During her show Thursday, news broke that Chinese President Xi Jinping would announce his country will launch a national carbon emissions trading market in 2017.
"This is the biggest news in climate policy, maybe ever," Maddow said.
"Happy visit to the United States, Pope Francis," she added, referring to the pontiff's current trip with stops in Washington, New York City and Philadelphia.
China's cap-and-trade program, which was first reported on by The New York Times, would mark a substantial step forward for the world's largest polluter.
"President Obama, of course, has tried to get the U.S. Congress to pass this kind of program, or some other program to limit carbon in the U.S.," Maddow said. "The congressional resistance has been in large part because China wasn't doing enough. Therefore, why should we?"
Perhaps, Maddow speculated, the move will push Congress to act.
"Well now, if China is forging ahead here and dealing with climate change using a U.S.-designed policy to do so, this could change everything in the political climate around the climate climate," she said. "This is huge news for the environment globally, and potentially for policy in the United States. This is almost as big as environmental news can get."