The oceans are rising at a pace that threatens coastal communities and are becoming more acidic as they absorb some of the carbon dioxide given off by cars and power plants, which is killing some creatures or stunting their growth, the report found.

Organic matter frozen in Arctic soils since before civilization began is now melting, allowing it to decay into greenhouse gases that will cause further warming, the scientists said.

And the worst is yet to come, the scientists said in the second of three reports that are expected to carry considerable weight next year as nations try to agree on a new global climate treaty. In particular, the report emphasized that the world’s food supply is at considerable risk — a threat that could have serious consequences for the poorest nations.

“Nobody on this planet is going to be untouched by the impacts of climate change,” Rajendra K. Pachauri, chairman of the intergovernmental panel, said at a news conference here on Monday.