Campaigners against the expansion of Bristol Airport said a decision to approve the plan was "disastrous" for the climate and "trashed" democracy.
The proposal has been granted on appeal by the government after being refused permission by the local council.
It was criticised locally, nationally and abroad and was denounced by Greta Thunberg who protested against it.
Bristol Airport chief executive Dave Lees said the expansion was "excellent news for our region's economy".
The plan would allow the airport to increase its capacity from 10 million to 12 million passengers per year.
Stephen Clarke, from Bristol Airport Action Network, said: "This is going to have a huge impact on the climate."
Mr Clarke, who is also a former Green Party councillor, said the decision was "wrong on many different levels" adding: "It totally trashes local democracy."
"11,000 people objected to this scheme, all of the local councils objected, the vast majority of the local politicians objected, and both the airport and the planning inspectorate have totally ignored that, so local democracy is a big loser in this decision.
'Climate biggest loser'
"But the main loser is the climate.
"Up to 20,000 extra planes a year, 10,000 extra car journeys a day, more night flights than Heathrow - how can that be right?
"Boris Johnson told us at COP26 we are one minute to midnight.
"Well, we are a little bit closer to midnight now because there are a lot of other regional airports expanding behind this decision."
Mr Clarke said the group was seeking urgent legal advice as to whether the decision could be challenged in the High Court.
Dave Harvey - BBC Points West Business Correspondent
This was an incredibly complicated process, the planning inquiry took three months and heard thousands of hours of testimony.
Reading directly from the 118 page report, the inspectors said there was and remained a significant level of opposition to the proposed scheme.
I have been to the meetings, at the protests, and hundreds of people turned out, thousands of complaints were made.
Objections, the inspectors say, were made at a local, regional, national and international level - the loudest international voice coming from Greta Thunberg, who specifically came to Bristol to oppose this.
And they say they realise the decision will come as a disappointment to those people who spoke so passionately in opposition.
But ultimately their decision was, quite simply, that national policy, both on aviation and on climate change, was generally in favour of the expansion of airports and doing other things to curb carbon emissions.
In a nutshell, it's not being banned everywhere else, so it shouldn't be restricted here. So, if you don't like it, you can't oppose it at local level, you've got to change the government policy.
The planning inspectorate said it recognised the "major disappointment" campaigners would feel, but said the benefits would outweigh the harm to green belt land.
North Somerset councillor, Steve Hogg, said the decision "makes no sense" adding "it has never made any sense".
He said: "The overwhelming majority of people here are going to bitterly regret the decision because it is going to have a material effect on their lives.
"They will be overflown, they will be woken up in the middle of the night with flights, and our roads and our environment is going to become materially worse over the next few years."
To accommodate the extra passengers, thousands of car parking spaces will be added.
The airport's chief executive Mr Lees said the expansion would "create thousands of new jobs in the years ahead and reduce millions of road journeys made to London airports each year".
Simon Earles, sustainability and corporate affairs director at Bristol Airport, added: "We do of course understand and respect that not everyone supports the plans that we put forward, but we have sought to listen to the concerns and take those into account.
"We've set out through these proposals our ambitious plans to become a net zero airport.
"And I must stress, this decision is not a free hit in terms of growth, inspectors have listened to the issues raised by the community and they have attached significant controls.
"The inspectors have given certainty to local communities in terms of how many flights there will be, they have limited the number both in total and the number of aircraft that fly at night.
"Our job now is to work with our airlines to ensure the cleanest, most modern aircraft are operating out of Bristol Airport."
Metro Mayor Dan Norris has blamed "out-of-date government policy" for the decision.
He said there needed to be an "urgent national conversation" about airports.
"I am dismayed but not at all surprised by this decision.
"The government's lack of green policy on UK airport expansion has resulted in inspectors ignoring the voices of local people.
"I have always thought the people are ahead of law makers on this issue.
"The public will severely punish politicians who ignore the climate emergency and I believe this decision will be overturned by a future new government within years."