Thanks. The results of that article cannot be applied directly to the situation we are in, because the initial temperature of their aqua-planet is 6 ºC higher than today’s mean global temperature. From note (6.93) of What We Owe to the Future (see here):
Hansen et al. 2013, 17. Popp et al. (2016) [the studied you linked to just above] found that if carbon dioxide concentrations reached 1,520 parts per million, a simulated planet would transition to a moist greenhouse state. If we burned all of the fossil fuels, then carbon dioxide concentrations would reach 1,600 parts per million (Lord et al. 2016, Figure 2).
However, the simulated planet’s initial climate was six degrees warmer than today’s Earth. This means that Earth would require a carbon dioxide concentration significantly higher than on the simulated planet to transition to a moist greenhouse.
Indeed, from the Discussion of the article you mention:
A recent study using the same model but in a different version found that the Earth’s climate remains stable for CO2 concentrations of at least 4,480 p.p.m. (ref. 17), whereas our study suggests that such concentrations would lead to a climate transition. Studies of Earth with other GCMs [global climate models] also found the climate to remain stable for higher CO2 concentrations than we do.
However, the initial climate of our aqua-planet is ~6K warmer than the one of present-day Earth.
...
If we account for the difference in the initial climates, the results of the two studies are not in contradiction. Indeed, the climate of the model version used in ref. 17 was recently shown to become unstable when the CO2 concentrations were increased from 4,480 to 8,960 p.p.m.
These concentrations of 4,480 and 8,960 p.p.m are 16.0 (=4480/280) and 32.0 (=8960/280) times the pre-industrial concentration, which suggests the existential CO2 concentration is 22.6 (= (16.0*32.0)^0.5) times as high as the pre-industrial one. Given the warming until now relative to pre-industrial levels of 1.04 ºC, and the current concentration of CO2 is 1.48 (= 414/280) times the pre-industrial one, it seems reasonable to expect the existential warming relative to the pre-industrial temperature is about 20 ºC (22.6/1.48*1.04 = 15.9), not 4 ºC.
The relation between warming and CO2 is exponential, s we need to count the number of doublings of CO2. Every doubling gives a constant increase of the temperature. Assuming that each doubling gives 2C and 22= 2exp4.5, we get around 9C above preindustrial level before we reach tipping point.
In the article the tipping point is above 4C (in the chart) plus 6C from warmer world = 10C, which gives us approximately the same result as I calculated above.
“Transition to a Moist Greenhouse with CO2 and solar forcing” https://www.nature.com/articles/ncomms10627
Thanks. The results of that article cannot be applied directly to the situation we are in, because the initial temperature of their aqua-planet is 6 ºC higher than today’s mean global temperature. From note (6.93) of What We Owe to the Future (see here):
Indeed, from the Discussion of the article you mention:
These concentrations of 4,480 and 8,960 p.p.m are 16.0 (=4480/280) and 32.0 (=8960/280) times the pre-industrial concentration, which suggests the existential CO2 concentration is 22.6 (= (16.0*32.0)^0.5) times as high as the pre-industrial one. Given the warming until now relative to pre-industrial levels of 1.04 ºC, and the current concentration of CO2 is 1.48 (= 414/280) times the pre-industrial one, it seems reasonable to expect the existential warming relative to the pre-industrial temperature is about 20 ºC (22.6/1.48*1.04 = 15.9), not 4 ºC.
The relation between warming and CO2 is exponential, s we need to count the number of doublings of CO2. Every doubling gives a constant increase of the temperature. Assuming that each doubling gives 2C and 22= 2exp4.5, we get around 9C above preindustrial level before we reach tipping point.
In the article the tipping point is above 4C (in the chart) plus 6C from warmer world = 10C, which gives us approximately the same result as I calculated above.