this topic, what would it be?

Zero emissions aviation.

It's not pie in the sky, but it's definitely going

to be a long haul journey.

Aviation is one of the hardest sectors to decarbonise.

Pre-pandemic, it accounted for about 2.4

per cent of total global emissions.

But many countries around the world

have decided they want to be net zero by 2050.

And that means that aviation has to work

hard and fast to get there.

By far the biggest factor when it comes to emissions

is how you power one of these through the air.

There's nothing more efficient or economical than jet fuel.

But fossil fuel emits carbon.

And by 2050 we're expecting 10bn passengers

to fly 20tn kilometres.

That's a lot of carbon.

There are alternatives being developed.

Electric aviation has caught lots of people's imagination.

The problem is simply that the batteries are still too heavy.

And that means that, frankly, you

can't pack the aircraft with very many passengers.

So when it comes to bigger aircraft and longer trips,

it's all about the fuel.

We have a range of options there.

First up, there's biofuels.

These are fuels that are made from feedstocks

that can range from plants to used cooking

oil, municipal waste, household waste.

Biofuels, like all sustainable fuels,

are two to four times the cost of jet fuel.

And in the wake of a pandemic that

has grounded the majority of the world's commercial aircraft,

airlines simply can't afford to pay that extra cost.

The second options are synthetic fuels, which are artificially

created to replicate kerosene.

Synthetic fuels are very new.

But they do exist.

KLM actually recently completed a flight using synthetic fuels.

But they are very expensive and very energy intensive.

One massive advantage of both biofuels and e-fuels

is that they can be just dropped straight into the tanks.

In fact, they're called drop-in fuels.

You don't have to change the aircraft.

You don't have to change the engines.

Minor adjustments, just tiny tweaks the engines.

You don't have to change the fuel supply system.

One thing to remember about these drop-in fuels

is that they do emit carbon when they're burned.

The advantage is that they only release the carbon

that they've already taken out of the atmosphere.

So they're what's called net zero, not true zero.

The only true zero option, at least

that we know about at the moment,

is hydrogen. It's got three times the energy

density of kerosene.

The problem is that its volume is four times bigger.

You will need tanks that are so big.

So like electric aviation, hydrogen

might be better suited to shorter flights.

But possibly the biggest obstacle

is that it would require trillions in investment.

Investment in new aircraft, in fuel storage systems,

in fuel distribution systems, and in production itself.

These sustainable aviation fuels will cost more.

Even the industry says it will be a very tough challenge

to get to net zero by 2050.

So we the passengers have a choice to make.

Either we fly less or we pay more.