Joe Biden this week pledged again to immediately rejoin the Paris climate agreement if he wins the presidential election, but ultimately meeting his ambitions for the U.S on the world stage would be much tricker.

Why it matters: Biden would face big challenges and complex decisions after announcing the U.S. is back on the climate diplomacy circuit.

• Remember his platform calls for not only getting back in but pushing other countries to boost their emissions goals and transform them into concrete steps.
• One thing to watch: how Biden interacts with China, the world's biggest emitter that recently pledged to become "carbon neutral" by 2060, but has not offered detailed plans.
• But given the long odds that Democrats will control the Senate next year, Biden will face checks on his ability to implement new U.S. cuts, even as he presses other nations.

What's next: Under the nuts and bolts of Paris, in which nations submit their own nonbinding CO2 targets, Biden would be expected to update the Obama-era pledge.

• Obama's submission called for cutting U.S. greenhouse gas emissions 26%-28% below 2005 levels by 2025.
• The next big UN climate conference is slated for late 2021, which would give the new administration a long window to prepare its plan.
• There's also a lot of discretion in how to structure the pledges, notes Jonathan Pershing, a senior Obama-era climate official. He said setting a 2035 target might be an option, given Biden's vow to move the U.S. power sector to 100% carbon-free sources by that date.

The intrigue:This good E&E News story ($) points out that "world leaders likely expect an impressive new climate commitment" from Biden.

• But given the long odds of moving a big climate bill through Congress, Biden's diplomatic leverage will depend on showing other policies will breathe life into the new pledge.
• "For increased ambition, they are going to have to scavenge for more cuts," one veteran of global climate diplomacy tells Axios.
• Options include stimulus provisions; tariffs on carbon-intensive goods; new regulations Biden's administration would seek to implement, and more, the source said.

What they're saying: Pershing cautioned against assuming there would no avenues for working with a GOP-controlled Senate.

• He points out that the early 2018 budget deal expanded key tax credits for carbon capture projects.
• Pershing also noted the potential for climate-friendly spending provisions in the pandemic recovery package expected to be an early priority.

Yes, but: Overall, Biden's climate agenda would need to rest heavily on executive actions (again, if he's indeed won and Republicans keep the Senate).

• Pershing, who now heads the environment program at the William and Flora Hewlett Foundation, sees "all sorts of ways you can create leverage" with executive powers.
• There are of course new regulations, but also support for states' climate policies, using the power of federal procurement, and more.
• “Executive authorities and actions shape investment, and those shape the direction of the U.S. economy,” he said.