Three years ago, Hansen and others argued in Science that (due to fast feedbacks) we would warm another "0.6 degrees 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 degrees C." 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 ppm, which is a function of not only 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 my blog posts.

On the other hand, the authors make it clear that reducing concentrations is not easy, even if we do not cross key 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 correct in stating 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 negative net 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:

Present policies, with continued construction of coal-fired power plants without CO2 capture, suggest that decision-makers do not appreciate the gravity of the situation. [Note to Hansen et al.: That is the understatement of the year.] We must begin to move now toward the era beyond fossil fuels. Continued growth of greenhouse gas emissions, for just another decade, practically eliminates the possibility of near-term return of atmospheric composition beneath the tipping level for catastrophic effects.

The most difficult task, phase-out over the next 20-25 years of coal use that does not capture CO2, is herculean, yet feasible when compared with the efforts that went into World War II. The stakes, for all life on the planet, surpass those of any previous crisis. The greatest danger is continued ignorance and denial, which could make tragic consequences unavoidable.

Okay, so we should have listened to Hansen two decades ago. The time to act is yesterday. He has been right longer than anyone I know.

One final point. Some pretty smart people think Hansen is wrong about the long-term climate sensitivity issue (start here). If I am reading that criticism correctly then I think Hansen responds to it in his new paper.

Also, if I am reading Hansen et al. correctly (and Lord knows I may not be), then I think he may be mostly right for a different reason than he thinks, which is to say, I think the carbon-cycle feedbacks (including the tundra melting and sink saturation) act as the equivalent of the amplifiers that he models ("loss of Greenland and Antarctic ice and spread of vegetation over the vast high-latitude land area in the Northern Hemisphere" -- I will come back to that vegetation issue in a future post). In other words, if you get near 450 ppm and stay there for any length of time, you will shoot up to 700 to 1000 ppm, which certainly gets you an ice-free planet. Or perhaps the simplest way to put this: the IPCC is right when it says this:

Climate-carbon cycle coupling is expected to add carbon dioxide to the atmosphere as the climate system warms, but the magnitude of this feedback is uncertain. This increases the uncertainty in the trajectory of carbon dioxide emissions required to achieve a particular stabilisation level of atmospheric carbon dioxide concentration. Based on current understanding of climate carbon cycle feedback, model studies suggest that to stabilise at 450 ppm carbon dioxide could require that cumulative emissions over the 21st century be reduced from an average of approximately 670 [630 to 710] GtC to approximately 490 [375 to 600] GtC. Similarly, to stabilise at 1000 ppm this feedback could require that cumulative emissions be reduced from a model average of approximately 1415 [1340 to 1490] GtC to approximately 1100 [980 to 1250] GtC.

We're at 8 GtC/yr and rising 3 percent annually. We need to average below 5 GtC/yr -- and maybe considerably less -- for the whole century to avert catastrophe. We need to be near or below zero by 2100.

My bottom line: Let's start working now toward stabilizing below 450 ppm while climate scientists figure out if we ultimately need to get below 350.