These statements are syntactically ambiguous.
The White House framed the announcement from the world's two largest polluters ahead of [a landmark U.N. conference to address the issue this fall].
The White House framed the announcement [from the world's two largest polluters ahead of a landmark U.N. conference] to address the issue this fall.
The White House framed the announcement from the world’s two largest polluters as move to position the countries in the fight against climate change ahead of [a landmark U.N. conference to address the issue this fall].
The White House framed the announcement [from the world’s two largest polluters as move to position the countries in the fight against climate change ahead of a landmark U.N. conference] to address the issue this fall.
In 1 and 3, a landmark U.N. conference is addressing the issue this fall.
In 2 and 4, it is the White House.
However, I would assert that it is likely to be the U.N. Conference that addresses the issue in both cases (1 and 3) because there's no causal link between the White House framing the announcements and then addressing the issue this fall.
Consider:
The White House framed the announcement to address the issue this fall.
At the very least, something like this is... unidiomatic, unless you were saying it in response to a question like "Who framed the announcement?" or "Why did the White House frame the announcement?"