Opponents of strong and necessary action on climate change like to say that it doesn’t matter what we do because China is spewing greenhouse gases like there’s no tomorrow. They’ll have to flail around for another excuse now that the United States and China have reached a landmark agreement that will pressure other major carbon-emitting countries to join the effort as well. After too many years of too little action since the Kyoto Protocol in 1997, the timing of the historic pact announced Wednesday is critical to break the stalemate.

The U.S. and China are by far the world’s largest carbon polluters, accounting for more than one-third of the global total. They have been at odds on the issue, and their inaction gave cover to India and other developing nations to stay on the sidelines. This deal will boost momentum going into a meeting next month in Peru, the last major negotiating session before a conference in December 2015 in Paris to enact a new international agreement to reduce carbon emissions. 

The entire world must take bold steps starting now — when there’s still a chance to slow global warming before it becomes too difficult and expensive. Otherwise, we should prepare for more frequent extreme weather, faster sea level rise and humanitarian disaster. 

The new targets are ambitious. Chinese President Xi Jinping agreed that his country would, for the first time, set a date when its carbon emissions would peak, after which it would reduce its use of coal and other fossil fuels. That date is around 2030 but could be sooner as China seeks to increase its use of cleaner energy sources. 

In return, the White House said the U.S. would cut its net greenhouse gas emissions 26 percent to 28 percent below 2005 levels by 2025. While not as fast a reduction as some environmental groups want, the new goal would double the current pace. If President Barack Obama is able to follow through on this commitment, that would be a worthy legacy. Unfortunately, you can bet obstructionists and skeptics will try to stand in the way, even though the evidence isn’t on their side. For instance, Senate Republican leader Mitch McConnell quickly claimed that the plan will be a job killer. In fact, clean energy is one of the fastest-growing sectors of the economy, in California and elsewhere. And in the longer term, global warming is a threat to economic growth, by making food, water and energy all more expensive. 

There will have to be some sacrifice by consumers, businesses and industry. But as California is showing with its climate change law and cap-and-trade system, it can be done without wrecking the economy. “Bold, necessary action from the U.S. & China on climate change,” Gov. Jerry Brown tweeted Wednesday. “California will continue to lead the way.”