meteorological
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  event as well.
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  A wildfire may begin as a single spark, but climate helps to determine the conditions waiting
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  when that spark arrives.
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  As our atmosphere continues to warm, understanding how heat, moisture and weather influence those
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  conditions are becoming increasingly important.
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  Because climate change doesn't have to light the fire to change the way it burns.
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  I'm Dr. Mac, and this has been the climate translation.
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  If you have a question about the climate you've been too afraid to ask, or if you have
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  a differing opinion, I want to hear from you.
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  I can use your viewpoints in future episodes.
• 20:30
  You can reach me at theclimatetranslation at gmail.com.
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  I'll see you next time.