Our expectations were all out of whack on climate change. For years, environmentalists and world leaders promised a “big bang” international agreement, a comprehensive, world-wide plan to reduce greenhouse gas emissions, to which pretty much every nation on the planet would be a willing party. A global problem, after all, requires a global solution.

President Obama and Chinese President Xi Jinping (Official White House Photo by Pete Souza)

But decades of work has merely resulted in soft promises from some nations to reduce their emissions, a nominal commitment to enforce an international agreement, its content not yet articulated, by the end of this decade, and clarity about the political obstacles in the way of a big accord.

The heart of the problem is that developing countries want the developed world to reduce its greenhouse gas output and pay them, and the West doesn’t like that idea absent real greenhouse-gas commitments from big developing nations with rising economies, such as China, India and Brazil. It’s hard enough for a few countries to work through that problem issue-by-issue, let alone all of them at the same time across the variety of contentious questions involved in world climate discussions. U.N. forums have been fractious, with national interest, global politics and concerns about reputation pulling the big players in various directions.

That’s why one bit of news from the G20 conference that just wrapped up in St. Petersburg, Russia, is encouraging. Outside of the cumbersome U.N. process, the United States and China, the world’s two largest greenhouse emitters, announced steps to “phase down” the use of hyrofluorocarbons (HFCs), potent greenhouse gases used in refrigerators, air conditioning units and other things. They agreed to use the machinery of Montreal Protocol — a preexisting international system that has successfully cut pollutants that harm the ozone layer — to do it. And the full G20 — which includes Brazil and India — issued a statement supporting the plan to roll back HFCs.

These nations have to fill in specifics. But if they arrest the global expansion of HFC use, the White House says that unified action could prevent the equivalent of 90 gigatons of carbon dioxide entering the atmosphere by 2050. That, The Post’s Steve Mufson points out, is about two years’ worth of the world’s current output.

On its own, that won’t do nearly enough. But this sort of agreement — among a few, big countries responsible for nearly all of the emissions that matter — represents another approach, outside the U.N. process, to achieving meaningful global action on climate change. The central problem doesn’t disappear with fewer players in the room. Yet, so far, it has been easier for Chinese leaders in particular to make climate-change commitments after huddling with President Obama to hash them out. This may be because such agreements require some level of mutual sacrifice and, therefore, trust that the other parties are serious. Americans are wary of moving before the Chinese do, and vice versa. The HFC announcement was also easier because it is a discrete issue that the G20 can address in isolation.

It hasn’t been clear that this strategy would work any better than the U.N. process. But, at the least, this sort of effort keeps climate change in world leaders’ minds. At best, meanwhile, the big emitters could see to a range of problems, pressure others to join in, and lead the way to a global treaty consisting of a bundle of smaller agreements already struck among themselves and various countries. That’s worth trying for.