LONDON — The only three living diplomats who have led the United Nations global warming talks said there’s little chance the next climate treaty will prevent the world from overheating.

The specific goal, to hold temperature increases to 2 degrees Celsius (3.6 degrees Fahrenheit), was endorsed by envoys from 190 nations in 2010. It’s considered the maximum the environment can bear before climate change becomes more dangerous. Delegates to the UN Framework Convention on Climate Change meet in Warsaw starting Nov. 11 to work on a treaty that could be agreed to in 2015.

The comments from the current and former executive secretaries to the UNFCCC add to the urgency of the Warsaw talks. Humans already have emitted more than half the greenhouse gases needed to surpass the 2-degree target, a panel of scientists brought together by the UN concluded in September. The World Bank last year said the planet is on track to warm by 4 degrees this century, a level that would raise the seas, worsen droughts and make storms more violent.

“There is nothing that can be agreed in 2015 that would be consistent with the 2 degrees,” said Yvo de Boer, who was UNFCCC executive secretary in 2009, when attempts to reach a deal at a summit in Copenhagen crumbled with a rift between industrialized and developing nations. “The only way that a 2015 agreement can achieve a 2-degree goal is to shut down the whole global economy.”

The Warsaw meeting will continue work toward a treaty limiting carbon dioxide emissions in all nations. The aim is to complete the text in 2015 and for targets to take effect in 2020. Even the current diplomat managing the process says success will require further steps beyond the treaty.