by Judith Curry
“Without significant cuts in emissions by all countries, and in key sectors, the window of opportunity to stay within less than 2 degrees [of warming] will soon close forever.” – UN Secretary-General Ban Ki-Moon
The United Nations Framework Convention on Climate Change (UNFCCC ) international environmental treaty (1992) states as its objective:
The ultimate objective of this Convention and any related legal instruments that the Conference of the Parties may adopt is to achieve, in accordance with the relevant provisions of the Convention, stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system.
Where does ‘2 degrees’ come from? At the 2009 UN Climate Change Conference in Copenhagen, international negotiators agreed to the following: Humanity must not let the planet get hotter, on average, than 2 degrees Celsius above preindustrial levels. 2 degrees has been the focal point of international agreements and negotiations ever since.
In 2009 Michael Mann published an opinion piece in PNAS Defining dangerous anthropogenic interference, which describes the burning embers diagram and the logic that went into the 2 degree determination.
For some background on the controversy surrounding this issue, see these previous CE posts:
Ditch the 2C warming goal
David Victor and Charles Kennel have published a provocative comment in Nature entitled Ditch the 2C warming goal. Subtitle: Subtitle: Average global temperature is not a good indicator of planetary health. Track a range of vital signs. Excerpts:
Bold simplicity must now face reality. Politically and scientifically, the 2 °C goal is wrong-headed. Politically, it has allowed some governments to pretend that they are taking serious action to mitigate global warming, when in reality they have achieved almost nothing. Scientifically, there are better ways to measure the stress that humans are placing on the climate system than the growth of average global surface temperature — which has stalled since 1998 and is poorly coupled to entities that governments and companies can control directly.
Since , two nasty political problems have emerged. First, the goal is effectively unachievable. Owing to continued failures to mitigate emissions globally, rising emissions are on track to blow through this limit eventually. Because it sounds firm and concerns future warming, the 2 °C target has allowed politicians to pretend that they are organizing for action when, in fact, most have done little. Pretending that they are chasing this unattainable goal has also allowed governments to ignore the need for massive adaptation to climate change. Second, the 2 °C goal is impractical. It is related only probabilistically to emissions and policies, so it does not tell particular governments and people what to do.
A single index of climate-change risk would be wonderful. Such a thing, however, cannot exist. Instead, a set of indicators is needed to gauge the varied stresses that humans are placing on the climate system and their possible impacts. The best indicator has been there all along: the concentrations of CO2 and the other greenhouse gases. A global goal for average concentrations in 2030 or 2050 must be agreed on and translated into specific emissions and policy efforts, updated periodically, so that individual governments can see clearly how their actions add up to global outcomes.
Policy-makers should also track ocean heat content and high-latitude temperature. Because energy stored in the deep oceans will be released over decades or centuries, ocean heat content is a good proxy for the long-term risk to future generations and planetary-scale ecology. High-latitude temperatures, because they are so sensitive to shifts in climate and they drive many tangible harms, are also useful to include in the planetary vital signs.
What is ultimately needed is a volatility index that measures the evolving risk from extreme events — so that global vital signs can be coupled to local information on what people care most about. A good start would be to track the total area during the year in which conditions stray by three standard deviations from the local and seasonal mean.
The window of opportunity for improving goal-setting is open. This autumn, a big push on climate policy begins — with the aim of crafting a new global agreement by late 2015 at the UNFCCC’s Conference of the Parties in Paris. Getting serious about climate change requires wrangling about the cost of emissions goals, sharing the burdens and drawing up international funding mechanisms. But diplomats must move beyond the 2 °C goal. Scientists must help them to understand why, and what should replace it.
Most of the reactions to this essay that I’ve seen are negative, see for example:
However the underlying agreement is that ‘urgent action is needed to reduce anthropogenic greenhouse gases’; the disagreement is on how to sell this politically.
David Victor responds at length at DotEarth, definitely worth reading.
For the moment, lets accept the premise that 2C (post industrial warming) is a useful danger threshold. When can we expect to reach 2C? According to climate model projections with the RCP8.5 scenario, we would reach 2C around 2040. However, if you consider a Transient Climate Response of 1.3C (e.g. Lewis/Curry), then you would not reach the 2C threshold in the 21st century for any of the scenarios except RCP8.5. It seems we are on track for RCP8.5 for CO2 emissions, and RCP6.0 for the other gases. So we would just be reaching 2C at the end of the 21st century. And the ‘pause’ is slowing all this down by at least a decade or two.
So in context of the current UN framework, personally I prefer to keep 2C for now as a policy target. There is simple political reality: the pause, combined with lower sensitivity estimates, are acting to ‘kill the cause’, i.e. urgent action needed to reduce emissions. Ross McKitrick has a new paper where he makes similar points Climate Policy Implications of the Hiatus in Global Warming.
Hence scientists who are urging action are looking for another metric that looks more seriously damaging than the pausing global surface temperature. Well CO2 is the one metric that unambiguously is going up and will keep going up; the only problem is that its difficult to care about all this if the CO2 is not changing the climate adversely in regions where people live. Using deep ocean heating (a few hundredths of a degree C in the last half century, inferred from climate models) seems fairly ludicrous; if we are indeed sequestering heat in the deep ocean, well that seems to be a pretty benign place for it. Developing some metric based on extreme weather events is thwarted by the inconvenient truth that there is no detection of an increase in most types of extreme weather events and it is extremely difficult to attribute any change to humans, given the dominant influence of natural variability on extreme events.
Here’s why I don’t like the 2C metric, and it has little to do with the issues raised by Victor and Kennel. The 2C metric is relevant only for the so-called linear model of decision making. Even if we had some understanding and agreement on what state of the global climate was regarded as dangerous, and we knew what to expect from natural climate variability in the 21st century (e.g. solar, volcanoes, ocean circulations), there is very substantial uncertainty in climate sensitivity to increased CO2, and hence the appropriate emissions target to avoid this level of warming. While the methods continue to be refined to determine sensitivity, none fully account for natural internal variability or for unforced ‘climate shifts’.
We just don’t know what the 21st century climate will be (I doubt we’ve appropriately bounded the possibilities), and hence we are faced with conditions of deep uncertainty for decision making. I’ve discussed in a series of previous posts how we can make good decisions under conditions of deep uncertainty:
- Decision making under climate uncertainty
- Can we make good decisions under ignorance?
- Coping with deep climate uncertainty
- World Bank on Understanding Climate Uncertainty
- Strategies for robust decision making for climate adaptation
A more realistic vulnerability based approach that is regional and tied to extreme weather events (whether or not they are exacerbated by human emitted greenhouse gases) is more likely to secure the common interests in each region and reduce the devastating impacts of these events on economic development. No or low regrets energy policies such as conservation and efficiency, research into new technologies, and implementations of other energy policies with ancillary benefits also make sense under conditions of deep uncertainty.
Dare we hope for the UN to move away from emissions targets and loss and damage mechanisms towards more more robust policies that reduce vulnerability in the short term, support economic development goals, and work towards cleaner energy in the long term?