Latest / The Climate Translation / Invisible Mirrors

Transcript

Invisible Mirrors · The Climate Translation

• 0:00
  When we talk about climate change we usually talk about gases, carbon dioxide, methane, things
• 0:14
  that trap heat, but there's another part of the story floating right alongside them.
• 0:20
  Tiny particles suspended in the air, so small you can't see them individually, but powerful
• 0:27
  enough to change how much sunlight reaches the surface and how much heat escapes back into
• 0:33
  space. Scientists call them aerosols. Some aerosols act like mirrors, reflecting sunlight
• 0:41
  and cooling the planet, others act like tiny heaters, absorbing energy and making warming
• 0:47
  worse. And unlike greenhouse gases many of them don't last for centuries, they can appear
• 0:54
  or disappear on the order of days, weeks or years. Today we're going to take apart this
• 1:01
  invisible haze. We'll look at how volcanoes can cool the earth, how aircraft can leave
• 1:07
  warming fingerprints across the sky, and why cleaning up air pollution can briefly increase
• 1:13
  warming before it reduces it. Welcome to the podcast, I'm Dr. Mac, and this is the Climate
• 1:21
  Translation.
• 1:28
  When a major volcano erupts like Mt. Pinatubo in 1991, it doesn't just throw ash into the
• 1:34
  air and call it a day. The most important climate impact comes from something you can't see
• 1:41
  very easily, sulfur dioxide gas. Pinatubo injected millions of tons of sulfur dioxide high
• 1:50
  into the stratosphere, that dry, stable layer of the atmosphere that's above the majority
• 1:57
  of our weather. That's important because once material gets into the stratosphere, it doesn't
• 2:03
  come out quickly. It hangs around. Up there, the sulfur dioxide reacted with the tiny amounts
• 2:10
  of water vapor and formed microscopic droplets of sulfuric acid. Those droplets spread around
• 2:17
  the globe, forming a thin hazy layer. These droplets acted like mirrors. They scattered
• 2:24
  and reflected incoming sunlight back into space before it ever reached the surface. The
• 2:30
  result was less energy getting in to the climate system. After the Pinatubo eruption, the
• 2:38
  average temperature of the entire planet dropped by about a half a degree Celsius for more
• 2:44
  than a year. It was an amazing effect, and one my students often react to when we talk
• 2:50
  about it in class. Usually as a result of our discussion, a very reasonable question comes
• 2:56
  up. If volcanoes cool the planet, why can't they help cancel out some of the human-induced
• 3:03
  global warming? The answer is because aerosols and greenhouse gases play completely different
• 3:09
  games. Volcanic sulfate aerosols are short-lived. Over time, gravity wins. The particles slowly
• 3:18
  settle out of the stratosphere and fall back to Earth. Within a few years, the cooling
• 3:23
  effect is gone. Carbon dioxide, on the other hand, sticks around for much longer - centuries.
• 3:31
  So volcanoes can temporarily dim the lights, but they don't change the wiring. This is
• 3:37
  why scientists call sulfate aerosols short-term climate forcers. They don't set the long-term
• 3:44
  direction of the climate, but they can strongly influence temperature for a brief time. And
• 3:51
  that influence comes with side effects. Volcanic aerosols can change rainfall patterns. They
• 3:58
  can weaken monsoons. They can alter ozone chemistry in the stratosphere. Cooling the planet doesn't
• 4:05
  mean cooling it evenly or gently either. That complexity is why the next idea makes scientists
• 4:12
  uneasy. And that is, if volcanoes can cool the planet by reflecting sunlight, some people
• 4:19
  have asked, what if humans try to do that on purpose? The idea is called solar geoengineering,
• 4:27
  deliberately adding reflective aerosols to the stratosphere to reduce warming. On paper,
• 4:35
  the physics works. In practice, it's incredibly risky. You wouldn't be adjusting a thermostat,
• 4:42
  you'd be altering global circulation patterns, precipitation, and regional climates,
• 4:48
  without any reliable way to stop it quickly if something went wrong. And critically, it won't fix
• 4:55
  the root problem. It won't reduce carbon dioxide. It won't stop ocean acidification. It would only
• 5:03
  mask some of the warming while leaving everything else unchanged. Still, some scientists believe it's
• 5:10
  a worthwhile thing to research. For example, researchers connected to Harvard's solar geoengineering
• 5:18
  research program have explored small-scale experiments to better understand how particles behave
• 5:24
  in the stratosphere. Not to deploy them, but to reduce uncertainty. International bodies like
• 5:31
  the National Academies of Science, Engineering, and Medicine have also called for limited research,
• 5:37
  strict governance, and global oversight, emphasizing that this is not a substitute for cutting emissions.
• 5:45
  The key point is this, studying a risk is not the same as deploying a solution.
• 5:52
  Most scientists working in this area agree on three things. We don't know enough to use it.
• 5:57
  It carries serious ethical and political risks, and it should not distract from reducing greenhouse
• 6:04
  gases as a whole. But because the stakes are so high, some research continues, albeit under intense
• 6:12
  scrutiny. So if sulfur dioxide acts like a mirror, that our next aerosol can be thought of as
• 6:19
  its dark opposite. Black carbon, also known as soot, is produced by incomplete combustion,
• 6:27
  diesel engines, coal burning, wildfires, residential cooking, and agricultural burning.
• 6:33
  Wherever fuel burns inefficiently, soot is released. If sulfate aerosols are mirrors,
• 6:42
  Soot is more like a strip of black asphalt suspended in the air. Because it's dark,
• 6:48
  soot doesn't reflect much sunlight, it absorbs it. That absorbed energy heats the surrounding
• 6:54
  air directly, adding to warming rather than offsetting it. An unlike greenhouse gases,
• 7:01
  which trap heat already in the system, black carbon increases heating by soaking up incoming
• 7:08
  solar energy. So is soot a greenhouse gas? No, greenhouse gases trap outgoing heat. Soot adds heat
• 7:17
  by absorbing sunlight on the way in. It's a different mechanism, but it has the same consequence,
• 7:24
  where soot ends up as just as important as how it behaves. When black carbon falls out of the
• 7:32
  atmosphere and lands on bright surfaces, snow and ice in particular, it changes the surface
• 7:38
  itself. Clean snow reflects more sunlight. Darkened snow absorbs it. This effect is especially
• 7:46
  concerning in places like the Arctic and the Himalayas, where glaciers act as natural water
• 7:52
  reservoirs. A thin layer of soot can dramatically increase melting. That meltwater may seem beneficial
• 8:00
  in the short term, but over time it threatens the stability of water supplies for hundreds of
• 8:06
  millions of people downstream. Alongside black carbon, combustion also releases organic carbon
• 8:14
  aerosols. These are lighter colored particles made of complex organic molecules. Organic carbon
• 8:21
  behaves differently. Some of it reflects sunlight, producing a cooling effect. Some of it absorbs
• 8:28
  light weekly. The net effect depends on the source. Wildfire smoke behaves differently than diesel
• 8:36
  exhaust, for example. This complexity is one reason aerosols are among the largest uncertainties
• 8:42
  in climate science. So if some pollution cools and some warms do they cancel each other out?
• 8:51
  No, because they don't line up neatly in space and time. Soot tends to warm strongly but briefly.
• 8:58
  Organic aerosols can cool weekly, but over broad areas. And both interact with clouds,
• 9:05
  humidity and sunlight in ways that vary from region to region. The result isn't balanced. It's
• 9:12
  patchwork. And here's a part that might surprise you. Black carbon doesn't last long on the
• 9:17
  atmosphere, typically days to weeks. That means if you reduce Soot emissions, you can produce a near
• 9:24
  immediate climate benefit, especially in sensitive regions. Cutting soot improves air quality. It
• 9:31
  reduces health risks, and it slows warming all at once. It's one of the few climate actions where
• 9:38
  benefits show up very quickly. But we can't always control how or when soot enters the atmosphere.
• 9:47
  The recent wildfire seasons in Canada make that painfully clear. Smoke from those fires
• 9:53
  travel thousands of miles, degrading air quality across much of North America. In the short term,
• 9:59
  that kind of soot load can also affect how much sunlight reaches the surface and how the
• 10:05
  atmosphere heats. Not enough to change the climate trajectory, but enough to temporarily shift
• 10:12
  regional conditions. It's a reminder that black carbon isn't just an emissions problem.
• 10:18
  It's also a consequence of a warming, drying world that makes large fires more likely in the first
• 10:25
  place. Now let's look at one of the strangest aerosols humans create, often without realizing it.
• 10:33
  Every time a jet flies at cruising altitude, it leaves behind those thin white lines that we call
• 10:39
  contrails, short for condensation trails. At first glance, they look like harmless clouds that
• 10:47
  slowly fade away in minutes. But under the right conditions, they don't fade, they spread,
• 10:54
  and they matter. At high altitudes, jet engines release hot exhaust that contained water vapor
• 11:01
  and tiny soot particles. An extremely cold, dry layers of the upper atmosphere that water vapor
• 11:09
  freezes, almost instantly onto those particles, forming ice crystals. That forms a contrail.
• 11:16
  Most contrails disappear quickly, but when the air is especially cold and humid, they can persist
• 11:22
  for hours and spread into wide, thin sheets of cloud that look almost indistinguishable from natural
• 11:29
  cirrus. And that's where the climate effect comes in. Cirrus clouds, high ice clouds,
• 11:37
  whether natural or human-made, have a dual personality. During the day, they reflect a small
• 11:44
  amount of incoming sunlight, which produces a slight cooling effect. But at night, they trap
• 11:50
  outgoing heat, acting like a thin blanket that slows down how fast the earth can release heat
• 11:57
  back into space. Because contrails tend to persist at night, and because their heat trapping
• 12:04
  effect lasts longer than their reflective effect, the net impact is warming. So how can something
• 12:12
  that looks like a cloud cause warming? It's because not all clouds act the same way. Low thick
• 12:20
  clouds, mostly cool, high thin ice clouds, mostly warm. Contrails fall into that second category.
• 12:30
  So are airplanes worse than cars? No, but they're different. Most of aviation's climate impact
• 12:38
  still comes from carbon dioxide, just like any other fossil fuel use. But contrails add an extra
• 12:45
  short-lived warming effect that happens immediately, especially in busy flight corridors.
• 12:51
  Some studies suggest at times, the warming from contrails can be comparable to the warming
• 12:58
  from aviation's carbon dioxide output, but only over short periods and in specific regions.
• 13:05
  That doesn't mean contrails replace carbon dioxide as a concern, but it means it stacks on top of
• 13:11
  it. The good news is that contrails are also one of the few aerosol effects we can potentially
• 13:18
  reduce without waiting decades. Because contrails only form under certain atmospheric conditions,
• 13:25
  small changes in flight altitude or routing can sometimes avoid them entirely. Researchers
• 13:33
  are actively studying whether slightly adjusting flight paths could significantly reduce contrail
• 13:39
  formation without minimal added fuel use. But here's where the uncertainty comes in.
• 13:46
  We still don't know exactly how contrails interact with natural clouds, how long their effects
• 13:52
  persist globally, or how those impacts will change as air travel continues to grow.
• 13:59
  And that's why contrails are such an active research topic right now. They sit at the intersection
• 14:06
  of climate physics, engineering, economics, and policy. It's a reminder that not all climate
• 14:13
  impacts come from smokestacks or tailpipes at the surface. But speaking of the surface,
• 14:20
  let's drop down and discuss some aerosols that have existed long before humans ever burn fuel
• 14:27
  or flew at cruising altitude. That's dust and sea salt. Let's start with dust.
• 14:34
  Every year, enormous plumes of fine dust are lifted out of the deserts like the Sahara
• 14:41
  and carry thousands of miles by the wind. Satellites regularly track these plumes crossing the
• 14:48
  Atlantic Ocean as they reach the Americas. That dust does several things at once. First,
• 14:55
  it shades the surface below, slightly reducing the amount of sunlight reaching the ocean.
• 15:01
  That produces a small cooling effect. But the dust also carries nutrients, especially iron.
• 15:09
  When it settles out over the ocean or reaches places like the Amazon Rainforest,
• 15:14
  it fertilizes ecosystems that would otherwise be nutrient poor. In other words,
• 15:20
  the same dust that dims sunlight helps fuel life. Now let's look at sea salt.
• 15:27
  When waves break, tiny droplets of sea water are launched into the air.
• 15:32
  As the water evaporates, microscopic salt particles remain suspended in the atmosphere.
• 15:39
  These particles are incredibly important because they act as cloud condensation nuclei,
• 15:44
  the seeds that water vapor needs to form cloud droplets. No particles, no clouds.
• 15:52
  Sea salt helps determine how bright clouds are, how long they last, and how much sunlight they reflect.
• 15:59
  In many ways, these natural aerosols are part of the Earth's built-in climate regulation system.
• 16:06
  So if aerosols cool the planet, wouldn't more of them be good, and the answer is complicated.
• 16:13
  For much of the last century, human-made aerosols, especially sulfates from burning coal,
• 16:19
  particularly mask warming caused by greenhouse gases. They act like a temporary sunshade
• 16:25
  reflecting some sunlight back into space. This helped cool the planet slightly, but at a huge cost.
• 16:32
  Those same aerosols caused severe air pollution, acid rain, and millions of premature deaths worldwide.
• 16:40
  Cleaning them up was one of the great public health successes of the last fifty years in many places.
• 16:47
  And here's the paradox. As we clean the air, which we absolutely should,
• 16:52
  we remove the temporary cooling effect, the mirrors disappear faster than greenhouse gases do.
• 16:59
  The result is that a warming can appear to accelerate, even though emission policies are improving.
• 17:07
  This doesn't mean that making air cleaner is a mistake. It means aerosols were never a solution.
• 17:13
  They were just a side effect. Climate change isn't caused by the absence of pollution.
• 17:19
  It's caused by the accumulation of greenhouse gases.
• 17:23
  Aerosols complicate the process because they operate on a short time scale and are very localized,
• 17:29
  while greenhouse gases shape the long-term global trend.
• 17:34
  That's why aerosols are one of the hardest pieces of the climate system to model and predict.
• 17:40
  They cool and they warm. They help and they harm. They appear and disappear quickly.
• 17:47
  And they matter a lot. Understanding aerosols helps us make sense of why
• 17:53
  temperature changes don't always track exactly emissions perfectly year-to-year.
• 17:59
  And why cleaning up the air can briefly reveal warming that was already there.
• 18:04
  Aerosols remind us that the climate system isn't governed by one single lever.
• 18:09
  Some particles cool the planet by reflecting sunlight. Others warm it by absorbing energy.
• 18:16
  Some linger for years. Others vanish in weeks. And many do things at the same time,
• 18:23
  depending on where they are and where they appear. They are the invisible mirrors of
• 18:28
  our atmosphere, shaping how much energy gets in, and how much gets out, and how unevenly those
• 18:35
  effects are felt across the globe. Understanding aerosols helps explain some of the climate puzzle
• 18:42
  that confuses so many people. They explain why temperatures don't rise smoothly year after year,
• 18:48
  why cleaning up air pollution can briefly reveal warming, and why short-term changes can never cancel
• 18:55
  out the long-term trends driven by greenhouse gases.
• 19:08
  Aerosols remind us that climate change isn't just about what we emit,
• 19:13
  but how the atmosphere responds in real time. And they show us why careful science matters,
• 19:20
  especially when small particles can have big effects. Clear skies is still the
• 19:26
  goal, cleaner air saves lives, and reducing greenhouse gases remains the only way to stabilize
• 19:33
  the climate long term. But along the way, aerosols help us see this system for what it really is,
• 19:40
  complex, connected, and sensitive to details we can't afford to ignore.
• 19:47
  I'm Dr. Mac, and this has been the Climate Translation. If you have a question about the climate,
• 19:54
  been too afraid to ask, or if you have a differing opinion, I want to hear from you.
• 19:59
  I can use your viewpoints in future episodes. You can reach me at theclimatetranslation
• 20:05
  at gmail.com. I'll see you next time.