12/15/2015 12:44 pm ET | Updated Dec 15, 2016

The Paris Climate Agreement: Deliverance or Disappointment?

By John Sterman

The most common question I was asked in Paris: Is the agreement a triumph, as the negotiators and heads of state declare, or another weak pronouncement that will do little to stave off climate catastrophe? The answer is both: The Paris agreement represents real progress. It also falls significantly short. If you are a "glass half full" person, there's much to celebrate. If you are a "glass half empty" person, there's much more work to do to turn the words of the diplomats into actions that will cut greenhouse gas emissions. Here's a quick guide to what the agreement really means -- and what we must now do.

Goals versus Reality: The agreement lowers the maximum warming we should allow, from 2°C (3.6°F) above preindustrial levels to "well below 2°C... and pursuing efforts to limit the temperature increase to 1.5°C" (2.7°F).

Half full: Holding warming to 1.5°C might slow sea level rise and prevent the loss of south Florida, New Orleans, London, the Netherlands, Shanghai and island nations such as Tuvalu. The lower limit recognizes there is no safe level of warming: the higher the global average temperature, the greater the harms and risks.

Half empty: We've already warmed the Earth about 1°C (1.8°F) since the Industrial Revolution. That warming may have already triggered the irreversible loss of Antarctic glaciers that will add more than three feet to long-run sea level rise, on top of melting in Greenland and around the world. Worse, the 1.5°C goal was not accompanied by stronger pledges for emissions reductions. We behave like a person trying to lose 50 pounds, but who isn't eating less or exercising more and then says, "my new goal is to lose 80 pounds."

Emissions Reductions: The agreement maintains the voluntary system of pledges, known as "INDCs" (Intended Nationally Determined Contributions") under which each country submits its own goals and plan to limit its greenhouse gas emissions.

Half full: So far more than 180 nations have submitted pledges. China, the world's largest emitter, pledged that their CO2 emissions would peak no later than 2030, a huge change from the failed 2009 Copenhagen meeting. If all pledges to date are fully implemented, expected warming by 2100 would fall about 1°C (1.8°F) compared to no action.

Half empty: Pledges are but promises. Yet taking the nations of the world at their word and assuming that their promises to date are fully implemented -- without stronger actions they have not yet pledged -- would lead to expected warming of 3.5°C (6.3°F), far above the 1.5-2°C (2.7-3.6°F) goal.

The need for stronger action: Recognizing that gap, the agreement "notes with concern" that "much greater emissions reduction efforts will be required" between now and 2030. That is both scientifically correct and a remarkable admission in an international agreement.

Half full: To address the emissions gap, the agreement establishes a review process under which the pledges will be updated, and, presumably, strengthened every five years. That is a significant breakthrough. Deeper, earlier cuts in emissions between now and 2030 would make it easier to close the emissions gap and limit warming to the 1.5-2°C goal.

Half empty: The emissions gap between now and 2030 is big, but it's even bigger after 2030. Even if emissions stopped growing today, global warming would continue. Why? Think of the atmosphere as a giant bathtub. Everyone knows that if you pour water into the tub faster than it drains out the water level will rise; unchecked, the tub will overflow and destroy your house. In the same way, we now spew greenhouse gases into the atmosphere far faster than they are removed. Stopping emissions growth will not balance the flows. The tub will continue to fill, the planet will continue to warm, and the closer we come to crossing dangerous tipping points that might cause rapid, irreversible warming and sea level rise.

Half Full: The agreement recognizes the need to balance the flows into and out of the 'carbon bathtub', stating that the "Parties aim to reach global peaking of greenhouse gas emissions as soon as possible, recognizing that peaking will take longer for developing countr[ies], and to undertake rapid reductions thereafter... so as to achieve a balance between... emissions... and removals..."

Half Empty: The contentious issue of who must cut and how much remains unresolved. Many developing nations continue to argue that the developed nations created the climate crisis and therefore bear historical responsibility to solve it by cutting their emissions, while the developing nations must continue to burn fossil fuels. That is simply not possible: To have any chance of limiting warming to 2°C (3.6°F), global emissions must fall essentially to zero before the end of the century. To do so, all nations must cut. China, the world's largest emitter, has pledged to cap their emissions no later than 2030, but has not yet pledged to cut them afterwards. India, the world's third largest emitter, plans to increase their emissions significantly between now and 2030.

Half Full: The debate over historical responsibility is shifting from who gets to burn to who should pay to stop the burning. The developed nations should help the developing world leapfrog the destructive fossil fuel economy through efficiency and renewable energy, just as Africa jumped straight to mobile telephony, leapfrogging landlines. To do so, the developed nations agreed five years ago to help developing nations pay for mitigation and adaptation. The goal is $100 billion per year by 2020. The Paris agreement reaffirms that goal.

Half Empty: Contributions to the $100 billion/year goal are neither mandated nor pledged. The agreement merely "strongly urges" the developed nations to "scale up their level of financial support."

Verification: Ronald Reagan famously quoted the Russian proverb, 'trust but verify,' when negotiating the nuclear arms treaty with the Soviet Union. Does the Paris agreement require verification of emissions reductions?

Half Full: The agreement takes a big step toward verification by establishing "an enhanced transparency framework..." that should use "metrics assessed by the Intergovernmental Panel on Climate Change" to "ensure methodological consistency."

Half Empty: The devil is in the details, specifically, whether scientifically sound accounting will be adopted versus self-reported emissions reductions that may include double counting and dubious claims. Questionable accounting is rampant: some developing nations count on unverifiable and controversial emissions reductions from forest preservation and land use change, while the European Union ignores the full lifecycle emissions from biofuels.

Half full or half empty? That's the wrong question. The right question: what are we now called upon to do? Let us praise the real progress made while we loudly and clearly tell our leaders that goals are not actions, pledges are merely promises, and time is running out. We must build on Paris to strengthen the pledges and implement the policies to realize them. We must organize against the counter-reaction and denial already underway. Success will come only if we get involved. Join any of the groups working to cut emissions, create jobs, and build a sustainable economy, from the progressive, to the nonpartisan Citizens Climate Lobby, Environmental Entrepreneurs and Mothers Out Front, to the conservative RepublicEn and ConservAmerica. If our leaders won't act, we must elect new leaders who will. We can do it. We've done harder things before. But we have to act, now.

John Sterman is the Jay W. Forrester Professor of Management at the MIT Sloan School of Management and faculty director of the MIT Sloan Sustainability Initiative. He is a member of the ARCS Board of Directors. His interactive climate policy models are freely available at