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 2^oC 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 2^oC 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 2^oC 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 2^oC 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 2^oC (it actually seems to range from 1^oC to 4^oC). 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 2^oC 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 2^oC 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 2^oC 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 2^oC, 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 2^oC 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 2^oC 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 2^oC 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 2^oC 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 2^oC 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 2^oC) 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 1^oC 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.