The NEW YORK TIMES correctly points out this:
But under the Constitution, a president may enter into a legally binding treaty only if it is approved by a two-thirds majority of the Senate.
And it then goes on to say how President Obama will get around that pesky rule:
To sidestep that requirement, President Obama’s climate negotiators are devising what they call a “politically binding” deal that would “name and shame” countries into cutting their emissions
NEW YORK TIMES splashes headline: Obama Pursuing Climate Accord in Lieu of Treaty