Almost nine out of 10 climate scientists do not believe political efforts to restrict global warming to 2C will succeed, a Guardian poll reveals today. An average rise of 4-5C by the end of this century is more likely, they say, given soaring carbon emissions and political constraints.

Such a change would disrupt food and water supplies, exterminate thousands of species of plants and animals and trigger massive sea level rises that would swamp the homes of hundreds of millions of people.

The poll of those who follow global warming most closely exposes a widening gulf between political rhetoric and scientific opinions on climate change. While policymakers and campaigners focus on the 2C target, 86% of the experts told the survey they did not think it would be achieved. A continued focus on an unrealistic 2C rise, which the EU defines as dangerous, could even undermine essential efforts to adapt to inevitable higher temperature rises in the coming decades, they warned.

The survey follows a scientific conference last month in Copenhagen, where a series of studies were presented that suggested global warming could strike harder and faster than realised.

The Guardian contacted all 1,756 people who registered to attend the conference and asked for their opinions on the likely course of global warming. Of 261 experts who responded, 200 were researchers in climate science and related fields. The rest were drawn from industry or worked in areas such as economics and social and political science.

The 261 respondents represented 26 countries and included dozens of senior figures, including laboratory directors, heads of university departments and authors of the 2007 report from the Intergovernmental Panel on Climate Change (IPCC).

The poll asked the experts whether the 2C target could still be achieved, and whether they thought that it would be met: 60% of respondents argued that, in theory, it was still technically and economically possible to meet the target, which represents an average global warming of 2C since the industrial revolution. The world has already warmed by about 0.8C since then, and another 0.5C or so is inevitable over coming decades given past greenhouse gas emissions. But 39% said the 2C target was impossible.

The poll comes as UN negotiations to agree a new global treaty to regulate carbon pollution gather pace in advance of a key meeting in Copenhagen in December. Officials will try to agree a successor to the Kyoto protocol, the first phase of which expires in 2012. The 2C target is unlikely to feature in a new treaty, but most of the carbon cuts proposed for rich countries are based on it. Bob Watson, chief scientist to Defra, told the Guardian last year that the world needed to focus on the 2C target, but should also prepare for a possible 4C rise.

Asked what temperature rise was most likely, 84 of the 182 specialists (46%) who answered the question said it would reach 3-4C by the end of the century; 47 (26%) suggested a rise of 2-3C, while a handful said 6C or more. While 24 experts predicted a catastrophic rise of 4-5C, just 18 thought it would stay at 2C or under.

Some of those surveyed who said the 2C target would be met confessed they did so more out of hope rather than belief. "As a mother of young children I choose to believe this, and work hard toward it," one said.

"This optimism is not primarily due to scientific facts, but to hope," said another. Some said they thought geoengineering measures, such as seeding the ocean with iron to encourage plankton growth, would help meet the target.

Many of the experts stressed that an inability to hit the 2C target did not mean that efforts to tackle global warming should be abandoned, but that the emphasis is now on damage limitation.