It was clear heading into the U.N. climate change conference in Lima that countries would punt all the toughest issues until next year in Paris, when a grand new global deal is due. All they really needed in Lima were a few procedural decisions setting the stage.

So why did it take more than 30 hours beyond the conference deadline to deliver something so modest?

The answer is that even a seemingly trivial procedural issue can be freighted with substantive implications, so countries fret over every nuance, lest they let something slip that will come back to haunt them later. In Lima, like so many times before, their biggest worry was how responsibility will be distributed across developed and developing countries.

At the start of the global climate effort, developed countries were comfortable with a stark division assigning most of the responsibility to them. But 20 years later, China is now the world’s largest carbon emitter, and developed countries no longer accept the so-called firewall between the two groupings.

The 2011 Durban Platform for Enhanced Action, which launched the current round of negotiations, said the Paris agreement would be “applicable to all.”  But just what that means was left to be sorted out later, and will likely be the central challenge in Paris.

The handwriting is on the famous firewall – it’s coming down. China’s willingness to stand side by side with the United States last month to jointly announce their post-2020 emissions goals is a tacit acknowledgement of that. The question is what if anything takes its place.

Developed countries, most notably the United States, are pushing for “self-differentiation.”  Countries have agreed to come forward next year with their “nationally determined contributions” to the Paris agreement, and these, they argue, will naturally fall along a spectrum reflecting countries’ varying national circumstances. There’s no need for set categories.

Although China’s not ready to go that far, it might in the end be able to live with something close to that. But a lot of other developing countries worry that self-differentiation lets the rich countries off the hook and eliminates their own safe harbor, so they’ll keep resisting it to the bitter end.

The question then becomes whether there’s some middle ground between the old firewall approach and full self-differentiation.

A recent proposal from Brazil calls for a “concentric differentiation” approach: Developed countries in the middle with absolute economy-wide emissions targets; better-off developing countries in the next circle with other types of economy-wide targets; and least developed countries in the outer circle with non-economy-wide actions. All countries would move to the center over time. But it’s not clear when or how, so to developed countries, it looks like the old firewall in a new guise.

In Lima, parties got through the differentiation issue largely by skirting it. Borrowing directly from the recent US-China announcement, they restated the long-established principle of “common but differentiated responsibilities and respective capabilities” with a slight addition: “in light of different national circumstances.”

Figuring out just how those words are translated into the provisions of a new global pact will no doubt mean more long nights next year in Paris.