U.N. climate negotiators will meet Monday for their last session ahead of a major conference in Paris to hammer out the details of what's envisioned to become the most ambitious agreement ever to fight global warming.

Delegates at the weeklong talks in Bonn, Germany, are expected to start line-by-line editing a 20-page draft that still contains multiple options on how to reduce the greenhouse gas emissions that scientists say are warming the planet.

Some 150 countries have already made voluntary pledges to cut or curb their emissions after 2020, when the deal is supposed to take effect.

But several analyses show those pledges won't be enough to prevent dangerous levels of warming so a key element of the Paris deal would be a mechanism to raise those commitments over time.