Editor’s Note: Robert Redford is an actor, director and trustee of the Natural Resources Defense Council. The opinions expressed in this commentary are those of the author. View more opinion articles on CNN.

CNN  — 

Earlier this week, Donald Trump notified the United Nations of his intention to pull America out of the most significant climate effort in history, the now-famous Paris Accord. It’s a decision that has shocked millions with its short-sighted foolishness. That he chose to do this on the first day he could legally do so under the UN’s rules for withdrawal simply reveals his immoral dedication to isolating us from the world community by willfully ignoring the biggest, most urgent threat facing human civilization.

Now begins a possible one-year extrication countdown that could become final on November 4, 2020 – just one day after Americans decide whether to grant Trump another four years in office, or to hand the reins of government over to someone else (I’m rooting for the latter).

Robert Redford

A newly-sworn-in future president, one who cares about America and its future, could immediately request re-entry into the agreement as a first order of business, and after 30 days our country would be allowed to rejoin as a full, participating member. By that point, other world leaders would have every right to be concerned about the depth of our commitments. We shouldn’t even risk it – we shouldn’t even be facing this horrible reality.

I was honored to be in Paris in 2015, as representatives from more than 170 countries forged the terms of an agreement that would become the cornerstone of a new modern and international approach to fighting climate change. On a stage at Paris City Hall, I sat with mayors from all over the world and heard them describe local actions they were taking to slash their cities’ carbon footprints – they weren’t waiting, because mayors are closest to the people and they are experiencing the effects of climate change real-time: wildfires, floods and rising seas. They can’t afford to wait.

On a different stage, I sat with indigenous leadership from highly vulnerable Pacific Island cultures and listened to their stories of devastation. It was a reminder that those who have contributed the least to climate change are often tragically the most affected by it.