The world's emissions of the greenhouse gases causing global warming should peak in 2020 and then start to decline, the British Government is proposing in the run-up to the global climate conference taking place at Copenhagen in December.

Emissions from developed nations such as Britain and the US should reach their highest point even earlier, by 2015, the Energy and Climate Change Secretary, Ed Miliband, suggested to other countries in a meeting in Mexico this week, in the first move to make the crucial issue of a "carbon peak" an official target of the Copenhagen agreement.

Emissions of gases such as carbon dioxide have been rising at a far faster rate than was predicted even a decade ago, and research published by the UK Met Office last year showed that the point at which they begin to decline as a whole is absolutely vital in bringing rising temperatures under control.

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A few years' delay in the peak can mean the world is committed to a significantly higher rise than would otherwise be the case, and computer simulations by the Met Office Hadley Centre indicate that for every ten years the peak is postponed, another half degree of temperature increase becomes unavoidable.

Hitherto, the issue of the "global peak" has largely remained a theoretical one, but this week Mr Miliband and British officials put it on the table at a meeting in Mexico of the Major Economies Forum on Climate and Energy (MEF), a new, pre-Copenhagen high-level discussion group which has been convened by the US President Barack Obama.

The MEF meetings will culminate in a world leaders' summit on climate change which will take place alongside the G8 meeting in Italy in July, and which will be a critical moment in the push towards a Copenhagen climate deal.

Yesterday Mr Miliband said the issue of a global peak in emissions had so far been "significantly under-emphasised". If it could be agreed, it would "irreversibly break the trend towards rising emissions," he said, adding: "It would show that something had changed. We are arguing very strongly for a 2020 global peak."

Dr Vicky Pope, the Hadley Centre's head of climate change advice, said yesterday: "Even if emissions peak in the next ten years and then decline rapidly, temperatures are still likely to rise to around two degrees Celsius above pre-industrial levels by the end of the century. Every 10-year delay in starting reductions will result in a further 0.5 degree increase in the most likely temperature rise, so the need for action is urgent."

The Hadley Centre's simulations last year indicated a most likely two degree rise with a 2015 peak (and world carbon emissions subsequently declining at three per cent a year to 2050), a 2.5 degree rise with a 2025 peak and a similar decline, and a three degree rise with a 2030 peak. In each case the temperature rise is a best guess - a 50-50 chance - and there are possibilities that it could be lower, or indeed, significantly higher.

All countries, including the UK, must be more ambitious in commitments to cut greenhouse gases, Mr Miliband said, ahead of the launch today of the Government's own manifesto on what needs to be achieved in Copenhagen.

Greater public pressure would play a part in ensuring the politics of negotiating a new deal catches up with the science of what needs to be done, he said. From today the Government is distributing pamphlets setting out the importance of Copenhagen, which will be sent to public bodies such as schools and hospitals, and launching a website www.ActOnCopenhagen.gov.uk.