For some time now the world’s emissions-reduction strategies have been guided by the belief that global temperatures must not be allowed to exceed two degrees C above the pre-industrial mean (or slightly more than 1˚C above current temperatures) if we are to avoid “dangerous interference with the Earth’s climate system”.
Politicians are in general agreement that limiting warming to less than 2˚C is necessary to avoid redlining the Earth’s climate:
“The world (must) keep the global temperature increase from climate change below two degrees.” Ed Davey.
“Our joint goal (is) limiting global warming to under two degrees celsius.” German Environment Minister Barbara Hendricks.
“The U.S. continues to support the 2˚C goal.” Todd Stern, America’s top climate diplomat
And also that bad things are likely to happen if the climate does redline:
“experiencing global warming of as much as three or four degrees (could) lead to catastrophe, if not war.” François Hollande.
“A rise of 4˚C would be enough to wipe out hundreds of species, bring extreme food and water shortages in vulnerable countries and cause catastrophic floods that would displace hundreds of millions of people.” Ban Ki-moon
Clearly a lot depends on whether the world can keep global mean temperatures below the 2˚C “danger threshold”, assuming that 2˚C really is the danger threshold. Which raises the question, how good is the 2˚C estimate? Given the trillions of dollars that have already been spent trying to stay below it one imagines that it must be very good – a product of exhaustive research and the application of hard scientific principles.
But is it?
Unfortunately, it isn’t.
The 2˚C threshold originated with a study published by the German Advisory Council on Global Change for the guidance of the delegates who attended the UN Framework Convention on Climate Change in Berlin in 1995, and it hasn’t been changed since. The study itself, however, can only charitably be described as scientific. It begins with two questionable assumptions:
- The late Quaternary period “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.”
Proceeds to a questionable conclusion:
- The “tolerable temperature window” is therefore 10.4˚C to 16.1˚C. (The authors then widened the window by 0.5˚C in both directions to be “generous”, raising the upper limit to 16.6˚C, 1.3˚C above the 15.3˚C global mean in 1995 and ~2˚C above the pre-industrial global mean).
And backs up its results up with no data:
- No evidence is presented to support the claim that 1.3˚C more warming will cause “dramatic changes in the composition and function of today’s ecosystems”.
- No evidence is presented to support the claim that the “Quaternary range of fluctuation” represents the “tolerable temperature window”.
- The source of the 10.4˚C mean minimum and 16.1˚C mean maximum Quaternary temperature estimates is not given.
- The justification for expressing Quaternary global mean temperatures to the nearest tenth of a degree is not discussed. (Realistically we would be lucky to get within a couple of degrees.)
By defining 9.9˚C-16.6˚C as the “tolerable temperature window”the study also implies that while the Earth and its ecosystems can’t tolerate more than 1.3˚C of additional warming they can weather an unusually frigid Ice Age without serious difficulty.
One would think that a study that reached conclusions like this without providing any backup data would have come under scrutiny, but the science, or lack of it, behind its 2˚C threshold estimate was never seriously questioned (although there was a lot of later wrangling over the amplitude, with some countries that felt particularly threatened, such as the Pacific Island nations, demanding 1.5˚C to be on the safe side). But after being formally adopted by the European Council in 1996 the rest of the world fell gradually into line, and the 2˚C threshold finally became the world’s official target at the Cancun Conference in 2010.
The study that gave birth to it, however, was published almost twenty years ago. What advances in scientific understanding of the “danger threshold” have there been since then? Well, science has concluded that the danger threshold is unquantifiable; you can’t put a number on it. Where you put it depends on how you define “dangerous” and how you assess risk, and risk assessments are sensitive to small changes. The “burning embers” diagrams of Smith et al 2011, for example, show risk levels increasing significantly between the 2001 IPCC Third Assessment Report and the 2007 Fourth Assessment report simply because the IPCC was more confident in 2007 than it was in 2001 that a risk existed (up from “more likely than not” to “likely” etc):
Smith et al. “burning embers” diagrams
And because risk assessments of this type are purely subjective (my purely subjective risk assessments are much less alarming than Smith et al’s, incidentally) they don’t give us any hard numbers, and even if they did these numbers wouldn’t be used because UNFCCC negotiators are already locked into 2˚C. As RealClimate puts it: If one wanted to sabotage the chances for a meaningful agreement in Paris next year, towards which the negotiations have been ongoing for several years, there’d hardly be a better way than restarting a debate about the finally-agreed foundation once again, namely the global long-term goal of limiting warming to at most 2°C.
Personally I think it would be an excellent idea to sabotage the Paris negotiations and start afresh with a global emissions-reduction plan that had some chance of working, but that isn’t going to happen. The politicians and policymakers are going to press on regardless, meaning that yet more trillions will be spent trying to stay under a 2˚C “danger” threshold that dates from a 20-year old study of highly dubious scientific merit and which for all we know may not be anywhere close to the true danger threshold. Hopefully the world’s emissions-cutters will at some point come to their senses and realize this, but don’t hold your breath.