It is now two and one-half minutes to midnight.

Our organization, the Bulletin of the Atomic Scientists, is marking the 70th anniversary of its Doomsday Clock on Thursday by moving it 30 seconds closer to midnight. In 2016, the global security landscape darkened as the international community failed to come to grips with humanity’s most pressing threats: nuclear weapons and climate change.

Making matters worse, the United States now has a president who has promised to impede progress on both of those fronts. Never before has the Bulletin decided to advance the clock largely because of the statements of a single person. But when that person is the new president of the United States, his words matter.

This is the closest to midnight that the clock has been since 1953, when it was moved to two minutes to midnight after United States and the Soviet Union tested their first thermonuclear weapons within 10 months of each other.

We understand that Mr. Trump has been in office only days, that many of his cabinet nominees are awaiting confirmation and that he has had little time to take official action.