A grim future

Pachauri pointed to scenarios sketched last year in the fourth assessment report by his organisation, the Intergovernmental Panel on Climate Change (IPCC).

In it, IPCC said that the evidence for human interference was now incontrovertible and that the climate was already starting to change.

In the past century, it said, Earth's mean global temperature rose by 0.74°C over pre-industrial times, and this had already triggered glacier erosion, loss of snow and ice cover in alpine regions and retreating permafrost.

It gave a "best estimate" of further warming of between 1.8°C and 4°C by 2100.

The higher the temperature, the greater the impact from drought, floods, rising sea levels and more intense storms, spelling hunger, sickness and hardship for many millions.

These temperatures themselves are chiefly dependent on atmospheric levels of global warming gases, disgorged by burning of oil, gas and coal.

Stop, before it's too late

Cedric Philibert, a French expert at the International Energy Agency (IEA) in Paris, said one of the biggest problems was that these gases had an inertial effect.

Even if all emissions were stopped today, the warming would continue for a number of years because of the build-up of existing gas concentrations.

"It takes between 15 and 20 years for any change to take effect," said Philibert.

Pachauri said that, by factoring in the 20th-century increase in temperature and CO2 pollution already pumped out this century, emissions would have to peak by 2015 to limit overall warming to 2°C to 2.4°C.

By postponing the cap until, say, the 2030s or the 2040s, Earth's temperature would stay higher and for longer, he said.

Added to this is the risk that higher temperatures could unleash poorly-understood triggers which could amplify the warming at a stroke.

Pep Canadell, executive director with the Global Carbon Project, an Australian government-backed research scheme, said that even if the G8 goal of 2050 were met, emissions would have to be further ratcheted down.

"If after 2050, greenhouse emissions were to remain stable for the rest of the century without any further emission cuts, the temperature (warming) of the planet would far exceed three to four degrees, so the 2050 emissions target is only the first step towards dealing with climate change."