[dragonwriter]

> A treaty is supposed to be ratified by the Senate with a 2/3 majority.

A treaty under international law can be any of the following in US law (in roughly descending order of the force of the agreement in domestic law):

1. A treaty.

2. A “Congressional-executive agreement” under which the executive makes an agreement supported by statutory authority, or

3. A “sole executive agreement” under which the executive makes an agreement, and then seeks to apply it through statutory and other executive authority even though the agreement itself is not specifically authorized.

IIRC, the legal theory of the Obama Administration was that Paris was within the scope of authorized international agreements under the Clean Air Act and/or other environmental laws, and thus they treated it as a Congressional-Executive Agreement; as such, the underlying statutory authority remaining in place, Biden would as free to rejoin without Senate ratification or additional Congressional action as Obama was to join in the first place.