Some 80 million passengers fly through London's Heathrow Airport each year, but to boost its capacity by another 50 million, the airport seeks to add a third runway. This could make Heathrow, by far, the busiest airport on Earth.

But three UK judges on Thursday blocked the construction of a new, more than 10,000-foot-long stretch of asphalt. In a lawsuit brought by environmental groups, the judges found the government's runway plan failed to consider how the added air traffic — which will emit prodigious amounts of heat-trapping carbon dioxide — would impact the UK's commitment to slash its carbon emissions, as agreed to in the historic Paris climate accords.

The lapse is "legally fatal" to the runway plans, the judges wrote.

"The Paris Agreement ought to have been taken into account by the Secretary of State in the preparation of the [runway plans] and an explanation given as to how it was taken into account, but it was not," they said.

The new runway, however, could still potentially get built one day — if Heathrow explains how it wouldn't hinder the nation's aims to ambitiously slash carbon emissions. (The judges emphasized they can't totally ban a third runway, only that the plans for the runway didn't take into account the nation's Paris commitments.)

But when it comes to carbon emissions, airplanes have a big problem. That's because big airliners — which make up around 95 percent of commercial aviation emissions — will continue to burn liquid fuels, at least through much of the century. "We will continue to use jet fuel for as far as the eye can see," Delta CEO Ed Bastian said earlier in Februray . And jet fuels are largely made from oil products.

A chart showing carbon emissions from planes. Image: INTERNATIONAL COUNCIL ON CLEAN TRANSPORTATION

BREAKING: We won! Today we blocked the Tory government plans to build a third runway at Heathrow Airport.



Today’s judgment is a major victory for all Londoners who are passionate about tackling the climate emergency and cleaning up our air. https://t.co/59MEn2X6Lw — Sadiq Khan (@SadiqKhan) February 27, 2020

If airlines want to boldly slash emissions, however, they have an option: fuels made from plants, commonly called biofuels. Yes, when burned in an engine, biofuels will still emit carbon dioxide into the atmosphere. But, critically, biofuels don't require that any new oil be fracked and drilled out of the Earth, adding new sources of CO2 to the air.

"Instead of releasing carbon as CO2 that’s been in the earth for billions of years, this is from a plant,” Andrew Sutton, who works on the chemical energy storage team at Los Alamos National Laboratory, told Mashable.

It is possible, though challenging, for airlines to ramp up their use of biofuels over the next 10 or 15 years, explained aviation engineer Timothy Takahashi. But there's very little infrastructure and production of biofuels today. These greener fuels made up less than one-tenth of one percent of aviation fuels burned in 2018.

If future planes passing through Heathrow ran on biofuels, perhaps the airport could justify the addition of a new runway, and tens of millions more annual passengers.

There's also a hot buzzword in the aviation industry that can play a smaller role in helping any airline, or airport, meet it ambitious climate targets: carbon offsets. The practice involves paying for projects that absorb carbon dioxide from the air (conserving land, planting trees) or technology that replaces fossil fuel burning (wind and solar projects) to offset carbon dioxide released.

The problem is that planting lots of trees, for example, doesn't slow civilization's rising carbon emissions, nor does it immediately soak up carbon from the air. Even if 1 trillion trees were planted on Earth by 2050, it wouldn't remotely solve Earth's growing carbon emissions problem. And, of course, there's the increasing problem of forests burning down in a hotter climate, which pumps carbon dioxide right back into the atmosphere.

Meanwhile, new wind farms are great, but those wind farms will not stop planes from emitting carbon into the atmosphere — and global carbon emissons must drop to zero for the planet to stop warming.

Importantly, the aviation industry today already emits more carbon into the atmosphere than nearly every nation on Earth, and aviation CO2 emissions have risen by 32 percent since 2013. Around 20,000 planes fly around Earth today, but some 50,000 planes are projected to take to the air by 2040.

Airplane traffic is expected to triple in the next 25 years.

Unless airlines rapidly transition to biofuels, more runways will almost certainly mean much more carbon dioxide entering the atmosphere.