Bruised and battered at the end of a tough week, it had all started so well for Mr Brown. In his first major speech on the environment as Prime Minister, he said on Monday that Britain needed to lead the way in cutting greenhouse gas emissions and make “tough decisions” to become a low carbon economy.

“Our vision has one overriding aim: holding the rise in global average temperature to no more than two degrees centigrade,” said Mr Brown at a meeting hosted by WWF. “This requires global greenhouse gas emissions to peak within the next 10 to 15 years and be cut at least by half by 2050.”

After reaffirming his commitment to an EU target of generating 20% of its energy from renewable sources by 2020 - and saying plastic bags should be banned - the PM then suggested he would consider upping the UK’s target for reducing CO2 emissions by 2050 from 60% to 80%, in line with the current scientific consensus.

But despite the stirring rhetoric, no final decision will be made until a new climate change advisory committee convenes next year.

Mr Brown’s hesitancy seems misguided at best. Only last week, the IPCC delivered its fourth stark warning this year that stalling on cutting CO2 emissions could be catastrophic.

“If you look at the scientific knowledge things do seem to be getting progressively worse,” said Rajendra Pachauri, chair of the IPCC, last Saturday. “Maybe before we were in a state of ignorance, but also we’ve seen much stronger trends in climate change. So you’d better start with the interventions even earlier. Now.”

And it went from bad to worse for the PM. Only days after his speech, analysts dismissed the EU’s renewables target as “unachievable”, based on the UK’s current performance, while a major report for the Renewables Advisory Board warned that the government’s plans to ensure all new homes are zero-carbon by 2016 are behind schedule – because of a shortfall in onsite renewable energy capacity.

With news of 25 million people’s personal details being lost in the post making Mr Brown sweat in the House of Commons on Wednesday, the publication of his backing for a third runway at Heathrow cranked up the heat some more.

The plan is for a 2,200 metre runway to be built north of Heathrow by 2020. This would almost double the number of passengers passing through the airport and increase the number of flights from 470,000 to 700,000 a year, much to the consternation of environmentalists and local residents.

Mr Brown’s Transport Secretary Ruth Kelly insisted the expansion was absolutely necessary to boost the economy.

“Heathrow supports 170,000 jobs, billions of pounds of British exports and is our main gateway to the global economy,” she said. “If nothing changes, Heathrow’s status as a world-class airport will be gradually eroded, jobs will be lost and the economy will suffer.”

And appearing on Channel 4 news, BA Chief Executive Willie Walsh painted a very clear picture for viewers. We must choose, he said, in favour of the economy, jobs and growth, because the aviation industry will agree to join the EU Emissions Trading Scheme and pay for the right to pollute.

“We are committed to ensuring that growth is sustainable,” Mr Walsh told the BBC. “If airlines want to fly more, they will have to pay for emissions reductions in other industries – so overall carbon dioxide in the atmosphere will not rise because of a third runway.”

Richard Dyer, from Friends of the Earth, took a rather different view.

“Allowing airports like Heathrow to expand seriously threatens targets for tackling global warming,” he said. “The government must curb the growth in flights if it is serious about significantly cutting Britain’s carbon footprint.”

Adding to the concerns of environmentalists, non-EU airlines responded by lobbying their governments to reject the proposed emissions trading compromise.

This rather neatly takes us back to the top of the week and Mr Brown’s “tough decisions”. The evidence tells us that anthropogenic CO2 emissions are threatening the planet’s climate, with potentially disastrous consequences if we fail to take action now. But at the end of a difficult week for the Prime Minister, it is clear there is currently a gap between the evidence and policy in the UK - a gap that can only be bridged by making tough decisions in politics, business and our daily lives.

Accordingly, we are left facing a clear choice: opt for more of the same in the form of ‘economic growth’ narrowly prescribed as building more runways and taking more flights; or understand that something much greater is at stake, accept that things must change, and take steps to significantly reduce CO2 emissions today.