Has recent drop in airline flights over the US had a measurable warming effect as was found after 9/11?

The Climate Impact of Fewer Flights: What We Learned After 9/11 and During the Pandemic

Remember when all flights were grounded after 9/11? It was a scary time, but it also gave scientists a rare chance to peek into how air travel messes with our climate. They found something pretty interesting: the absence of those familiar white streaks in the sky – contrails – actually changed daily temperatures. So, with fewer planes in the air recently, you might wonder, are we seeing the same thing happen again? Well, buckle up, because the answer is, as usual with climate stuff, a bit complicated.

9/11 and the Curious Case of the Missing Contrails

After the 9/11 attacks, the FAA did something unprecedented: they grounded all flights for three whole days. It was during this eerie quiet that researchers noticed something odd. Without planes constantly crisscrossing the sky, the daily temperature range – the difference between the hottest and coldest part of the day – went up. Turns out, contrails, those icy lines planes leave behind, act like a bit of a climate seesaw.

During the day, they bounce some sunlight back into space, which cools things down. But at night, they trap heat, like a blanket, keeping things warmer. So, no contrails meant warmer days and cooler nights, making the temperature swing bigger. Some studies even pegged this increase at around 1.1°C (2°F). Pretty wild, huh?

Now, before we get carried away, it’s worth mentioning that not everyone agrees on the 9/11 contrail effect. Some experts think those temperature changes could’ve just been normal weather doing its thing.

Contrails: A Climate Double-Edged Sword

The big takeaway from 9/11 is that contrails definitely have a real, measurable impact on our climate. And it’s not as simple as “planes bad, no planes good.” While the CO2 spewed out by airplanes is a major climate headache, contrails can actually have an even bigger short-term effect. In fact, some recent studies suggest that the warming caused by contrails might outweigh the warming from all that jet fuel carbon!

Think of it this way: contrails are like a temporary lid on the atmosphere, trapping heat that would otherwise escape. Sure, they reflect some sunlight during the day, but overall, they seem to be making things warmer. The IPCC, those climate gurus, even estimated that contrails could be responsible for over a third of aviation’s total climate impact. That’s a lot!

COVID-19 and the Great Air Travel Dip

Then came COVID-19, and suddenly, the skies were empty again. Another chance to study what happens when planes stay on the ground! But this time, the story got even more twisty. Some research suggests that less soot from planes might actually lead to more warming. Confused? I don’t blame you.

See, soot particles in plane exhaust can actually increase the number of ice crystals in those high-altitude cirrus clouds. And the more ice crystals, the more the clouds reflect solar energy back into space, which keeps the temperature down. So, when there’s less soot, there are fewer ice crystals, and the clouds don’t reflect as much sunlight.

The Aerosol Masking Effect: A Climate Plot Twist

And here’s another curveball: the “aerosol masking effect.” Aerosols, those tiny particles floating around in the air, can bounce sunlight back into space, too. They come from all sorts of places, from volcanoes to factories.

The thing is, aerosols can actually hide some of the warming caused by CO2 and other greenhouse gases. So, when we cut down on air travel, we also cut down on aerosols, which means we’re essentially taking off a mask and revealing the warming that was already happening underneath.

So, What’s the Bottom Line?

While the 9/11 flight grounding gave us a glimpse of what happens when planes disappear, the truth is way more complex than a simple cause-and-effect. The recent drop in flights during the pandemic gives us a chance to really nail down how air traffic affects cloud cover. But we’ve got to consider everything: contrails, aerosols, even changes in cirrus clouds. It’s like trying to solve a giant climate puzzle!

And let’s not forget, planes don’t just pump out CO2. They also release other stuff, like nitrogen oxides, that contribute to warming. Aviation is responsible for a small percentage of global CO2 emissions, but when you factor in all those other effects, its total impact on global warming is a lot bigger.

The ultimate goal? Fewer flights, period. We can get there by using cleaner fuels, finding flight paths that avoid contrail formation, and, yeah, maybe even taking the train once in a while. It’s a complicated problem, but every little bit helps.