The dubious 2°C limit

No comment on the Paris (and newest Bonn) climate meetings are written without raising the warning finger that everything should be done to limit global warming at +°2C “above pre-industrial levels“. Sebastian Lüning and Fritz Vahrenholt have published a new paper in “Frontiers in Earth Science” that tries to find the origin of the number, and how scientifically correct is the baseline yielding this maximum permitted warming.

This paper is very readable; they are not the first to point to the astounding vagueness of the foundations of this internationally agreed limit, but their paper has the big merit of being short and easy to understand. I will discuss just some major findings, and suggest a careful lecture of the full paper.

  1. The sea surface temperatures (SST) of the last 10000 years.

The last 10k years correspond to the ongoing interglacial period. The (annotated) figure shows how the SST changed during that period:

If we take as a baseline the average SST temperature of the last 10k years (up to 1850, the year that is commonly taken as the start of the industrialization), we see that the current global temperature is just +0.4°C higher. The blue box shows that the baseline period for the Paris aim is about 0.35° lower that this average (which corresponds to a period with only natural climate changes).

Taking the Holocene average SST as a baseline, +2°C would allow a supplementary warming (compared to today) by about 1.6 °C !

2. The land + sea global temperatures of the last 2000 years.

The next figure uses both land and sea temperature reconstructions:

It is obvious that taking 1850 as a baseline is extremely dishonest, even silly: the end of the Litte Ice Age corresponds to the coldest period during the last 10000 years. How could any intelligent person choose a baseline that corresponds to an extreme value, and scientifically speaking is at least 0.6°C too low?

3. Conclusions

1. The Paris convention suggests that the pre-industrial climate was more or less constant, and that it is mostly human activity which caused the observed warming since 1850. This is blatantly false, as during the Holocene the climate was highly variable, and has reached or even exceeded the actual global temperatures.
The following pictures shows how the Greenland temperature changed during that period:


2. A big chunk of the observed +0.8°C warming (w.r. to the 1850-1900 period) is of natural origin.
3. Using a 2°C target could be valid, but choosing and imposing the coldest period of the last 10000 years as a baseline is silly, dishonest and unscientific.




  • added Greenland graph (16Dec2017, 15:26 UTC

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