My Lukewarmer post, or how to lose friends on both sides in the AGW debate:

So in my first couple posts I have talked about pipelines and fossil fuels, but people seem unwilling to accept my views on pipelines until I answer the big question: where do I sit on the topic of anthropogenic global warming (AGW). 

Way back in 1859 John Tyndall demonstrated that selected gases including, water vapour, carbon dioxide and methane can intercept/absorb Infra-red (IR) radiation. A few years later Swedish chemist Svante Arrhenius did the first experiments/calculations to estimate how changes in the concentrations of carbon dioxide in the atmosphere should affect global temperatures. Since then the field of chemistry has refined the number and it is now generally accepted that that the direct climate sensitivity to carbon dioxide (excluding all positive or negative feedbacks) is roughly 1.2°C of warming per doubling of carbon dioxide concentrations. What that means is in the absence of feedbacks (positive or negative) every doubling of carbon dioxide concentration in the atmosphere should result in an increase in the global mean temperature of approximately 1.2 ^oC. 

So what does this mean? Well overwhelming consensus in the field is that a majority of the observed warming from 1951 to 2010 (numbers from the IPCC 2013 Physical Sciences Report) can be attributed to anthropogenic increases in the concentrations of Tyndall gases in the atmosphere (I, and many others, find that term “greenhouse gases” imprecise. A recent suggestion has been to refer to these gases as “Tyndall Gases” which I find both appropriate and clean as the name carries no additional baggage). There are some contrarians who challenge this basic consensus, but it would be difficult to be considered a serious thinker in the field if you did not accept the basic chemistry of AGW. 

Now given the chemical reality of AGW; the next research issue to be addressed deals with the equilibrium climate sensitivity of the atmosphere/biosphere to increases in Tyndall gases. The Intergovernmental Panel on Climate Change (IPCC) has this to say:

Equilibrium climate sensitivity is likely in the range 1.5°C to 4.5°C (high confidence), extremely unlikely less than 1°C (high confidence), and very unlikely greater than 6°C (medium confidence). The lower temperature limit of the assessed likely range is thus less than the 2°C in the AR4, but the upper limit is the same. This assessment reflects improved understanding, the extended temperature record in the atmosphere and ocean, and new estimates of radiative forcing.

– The IPCC includes an important footnote: No best estimate for equilibrium climate sensitivity can now be given because of a lack of agreement on values across assessed lines of evidence and studies.

So the IPCC says they do not have a best estimate for climate sensitivity and the manner in which models handle sensitivity and feedbacks remains a topic of furious debate and a tremendous amount of research. Every month a new paper is published that changes the best science on the topic.

On the extreme end of the warming side are those who believe in catastrophic anthropogenic global warming (CAGW). These individuals believe that climate sensitivity is at the highest end of the spectrum and a tipping point will be reached at some, still-to-be-defined, concentration of Tyndall gases in the atmosphere causing a positive feedback loop that will result in cataclysmic heating. The result will be a substantial change occurring over a very limited time frame (in tens of years rather than hundreds or thousands of years). That is how they translate the naturally expected 1.2°C of warming per doubling of carbon dioxide concentrations into increases ranging from 4.5 °C to over 6 °C per doubling. Proponents of CAGW insist that the tipping point is nigh and that we must decarbonize ASAP or risk probable extinction as a species.  

I look at the issue differently, my graduate-level courses in global biogeochemical cycles identified that most geochemical cycles involve negative feedback loops that buffer changes. People like me are called “lukewarmers”. We do not assume that positive feedback loops dominate the system but rather a balance of positive and negative cycles are at play with the resultant effect being a relatively minor positive effect. Unsurprisingly, an examination of the historical record demonstrates that changes in global temperature have historically mirrored the direct IR absorbtion characteristics of the increases in the various Tyndall gas concentrations. 

So when you read someone talk about the “consensus” understand what that word really means. There is an overwhelming consensus that humans are having an effect on the global climate, but the IPCC (the organization who define the consensus) says: 

No best estimate for equilibrium climate sensitivity can now be given because of a lack of agreement on values across assessed lines of evidence and studies.

 

The issue is that in the scientific community the issue isn't settled. The absence of consensus on sensitivity doesn’t stop political and social activists who insist they know better than the climate scientists. As a numerate and scientifically literate individual, I am skeptical, not of the science, but of the purveyors of these political messages. I strongly believe we are affecting our climate and strongly believe we need to decarbonize our economy but that is definitely a topic for a different post.