Understand decades of climate change negotiations and what's at stake in Paris in two minutes. (Gillian Brockell/The Washington Post)

The climate negotiations that are taking place this week in Paris are buoyed by optimism. If the negotiations succeed, it will be in large part because of the 2009 Copenhagen climate negotiations, which were regarded at the time as a failure. Six years on, it is clear that the reverse is almost certainly true — Copenhagen was a tremendous success.

Why was a Copenhagen a success?

Copenhagen largely killed off the top-down treaty-based approach to climate change in favor of a new bottom-up regime based on country pledges, what have come to be called Intended Nationally Determined Contributions (INDCs). Coming in to Paris, more than 150 INDCs representing 179 countries have been submitted to the United Nations, including from China and the United States, whose emissions constitute about 40 percent of the global total.

Why did we need to kill off Kyoto?

For two decades, environmentalists’ theory was that we could tackle climate change only through treaties, which to them demonstrated a deep commitment by states to action.

The 1997 Kyoto Protocol included a global cap on emissions as well as legally binding targets and timetables that shared out reductions among a subset of rich countries but none for fast-growing countries such as China. That arrangement, underpinned by the principle of “common but differentiated responsibilities,” was unacceptable to the United States, which did not ratify Kyoto. By the mid-2000s, the regime had become “ossified,” as Joanna Depledge describes it. Copenhagen fundamentally rebooted the policy approach to climate change.

Will Paris deliver a new treaty?

No. In the United States, most controversial treaties have to be submitted to the Senate, where two-thirds of the senators have to provide their advice and consent. That high bar has meant that relatively innocuous treaties such as the Law of the Sea have not been ratified. Andrew Moravcsik described this form of American “exemptionalism” as pervasive.

At Copenhagen, the Obama administration set the stage for Paris by seeking an agreement that would create obligations for all countries but with some ambiguity about the precise legal form of the agreement. Pledges of intent would allow countries to participate based on diverse national circumstances and what they could implement at home. While some activists and some European countries still want the Paris agreement to be formalized and legal, the United States has to avoid a treaty, which would be a dead letter that would have no chance of Senate approval in the foreseeable future.

As former undersecretary of energy David Sandalow noted, the United States can agree to certain “procedural requirements” to address climate change under the 1992 Framework Convention on Climate Change, to which it is already a party, without returning to the Senate. These could include procedures for measurements and reporting of emissions. New legally binding commitments, involving emissions reductions of a certain amount by a certain date, would require Senate approval.

What this means is that the agreement that will come out of Paris is likely to be legally formalized in some respects (such as measurement and reporting) but not others (emission-reduction pledges).

How to get more ambition

The key defect of a bottom-up regime is that it may not be ambitious enough. Analysts worry that the pledges on offer do not add up to enough emission reductions to avoid dangerous climate change, defined as a 3.6 Fahrenheit (or 2 degrees Celsius) increase above pre-industrial levels. A recent report from the U.N. Environment Program suggests that the pledges, if fully implemented, amount to about half of what is needed.

What advocates want to see, therefore, is a process that would allow the global community to “ratchet” up ambition with frequent review cycles, as the science demands and as states gain experience with how hard or easy it is to meet the first set of commitments.

The sticking points are how often the review cycles will take place and how soon the first one will take place. Proponents would like the first to take place in 2020. France and China recently blessed a five-year review cycle.

A big fear at Paris is that some countries, India in particular, may not be keen on this level or regularity of intrusiveness. India still has 300 million citizens without electricity. As part of its INDC, India made an ambitious pledge to increase its non-fossil electricity to 40 percent of its overall total by 2030. The non-fossil share is currently about 30 percent, and the country aims to increase its electricity use by several orders of magnitude.

Given its need to extend energy access, India is concerned that climate mitigation will hurt its economic development. At Copenhagen, rich countries pledged to mobilize $100 billion per year in climate finance for developing countries from both public and private sources. Unfruitful discussions about technology transfer have long been part of the climate negotiations. Going into Paris, India is concerned that promises of climate finance and technology transfer won’t materialize.

In a sense, India is taking up the role once occupied by China, which has become more constructive, as its own air pollution levels have become intolerable. Thus, if a satisfactory agreement is going to be reached at Paris, it has to go a long way to assuage India’s concerns.

In contrast to 2009, when world leaders met in the wake of the financial crisis, the setting is more favorable this time around, and the science about the urgency of the problem has become only more compelling.

Climate change policy is now more pragmatic

The climate regime that will be further institutionalized in Paris is based on pragmatic acceptance of how the world works. Since there is no overarching world government, no one can make states do things that they do not want to. Environmental regimes are notably weak on enforcement capacities.

As a consequence, climate negotiations and international instruments serve as focal points for countries to align their actions for common purposes (Beth Simmons/Daniel Hopkins and Jana von Stein had a fantastic debate years ago on whether treaties constrain state action or just validate what states plan on doing anyway. They were both right).

Since climate protection is a global public good, there is always the potential for free-riding behavior, cheating and disputes over burden-sharing. International organizations can serve as negotiating platforms to ensure costs are distributed to the satisfaction of the participating parties, if not fairly.

The regime can also serve to publicize the actions of states and hopefully detect and deter cheating. This is international relations theory 101, but the climate community had to learn some of these lessons the hard way. The important point is that negotiators now appreciate them, and they finally have the wind at their backs. Without Copenhagen, they wouldn’t be there at all.

Joshua Busby is an associate professor at the Lyndon B. Johnson School of Public Affairs at the University of Texas at Austin. His last book, “AIDS Drugs for All” (with Ethan Kapstein), examined the challenges that social movements face changing global markets.