“When I was growing up in Seattle, planes were everywhere,” writes Bill Gates.

“Boeing was the biggest employer in the area, and it felt like almost every adult I knew had some connection to the company,” the Microsoft founder adds.

Gates says his Neighbors, family friends, and even several of local teachers all worked in aviation at some point.

“I remember looking up at the planes that flew over my house and thinking about the engineers who put them there.”

Today, many decades later, Gates finds himself thinking about planes a lot.

“They’re a major contributor to climate change and a key focus of our work at Breakthrough Energy.”

This is what Bill Gates wrote about planes and climate

Aviation is one of the hardest parts of the economy to decarbonize.

Keeping a 90,000 lb. passenger jet in the sky is an engineering miracle on its own.

Finding a way to do that without adding significant weight and while maintaining necessary safety standards is no easy feat.

Everyone knows planes create carbon emissions when they burn jet fuel.

To reduce emissions from shorter and regional flights, electric and hybrid aircraft are in development.

For longer flights, we need sustainable aviation fuels, or Sustainable Aviation Fuel (SAF), which can drop into existing engines to power them with substantially lower lifecycle emissions.

Amazing work is being done on both fronts, but we’re still years away from either being used at scale.

But jet fuel isn’t the only way that planes are warming the planet. Condensation trails—commonly called “contrails”—are the lines you’ll sometimes see left behind a plane as it flies, and they are also a significant contributor to climate change.

For an affordable climate win, look to the sky

“If we got rid of some contrails, we could eliminate up to two percent of all global warming.”

There are a lot of misconceptions about contrails. Some people refer to them as “chemtrails” and believe they contain toxic chemicals. The reality is much less dramatic: They’re just clouds.

Contrails are an unintentional byproduct of plane travel, and no one is putting lines in the sky on purpose.

In fact, researchers are looking for ways to stop them from forming—not because they’re poisonous, but because they make the planet hotter. The science behind how that works is fascinating.

You may have noticed that, while some planes create contrails, others do not.

A plane needs to fly through a pocket of air with the right temperature and the right humidity.

If it does, water nucleates around the tiny soot particles that come out of the plane’s engine and freezes into ice crystals.

Most contrails dissipate quickly and don’t interfere with the climate.

But in the right conditions, some can linger, spread for hours, and become cirrus clouds.

Cirrus clouds are a natural phenomenon.

If you look outside your window right now, there’s a good chance you can see them in the sky. (They’re the high, wispy clouds that look like brush strokes.) Cirrus clouds are one of the most common types of cloud, and they play a critical role in helping the planet regulate its temperature.

Like natural cirrus clouds, contrails reflect sunlight entering our atmosphere and absorb heat before it can leave.

That’s fine during the day, but at night, contrails trap heat that would otherwise escape to space. These warming contrails are the ones we need to prevent.

Persistent contrails are responsible for about half of the temperature increase caused by aviation.

If we were to get rid of them, we would eliminate up to two percent of all global warming.

Doing this would cost less than ten dollars a ton of emissions avoided (probably around a dollar or two), compared with US$130 or more per ton for Sustainable Aviation Fuel (SAF).

That’s a mind-blowing impact for such a relatively cheap intervention.

Compared to other climate interventions, it’s relatively simple to stop contrails from forming in the first place.

We know what conditions are required to create contrails, and we know how to find them. If we can change the route a plane takes to avoid those zones, we can prevent the kind that contribute to climate change.

Rerouting a plane slightly to avoid atmospheric conditions is nothing new.

Every flight you’ve ever taken has followed a path designed to minimize turbulence and stay away from bad weather.

Sometimes this path is the shortest distance between your departure city and your destination, and sometimes it involves a little extra flight time.

Airlines factor this deviation in when scheduling flights, so you rarely notice it. We can use the same approach with contrails.

To avoid the majority of warming contrail regions, we would need to adjust the routes of just five percent of flights globally.

When an airline files the flight plan for a specific flight—usually about 4-8 hours before departure—they can include any adjustments needed to avoid areas that will create warming contrails.

These areas of the sky tend to be thin.

So instead of steering around them, the most efficient approach is usually to fly either a little bit below or above the zone.

You might be wondering, “But what about the extra fuel burned by avoiding these contrail zones?”

It’s a good question, and it’s an important concern to consider.

While it’s true that rerouting to avoid contrails may burn more fuel, the reality is that it uses such a minuscule amount—as little as 0.1 percent more fuel when averaged across all flights—that the contribution to climate change is far smaller than that of the potential contrail.

And that extra fuel matters even less as cleaner fuels like SAF scale up.

The benefits of preventing contrails are undeniable.

The question is how we make contrail avoidance standard practice.

I helped start a nonprofit called Contrails.org that is doing amazing research into the problem and developing open-source software that airlines can use to plan routes. (I recommend playing around with the contrails map they have on their website.)

It’s really fantastic to see all the partnerships that have been built to do this work.

Last year, Contrails.org teamed up with American Airlines and Google Research to fly more than 100 flights using contrail avoidance, and now they’re working with partners on a landmark study backed by the UK government called Operation Blue Skies.

This winter and next, some flights passing through the Shanwick airspace off the west coast of the UK will have their routes adjusted slightly by air traffic control to avoid creating contrails. I’m excited to see the results.

The more research is done, the clearer the importance of this work becomes. With the right cooperation, we can scale this effort out and have a massive impact.

At the same time, I recognize that I am an imperfect messenger to write about this topic.

I fly in private planes, after all.

Shrinking those contributions is a responsibility I take seriously, which is why I make sure that any flight I take follows a route that avoids creating warming contrails. I also use SAF when I fly and fully offset my family’s aviation emissions.

The big question around this work is whether there’s enough of an incentive for airlines to buy into it.

Avoiding turbulence is a safety issue, and it’s also good business—passengers don’t like a bumpy ride and, in the worst case scenario, severe turbulence can cause injuries.

The economic benefits of avoiding contrails are less obvious, which is why Contrails.org offers its software for free.

(Airlines pay to subscribe to flight planning systems that, among other things, help them plan for weather.)

Still, I’m hopeful that contrail avoidance will become part of the air traffic modernization process that’s happening in countries around the world right now.

When I first learned about contrails and saw the math, I couldn’t believe it.

What do you mean that, for a very modest cost, we can eliminate up to two percent of all global warming? For just a dollar or two per ton, we can get rid of something that makes the world hotter.

That’s the kind of progress we need to make now while other innovations scale up and deploy.