Former Secretary of State John Kerry on Sunday dismissed President Donald Trump’s claims that he will negotiate a better version of the Paris climate agreement and said “America has unilaterally ceded global leadership on this issue.”

Kerry, who helped negotiate the 2015 climate pact under President Barack Obama, compared Trump’s promise to get better terms for the U.S. to the O.J. Simpson murder trial.

“When Donald Trump says to the world, ‘Well, we’re going to negotiate a better deal,’ I mean, you know, he’s going to go out and find a better deal?” Kerry told host aid.Chuck Todd on NBC’s “Meet the Press.” “I mean, that’s like O.J. Simpson saying he’s going to go out and find the real killer. Everybody knows he isn’t going to do that because he doesn’t believe in it. Because if he did believe in it, you wouldn’t pull out of Paris.”

“America has unilaterally ceded global leadership on this issue, which, for years, even Republican presidents, George H.W. Bush, you know, pushed in this direction,” he added.

The 2004 Democratic presidential nominee also charged that Trump is ignorant of the science behind climate change.

“I mean, the fact is that, still, his whole staff cannot tell you whether or not he believes that climate change is a hoax,” Kerry said. “And I will say to you if you truly understand the science, if you have done your due diligence and homework, there is no way that you cannot conclude that there’s an urgency to doing something. And you would not pull out of Paris.”

The 2015 agreement, signed by more than 190 countries, seeks to combat climate change by setting targets to curb carbon emissions. Asked whether he regrets that the Obama administration did not ask the Senate to ratify the agreement, which would have prevented Trump from unilaterally exiting it, Kerry said he did not.

“It wouldn’t have happened. It’s very simple. Let’s be realistic about it,” Kerry said. “The president made an executive agreement because that was the best that we could do. And we presumed that common sense, that basic economics, that science would ultimately prevail.”