David Robert's Blog

This is a short letter in response to a Reddit post on climate change skepticism (see below):

Until recently, I never read much about climate change other than standard newspaper stuff. I think I have heard most of the arguments for it. I’ve also heard people say stuff like “everyone who actually studies/is an expert on climate change agrees”. Well, Judith Curry studies it and doesn’t. As I read through this presentation, it seems balanced and well thought out. In fact, searching through “climate sceptic” sites on the web, I find surprisingly sensible-seeming people with logical arguments and data references. This [Curry, 2018] walkthrough is a good example. So can anyone tell me; am I missing something obvious? Why are the arguments for climate scepticism wrong?

Any feedback is appreciated (u/HCAndersAnd, 2018).

Dear Skeptic,

I think that you are honest and fair-minded. Your skepticism of consensus climate change science is understandable, but it is misplaced. Since you are not an expert on the science of climate change, you are rationally required to believe consensus climate change science.[1] But you might ask, “Why should I believe that the climate science claims made by proponents of climate change mitigation are consensus climate science?”

Signatory governments to the Paris climate change agreement base their support for climate change mitigation on the Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC) (The Paris Agreement, n.d.).[2] A key finding of the AR5 is quoted below:

Projections of changes in the climate system are made using a hierarchy of climate models ranging from simple climate models, to models of intermediate complexity, to comprehensive climate models, and Earth System Models. These models simulate changes based on a set of scenarios of anthropogenic forcings. […]

Relative to the average from year 1850 to 1900, global surface temperature change by the end of the 21st century is projected to likely exceed 1.5°C for RCP4.5, RCP6.0 and RCP8.5 (high confidence). Warming is likely to exceed 2°C for RCP6.0 and RCP8.5 (high confidence), more likely than not to exceed 2°C for RCP4.5 (high confidence), but unlikely to exceed 2°C for RCP2.6 (medium confidence). Warming is unlikely to exceed 4°C for RCP2.6, RCP4.5 and RCP6.0 (high confidence) and is about as likely as not to exceed 4°C for RCP8.5 (medium confidence). {12.4} (IPCC, 2013, p. 18)

You should believe that the above statement (call it X) is consensus climate change science because (i) X is clear and unambiguous and (ii) X is quoted from the work of the IPCC, and there is a consensus among recognized organizations of experts with expertise in climate science that “the work of the IPCC represents the consensus of the international scientific community on climate change science.” (The Science of Climate Change, 2001, p. 1261)[3]

Suppose however that X was consensus climate science at the moment of publication of the AR5, but X is no longer consensus climate science, and you have polling or other unequivocal evidence to support this. It then follows that (i) you should disbelieve X if consensus temperature projections have shifted since publication of the AR5 or (ii) you should withhold belief about X if there are no longer any consensus temperature projections and no such projections are believed by even the majority of climate scientists.

Suppose instead that X was not consensus climate science at the moment of publication of the AR5 (and that X is not currently believed by even the majority of climate scientists). This would be the most significant and egregious failure of science communication in history. All the climate scientists who were involved in signing off on X’s wording would have experienced a lapse or would have engaged in deception. This would have happened in a report, the AR5’s Summary for Policymakers, that is designed to guide laypersons in making informed policy decisions on climate change. This is very improbable.

Therefore, unless you have polling or other unequivocal evidence that X is no longer consensus climate science, the likelihood that X is currently consensus climate science is significantly greater than the likelihood that X is not currently consensus climate science. And so, it is rational to believe that X is consensus climate science.

In conclusion, if you were a dissenting expert on the science of climate change—that is, if you regarded yourself as such or if you were a practicing climate scientist who disagreed with the conclusions of your peers on climate change science—then you would be entitled to your own view on those conclusions. But, judging from your question, you are clearly not a dissenting expert. As such, I hope to have convinced you that, unless and until you become an expert, you are rationally required to believe the statement quoted above from the IPCC’s AR5.[4]

Bibliography

Coady, D., & Corry, R. (2013). The Climate Change Debate: An Epistemic and Ethical Enquiry. London: Palgrave Macmillan. https://doi.org/10.1057/9781137326287

Curry, J. (2018, June 12). The Debate: My Presentation. Climate Etc. https://web.archive.org/web/20180630062836/https://judithcurry.com/2018/06/12/the-debate-mann-titley-moore-curry/

Gao, Y., Gao, X., & Zhang, X. (2017). The 2 °C Global Temperature Target and the Evolution of the Long-Term Goal of Addressing Climate Change—From the United Nations Framework Convention on Climate Change to the Paris Agreement. Engineering, 3(2), 272–278. https://doi.org/10.1016/J.ENG.2017.01.022

IPCC (2013). Summary for Policymakers. InT. F. Stocker, D. Qin, G.-K. Plattner, M. Tignor, S. K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex & P. M. Midgley(Eds.), Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge: Cambridge University Press. http://www.climatechange2013.org/images/report/WG1AR5_SPM_FINAL.pdf#page=18

Kaulbars, M. (2010, June 14). Letter To A Climate ‘Skeptic’. News Junkie Post. https://web.archive.org/web/20180630073421/http:/newsjunkiepost.com/2010/06/14/letter-to-a-climate-%E2%80%9Cskeptic%E2%80%9D/

Morrison, D. (2011). Letter to Climate Skeptics. Skeptic, 16(2), 10. http://go.galegroup.com/ps/anonymous?id=GALE%7CA252289589

Newman, S, (2016, July 1). An open letter to climate change sceptics. 1 Million Women. https://web.archive.org/web/20180423160439/https:/www.1millionwomen.com.au/blog/open-letter-climate-change-sceptics/

Scientific consensus: Earth’s climate is warming. (n.d.) Global Climate Change: Vital Signs of the Planet. Earth Science Communications Team at NASA’s Jet Propulsion Laboratory. Retrieved April 8, 2021, from https://climate.nasa.gov/scientific-consensus/

The Paris Agreement (n.d.). United Nations Climate Change. Retrieved April 8, 2021, from https://unfccc.int/process-and-meetings/the-paris-agreement/the-paris-agreement

The Science of Climate Change (2001). Science, 292(5520), 1261. https://doi.org/10.1126/science.292.5520.1261

u/HCAndersAnd (2018, June 12). What is wrong with these arguments? Reddit (r/climatechange). Retrieved April 8, 2021, from https://www.reddit.com/r/climatechange/comments/8qnq31/what_is_wrong_with_these_arguments/

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[1] For any proposition, p, a layperson with respect to p is rationally required to believe p if and only if p is believed by the majority of experts on p. See Coady and Corry (2013).

[2] Gao, Gao, and Zhang (2017) note that,

In 2008–2014, the IPCC’s Fifth Assessment Report (AR5) made a comprehensive assessment of the climate system change, risks, emission budget, and mitigation pathway choice of 2°C global warming on the basis of the research results available. After scientific assessment and a series of political pushes, one of the three goals reached at the 2015 Paris Climate Change Conference was stated as “Holding the increase in the global average temperature to well below 2°C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5°C above pre-industrial levels” [3].

[3] See also Scientific consensus: Earth’s climate is warming (n.d.).

[4] I noted three earlier uses of this “Letter to a climate skeptic” format: Kaulbars, 2010; Morrison, 2011; and Newman, 2016.