Monday, June 6, 2016

Contemporary Issues in Climate Change Law & Policy, Part 2: Creating Legal Pathways to a Zero Carbon Future, by John C. Dernbach

Land Use Prof Blog is hosting a series of posts that are excerpts from book chapters in the recently released Contemporary Issues in Climate Change Law and Policy:  Essays Inspired by the IPCC.  The book was co-edited by Robin Kundis Craig (Utah) and me.  The posts will progress in the order of the book's chapters.  This second post is an excerpt from Prof. John C. Dernbach's chapter, "Creating Legal Pathways to a Zero Carbon Future."  Read Prof. Dernbach's entire chapter here.  Links to previous excerpts are at the bottom of this post.

Buy the book here.



The Challenge of the Carbon Budget

The challenge posed by climate change is both urgent and enormous. It is also daunting: it requires that the world, as a whole, move as soon as possible from the current situation of increasing greenhouse gas emissions to rapid reductions in greenhouse gas emissions. A recently developed concept—the carbon budget[1]—provides a way of understanding both the magnitude of this challenge and possible pathways for an effective response.

The objective of the U.N. Framework Convention on Climate Change is “stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system.”[2] In 2010, the Conference of the Parties to the Convention translated the stabilization objective into a maximum permissible surface temperature increase—2 degrees Celsius (or 3.6 degrees Fahrenheit) above preindustrial levels.[3] Parties, it said, “should take urgent action to meet this long-term goal, consistent with science and on the basis of equity.”[4] In addition, it stated the importance of “strengthening the long-term global goal on the basis of the best available scientific knowledge, including in relation to a global average temperature rise of 1.5 °C.”[5] The Paris Agreement, which was adopted unanimously by the Conference of the Parties in December 2015, stated the objective in terms of both temperatures—to hold “the increase in the global average temperature to well below 2 °C above pre-industrial levels,” and to “pursue efforts to limit the temperature increase to 1.5 °C above pre-industrial levels, recognizing that this would significantly reduce the risks and impacts of climate change.”  

The IPCC has translated the 2°C limit into a carbon “budget”—a numerical limit on all additional emissions, cumulatively, for the rest of the century. It concluded that this budget is between 630 and 1,180 gigatons of carbon dioxide equivalent.[7] That range represents the cumulative total of all new emissions of carbon dioxide equivalent between 2011 and 2100.[8] If cumulative emissions do not exceed the figures in that range, the IPCC states, it is “likely” that global average temperatures will stay below a 2°C increase.[9] To have a “likely” chance of staying within this budget, IPCC says, global greenhouse gas emissions need to be 40% to 70% lower by 2050 and “near zero” gigatons of carbon dioxide equivalent or “below” by 2100.

Several points of caution are needed to understand this carbon budget. First, there is a one in three chance that, on its own terms, the budget will not succeed. The term “likely”—as used by both the Conference of the Parties and the IPCC—means that the chance of a particular outcome is greater than 66%,[11] or two out of three. To put this probability in perspective, it helps to recall that the U.S. Environmental Protection Agency (EPA) has traditionally regulated chemicals under its major statutes when they create a risk of cancer of between one in 10,000 and one in 10 million.[12] Cancer risks from chemicals are different from the risks of climate change, of course, but the contrasting probabilities are striking nonetheless. Even in Russian roulette, a player has only a one in six chance of dying. 

Second, other calculations of a carbon budget provide even less time to reduce emissions that low. The writers of a frequently cited 2009 paper in Nature, for example, focused on the time period between 2000 and 2050, not 2000 and 2100, and calculated carbon budgets to avoid exceeding a 2°C increase based on cumulative emissions in the first half of this century.[13] Given past and projected emissions, they conclude, “we would exhaust the CO2 emission budget by 2024, 2027 or 2039, depending on the probability accepted for exceeding 2°C (respectively 20%, 25% or 50%).”[14] The International Energy Agency states that, with business-as-usual emissions, the remaining carbon budget (based on a 50% chance of keeping the temperature increase below 2°C) will be exhausted around 2040.[15] Others, including James Hansen, are less certain that the world can increase global temperatures by 2°C without severe adverse consequences. They argue that 1.5°C, or an even lower temperature limit, would be even better.[16] The Paris Agreement appears to be based on a recognition of these concerns, aiming to keep the temperature increase “well below 2 °C” and indicateing the desirability of holding the increase to 1.5°C.  Of course, the carbon budget to stay below a 1.5°C increase is even smaller, and hence it is more likely that the world will exceed it.

      Third, operationalizing this budget requires that it be allocated by nation based on population, historical contribution to global atmospheric greenhouse gas concentrations, development status (developed vs. developing), equity, and other factors. The question of each nation’s “fair share” of the budget is both essential and highly contested.[17] 

At the same time, if business as usual continues, and the growth of greenhouse gas emissions continues to accelerate, the world will simply blow by the budget and considerably exceed global average temperature increases of 2°C. According to the IPCC, emissions of carbon dioxide equivalent are increasing by about 1 gigaton annually, were the highest in human history between 2000 and 2010, and in 2010 alone reached 49 gigatons.[18] Half of cumulative anthropogenic (human-caused) carbon dioxide emissions have occurred in the last 40 years.[19] These increases are occurring in spite of the efforts that have been made thus far to reduce greenhouse gas emissions.[20] The IPCC thus concludes:

Without additional efforts to reduce GHG emissions beyond those in place today, emissions growth is expected to persist driven by growth in global population and economic activities. Baseline scenarios, those without additional mitigation, result in global mean surface temperature increases in 2100 from 3.7 °C to 4.8 °C compared to pre-industrial levels. . . .[21]

A variety of other projections based on business-as-usual emissions growth also put the world on track for a temperature increase of at least 4°C.[22] 

A 2012 report for the World Bank by the Potsdam Institute for Climate Impact Research and Climate Analytics describes the impact of a 4°C temperature increase by 2100 as disastrous.[23] Such a world, the report said, would be “one of unprecedented heat waves, severe drought, and major floods in many regions, with serious impacts on ecosystems and associated services.”[24] The report adds:

[G]iven that uncertainty remains about the full nature and scale of impacts, there is also no certainty that adaptation to a 4°C world is possible. A 4°C world is likely to be one in which communities, cities and countries would experience severe disruptions, damage, and dislocation, with many of these risks spread unequally. It is likely that the poor will suffer most and the global community could become more fractured, and unequal than today.[25]

      In the 2015 Paris Agreement, “Parties aim to reach global peaking of greenhouse gas emissions as soon as possible…and to undertake rapid reductions thereafter…so as to achieve a balance between anthropogenic emissions by sources and removals by sinks of greenhouse gases in the second half of this century….”[26] This “balance” means that net greenhouse gas emissions should be zero by that time.  Serious efforts to address the carbon budget must begin as soon as possible.   As economist Nicholas Stern summarizes the available scientific literature, the window for keeping temperatures under 2°C “is still open, but is closing rapidly.”[27]


[1] Fred Pearce, What Is the Carbon Limit? That Depends Who You Ask, environment360, Nov. 6, 2014,   

[2] United Nations Framework Convention on Climate Change, art. 2, May 29, 1992, S. Treaty Doc. No. 102-38, 1771 U.N.T.S. 107. U.N. Doc. A/AC.237/18 (Part II)/Add.1; 31 I.L.M. 849.  

[3] Conference of the Parties, United Nations Framework Convention on Climate Change, Decision 1/CP.16 (The Cancun Agreements: Outcome of the Work of the Ad Hoc Working Group on Long-term Cooperative Action under the Convention) ¶ 4, in Report of the Conference of the Parties on its Sixteenth Session, Held in Cancun from 29 November to 10 December 2010, Addendum, Part Two: Action taken by the Conference of the Parties at its sixteenth session, FCCC/CP/2010/7/Add.1 (2011), available at  

[4] Id. 

[5] Id. That translates to 2.7 degrees Fahrenheit. 

[6] United Nations Framework Convention on Climate Change, Conference of the Parties, Paris Agreement, art. 2.1(a), in Decision 1/CP.21 (Adoption of the Paris Agreement) (2015), U.N. Doc. FCCC/CP/2015/L.9/Rev.1, available at

[7] 2014 IPCC Mitigation Report, supra note 3, at 431. A gigaton is one billion tons. Carbon dioxide equivalent includes all greenhouses gases measured according to the warming potential of carbon dioxide.

[8] Id.

[9] Id. at 441. Working Group I reached a slightly different estimate about the budget—1,010 additional gigatons of carbon dioxide equivalent. 2013 IPCC Physical Science Report, supra note 1, at 27. Working Group I used a slightly different methodology and did not use ranges. 2014 IPCC Mitigation Report, supra note 3, at 441.

[10] 2013 IPCC Physical Science Report, supra note 1, at 13. 

[11] Id. at 4, note 2. 

[12] John D. Graham, The Legacy of One in a Million, Risk in Perspective 1-2 (1993) (Harvard Center for Risk Analysis), available at

[13] Malte Meinshausen et al., Greenhouse-Gas Emission Targets For Limiting Global Warming To 2 °C, 458 Nature 1158 (2009). 

[14] Id. at 1159. 

[15] International Energy Agency, Energy and Climate Change: World Energy Outlook Special Report, Executive Summary 2 (2015), available at   

[16] James Hansen et al., Assessing “Dangerous Climate Change”: Required Reduction of Carbon Emissions to Protect Young People, Future Generations and Nature, 8 PLOS One e81648 (2013). See also Jeff Tollefson, Global-warming Limit of 2 °C Hangs in the Balance, 520 Nature 14 (Apr. 2, 2015). 

[17] Donald A. Brown, Climate Change Ethics: Navigating the Perfect Moral Storm (2012); Fred Pearce, The Trillion-Ton Cap: Allocating The World's Carbon Emissions, environment360, Oct. 23, 2013, at \ 

[18] 2013 IPCC Physical Science Report, supra note 1, at 6.   

[19] Id. at 7. 

[20] Id. at 6. 

[21] Id. at 8.

[22] Sustainable Development Solutions Network & Institute for Sustainable Development and International Relations, Pathways to Deep Decarbonization 4 (2014), available at   

[23] International Bank for Reconstruction and Development/World Bank, Turn Down the Heat: Why a 4°C Warmer World Must be Avoided (2012), available at                    

[24] Id. at ix. 

[25] Id. at xviii. 

[26]  Paris Agreement, supra note 9, art. 4.1. 

[27] Nicholas Stern, Why are We Waiting? The Logic, Urgency, and Promise of Tackling Climate Change 32 (2015). 


Below is a list to previous excerpts in this series:

Part 1:  Climate Change, Sustainable Development, and the IPCC’s Fifth Assessment Report, by Robin Kundis Craig

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