What is so Special about 2 degrees C in the Climate Change Debate?

In my last post I promised to take a bit of time to discuss the 2C target. You would expect that such an important target was picked through the use of a detailed scientific process with input from the brightest minds on the planet. In this you would be wrong. The acceptance of the 2C target is actually one of the most deliberately muddied topics in the climate change debate. The reason for this is because one of the not-very-secret secrets of the climate change debate is that the 2C target was originally chosen based on the thinnest of technical rationales. It was then backstopped with a thin veneer of research and buffed up to look like it had a stamp of approval marked “SCIENCE” before being trotted out for the public. Most distressingly, unlike a true science-based target, which would be refined as our knowledge base has improved and we have become more capable of understanding the climate system, this target has held absolutely steady since it was suggested in the late 1990’s. Admittedly, since the target was not chosen based on the science, advances in science really should not affect its value.

Now that I have written an introduction that should get every climate activist on the planet in a tizzy let me tell you a second secret. As targets go, the 2C target ends up being a pretty reasonable first guess. While a lot of science supporting the target is pretty flaky (and created using some pretty interesting premises) a lot of the research indicates that the most likely range certainly is somewhere around 2C (it actually seems to range from 1oC to 4C). I’ll go more into that later but wanted to reassure readers that the policy folks aren’t completely off their rockers.

For those of you really interested in reading deeper into the debate around the 2oC target there was a fascinating run of articles/blog postings in October 2014 that will provide interested readers with far more details than I have room to provide here. The discussion started with a comment in Nature by Drs. David Victor and Charles Kennel titled:Climate Policy: Ditch the 2oC Warming Goal. This was countered almost immediately at RealClimate.org (a blog for and by some of the big names in Climate Science) with an article by Dr. Stefan Rahmstorf titled Limiting global warming to 2 °C – why Victor and Kennel are wrong. The New York Times Dot Earth blog (another biggie in the climate blogosphere) then provided Victor and Kennel an opportunity to respond, which they did at length. For those of you used to policy discussions in the political sphere, this sounds like a pretty normal state of affairs. Informed professionals present their thoughts in a major journal, interested observers/experts provide counter-arguments and then the authors reply in kind. When done consistently this type of discussion would provide interested parties an opportunity to become informed without having to drown themselves in the primary literature. Sadly in the field of climate science this quality of back and forth discussion is seldom seen. Of note, while I present this discussion as a model of how the debate might be carried out in the climate field, my more sensitive readers will note that the RealClimate blog posting starts with a pretty nasty run of ad hominems and mean-spirited prose before the author actually gets down to discussing the topic at hand. Believe it or not behaviour that would get you shunned in virtually any non-academic, professional environment is actually considered a step up from the normal in this field. Having read all the documents I feel that Drs. Victor and Kennel made a far better case.

To go back to my promise, let me quickly summarize the history of the 2oC target. At the 1992 Rio Summit it was agreed that there was a need to “stabilize greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system (ref)“. The problem is that at the time they really had no idea what that would entail. It was understood that “global warming” was the next serious environmental challenge but no one knew exactly what that meant or even if we might be too late to make a difference. Coming out of the Summit a number of meetings and technical documents were sought/produced to establish what it would take to actually “prevent dangerous anthropogenic interference in the climate“. The problem was that at that point in time the climate field was really in its infancy. The first integrated assessment global climate models were still being developed and the computer power needed to do the necessarily complex calculations was not readily available. Just consider that an iPad2 would have made the list of the world’s speediest supercomputers until 1994; so at that time the models were still pretty basic. In the absence of the tools we take for granted these days a number of organizations and governmental bodies went to work formulating approaches that could be used in subsequent assessments. As described at the RealClimate blog, the critical one ended up being from the German government’s Advisory Council on Global Change. This document took a very qualitative look at the problem. It identified a “tolerable temperature window” based on historic reconstructions of temperature regimes and came up with the following argument:

This geological epoch has shaped our present-day environment, with the lowest temperatures occurring in the last ice age (mean minimum around 10.4°C) and the highest temperatures during the last interglacial period (mean maximum around 16.1°C). If this temperature range is exceeded in either direction, dramatic changes in the composition and function of today’s ecosystems can be expected. If we extend the tolerance range by a further 0.5°C at either end, then the tolerable temperature window extends from 9.9 °C to 16.6 °C. Today’s global mean temperature is around 15.3°C, which means that the temperature span to the tolerable maximum is currently only 1.3 °C.

So starting with the approximate 0.7°C that had already been observed by 1995 they ended up with a number of 2C, on which to hang their hat. As described at the RealClimate blog, the rest was history. The German position was adopted by the European Union and it became the de facto number we see and love. Kinda scary eh! The 2oC target is simply a sensible-sounding qualitative number based on approximate temperature ranges for interglacial periods, buffered with a fudge factor.

The next obvious question is: what is inherently wrong with the 2oC target? Well, assuming you are okay with its qualitative derivation, the obvious next issues are: 1) the question as to whether the number is the right one? and 2) the question as to whether it makes sense using a lagging indicator as your target? Let’s deal with the second issue first. As everyone involved in the climate change debate understands, temperature is a lagging indicator. Moreover, as demonstrated by the “pause” temperature can be a seriously delayed lagging indictor. Since we don’t have a good handle on climate sensitivity 2C could happen at 450 ppmv, it might occur at 800 ppmv or in an even less likely scenario it could take until 1200 ppmv? As we have seen with the pause, temperature has essentially held still for the better part of 20 years while carbon emissions have continued pretty much unabated. This point seems to be completely missed by Dr. Rahmstorf in his article. He harps on the political value of the target without ever acknowledging that it is less than a target than a shimmering mirage which may or may not exists somewhere out there somewhere, if you squint just right. Under the current target approach we will only be sure we have reached the edge of the chasm after we have started falling…essentially the Wile-E-Coyote approach to policy. If you truly believe that 2C represents a danger point then it would be much better to establish what emissions will get you to that point and ensure we do not exceed those numbers. This is the approach suggested by Meinshausen in his letter to Nature where he suggests carbon budgets as a much better policy tool. While I cannot attest to the validity of his actual numbers, the whole idea of negotiating carbon budgets makes a lot more sense than negotiating to keep ourselves below some nebulous post hoc target.

Another consideration in the debate is whether 2oC even represents an appropriate target. Since this post is already getting quite long instead of going into extreme detail on the topic I would simply direct you to a very interesting paper by Dr. Richard Tol from the University of Sussex. Since the article is pay walled, I will link to a figure from the paper which I found online.




What the paper indicates (and is illustrated in the figure) is that minor amounts of heating (less than 2C) may actually result in improvements in quality of life (as indicated by effect on % GDP). The conclusions of the Dr. Tol paper were not surprising to me as any plant biologist will tell you: plants grow better and have more drought tolerance under conditions of higher atmospheric CO2 concentrations. While the exact point at which the baseline is crossed remains under debate the positive effects of small increases in atmospheric CO2 concentrations are undeniable.

An interesting consideration of the Tol graph, that I have not seen discussed, is how the improvement in quality of life derived from the first 1oC heating might actually be hindering attempts to slow the growth in carbon emissions. Every week we see another attempt by alarmists to link higher CO2 concentrations to wild and woolly weather and unexpected global events. As I’ve written elsewhere, all this crying wolf does little to help their cause and runs contrary to the peer-reviewed research. Perhaps the policy people out there might want to acknowledge that increased CO2 concentrations are actually expected to improve things in the short run. Framing the current conditions as a “calm before the storm” would better allow the public to understand the risk. Instead they keep talking about Hurricane Katrina which only reminds us about how long it has been since Hurricane Katrina actually happened.

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6 Responses to What is so Special about 2 degrees C in the Climate Change Debate?

  1. Good question.

    But bringing Tol's work in evidence brings me to a better one – what is the epistemic status of economic models as compared to climate models. Are they similarly informative? I think the answer is, demonstrably, no.

    So seeing as your argument seems to be based in large measure on Tol's work (which is an outlier even among the published economics) should we begin with whether we believe it?

    Please see



  2. I guess we could sit here and throw rocks at integrated asset models all day. I only want to point out these IAMs are used to prepare the IPCC RCP's. This tells me all these predictions about future states are indeed fuzzy.

    I don't believe the IPCC can structure a work flow to get useable products. So my suggestion is that you start looking for improvements on how you go about making these predictions outside the IPCC community.

    Given the high dose of politics and propaganda being used, and the degree of personal animosity and hostility in the various “climate science” camps, plus economic reality, I don't think Obama, the EU, or climate science/economics can come close to reaching a successful outcome with current policies.

    This means I'll keep advocating geoengineering research. It seems to be a pragmatic solution.


  3. Slywolfe says:

    I am not yet convinced that “average” temperature is a meaningful metric. Nobody experiences (or suffers from) “average” temperature. The real consequences are from the highs and lows (and frequencies and durations). A 2 degree increase in “average” temperature could be essentially meaningless.


  4. It could be, conceivably.

    But that would be an astonishing coincidence, as the reason for the rise would be shifts in the way energy flows though the system. For this to result in very similar weather at all places on the planetary surface (given the complexity and mutability of fluid flows) seems extraordinarily unlikely.

    There is surely some temperature difference for which this similarity expectation wholly fails; an 80 c increase would turn the oceans into gumbo.

    We think it's hard to say whether that's this side of 2 C. What number would you put on it and why?


  5. Pingback: On Linda McQuaig’s comments, Carbon budgets, and keeping oil sands “in the ground” | A Chemist in Langley

  6. Pingback: On inane criticism in the climate change debate – the Ridley affair | A Chemist in Langley

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