"If you (don't want) run-away climate change, you need to be at about 350 parts per million (ppm) of CO2 ... We're currently at 387 ppm CO2, going up at 2 per annum," said King, director at Oxford University's Smith School of Enterprise and Environment.

Carbon dioxide (CO2) is the most common greenhouse gas, and atmospheric levels are sometimes measured as CO2 in parts per million. Collectively, all greenhouse gases can also be expressed as CO2 equivalent (CO2e).

King said that maintaining atmospheric CO2 levels at 450 ppm risked a 20 percent chance of global temperatures rising nearly 4 degrees Celsius.

"If you include all greenhouse gases, we're at around 420 ppm CO2e," he said, speaking at a climate change workshop hosted by Thomson Reuters in London.

He said Europe needed to reduce its annual per-capita emissions by 80 percent, or from 11 tonnes of CO2e, to India's current level of 2.2 tonnes per person by 2050.

The United States, emitting an average of 27 tonnes of CO2e per person every year, also needs to fall to these levels if the world is to avoid a dramatic rise in temperatures, he said.

"I think that encapsulates the challenge, to move from where we are now to where the Indians are today, while growing the global economy at the same time," said King.

RISKING DISASTER

Failure to do so courted environmental disaster, he said, explaining that melting Arctic sea ice heated up the ocean in the far north much faster because ice reflects a large portion of the sun's radiation, while open ocean absorbs the sun's heat.

A rise of several degrees Celsius could also mean the Amazon rainforest drying out, turning it into a big source of carbon dioxide emissions rather than a vast sink for the gas as it is now.

The first round of the UN's Kyoto Protocol, which seeks to cut greenhouse gas emissions from 37 industrialised nations, expires in 2012 and governments are scrambling to agree a successor agreement by the end of 2009 at a UN meeting in Copenhagen.

If governments fail to reach consensus, King thinks another solution to climate change might be so-called geo-engineering, which uses technology to deliberately modify the environment and to promote human habitability.

"We need to remove the carbon dioxide, I suspect not from the atmosphere because it's too expensive ... but possibly from the oceans as they are acidifying," King said.

Oceans absorb large amounts of CO2 but increasing levels of the gas in the atmosphere is causing oceans to become more acidic, threatening the food chain and marine ecosystems such as coral reefs.

Making geo-engineering profitable for the private sector by establishing a market price for carbon dioxide might promote research and development in the new technology.

"I haven't worked out what the price of carbon dioxide would have to be to encourage companies to start pumping it out of the oceans, but that is the way we need to move forward."