This four-part post originally appeared on the Net Impact Boston blog.

• Part 1: COP21: The UN Climate Change Conference
• Part 2: COP21: Why This Year is Different
• Part 3: My Impressions from COP21
• Part 4: What’s In the Paris Agreement?

Excitement is the highest here in Paris for this year’s United Nations Conference on Climate Change, or “Conference of the Parties” (COP) 21, than any other single event I’ve been involved with in my sustainability career. It’s perhaps the most important COP since the one held in Kyoto in 1997, during which the Kyoto Protocol was signed. The conference has been repeated annually since the UN’s Framework Convention on Climate Change (UNFCCC, called the “UNF triple C”) was established in 1992. But this one is different—for a number of reasons.

First, some background for the less climate-geeky among us. The Kyoto Protocol was a seminal international agreement that recognized mankind’s role in warming and destabilizing the global climate. Signed in 1997 and taking effect in 2005, it was a pioneering protocol (meaning, a legally-binding agreement) that set reduction targets for signatory countries.

It wasn’t considered successful, though. First, India and China were categorized as developing countries under the Kyoto Protocol; the reality of their explosive development in the last two decades has shown that their treatment as modeling the emissions of poor, developing economies is way off the mark. Second, the world’s historically largest polluter, our fair United States, signed the protocol in 1998 under President Bill Clinton, but did not ratify it; any treaty must be ratified by the US Congress to be considered binding. Since Congress—then as now—refused to sign it, Clinton didn’t even put it to vote, and the US has remained unbounded by Kyoto.

This chart of current emissions shows clearly why Kyoto failed to put a halt to global warming: taking into account the US (didn’t ratify), and China and India (the Protocol did not impose any emissions targets on developing nations), and Canada (who later pulled out), a full 50% of emissions weren’t counted under the Protocol—even before we get to the long tail of “Other” countries.

(At the time of Kyoto, the US was the largest annual emitter of greenhouse gases; we’ve since been eclipsed by China, but still retain the dubious distinction of having the most cumulative historic emissions. Plus, with one-quarter of the population of China, the US remains the highest per capita emitter.)

So the world needs a new agreement. There have been several milestones leading up to COP21. At COP13 in Bali in 2007, negotiators—led by, surprisingly, the climate negotiation team under President George W. Bush—won a major victory in the Bali Accord, in which all countries—developed and developing—agreed to be responsible for reducing emissions or curbing emissions growth. (Whether the rich countries will subsidize the poor countries’ sustainable development has remained a sticking point.) Then at COP15 in Copenhagen in 2009, world leaders got very close to an agreement, but failed—and it nearly derailed international climate efforts. But subsequently at COP20 in Lima last year, the UN drafted an outline of an agreement that would commence in 2020. (Copenhagen’s outcome covered targets through the year 2020.)

So after years (decades!) of negotiations, hopes for a Paris agreement are high. In the next post, I’ll highlight some of the ongoing advances during this week’s negotiations, as well as my impressions of the week in Paris.