Per the UN, 31 countries ratified or accepted the  Paris Climate Agreement today, September 21.  That means that with the previous ratifications/acceptances, the Paris Agreement has now 60 parties, crossing the 55 party threshold that is necessary for the agreement to enter into force, i.e. become legally effective. Since today’s ratifications/acceptances included some large countries who are significant GHG emitters, such as Brazil, it also means that the agreement now covers 47.76% of the world’s GHG emissions (per the Paris agreement tracker of ratifications).  That puts the agreement within striking distance of reaching 55% coverage of worldwide GHG emission, the second critical threshold for entry-into-force.

If this second threshold is met before the next President enters office, it could make the Paris Agreement’s legally effective before that time.  With the Agreement legally in effect, it would become significantly more difficult for any subsequent President opposed to the Agreement to withdraw immediately from it (as GOP Presidential candidate Trump has previously suggested he might do).  Per Article 28, a three year waiting period would then apply.  [Edit:  As it was kindly pointed out by Steve Wolfson and others, Article 28.3 provides that withdrawal from the underlying UN Framework Convention on Climate Change automatically also withdraws a party from the Paris Agreement. (“Any Party that withdraws from the Convention shall be considered as also having withdrawn from this Agreement.”)  The Convention only has 1 year waiting period for withdrawal.  Hence, using the Convention’s  withdrawal process could allow a country to withdraw from the Paris Agreement within 1 year, rather than 3 years.]

However, for the agreement to be effect on January 20, 2017, when the next President is inaugurated, that second, 55% threshold would have to be crossed (by additional countries depositing ratifications/acceptances) by December 22, 2016.  That is due to the 30 day delay for entry-into-force imposed by Article 21.  Hence, December 22, 2016 will be the day to watch for this contingency.