Here is the draft of the long-awaited defense of why we need an ultimate target of 350 ppm for atmospheric carbon dioxide, by NASA’s James Hansen et al., “Target Atmospheric CO2: Where Should Humanity Aim?” [Yes, they know we're already at 385 ppm and rising 2 ppm a year.]
The paper does suffer from one inherent analytical weakness that makes it (a tad) less dire than it appears — and some people believe the core element of this analysis is wrong (see very end of post), although I don’t.
This paper is really just a continuation of Hansen’s earlier analysis arguing that the real-world or long-term climate sensitivity of the planet to doubled CO2 [550 ppm] is 6°C — twice the short-term or fast-feedback-only climate sensitivity used by the IPCC. [You might want to read this post first since it is a bit clearer on the difference between the two sensitivities.]
The key paleoclimate finding of the article:
We infer from the Cenozoic data that CO2 was the dominant Cenozoic forcing, that CO2 was only ~450 ppm when Antarctica glaciated, and that glaciation is reversible.
That is, if we stabilize at 450 ppm (or higher) we risk returning the planet to conditions when it was largely ice free, when sea levels were higher by 70 meters — more than 200 feet!
Three years ago, Hansen (and others) argued in Science that [due to fast feedbacks], we would warm another “0.6°C without further change of atmospheric composition” [i.e. with no more CO2 emissions]. Now he’s saying “Warming ‘in the pipeline’, most due to slow feedbacks, is now about 2°C.” So the paper concludes:
An initial 350 ppm CO2 target may be achievable by phasing out coal use except where CO2 is captured and adopting agricultural and forestry practices that sequester carbon. If the present overshoot of this target CO2 is not brief, there is a possibility of seeding irreversible catastrophic effects.
The inherent weakness of the paper from a policy perspective is that even if you accept their analysis (which many will not), the authors do not know how long we can overshoot 350, which is a function of not just the duration of the overshoot, but the magnitude (i.e. how high concentrations go). They note: “The time needed for slow feedbacks to ‘kick in’ is uncertain. Current models are inadequate and no paleoclimate analogue to the rapid human-made GHG increase exists.” We are truly running a first-of-a-kind experiment on the climate.
The authors write “paleoclimate and ongoing changes, and the ocean response time, suggest that it would be foolhardy to allow CO2 to stay in the dangerous zone for centuries.” Well, of course, but centuries is a long time. The authors argue:
Humanity’s task of moderating human-caused global climate change is urgent. Ocean and ice sheet inertias provide a buffer delaying full response by centuries, but there is a danger that human-made forcings could drive the climate system beyond tipping points such that change proceeds out of our control.
That, of course, is a central point of this blog.
On the other hand, the authors make clear that reducing concentrations is not easy even if we do not key cross carbon cycle feedback tipping points. Moreover, recent analysis suggests that “if emissions were eliminated entirely, radiative forcing from atmospheric CO2 would decrease at a rate closely matched by declining ocean heat uptake, with the result that while future warming commitment may be negligible, atmospheric temperatures may not decrease appreciably for at least 500 years.”
So I suspect the authors are right that 450 ppm is too high if maintained for even a few centuries. On the other hand, realistically, 350 ppm is simply not going to be seen again this century. The authors write:
This target [350 ppm] must be pursued on a timescale of decades, as paleoclimate and ongoing changes, and the ocean response time, suggest that it would be foolhardy to allow CO2 to stay in the dangerous zone for centuries.
The ill-defined difference between decades and centuries is key. What if we could keep the peak below 450 ppm, and start concentrations declining by 2100, which would almost certainly require near-zero if not net-negative global emissions, and then get back to near 350 ppm by, say 2150 and then even lower by 2200? Would that be good enough? As I argued in my book, I believe that with a World War II scale effort for the next few decades, we could stay below 450. My take away from this paper is that we would need to keep up that level of effort through 2100 — to get back below current levels.
The final point of the paper deserves reprinting: