The United States could not actually nullify the global agreement on its own, and withdrawing from the agreement would take one to four years, depending on the approach. Because the Paris Agreement does not levy sanctions or fines against countries that fail to meet their pollution-reduction pledges, it's possible the Trump administration could choose to remain in the agreement while basically polluting as usual. Yet, for all those caveats, this is also true: The Paris Agreement is bigger than Donald Trump. The Trump administration could choose to withdraw from the Paris Agreement, but it wouldn't be able to start that process until November 2019 , three years after the agreement "came into force," to use UN-speak. The Paris Agreement stipulates there is a one-year delay from that point, which would put withdrawal in late 2020.

Source: CNN February 22, 2017 12:33 UTC