Parting company with McKibben and, maybe, Hansen:
The nation’s top climate scientist, NASA’s James Hansen, apparently now believes “the safe upper limit for atmospheric CO2 is no more than 350 ppm,” according to an op-ed by the the great environmental writer Bill McKibben. Yet while preindustrial levels were 280, we’re now already at more than 380 and rising 2 ppm a year!
Like many people, in the 1990s I believed 550 was the target needed to avoid climate catastrophe — but now it’s clear that
1. 550 ppm would lead to the greatest disaster ever experienced by human civilization — returning us to temperatures last seen when sea levels were some 80 feet higher. This is especially true because….
2. Long before we hit 550, major carbon cycle feedbacks — the loss of carbon from the tundra and the Amazon, the saturation of the ocean sink (already beginning) would almost certainly kick in to high gear, inevitably pushing us to much, much higher CO2 levels (see here and here and my book).
Exactly when those feedbacks seriously kick in is the rub. No one knows for sure, but based on my review of the literature and interviews of leading climate scientists, somewhere between 400 and 500 ppm seems most likely. It could be lower, but it probably couldn’t be much higher.
…
But at least we’re homing in on the right number. Three hundred and fifty is the number every person needs to know.I part company with him here. I haven’t talked to Hansen yet and I’ll reserve further judgment until I see a paper or PPT by him.
Since beating 450 ppm is doable and certainly necessary — that’s where I draw the line. One advantage of pursuing 450 is that if we do get some sort of unexpected breakthrough — a cheap and practical way to draw CO2 out of the air (that doesn’t use a lot of land, water, or energy) and stick it someplace permanent — then we would have a system in place to deploy it fast enough to perhaps get to below 400 ppm. And even if turns out 450 doesn’t avert catastrophe, it will surely slow down the impacts enough to make adaptation more viable.
The author of the above text is Joe Romm, writing at Climate Progress. I have a lot of respect for Joe and his work (as in the countless times I’ve referred to and plugged his book The Hype About Hydrogen), but this time around I think I have to part company with him.
Please don’t misunderstand my position: I’m most definitely not saying that I think the 350 ppm CO2 number is correct and 450 is wrong, merely that there’s a very strong possibility that the actual number is much closer to 350 than it is to 450. (And for the record, we’re currently at approximately 382 ppm.)
My point, as I’ve said here before (and on Joe’s blog just minutes ago) is that the CO2 we’ve been pumping into the atmosphere for over 100 years has been far ahead of the science all along. The climate scientists are doing a heroic job of closing that coceptual gap, but considering the terrifying surprises we keep seeing (many documented in Joe’s current book, Hell and High Water), it’s clear we have a distinctly imperfect understanding of what the hell’s going on out there in the real world.
Specifically, the problem is positive feedback loops. As Joe points out, there are several potentially game changing feedbacks in nature, such as the possibility of massive CO2 and/or methane releases from melting tundra and no-longer-permafrost and the thermal stratification of the oceans, which would significantly reduce their ability to absorb CO2 from the atmosphere. I’m not even sure it’s fair to say that scientists have a reasonably complete and accurate view of how the Greenland and Antarctic ice sheets respond to a given level of warming, another source of scary surprises, as Joe points out in H&HW.
As long as we’re still discovering new feedbacks or further quantifying known feedbacks (both sources of nasty surprises), the only prudent thing we can assume is that the conventional wisdom–the conclusion that the magic number is 450 ppm–is wrong. It’s entirely possible that we’ll find some negative feedbacks that mitigate the situation, beginning at, perhaps, 400 or 420 ppm. Or maybe we’ll find that the effects of the warming that does occur won’t be as bad as predicted. But I’ve seen no serious discussions of our catching that kind of break.
Until we know more, my view of this will be: The number the scientists feel confident in, today, is 450, but it’s very likely lower than that, possibly much lower.
One last thing on Joe’s post above: He says, “Since beating 450 ppm is doable and certainly necessary — that’s where I draw the line.” That sounds eerily reminiscent of the scene in Apollo 13 where a frustrated engineer says, “You’re telling me what you need, and I’m telling you what you’ve got.” In other words, “doable” has zip to do with what the real number is, and it shouldn’t even be brought into the discussion. Reality doesn’t give a flying fig about what is or isn’t within our capabilities, and the less convenient we think the truth is the more urgently we should be working to determine the number.
By the way, I plan to write a review of H&HW as soon as I finish it. So far, it’s shaping up to be a must-read title for anyone interested in e+e issues.
You must be logged in to post a comment.
December 30th, 2007 at 1:59 pm
I can’t agree with a lot of what is discussed in this thread. I wonder, in today’s world, if 450 IS doable. I think it is going to take a lot of people agreeing for this to be doable, and the contribution of the US toward reduction will not begin until at least late Jan of 2009, so just add a couple of more points to the present # of 382 or whatever. I have absolutely no professional training in this, but I do think from my reading that we are past some of the tipping points, possibly only in some specific areas, but I do think that some of the stories on methane releases in the Alaskan lake, last summer, and the thawing of the Siberian tundra before that, and the thawing of the Alaskan Tundra are indicators that we will see substantial contributions to CC from sources over which nobody has any control. Hopefully, we will see the absence of J. Inhofe from the Senate floor and have someone of serious moral fiber in the White House by 2/2009 and get a start on the future. Even then, the Chinese and their economic growth, the same for India, and the other third world growth will continue beyond our efforts even if a voice of reason is heard (hopefully loudly) from Washington, DC.
Hence, I do not think 450 will be doable.
As to sequestration of atmospheric CO2, the stuff that is really causing the problems is extremely high in the atmosphere and is not something we can reach from the ground. Hence, capturing and liquefying it and then storing it, as best we can, is highly unlikely. Water vapor on the other hand, cycles throught the lower atmosphere, and if it were not for the energy cost of capturing it, we already know how. It is just a matter of having the will to do it. And, unlike Apollo 13, we do not have people who would be willing to cooperate in the task. Even if we start to do something about water vapor, we still have to do what we can to control CO2, so it is not the sole answer.
First, however, we have to deal with the energy crisis anyway which will complicate our ability to do any of this. That is what I am most worried about for my generation’s heirs to the earth.
January 1st, 2008 at 4:15 pm
I agree very much with your approach, that we should look at what the facts are telling us rather than adopting beliefs just because they would be more politically convenient and effective. In the past I have been somewhat critical of Hansen with his claim that the tipping point is ten years in the future, because that just seemed too politically expedient - close enough to be motivating, but distant enough to give us time to do something. However now with Hansen in effect saying that the tipping point was ten years (or more) in the past, I have to salute him for speaking the truth even if it is harder to spin politically. Of course in theory Hansen’s position should motivate us to work even harder, knowing that we are already in deep trouble and it is only getting worse, but in practice, human psychology makes it harder to motivate people with that kind of warning than with Romm’s more politically calculated message.
January 29th, 2008 at 11:43 am
[…] The “magic ppm number issue” is one I’ve talked about before–just last month, in fact. Then I quoted an article in which James Hansen says that the magic concentration level at which global warming turns into an all-out disaster is not 550 ppm of CO2 or even the more recently quoted 450, but 350, which is less than the current level of about 383. […]