The environmental challenges ahead for Heathrow's third runway

So the government has finally given its backing to a third runway at Heathrow. But now the mother of all balancing acts begins.

How do you accommodate 260,000 more aircraft and 45 million more passengers each year without breaching legally binding environmental targets?

The government has listened to the economic case for expansion. The airport's owners have argued the £18bn runway could benefit every family in Britain to the tune of £24,500 over its 60-year lifespan.

It has also been argued that our departure from the EU makes expansion all the more necessary to make Britain connected and competitive with the rest of the world.

That's not to mention thousands of jobs locally both during and after the construction phase.

A third runway would make Heathrow the UK's largest source of carbon emissions. Credit: PA

But at what environmental cost?

Today's announcement went a long way to diffuse local concerns about noise pollution with a proposal for a £2.6bn compensation package for affected residents.

It will, however, be harder to buy a way out of other environmental impacts. The UK is already in annual breach of nitrogen oxide limits. Heathrow is already a major hot spot for the gas - mainly produced by diesel traffic approaching the airport.

The government's own analysis suggests a third runway "risks" breaching them further. Especially if the airport is completed on time, before targets for low carbon vehicles are expected to bite in the 2030s.

The government could risk breaching legally binding environmental targets. Credit: PA

Air quality targets are also hard to ignore. The government has lost several high court challenges over its existing policy. It's possible subsequent court challenges could slow Heathrow expansion.

Then there's the rest of the planet to think about. Aviation injects carbon emissions directly into the upper atmosphere.

According to the Committee on Climate Change, the government's independent advisory committee, in order to meet our domestic carbon targets, emissions from planes have to stay at 2005 levels. Yet a third runway would make Heathrow the UK's single largest source of carbon emissions.

The airport says it has an ambitious plan to offset the 15% increase in emissions that a third runway will bring, but environmentalists are already saying the damage from expanding Heathrow far outweighs the benefits.