Paris – Yesterday I noted the optimism that seems to have affected the climate campaigners and negotiators here at the U.N. climate change conference. This got me to wondering about why that might be. I think that part of the reason is that process of putting together whatever turns out to be the Paris Accord is largely a non-zero-sum bottom-up exercise. Countries are not being told what to do, but each one gets to propose for itself what it plans to do about man-made global warming. In addition, thousands of states, provinces, regions, cities, and businesses have piled on to make voluntary pledges to reduce their greenhouse gas emissions. (This is not to say their electorates agree with the decisions being made by their governors and mayors.)

This pledging process avoids the divisive zero-sum gaming that characterized previous climate negotiations and which resulted in failed climate treaties. Both the 1997 Kyoto Protocol and the successor agreement that was supposed to be approved at the Copenhagen climate conference in 2009 were conceived as top-down legally binding regulatory systems. They aimed to impose very specific carbon dioxide limits on developed country signatories. For example, the U.S. target under the Kyoto Protocol was to cut its emissions 7 percent below the levels it emitted in 1990; total emissions from developed countries were supposed be cut by 5.2 percent below 1990 levels by 2012.

The Kyoto Protocol was never ratified by the U.S. and the 2009 Copenhagen conference collapsed because countries, especially rapidly developing emitters like China and India, would not accept top-down imposed greenhouse gas emissions cuts. The hastily cobbled together face-saving Copenhagen Accord was styled as a "political agreement" not a treaty. It did set the goal of keeping future temperature increases below 2°C of the pre-industrial average. Under the Copenhagen Accord, rich countries committed to voluntarily adopting economy-wide emissions targets for 2020. How much to cut was individually up to each country to decide. This has become the model for the hoped-for Paris Accord in which each country submits its intended nationally determined contribution (INDC) to the U.N. Framework Convention on Climate Change (UNFCCC) secretariat. INDCs are basically promises countries make regarding how they will address man-made global warming.

In 2010, the Obama administration pledged to cut U.S. emissions 17 percent below their 2005 levels by 2020. According to the latest Environmental Protection Agency figures U.S. emissions have so far dropped to about 11 percent below their 2005 levels. In March, the Obama administration submitted an INDC pledging to reduce U.S. greenhouse gas emissions up to 28 percent by 2025.

In November, Secretary of State John Kerry warned that whatever else was agreed upon in Paris there were "not going to be legally binding reduction targets like Kyoto." However, President Barack Obama at the Paris conference did concede, "Although the targets themselves may not have the force of treaties, the process, the procedures that ensure transparency and periodic reviews, that needs to be legally binding."

The U.S. seems to be adopting New Zealand's "bounded flexibility" approach in which each country's emissions reduction "contributions" – not "obligations" – are not formally included in the core of the legal agreement. The Paris Accord would be legally binding with respect to reporting, monitoring, and review of the voluntary pledges. While countries cannot be told what they must contribute, each would be required to accurately tell the others how it's doing with regard to its pledges.

President Obama apparently plans to argue that the division between voluntary targets and obligatory reporting procedures is enough to avoid having to submit the new accord to the Senate for its advice and consent. The legally binding accounting provisions will supposedly be merely an extension of U.S. obligations under the U.N. Framework Convention on Climate Change ratified in 1994. It is true that the U.S. government already does a pretty comprehensive job of reporting on greenhouse gas inventories and the actions taken to reduce emissions. So maybe the Obama administration's interpretation is legally plausible, but it's doubtful that Congressional Republicans will agree. In any case, U.S. Special Representative on Climate Change Todd Stern said here that the issue of U.S. domestic politics is not coming up much during the negotiations here.

The legally binding part of the accord would require that parties come back periodically to review and evaluate each other's pledges. In light the reviews countries are supposed update their INDCs in accordance with what the science is saying about the pace and intensity of man-made warming. This may, however, not work out as the negotiators and activists here in Paris expect. They believe that pledged cuts will need to be ramped up significantly as the globe grows ever warmer. But it is possible that the science may tell them something else in the coming years.

What if instead of accelerating as predicted the pace of warming were to continue at the rate of +0.11°C per decade reported by the satellite measurements? What if further research confirms that the climate sensitivity for a doubling of atmospheric carbon dioxide is lower than earlier more alarming estimates? That would mean that future warming would be lower than most climate models now predict. In other words, what if the globe doesn't have the climatic equivalent of Ebola, but instead is suffering from a mild cold?

If climate science does validate such happy outcomes, the Paris Accord's mandated review and update processes would afford countries the opportunity to scale back their INDCs. The flexible structure of the Paris Accord captures some of the elements of the intriguing proposal for a temperature indexed carbon tax by University of Guelph economist and statistician Ross McKitrick. McKitrick, a skeptic of predictions of catastrophic warming, advocates putting a small tax on carbon dioxide emissions, tied to a suitable measure of atmospheric temperatures. "If temperatures go up, so does the tax," explains McKitrick. "If they do not, the tax does not change. In this way everybody will expect to get the policy they think best, and whoever turns out to be right deserves to be so." If the skeptics are right about the direction of man-made warming, it is possible that they may end up (at least faintly) cheering the Paris Accord.

Note: I am filing daily dispatches from the Paris climate change conference, and will keep readers informed on just how onerous the new accord turns out to be.