US ignorance about the risks and reality of global warming could sink hopes of a new global deal to control greenhouse gas emissions at December's climate talks in Copenhagen, an advisor to the German government has said.

Professor John Schellnhuber, of the Potsdam Institute for Climate Impact Research in Germany, said the US was "climate illiterate" and that the rest of the world may be forced to agree a new deal without it.

"Nobody should dream of the possibility that numbers and targets for countries will be sealed in Copenhagen," said Schellnhuber, one of the world's foremost climate scientists. "If the US doesn't move then nothing will happen."

He added: "The US in a sense is climate illiterate. It is a deeper problem in the US, if you look at global polls about what the public knows about climate change. Even in Brazil and China, you have more people who know the problem, who think that deep cuts in emissions are needed."

He predicted that it could be several years before the US would be willing to take on carbon cuts that were ambitious enough to persuade countries such as China to set targets of their own. At UN talks last week, China and India made small steps forward on this issue, but Obama was unable to do the same.

"The political chances seem very slim that something will happen in Copenhagen and even in the years after," he said. "Maybe in the conferences following Copenhagen some countries – including China and the EU – whatever the US does, will say: we go ahead now. Why can't we save the world without the US? Why should that not happen?"

The US has some of the highest carbon emissions per capita in the world, and any deal without it would be significantly less effective at curbing global temperature rise.

Speaking on the fringes of a climate science conference at Oxford University today, Schellnhuber said the former US president, George Bush, was to blame for a decade of inaction on climate change, and that many in the Republican party and the wider US population still did not understand the need to act. European nations and others have been waiting for President Obama to engage with the issue in a way that Bush refused to.

Schellnhuber said: "Obama is aware of the problem and he personally wants to do something. The problem is: can he provide the leadership to overcome the system? Every top politician gets to do two or three unpopular things, and the right politicians choose the right things."

To convert a global deal on climate change into US law would require a two-thirds majority vote in the US Senate, something that many in Europe believe is unrealistic given Obama's ongoing troubles with healthcare reform. "It just may not be possible to overcome the American inertia," Scellnhuber said.

Kevin Anderson, director of the Tyndall Centre for Climate Change Research, said: "It's vital for the climate that we get the Americans on board, but if we can't, then we can't just do nothing – we still need to make the biggest emission cuts that we can. If that means China and Europe and others going on without the Americans, then that may be the price we need to pay."

Schellnhuber's comments come as UN talks on a possible Copenhagen deal continue in Bangkok. Negotiators from 190 countries are wrestling with a draft treaty text, which runs to 200 pages and is riddled with alternate options and provisional text in square brackets.

His words reflect growing suspicion in Europe that the talks are crawling towards a unsatisfactory outcome, and that little progress is being made in the US. Earlier this month, the Guardian revealed a growing rift between Europe and the US over the latter's approach to how carbon targets could be set and met.

The Oxford conference is centred around the implications of a 4C rise in global average temperature, which scientists believe could be a possibility if serious carbon cuts are not agreed in Copenhagen.

Richard Betts, head of climate impacts at the Met Office Hadley Centre, presented a study demonstrating that the world could see a 4C rise as soon as 2060-2070 – within the lifetime of many people alive today.

Nigel Arnell, a climate expert at the University of Reading, said a temperature rise on this scale would bring about colossal changes in weather conditions, affecting billions of people. Some 15% of land worldwide that is currently suitable for agriculture would become useless, he said. Available land would shift north, to regions such as Siberia, which is currently covered in forest.

• This article was amended on Tuesday 29 September 2009. In the article above we said that the US has by some distance the largest carbon emissions per capita in the world, what we should have said is the US has some of the highest carbon emissions per capita in the world. This has been changed.