From climate point of view, we need to estimate not only the warming, but also the speed of warming, as higher speed gives high concentration of methane (and this differential equation has exponential solution). Anthropogenic global warming is special as it has very high speed of CO2 emission never happened before. We also have highest ever accumulation of methane hydrates. We could be past tipping point but do not know it yet, as exponential growth is slow in the beginning.
From SIA counteragrument follows that anthropic shadow can’t be very strong: we are unlikely to observe the world with a very strong anthropic shadow. However, some anthropic effects on climate likely to exist as we observe the preservation of habitability of the Earth despite changes а Sun luminosity. This gives us some range of values there anthropic shadow can be, and 0.1 per cent seems to be a reasonable estimate inside it. Though exact number or range is difficult to estimate. May be Sandberg’s work on near-misses in nuclear war would help—when we will have a chance to see it.
I feel that I didn’t answer the whole your question, so can you point what exactly is your point of disagreement.
The maximum temperature which would be achieved if we reached net zero today (T1).
The 2nd of these is higher, so the lower bound for the existential additional warming is smaller than the 26.0 ºC I estimated above (for an anthropic shadow larger than 50 %). I also understand T1 may be a function of not only T0, but also of the current composition of the atmosphere, and the rate at which it has been changing.
However, how large do you think is the difference between T0 and T1? If it is of the order of magnitude of the warming until now relative to pre-industrial levels of 1 ºC, there is still a margin of about 25.0 ºC (= 26.0 − 1) to the existential tipping point.
You mention that we may already have passed the existential tipping point, but that would imply a difference between T1 and T0 of more than 25.0 ºC, which seems very hard to believe.
I think that the difference between tipping point and existential temperature should be clarified. Tipping point is the temperature after which self-sustaining loop of positive feedback starts. In the moisture greenhouse paper it is estimated to be at +4C, after which the temperature jumps to +40C in a few years. If we take +4 C above preindustrial level, it will be 1-3 above current level.
Thanks for clarifying. I had understood that difference, but for me it is unclear from what you discuss here that the tipping point is only 4 ºC above pre-industrial temperature. Could you link to the specific paper you are referring to?
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.
From climate point of view, we need to estimate not only the warming, but also the speed of warming, as higher speed gives high concentration of methane (and this differential equation has exponential solution). Anthropogenic global warming is special as it has very high speed of CO2 emission never happened before. We also have highest ever accumulation of methane hydrates. We could be past tipping point but do not know it yet, as exponential growth is slow in the beginning.
From SIA counteragrument follows that anthropic shadow can’t be very strong: we are unlikely to observe the world with a very strong anthropic shadow. However, some anthropic effects on climate likely to exist as we observe the preservation of habitability of the Earth despite changes а Sun luminosity. This gives us some range of values there anthropic shadow can be, and 0.1 per cent seems to be a reasonable estimate inside it. Though exact number or range is difficult to estimate. May be Sandberg’s work on near-misses in nuclear war would help—when we will have a chance to see it.
I feel that I didn’t answer the whole your question, so can you point what exactly is your point of disagreement.
I agree there is a difference between:
The current temperature (T0).
The maximum temperature which would be achieved if we reached net zero today (T1).
The 2nd of these is higher, so the lower bound for the existential additional warming is smaller than the 26.0 ºC I estimated above (for an anthropic shadow larger than 50 %). I also understand T1 may be a function of not only T0, but also of the current composition of the atmosphere, and the rate at which it has been changing.
However, how large do you think is the difference between T0 and T1? If it is of the order of magnitude of the warming until now relative to pre-industrial levels of 1 ºC, there is still a margin of about 25.0 ºC (= 26.0 − 1) to the existential tipping point.
You mention that we may already have passed the existential tipping point, but that would imply a difference between T1 and T0 of more than 25.0 ºC, which seems very hard to believe.
I think that the difference between tipping point and existential temperature should be clarified. Tipping point is the temperature after which self-sustaining loop of positive feedback starts. In the moisture greenhouse paper it is estimated to be at +4C, after which the temperature jumps to +40C in a few years. If we take +4 C above preindustrial level, it will be 1-3 above current level.
Thanks for clarifying. I had understood that difference, but for me it is unclear from what you discuss here that the tipping point is only 4 ºC above pre-industrial temperature. Could you link to the specific paper you are referring to?
“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.