Commonly 2°C has been seen as the threshold for dangerous climate change, although last year the IPCC report on 1.5°C revealed that at that lower level we enter a zone where tipping points may take us to 4°C and beyond.
Levels of 4°C threaten civilisation as we know it. At 6°C we worry about the survival of the human race. However, at that point Tapio Schneider and his team at the California Institute of Technology, Pasadena, have found that there may be a further tipping point which heats the planet another 8°C to make it 14°C above pre-industrial levels (New Scientist, so probably pay-walled.)
How is this so?
- Schneider’s team modelled stratocumulus clouds over subtropical oceans, which cover around 7 per cent of Earth’s surface and cool the planet by reflecting the sun’s heat back into space. They found there was a sudden transition when CO2 levels reached around 1200 parts per million (ppm) — the stratocumulus clouds broke up and disappeared.
The loss of the reflective effect of the clouds would lead to runaway warming which would start as CO2 levels reached 1200. If we do nothing to curb emissions, CO2 levels would could expect to pass 1200 ppm some decades after 2100.
Other scientists not involved in the research have said the findings make sense, but no-one is too worried about it as they assume that we will in fact curb emissions.
If they looked at the issue of tipping points cutting in from 1.5°C, a point where the whole scenario is taken out of our hands, they might worry a bit more.
The researchers believe that dissipating clouds could explain the extreme warming event of the Paleocene–Eocene Thermal Maximum (PETM) 55m years ago, when global temperatures increased by 5-8°C and drove widespread extinction of species on both the oceans and land. CO2 levels then were not high enough to produce the warming that actually happened, so the dissipation of clouds may indeed have played a pert.
While there were extinctions generally, there was an increase in mammalian abundance, including primates during the PETM. However, we are putting CO2 into the atmosphere somewhere between 20 and 120 times as fast as happened in the PETM, which makes evolutionary human adaptation extremely unlikely if we reach the tipping point this study indicates.
Moreover, in the post CO2 is scrambling our brains, but will it kill us all? we found that it would be extreme folly on health grounds alone to allow CO2 levels to exceed 600 ppm. As a species we have flourished with CO2 levels mostly at 180 to 280 ppm.
I would also caution that the continents and oceans are differently disposed on the surface of the earth during the PETM 55 mya. 14°C warming would indeed be cataclysmic in our world.
In that post, I included a graph from October 2015, when the participating countries had put in their intended actions prior to the Paris Agreement:
It’s not clear which path we are taking, except it is not the green line.
Here again is our current record from the Australian BOM State of the Climate 2018 report:
There is a longer article on the paper at Carbon Brief.
The article Possible climate transitions from breakup of stratocumulus decks under greenhouse warming can be purchased at Nature Geoscience.