2C or not 2C

Overheated Thermometer

A recent article in Nature has questioned whether 2°C of global warming (beyond pre-industrial levels) is the right limit for achieving the United Nations Framework Convention on Climate Change (UNFCCC) objective of “stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system.” With its publication, authors David G. Victor and Charles F. Kennel have returned the scientific and policy questions behind the 2°C limit to the climate change debate table with gusto.

First adopted within the EU in 1996 (EU Council, 1996, item no. 6), and formalized globally in COP15’s “Copenhagen Accord,” the 2°C limit, while not legally binding, has been the mantra for governmental and institutional climate change efforts since. According to Victor and Kennel, however, this limit is “wrongheaded” because it fails on both political and scientific terms. First, it is sufficiently divorced from emissions and energy use so that it is ineffective in driving serious mitigation. Second, it doesn’t adequately characterize the stress with which the climate system is contending due to human activities. Thus, the 2°C mantra provides governments and institutions with little policy guidance, while at the same time failing to ensure accountability. polar-532_1527216a

Even before comparison of the IPCC’s 3rd (2001) and 4th (2009) Assessment Reports on climate change impacts alarmed the world, scientists were questioning the 2°C limit. Well-known former NASA scientist, James Hansen, began sounding the alarm in 2008, warning that the consequences of 2°C warming would be “disastrous.” Last December, Hansen offered (with a host of collaborators) his latest treatise on the topic, indicating that the cumulative 1,000 GtC in the atmosphere implied by a 2°C limit (based on multiple models, including those reported in the IPCC’s 5th Assessment Report) would actually generate “eventual warming of 3-4°C,” due to feedback loops. Even with 0.8°C warming, our planet is already experiencing climate extremes and other negative impacts.

Victor and Kennel acknowledge the attractiveness of the simplicity of the 2°C figure, but believe the global community is capable of conceptualizing and using a set of more accessible “vital signs” that can readily be translated into effective parameters for policy and action. In particular, they suggest atmospheric concentrations of CO2 and other GHGs as the best indicator of climate system health. They call for agreement on a global average GHG concentration goal on which “specific emissions and policy efforts” can be based. They also recommend tracking indices like ocean heat content and high-latitude temperature, both of which are currently measured. They base their recommendations on the success of the Millennium Development Goals and the Montreal Protocol, which they believe is due to the specific linking of goals to indicators that actually respond (within a few years) to human decision-making.

Many have criticized the Nature essay. Adam Vaughan, writing in the Guardian, reports a host of negative responses from scientists and international climate policy veterans, albeit not all totally damning. The climate activist community (ThinkProgress, Grist, and others) isn’t too happy, either. The timing of the article — less than 18 months before the deadline for the next international climate agreement, due to be inked in Paris — and it appearing in such a prominent journal have prompted the most anger. Many of these critics fear that doing away with the 2°C limit will let governments off the hook — exactly the result Victor and Kennel feel having the 2°C limit has done. (Victor’s rebuttal, available here, interestingly notes that many authors have articulated the need for multiple indicators.)

But do politicians feel that they’re “on the hook?” Current national energy policies have allowed a 34% increase in global GHG radiative forcing between 1990 and 2013, with atmospheric concentration of CO2 in 2013 exceeding the pre-industrial level (1750) by 142%. The World Meteorological Association’s September report of this data (see our September 9 blog posting on it) included the disturbing news that CO2 increase between 2012 and 2013 exceeded that of any single year since 1984.


With nearly universal agreement on the policy end (reduce anthropogenic emissions ASAP), but much disagreement over the policy driver (the line(s) in the sand beyond which there is no return), perhaps this catastrophe-based framing is poorly suited to policy-making. Victor and Kennel don’t suggest abandoning the 2°C limit today. Instead, they call for the UNFCCC to “chart a path” to designing a new set of indicators at that upcoming momentous Paris 2015 COP21 meeting.

2C or not 2C? We shall see.