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Irreversible Does Not Mean Unstoppable

Filed under: — david @ 1 February 2009 - (Italian) (Finnish) (Chinese (simplified)) (Español)

Susan Solomon, ozone hole luminary and chair of the Nobel Prize winning IPCC, and her colleagues, have just published a paper entitled “Irreversible climate change because of carbon dioxide emissions” in the Proceedings of the National Academy of Sciences. We at realclimate have been getting a lot of calls from journalists about this paper, and some of them seem to have gone all doomsday on us. Dennis Avery and Fred Singer used the word Unstoppable as a battle flag a few years ago, over the argument that the observed warming is natural and therefore there is nothing that humanity can do to alter its course. So in terms of its intended rhetorical association, Unstoppable = Burn Baby Burn. But let’s not confuse Irreversible with Unstoppable. One means no turning back, while the other means no slowing down. They are very different words. Despair not!

Solomon et al point out that continued, unabated CO2 emissions to the atmosphere would have climatic consequences that would persist for a thousand years, which they define operationally as “forever”, as in the sense of “Irreversible”. It is not really news scientifically that atmospheric CO2 concentration stays higher than natural for thousands of years after emission of new CO2 to the carbon cycle from fossil fuels. The atmospheric CO2 concentration has a sharp peak toward the end of the fossil fuel era, then after humankind has gone carbon neutral (imagine!) the CO2 concentration starts to subside, quickly at first but after a few centuries settling in a “long tail” which persists for hundreds of thousands of years.

The long tail was first predicted by a carbon cycle model in 1992 by Walker and Kasting. My very first post on realclimate was called “How long will global warming last?”, all about the long tail. Here’s a review paper from Climatic Change of carbon cycle models in the literature, which all show the long tail. A number of us “long tailers” got together (electronically) to do a Long Tail Model Intercomparison Project, LTMIP, just like the big guys PMIP and OCMIP (preliminary results of LTMIP to be appearing soon in Annual Reviews of Earth and Planetary Sciences). I even wrote you guys a book on the topic.

The actual carbon-containing molecules from the fossil fuel spread out into the other carbon reservoirs in the fast parts of the carbon cycle, dissolving in the oceans and getting snapped up by photosynthetic land plants. The spreading of the carbon is analogous to water poured into one part of a lake, it quickly spreads out into the rest of the lake, rather than remaining in a pile where you poured it, and the lake level rises a bit everywhere. In the carbon cycle, translated out of this tortured analogy, the atmospheric carbon dioxide content rises along with the contents of the other carbon reservoirs.

Ultimately the airborne fraction of a CO2 release is determined largely by the buffer chemistry of the ocean, and you can get a pretty good answer with a simple calculation based on a well-mixed ocean, ignoring all the complicated stuff like temperature differences, circulation, and biology. The ocean decides that the airborne fraction of a CO2 release, after it spreads out into the other fast parts of the carbon cycle, will be in the neighborhood of 10-30%. The only long-term way to accelerate the CO2 drawdown in the long tail would be to actively remove CO2 from the air, which I personally believe will ultimately be necessary. But the buffering effect of the ocean would work against us here, releasing CO2 to compensate for our efforts.

As a result of the long tail, any climate impact from more CO2 in the air will be essentially irreversible. Then the question is, what are the climate impacts of CO2? It gets warmer, that’s pretty clear, and sea level rises. Sea level rise is a profound consequence of the long tail of global warming because the response in the past, over geologic time scales, is tens of meters per °C change in global mean temperature, about 100 times stronger than the IPCC forecast for 2100 (about 0.2 meters per °C). The third impact which gains immortality from the long tail is precipitation. Here the conventional story has been that climate models are not very consistent in the regional precipitation changes they predict in response to rising CO2. Apparently this is changing with the AR4 suite of model runs, as Solomon et al demonstrated in their Figure 3. Also, there is a consistent picture of drought impact with warming in some places, for example the American Southwest, both over the past few decades and in medieval time. The specifics of a global warming drought forecast are beginning to come into focus.

Perhaps the despair we heard in our interviewers’ questions arose from the observation in the paper that the temperature will continue to rise, even if CO2 emissions are stopped today. But you have to remember that the climate changes so far, both observed and committed to, are minor compared with the business-as-usual forecast for the end of the century. It’s further emissions we need to worry about. Climate change is like a ratchet, which we wind up by releasing CO2. Once we turn the crank, there’s no easy turning back to the natural climate. But we can still decide to stop turning the crank, and the sooner the better.

Walker JCG, Kasting JF. 1992. Effects of fuel and forest conservation on future levels of atmospheric carbon dioxide. Palaeogeogr. Palaeoclimatol. Palaeoecol. (Glob. Planet. Change Sect.) 97:151–89

210 Responses to “Irreversible Does Not Mean Unstoppable”

  1. 201

    Here’s a small but cheerful straw in the wind–so to speak:

    “Ontario’s energy mix could be virtually 100 per cent coal-free by next year, especially if the government boosted conservation by residents and acted boldly with its upcoming green energy act, suggests a new report.”

    Of course, more of the replacement megawatts are coming from gas than wind, but still. . .

  2. 202

    On a related topic, here is a study on siting and generation consistency, based on Ontario’s wind capacity last year:

    (Ontario has the advantage of geographical extent in this regard, compared to most American states–1,076,395 km2, versus 696,241 km² for Texas.)

  3. 203
    James Cyr says:

    The terms “irreversible” and “unstoppable” are two sides of the same coin. Taken within the context of climate change, they both mean the same thing, i.e. a process that is continually evolving independent of man-made influences. The climate is changing, and it will continue to change. It is a process that can not be stopped, and to tinker with it is to invite disaster. The answer to the anthropogenic component is to reduce or eliminate sources of pollution; persue the development of alternate energy sources and minimize all forms of wastage.

  4. 204
    Hank Roberts says:

    Are you the James Cyr from Canada who’s so active online opposing the militant soc-ia-lists there? thinks CO2 isn’t pollution? thinks burning fossil fuels isn’t tinkering with climate, but limiting use of them would be? Just curious if there’s any relation, or if you’re someone with similar misconceptions but interested in learning.

  5. 205
    John Bartlett says:

    I like the suggetion that climate models be provided over the internet for people to play with. My only thought is that that strategy is a good one for those of us who are suffuciently educated in Maths and science to be able to appreciate the implications of playing with the models and indeed with the concept of modelling in this way. The vast majority of people are not so well informed. These people are only going to learn their information from the TV News and maybe newspapers, particularly the lower quality ones. What can we do to help them?

  6. 206
    Mark says:

    205: there is one. One that can be run on a desktop PC.

    I can’t remember the linky, but someone here will do.

    And its existence hasn’t apparently helped (apparent may be more true than reality, since all that’s happened is that instead of wavering and not saying anything, they decided and aren’t saying anything).

  7. 207
    Tim Curtin says:

    I am amused by the above discussion. Nobody seems to have noticed that Fig 1. and Fig 2. in Solomon et al equate growth of CO2 emissions (over 3% pa until early 2008, already falling fast) with growth of the atmospheric concentration of CO2 at the same rate, although the actual growth of the latter has been only 0.4% p.a. over the whole period since 1958, and was slightly below that from Jan 08 to Jan 09. What has been happening since 1958 is that global biospheric absorption of CO2 emissions has grown roughly pro rata with emissions, resulting in the relatively slow growth of [CO2]. In physical terms, the absorption or uptake of emissions was 1.8 GtC in 1958-59, 5.29 GtC in 2006-07, and prelim est. 5.8-6.0 in 2007-2008. I do not know why Solomon et al chose to ignore this data (taken from the GCP which Schneider helped to set up). Any suggestions?

    [Response: Try reading the paper? If they had concentrations growing at 2% a year, then by 2100 they’d have 2300 ppmv. They don’t, therefore growth rates of concentrations are less, exactly as you would expect if you use the Bern CC model. Oh look! That’s what they say they did. Please note that we are singularly uninterested in your ‘amusing’ attempts to make stuff up. – gavin]

  8. 208
    Hank Roberts says:

    You want amusing, recollect this is the Tim Curtin who thinks that for CO2 to increase some other gas has to decrease by the same amount, because it’s “parts per million” — he’s never retracted that claim:

    Starting here:

    Digging himself deeper:

    The low point: where you’ll see:

    Posted by: Tim Curtin | June 24, 2006 9:00 AM. #48. Ian Gould: what do you understand by “parts per million”? Lesser earthlings like me think it means that …

    This is Curtin’s interpretation of Gresham’s Law as though it could be applied to atmospheres: bad molecules drive good molecules out of circulation. He’s said to be an economic adviser.

  9. 209
    Bernard J. says:

    Tim Curtin has more ‘alternative’ takes on science than even Hank’s links at #208 suggest. Amongst these notions are his claim that increased CO2 emissions are universally beneficial to the biosphere, and that if humans cease to emit carbon dioxide, the living world faces an imminent crisis of existence. He also believes that climatology and ecology are peopled by liars, frauds and incompetents.

    This thread on Deltoid is a long and painful case history of Curtin’s ideas, and his refusal to provide any scientific basis for his claims:

    Curtin has lately had an enormous bee in his bonnet about this paper of Solomon’s, about Chris Field, and about NAS and PNAS in general. In fact, he is so disturbed by these people, and by a number of other scientists (including Gavin) and by scientific journals, that he has been extremely free with what are surely libellous comments. If anyone is curious about the slander that he has produced, and doesn’t want to waste the time wading through the entire thread to find them, a sample is gathered at:

    According to Curtin, he stands by his disparagements, and is happy to confront lawyers. If it wasn’t for the fact that Curtin is an economics emeritus at the Australian National University, and that he fancies himself as a publishing author in ‘sceptical’ climatology and climatological economics, it would all just be tragically comic…

  10. 210
    Jim Eager says:

    Oh, come on, Gavin, if they can’t make stuff up then what would they have left to post?