STOCKHOLM — The world’s top climate scientists on Friday formally embraced an upper limit on greenhouse gases for the first time, establishing a target level at which humanity must stop spewing the gases into the atmosphere or face irreversible and potentially catastrophic climatic changes.

They warned that the target was likely to be exceeded in a matter of decades unless steps are taken soon to reduce emissions.

Unveiling the latest U.N. assessment of climate science, the experts cited a litany of changes that are already underway, warned that they are likely to accelerate, and expressed virtual certainty that human activity is the main cause.

“Climate change is the greatest challenge of our time,” said Thomas F. Stocker, co-chairman of the Intergovernmental Panel on Climate Change, the U.N.-sponsored group of scientists that produced the report. “In short, it threatens our planet, our only home.”

The panel, in issuing its most definitive assessment yet of the risks of human-caused warming, hoped to give impetus to international negotiations toward a new climate treaty, which have languished in recent years in a swamp of technical and political disputes. The group made clear that time was not on the planet’s side if emissions continued unchecked.

“Human influence has been detected in warming of the atmosphere and the ocean, in changes in the global water cycle, in reductions in snow and ice, in global mean sea level rise, and in changes in some climate extremes,” the report said. “It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.”

Despite the findings, the course of future international negotiations remains unclear.