When booking flights online you may be offered the option to offset your share of carbon emissions for a few extra dollars.

But where does the money go, what is it used for, and is it worth ticking that carbon offset box?

The option is there because aviation is responsible for about 2 per cent of global carbon dioxide emissions.

In 2016, Australia's domestic and international civil aviation sector released the equivalent of 22 million tonnes of carbon dioxide.

As a result, about a third of airlines offer some form of carbon offsetting, where customers pay a few extra dollars into a scheme which carries out environmental improvement projects.

How are carbon offsets calculated?

Calculations vary between airlines. Usually they take the total carbon emissions produced by your flight, based on past fuel usage, and divide it by the number of seats on the plane.

This is then multiplied by the cost of offsetting one tonne of carbon, according to their carbon offset scheme. So shorter trips are cheaper.

You'll be stung less than a dollar for a one-way Qantas flight between Melbourne and Sydney. A return ticket from Melbourne to London will set you back $50.96 in carbon offsets.

Where does the money go?

Airlines don't keep it, but direct it to carbon offset projects.

There are broadly two types of projects offered by airline offset schemes, the University of Queensland's Brent Ritchie said.

"There's the general forestry types, stopping cutting down trees or replanting trees.

"The others are related to energy efficiency, so they might be renewable energy projects."

Energy projects, Professor Ritchie said, usually help the community as well as the environment.

In interviews and focus groups, he and his colleagues found "people usually like those ones with the human element better".

Planting trees is a long-term project, involving years of care. ( Freepik: welcomia )

Some projects are verified by rigorous standards. The National Carbon Offset Standard verifies carbon-neutral projects in Australia, for instance.

Still, not many travellers buy carbon offsets.

Qantas (including Jetstar) and Virgin Australia report about 10 per cent of passengers opt in.

Reasons for not buying carbon offsets are wide and varied, such as climate change denialism, unwillingness to pay more money and a mistrust of airlines.

The biggest barrier, though, is people simply don't have the information.

Professor Ritchie's interviews with travellers found they had little understanding of what happened to carbon offset payments.

Griffith University's Susanne Becken said airlines might see more people opt in by renaming carbon offsets to "environmental donations".

"Airlines haven't quite got around to moving away from that 'offset' term," Professor Becken said. "It's confusing."

So, is it worth paying for carbon offsets?

Yes ... insofar as it's better than nothing, said James Higham from the University of Otago in New Zealand.

"On the positive side of things, carbon offsetting does, perhaps, prod users to think about their personal carbon profile."

But even if you pay, you're not stopping carbon dioxide produced by your flight from entering the atmosphere.

Land-based carbon offsetting, such as planting trees, might help suck in some atmospheric carbon dioxide, but nowhere near as much as flights churn out.

"If you plant a tree, or plant a million trees, that doesn't really solve the problem because the carbon has been emitted and it's in the atmosphere," Professor Higham said.

"[The trees] may absorb some of the carbon dioxide, but then you have to maintain those trees.

"If they die, the carbon is released again and you're back to square one."

What else are airlines doing to cut emissions?

The obvious answer is to use renewable energy. So when can we expect to fly overseas on solar-powered planes?

"Not in our lifetime, I'm afraid," Professor Higham said.

"I'm sorry to say that technology ain't going to save us on this one.

"People talk about electric and solar flight, but it's just not going to happen, certainly not in the timeframes needed for urgent climate stabilisation."

And because aviation emissions aren't covered by the 2015 Paris Climate Agreement, the UN's International Civil Aviation Organisation and its member states signed a carbon offsetting deal in 2016.

Coming into effect in 2021, the Carbon Offsetting and Reduction Scheme for International Aviation, or CORSIA, will have airlines offset carbon emissions over 2020 levels by buying carbon credits from environmental projects.

CORSIA is separate to voluntary consumer carbon offsets.

But it's been criticised. For a start, the scheme will be voluntary for the first six years, Professor Higham said.

And in as-yet unpublished research, he and colleagues calculated that in the long run, CORSIA won't reduce emissions — there are too many loopholes for airlines to exploit.

An obvious airline emission-cutting measure is to get the biggest bang out of their fuel buck.

Each new generation of plane is more fuel efficient than the last, but that's an expensive measure, Professor Becken said.

So you may have noticed a few aircraft have implemented weight-loss measures — smaller tray tables, anyone?

Many Australian domestic flights don't have screens on the back of seats anymore.

This doesn't just free up the weight of the screens, it does away with hundreds of metres of cabling.

Airlines are also using high-tech, real-time weather tracking systems to take advantage of tail winds, Professor Becken said.

What else can flyers do to reduce their carbon footprint?

"We shouldn't be flying as much," Professor Ritchie said. "Let's be clear about that."

Professor Higham agrees.

"We need to really think about air travel," he said.

"I'm not saying for a minute that people stop flying and I'm not saying there should be less tourism.

"Say if I'm taking a holiday, I might go somewhere beautiful in New Zealand, instead of somewhere beautiful in Europe.

"And if I do want to go to Europe, I'll save up my 'carbon budget' and my financial budget, and travel in a few years' time."

If you must fly, try to do a bit of research and choose to fly with an airline that has a good emission reduction record, Professor Becken said.

And if your airline doesn't have a reputable scheme, you can source your own airline carbon emission program.

There are plenty of not-for-profit organisations such as Atmosfair in Germany, MyClimate in Switzerland and Climate Care in the UK that calculate your carbon offset payments and direct that money to their own projects.

"They have verified standards too," Professor Becken said. "You can even offset your car use."

An alternative to offsetting is "insetting", or start your own carbon scheme, she added.

"You can apply it to yourself. Do you offset every one of your flights, or do you save the money to help buy solar panels for your house?"