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.
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.