How much CO2 is too much?

How much CO2, is to much for our atmosphere to handle? What CO2 concentration should we aim for? There are very important questions that need to be answered.

According to a recent paper by Hansen et al. there is already to much CO2 in the atmosphere. Their paper states that a longterm goal for CO2 concentration should be 350ppm, unfortunately we are already at 385ppm and rising by 2ppm per year. It is also 100ppm lower than the 450ppm value that many other climate scientists have more or less agreed upon.

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.”

However the situation is not as dire as it seems. While the authors argue that 350ppm is the necessary longterm goal, if we wish to avoid and ice-free planet we don’t have to get to this goal immediately. In other words if we stabilize at a higher CO2 concentration we can gradually reduce the CO2 concentration until we achieve 350ppm. However:

If the present overshoot of this target CO2 is not brief, there is a possibility of seeding irreversible catastrophic effects.

The reason for this is that even if we stop and stabilize all emissions today at 385ppm, at this level there is enough warming in the pipeline to eventually trigger climate tipping points (such as the meting of the Arctic sea ice, the thawing of the northern permafrost, etc…) that will cause unavoidable warming. In other words if we stay at 385ppm (not likely) or get near 450ppm (more likely but still difficult) and stay there for any length of time, CO2 concentration may unavoidably shoot up to 700 to 1000ppm, which certainly gets us an ice-free planet. This means that if we wish to avoid these tipping points we need to lower CO2 concentrations before we hit them. The bad news is we really don’t know how much time we have before these tipping points are hit.

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.

These last two points to make should be self-evident to most, but any post I make on the topic of climate change seems to bring out the wackos, so I thought I would address them explicitly. First is that Hansen et al. are advocating 350ppm from a purely ‘scientific’ perspective. They are advancing a position, that based on their analysis of the data in order to avoid results ‘X’ (in this case an ice-free planet) we need to accomplish ‘Y’ (in this case CO2 concentrations of 350ppm ). That is their position, and it is up to policy makers to weigh the costs of implementing policies to achieve ‘Y’ against the costs of result ‘X’.

The second point to make is that the Hansen et al. paper is not part of the scientific consensus. In other words, while scientists all agree in the IPCC conclusions that climate change is real and caused by our greenhouse gas emissions, this paper by Hansen et al. is beyond that very conservative consensus, and thus there is healthy debate about it in the scientific community. Expect to see response papers.

Achieving the CO2 concentration of 350ppm that Hansen et al. are calling for will likely take a monumental effort, but if they are right the costs of not doing so would be disastrous.

It is be possible to achieve 350pp, though not with the current leadership.

UPDATE: RealClimate has a detailed explanation of the Hasen paper, concluding that:

However, even with the (substantial) uncertainties in the calculations and underlying assumptions, the conclusion that the Earth System sensitivity is greater than the Charney sensitivity is probably robust. And that is a concern for any policy based on a stabilization scenario significantly above where we are now.

Also Hansen himself has written a non-technical paper on the concept of tipping points and why they are so important. It is a must read for anyone confused on this issue.

4 thoughts on “How much CO2 is too much?

Add yours

  1. Essentially I read similar comments from Joseph Romm recently in the GristMill. I have heard the 450ppmv CO2 figure before, but no one had bother to mention that it comes from some research on Cenozoic CO2 levels associated with the glaciation events of that Era. Do you know of any papers, or sources that give a layman’s overview of the reasons behind that glaciation transitions; the reversal around 25mya to an ice free planet and then the re-glaciation around 15 mya? How solid is the evidence for 450ppmv of CO2 being the tipping point for the middle event back to an ice free planet?? Or is this 450ppmv of atmospheric CO2 just a SWAG, a scientifically wild-assed guess? Global Warming Art has a lovely chart of the temperature changes in the Cenozoic, but I wish they also had one for the CO2 transitions as well. Romm is hoping we can muster the global political will to stop the rise of CO2 at 450ppmv and then bring it back down towards 350ppmv in a 100 years or so. Now that’s a serious WAG about the political will and a SWAG about whether or not that time frame will stop all the positive feedback loops from exploding the atmospheric CO2 way beyond the 450 mark, guaranteeing an ice free planet yet again. Romm quoted Hansen, “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.”

    Playing around with a chart of the last Ice Age from Lonny Thompson’s lecture at the AAAS annual gathering in 2007, it looks like it took roughly 7,000 years for the transition from the bottom of last Ice Age to the beginning of the current Inter Glacial Warm period. That change equated to an average rise of about 6 degrees Centigrade globally. In the last 150 years, we’ve managed to induce a 1 degree rise and it’s estimated that we could add another 2 degrees over the next 100 years or 3 degrees in 250 years versus 6 degrees in 7,000 years. That’s roughly like comparing a speeding train to walking. We are in totally uncharted territory. We have been unconsciously imposing a massive climate experiment on the planet, and as Lovelock has indicated Gaia may get her revenge on us for messing around with her so thoughtlessly.

    A lot of consciousness raising to be done and not much time to do it in…


  2. How solid is the evidence for 450ppmv of CO2 being the tipping point for the middle event back to an ice free planet?? Or is this 450ppmv of atmospheric CO2 just a SWAG, a scientifically wild-assed guess?

    From what I understand the evidence for 450ppm (or 350ppm) for that matter is better than a SWAG (great term BTW) but not without controversy. As usual scientists need to continue to study the issue and refine their conclusions.

    Oh you might also be interested in

  3. Thanks Dan for the tip on Hansen’s other paper about Tipping Points. That paper clarified the scary fact that GHG in prehistory were always a Feedback mechanism, lagging Temperature Forcing by several hundred years. Now thanks to us, GHG are the Forcing mechanisms and Temperature is a Positive Feedback mechanism. Hansen’s discussion on the huge inertia of the Oceans and Ice Sheets was also helpful to my understanding but it seems these factors are way beyond predictable quantitative analysis. There are so many overlapping mechanisms and feedback systems that no one has a real clue as to when serious, irreversible events will happen, or what those events even are and what events will cascade inexorably after those tipping points. Reading Krupp’s Earth: The Sequel, I can almost begin to feel we can change this foreboding calamity around, but in reality, we are now dumping 8 gigatons of CO2 each year into the atmosphere and that is growing, not shrinking. And last year the rate of atmospheric build up of CO2 jumped to 2.4ppm/yr. up from 2ppm/yr. For all we really know about tipping points, we may have already passed them and all of this environmental call to arms, may just be a really sad last stand against the inevitable.

    McKibben postulated as much on KQED recently. He said we are really in the Hail Mary phase of this game, but we just don’t realize that yet. When I saw his City Arts and Lecture seminar on, he looked like a beaten down old man. Hansen may be right with his latest “350” figure, but sorry, it is just a SWAG. I have a hard enough time trying to convince people to look past their daily problems and see that Climate Change is real. It is so gigantic and so nebulous that few people have the energy and ability to think beyond the almighty NOW to see it. I don’t have the technical or mathematical or dramatic skills necessary to convince people that 350 is a meaningful number. Aiming for 350 would certainly give the rest of Life on the planet and “Civilization” whatever that means a better chance of surviving our hubris and ignorance, but 450 is already virtually scientifically impossible to prevent even if we had the political will, (assuming Hansen is right and 2C is already in the pipeline). Essentially our slowly dawning conscious selves are realizing that our grubby greedy selfish selves are shitting all over our House; Raping the Planet. And so far we have maybe 10% Consciousness vs. 90% Selfishness. Global Warming is not the Nazis; it is not the Atomic Bomb (it is actually much much worse). It has no immediacy to threaten people. How do we go to War against a Phantom?

  4. Hansen may be right with his latest “350” figure, but sorry, it is just a SWAG.

    I think our disagreement on this comes down to our definition of SWAG. I am fully aware that not all of Hansen’s colleges agree with his 350ppm figure, but that being said it is more than a guess. I wouldn’t call that a SWAG, but perhaps it fits under your definition of SWAG. Either way it doesn’t really mater.

    As for your other points, I am pretty much in full agreement with you.

Leave a Reply

Proudly powered by WordPress | Theme: Baskerville 2 by Anders Noren.

Up ↑