Runaway growth in the emission of greenhouse gases is swamping all political efforts to deal with the problem, raising the risk of “severe, pervasive and irreversible impacts” over the coming decades, according to a draft of a major new United Nations report.

Global warming is already cutting grain production by several percentage points, the report found, and that could grow much worse if emissions continue unchecked. Higher seas, devastating heat waves, torrential rain and other climate extremes are also being felt around the world as a result of human-produced emissions, the draft report said, and those problems are likely to intensify unless the gases are brought under control.

The world may already be nearing a temperature at which the loss of the vast ice sheet covering Greenland would become inevitable, the report said. The actual melting would then take centuries, but it would be unstoppable and could result in a sea level rise of 23 feet, with additional increases from other sources like melting Antarctic ice, potentially flooding the world’s major cities.

“Human influence has been detected in warming of the atmosphere and the ocean, in changes in the global water cycle, in reduction in snow and ice, and in global mean-sea-level rise; and it is extremely likely to have been the dominant cause of the observed warming since the mid-20th century,” the draft report said. “The risk of abrupt and irreversible change increases as the magnitude of the warming increases.”