U.N. Sec. Gen. Seeks Quick Ratification of Climate Accord

This article in The New York Times describes how U.N. Secretary General Ban Ki Moon is making a huge effort to have the Paris Climate Change Accord ratified before a new president can be inaugurated in the U.S. Note the timelines for ratification, and the conditions precedent to the Accord coming into force, discussed in the article. Why is it that the next president would be bound for at least 4 years if the Accord comes into force by January?