Earlier this week, before his government lurched into its now weekly crisis, Gordon Brown gave a speech to the CBI in which he declared his support for a third runway at Heathrow, claiming "we have to respond to a clear business imperative and increase capacity at our airports".

Cue much wailing and gnashing of teeth from environmentalists arguing that Brown can’t make an impressive speech on climate change one week and commit to expanding airports the next and expect to be taken seriously.

The CBI, meanwhile, welcomed the move, but those members of its climate change task force who have looked at the various reports on the aviation industry's rapid expansion would be forgiven some private doubts, despite the conclusion in their new report that "air travel can be part of a low carbon economy".

Of course, even with its new commitment to "do what it takes" on climate change keeping the CBI and the treehuggers happy is never going to be easy, but Brown could have done a far better job. Here's what he could of, or, if I may be so bold, should have said:

"You have rightly called for action at Heathrow. Our prosperity depends on it: Britain as a world financial centre must be readily accessible from around the world. And this week we demonstrated our determination not to shirk the long term decisions but to press ahead with a third runway.

I know this commitment will attract the ire of environmentalists and may not even win full approval from yourselves, committed as you now are to transitioning to a low carbon economy, but this third runway will help secure London's position as the world's financial capital and drive the economic growth that is essential if we are to make the investments required to decarbonise the UK's economy.

However, while I will not make this government a hostage to fortune and say this type of airport expansion will never happen again it will become rare to the point of extinction over the next few decades.

In a speech to the WWF last week I pledged that every new policy will be examined for its impact on carbon emissions, and while this does not mean we will cease carbon intensive projects altogether it does mean that where such projects are authorised we will have to deliver a greater cut in emissions in other sectors.

This is a hard truth to face, but the rate of growth within the aviation industry is now so rapid that it threatens to cancel out all the carbon savings we make in other sectors of the economy.

We have attempted to sidestep this truth by excluding aviation and shipping from inclusion in the climate change bill, but to continue to do so will undermine the credibility of this important and groundbreaking piece of legislation. More importantly it will also make it nigh on impossible for us to avert dangerous climate change – sadly the atmosphere does not care whether the emissions come from a UK power station or a plane midway over the Atlantic and the legislation should account for that.

Ignoring this truth any longer would be foolish in the extreme and so I have instructed the new climate change committee to assess whether aviation and shipping should be included in our emission targets as its first duty of business.

Far be it from me to second guess the committee's conclusions, but any sensible observer would accept that ultimately these emissions will have to be accounted for in some way.

Once the emissions associated with any new runaway have to be subjected to a carbon impact assessment it will become extremely difficult, if not impossible to authorise further projects and still hit our emission reduction projects.

I am loath to criticise the report this morning from the CBI's Climate Change Taskforce, because I truly believe that it is one of the most important documents on the challenges we face ever produced. But in my opinion it fails to take the issue of aviation sufficiently seriously.

The report accepts "that continuing growth in demand means that aviation will account for a bigger share of emissions over time" but fails to quantify how much it will grow or how big a share of emissions it will eat up. Let me fill you in.

The Department for Transport has estimated that the number of passengers passing through our airports will rise from around 200m a year now to between 400m and 600m by 2030. Without a miraculous revolution in aircraft design improvements in fuel efficiency will not be able to cancel out this increase in demand and emissions will soar. According to research from the Tyndall Centre, if we are to stabilise emissions at a level we have deemed safe while continuing to allow aviation to grow at its projected rate then the airline industry will account for half of the UK's emissions by 2050.

Such a share for aviation would make it all but impossible to hit our carbon targets and dangerous climate change and economic catastrophe will become increasingly inevitable.

This means that based on current trends we have no choice but to cap, or even scale back, airport capacity at some point in the future.

So, how can this be done without damaging the economic growth that as I explained earlier is essential to low carbon investments and is the justification for Heathrow's third runway?

If we are to mitigate the risk of climate change two things must happen: firstly, we must engineer the biggest technical revolution the aviation industry has seen since its birth as we attempt to move as far as possible towards decarbonising flight. And secondly, we must fly less.

I would argue that both of these things can be done without damaging UK plc, and if done properly they should even stimulate growth.

The aviation industry has made good progress in enhancing fuel efficiency and limiting carbon emissions, but it must go much, much further.

In order to drive this progress we have corrected the mistake we made in taxing air passengers - a move I now accept had little impact on emissions - and instead plan to begin taxing flights from November 2009. The level of the tax has not been agreed yet, but let me warn you now - it will be significant.

I understand your mutterings, but when introduced this tax will be accompanied by major tax cuts on energy efficient products and vehicles that will make the move revenue neutral. It will also provide a clear financial incentive for airlines to operate fuller planes and send a clear price signal to passengers that they should look to avoid flights wherever possible.

We are also committed to bringing aviation into the European emissions trading scheme – a move that will provide airlines with a financial incentive to cut emissions and only operate the most fuel efficient aircraft. Many within the aviation industry have signalled their support for this scheme, but many others in resisting these plans are displaying a short sightedness and irresponsibility that I find astounding. I implore you as some of their most influential customers to join with both the government and the CBI Climate Change Task Force in calling for them to drop their opposition to the proposals and join a scheme that already impacts many other sectors of the economy.

This will lead to an increase in ticket prices, but if it makes you as business leaders think more carefully about whether or not a flight is necessary then it is a price worth paying.

However, the stick alone will not be enough.

I will not hypothecate the taxes raised from flights because, as the Northern Rock episode has painfully proved, a government needs a certain flexibility in its spending decisions. However, I will pledge that we will dramatically increase our investment in R&D projects working to deliver lower carbon aviation.

The CBI's task force report notes that the Society of British Aerospace Companies has called for government investment in civil aviation technology of £130m to £150m per year. I will go far further than that and invest £300m in aviation R&D on the understanding that the focus of the research will be on limiting carbon emissions.

There are many exciting technology avenues open to the aviation industry, ranging from biofuels to new engine designs and lighter planes, and if the UK can deliver these innovations first then it will be able to tap into a global market desperate for more energy efficient technologies.

Furthermore, the CBI report outlines how 73m tonnes of carbon dioxide are wasted every year around the world due to inefficient use of airspace and infrastructure, such as the stacking of aircraft in the skies around Heathrow. Streamlining air traffic management systems in Europe alone could cut fuel consumption by 12 per cent and the case for our bringing airlines into the ETS is undermined by the EU's failure to deliver these reforms. I will make it a priority for our negotiations in Europe that these reforms are achieved.

However, with these legislative and technical improvements years away we must also act now to limit aviation's emissions and that is why I am asking you not to fly where possible.

I am not saying don't fly at all – because, as I said, I understand aviation's importance to business.

However, I am asking you to think back to the last time you flew to an international conference or business meeting and found yourself thinking it was a waste of your time. If you are anything like me you will not have to think back too far.

We all now have a responsibility to accept that flights are one of the biggest contributions we make as individuals to climate change and should think more carefully about when we take them.

Again, the government will move to help you change your company's approach to corporate flights. The latest video conferencing technologies are a wonder to behold and can genuinely cut the need for business people to fly and improve productivity. So to help drive their adoption we will offer both 100 per cent tax breaks on the technology and interest free loans to all firms installing them.

I also accept that our rail system is the laughing stock of Europe. With the high speed link to Europe now open the poverty of the rest of our network becomes even more apparent and it is an issue that must be resolved. That is why I am today launching an investigation into how we can deliver a genuinely world class high speed rail network that will make domestic flights completely unnecessary.

It will take a huge effort from everyone in this room as well as the fourth technology revolution that I spoke of in last week's speech to maintain economic growth while cutting emissions across the economy, including in aviation. But it is my belief that these measures, coupled with your ingenuity and entrepreneurial spirit, can help us increase prosperity while cutting aviations emissions."