The U.N. Environment Programme chief says a treaty on climate change wouldn’t be enough to avert a dangerous rise in global temperatures, and countries need to voluntarily make deeper cuts in carbon emissions.

Achim Steiner spoke on Tuesday hours before U.N. Secretary General Ban Ki-moon is due to open the decisive ministerial stage of a two-week 194-nation conference on climate change.

The conference is focussing on efforts to move toward a future agreement to legally bind all nations to emissions targets.

Mr. Steiner presented UNEP’s report showing the gap is widening between pledges by nations to reduce greenhouse gases and the targets set by scientists for preventing runaway global warming.

“We are losing time,” he said.