On December 12th, bleary-eyed negotiators walked out of the Paris-Le Bourget conference center to announce a global agreement to fight climate change. Reactions to the agreement have generally taken two forms - overheated claims about the historic nature of the agreement from many proponents and dismissal from both those demanding stronger action and those opposed to any action at all, on grounds that the agreement represents little change from business as usual.

Both reactions are, to some degree, correct. The agreement is historic, although not for the reasons that many suggest. But it also does not commit the world to legally binding emissions reductions of any sort. Nor are the commitments that have been made even remotely consistent with stabilizing atmospheric concentrations at anywhere close to 2 degrees Celsius, much less 1.5.

So why historic then? For over a decade, we along with a number of other folks have argued that progress on international climate mitigation efforts would require a fundamental shift. Where the top down approach focused on legally binding emissions targets and timetables has had no appreciable effect on global emissions, a bottom up bilateral and multilateral approach focused on real commitments to put clean energy infrastructure in the ground might begin to move the needle. Don't tell me what your emissions are going to be in 2050, tell me how much clean energy infrastructure you are actually prepared to build today. That shift was ratified in Paris and does in fact mark a historic departure from the framework convention established in 1992 and ratified in the Kyoto Accords in 1997.

The original framework failed for multiple reasons. While the framework established that developing nations would have the right to develop, it also envisioned that development in places like China and India might take a fundamentally different, and less carbon-intensive path than that taken by rich nations.

If that idea might have been plausible back in 1992, the last 25 years have decisively established that emerging economies around the world are developing exactly the same way that rich countries did, by urbanizing, industrializing, and consuming much more energy. Given currently available energy technologies, that has brought with it accelerating growth of carbon emissions. And with that, hopes that global emissions might be stabilized below 450 ppm have realistically vanished.

The rapid emergence of China, India, and other developing economies as formidable economic competitors to OECD economies has also rendered two further pillars of the old framework untenable: first, the notion that rich countries would agree to very deeply cut their own emissions to create more atmospheric space for poor nations emissions to grow or, alternatively, that they would heavily subsidize the deployment of cleaner but more expensive energy technologies in the developing world.

In the former formulation, rich countries would effectively penalize their own economies so that their competitors could grow. Under the latter, rich countries would subsidize the energy economies of their competitors. Both approaches have become non-starters in developed economies politically, as emerging economies around the world over the last several decades have experienced explosive economic growth while OECD economies have struggled.

The failure of cap and trade legislation in the US Congress and the collapse of international negotiations at Copenhagen marked the death knell for the old approach. But it is important to keep in mind that even if legislation had passed the US Congress and negotiations at Copenhagen ended less rancorously, the old framework would have been unlikely to deliver real emissions reductions much below business-as-usual, for all of the reasons delineated above.

The shift from binding and long term emissions targets to voluntary Intended Nationally Determined Contributions (INDCs) made inevitable the second historic shift in international climate mitigation efforts, which is the formal and explicit recognition that we do not, in fact, have all the technology we need to achieve deep reductions in emissions. While making legally binding commitments to reduce emissions is hard, promising to achieve those reductions decades in the future is not. So long as the focus of negotiations was long-term temperature targets, the disconnect between what anyone was actually doing and what they were saying about long-term emissions was easily overlooked.

With the shift of focus from long-term emissions and temperature targets to real commitments to deploy energy infrastructure over the near-term, that disconnect has become much harder to submerge. When it came down to making real and specific commitments to transform their energy economies, no major economy in the world was prepared to make commitments consistent with the long established goal of stabilizing global temperature at two degrees above pre-industrial levels.

The reticence to do so was not a failure of political courage but simply reflects the current state of low-carbon energy technology. While renewable energy technologies are getting better and cheaper, no large economy in the world has yet succeeded in making a big dent in emissions with renewables. Present day nuclear has in some places driven deep reductions in emissions but has been taken off the table by most of the developed world and is growing too slowly in places like China to really make a big difference. And we still lack proven low-carbon technologies for huge carbon intensive sectors of the global economy such as transportation and heavy industry. Big reductions in emissions, practically, are going to require low-carbon technologies that are better and cheaper. Hence, finally, the acknowledgement that stabilization at two degrees will require radical technological innovation.

For some, that recognition may seem depressing. We are nowhere near being able to stabilize emissions at levels that many people believe will be necessary to avoid catastrophic impacts on human societies and the environment. But there is another view, which is that real progress on mitigating and managing the long term risks associated with climate change cannot truly commence until we seriously acknowledge the scale of the challenge and the basic boundary conditions that will be necessary to address it. Those include recognizing three realities: first that billions of people are on an irreversible course toward living something that looks like a modern life, replete with the choices, comfort, and security that those of us in the rich world take for granted; second, that everyone on the planet and billions more likely to come can and should follow that path if they choose it; and third, that achieving that outcome while limiting global temperatures to something likely above two degrees but well below the business-as-usual scenario will require developing zero-carbon technologies capable of powering that world. With the agreement reached this month in Paris, the international community appears to have begun to turn its attention to the right challenge, even as we recognize that we are still a long way away from fully addressing it.

Correction: An earlier version of this post referenced the Maldives very modest emissions reduction commitments in its INDC. Several people have pointed out that the Maldives limited commitment may be more a function of an illegitimate political regime that profits from diesel imports than any iron law of climate policy. The sentence has been removed.

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