"I think that making policy based on facts, advancing issues based on analysis, is central to the effective functioning of government. And so I'm very concerned by the repeated tendency to engage in alternative facts and to assert things that are not supported by reality," Summers told CNBC from the China Development Forum in Beijing.
"And I think over time that will do great damage to credibility, and in many ways in governing, credibility is the coin of the realm."
Summers also addressed Saturday's communique from the G-20 finance ministers and central bankers, which appeared to break the long-standing practice of strongly endorsing open trade. That outcome indicated a victory for Trump's representative, Treasury Secretary Steve Mnuchin, and a defeat for nations like Germany who sought a strong defense for free trading principles.
"I certainly wasn't encouraged and it certainly wasn't a favorable development," Summers said of the communique. "Having been involved in writing many G-7 and G-20 communiques, I don't think the world is hanging on that prose, so I think the real question is not what words there were in the communique, but what will happen over time."
If there arises a global acceptance of protectionism, Summers said, then "that could be really quite damaging." But the G-20 message won't be "hugely consequential," he explained, if there is no movement toward increased tariffs.