A clause tucked away at the bottom of the draft agreement handed out at the Paris climate talks could blow up the agreement even if it is signed Saturday.

Article 18 Section 1 says the agreement will come into effect after at least 55 countries that have signed the agreement ratify, accept or approve the agreement domestically. But, crucially, for the agreement to take effect in 2020, countries that account for 55 percent or 70 percent — the percentage is still being negotiated — of global greenhouse gas emissions must be on board.