World leaders opened a pivotal climate summit in Scotland on Monday with apocalyptic warnings about the scarce time left to avert catastrophic global warming, yet offered few new commitments to more aggressively cut greenhouse gas emissions.

The gathering, called in the hope that the world could at last agree on meaningful steps to put a rapidly warming planet back on course, is scheduled to last nearly two weeks, but it took only hours for the first bumps to appear.

Long-running fault lines in the global debate over who should be the most responsible for cutting emissions emerged in the opening speeches of the assembled heads of state. So did barbs aimed at two major greenhouse gas emitters, China and Russia, whose leaders did not attend. And so did the tensions between the globe’s rich and poor, as less-developed countries demanded more aid and swifter action from wealthier ones.