The 2 degree limit

How many times have you heard about the two degree limit?  The Paris Accord was all about the two degree limit.

When researching another post I wondered what temperature was represented by this two degree limit and was surprised to find that the answer is not easily obtained.

The two-degree limit is “above pre-industrial” because back then before the evil capitalists started making things much cheaper to buy for the masses and making transportation much more affordable and generally improving the lives of everybody, the climate was perfect. Absolutely peachy, something nobody in their right mind would ever want to adjust by letting loose the evil demon of climate change.

But the climate change genie, a.k.a. global warming is out of the bottle and cannot be lured back in unless considerable sums of money are paid to the climate gods and considerable hardship is visited upon the developed nations of the world.

Only then can we have the nirvana of a pre-industrial world and its attendant perfect climate.

Back to the question, then. What is this perfect pre-industrial temperature?

Not quite a year ago, Ed Hawkins, Pablo Ortega, and Emma Suckling published a paper Estimating Changes in Global Temperature since the Preindustrial Period.  This seemed a good place to start. Quote.

The United Nations Framework Convention on Climate Change (UNFCCC) process agreed in Paris to limit global surface temperature rise to “well below 2°C above pre-industrial levels.” But what period is pre-industrial? Somewhat remarkably, this is not defined within the UNFCCC’s many agreements and protocols. Nor is it defined in the IPCC’s Fifth Assessment Report (AR5) in the evaluation of when particular temperature levels might be reached because no robust definition of the period exists. […] End of quote.

Whoa, whoa, whoa! Back up the truck!

We are beating ourselves up to ensure that we do not exceed a temperature limit of two degrees above pre-industrial but no one knows exactly when “pre-industrial” was and therefore no knows exactly what the temperature was at that particular time and therefore no one knows exactly how long we have got until we reach the two degrees tipping point?

This also means that no one knows who should be changing their ways to stave off disaster and no one knows why anyone should be changing their ways. Quote.

Ideally, a preindustrial period should represent the mean climate state just before human activities started to demonstrably change the climate through combustion of fossil fuels. […] End of quote.

As far as I know, this assumption has still not been demonstrably proven. Quote.

In the absence of a formal definition for preindustrial, the IPCC AR5 made a pragmatic choice to reference global temperature to the mean of 1850–1900 […] End of quote.

As with many things in climate ‘science’, someone made a SWAG (scientific wild-arsed guess) at a date. Quote.

However, some anthropogenic warming is estimated to have already occurred by 1850 (Hegerl et al. 2007; Schurer et al. 2013; Abram et al. 2016) as greenhouse gas concentrations had started increasing around a century earlier. On the other hand, the 1880s and 1890s were cooler than the preceding decades because of the radiative impact of aerosols from several volcanic eruptions, which may have compensated for the earlier anthropogenic influence. It is therefore plausible that a “true” preindustrial temperature could be warmer or cooler than 1850–1900, depending on the balance of these two factors. […] End of quote.

In other words, we have not got the faintest idea! But, this is a scholarly paper, so we will press on …

The conclusions of the paper include Quote.

We have assumed in the motivation for this discussion and choice of reference periods that the UNFCCC agreements on temperature limits refer to anthropogenic increases only, but this is not explicitly stated. We have not attempted to attribute the observed increase in global temperatures; nonanthropogenic factors (including internal variability) may have either offset or contributed to the warming. We have attempted to minimize issues of varying natural forcing and internal variability, but this effect cannot be removed entirely. End of quote.

Nope, we still don’t have a clue.

So, if you are at all concerned about the two degrees issue, I suggest you ask here for guidance, they are more likely to understand their subject than the IPCC.


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WH is a pale, stale, male who does not believe all the doom and gloom climate nonsense so enjoys generating CO2 that the plants need to grow by driving his MG.

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