5000 GtC

The advantage of working at a university, is that you get the opportunity to attend interesting seminars, especially if Kit Carruthers reminds you (on which note, I also discovered that Kit submitted his PhD thesis last month). The talk today was by Nathan Gillett and discussed what would happen if we emitted, in total, 5000GtC. This would be a really large amout of CO2; a bit less than 10 times as much as we’ve emitted already and about twice as much as would be emitted along RCP8.5 by 2100. However, it is possible to recover and burn as much as 5000GtC, so it’s not impossible that we would do so. I suspect, however, that it is highly unlikely, one reason for which being that it would be extremely stupid to actually do so.

The bottom line from the talk was that if we were to emit 5000 GtC the world would warm by between 6.6 and 11oC, with the Arctic warming by between 15.3 and 19.7oC. However, as I said above, it seems highly unlikely that we would actually be stupid enough to emit anything close to 5000GtC. However, there were a couple of other aspects that I found interesting. Something I’ve discussed a few times is the idea that how much we warm probably depends, largely, on our cumulative emissions. Essentially the transient climate response to cumulative carbon emissions (TCRE) is thought to be between 0.8oC and 2.5oC per 1000GtC. However, this is thought to only hold till around 2000GtC, at which point the warming was thought to decreases slightly. This more recent work suggests, however, that it continues linearly out to 5000GtC.

In a sense this linear dependence on cumulative emissions is surprising, given that warming depends logarthimically on atmospheric CO2 concentration. The reason, however, is largely because as we continue to emit more and more CO2, the airborne fraction is expected to increase (as the efficacy of the natural sinks decreases) and this compensates for the logarithmic dependence. Recently, however, it was suggested that the sensitivity to cumulative emissions was not only not linear, but was also lower than the other models suggested.

This appeared to be largely because the model assumptions lead to the airborne fraction remaining roughly constant and that, hence, the logarithmic depdence dominated (and, also, using a model with a low climate sensitivity). However, from the talk today it seems that all major models suggest that the airborne fraction should increase as we continue to emit CO2. Furthermore, these models – apparently – don’t include all possible carbon cycle feedbacks (such as permafrost release).

Anyway, that’s all I was going to say. It was an interesting talk and seemed to further strengthen the idea that warming is likely linear with respect to cumulative emissions. Essentially, if we have some warming target, and we want to give ourselves some chance of staying below that without requiring negative emissions, then we have a carbon budget. For our current targets (staying well below 2oC), we’ll use up the carbon budget within a couple of decades.

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10 Responses to 5000 GtC

  1. Roger Jones says:

    Because simple trend analysis keeps getting gamed by contrarians, it’s worth repeating that this linearity, if it holds, is only applicable over half-century or perhaps even longer time scales. Changes over the shorter term are non-linear.

  2. Tom Curtis says:

    Anders, I dislike the “don’t include all possible factors” arguments unless we have a good idea of the likely bias. It is true the models don’t include all factors, but they also don’t include factors like the apparent effect of icebergs. Nor do they include the probably fact that volcanic emissions may be 1.5 to 2 times what had previously been estimated, from which it would follow that background sequestration rates are higher too as they are in balance over the geological ages. (Note, estimated volcanic emissions used to be less than estimated sequestration, so if the higher estimates of volcanic emissions are not corrected or a short term aberration, the increase in expected sequestration rates will be significantly less than 1.5 to 2 times.)

  3. jyyh says:

    This likely doesn’t include burning all the foodplants, other plants and animals, and the soil that can burn. The estimate of warming is way too low.

  4. Roger,
    Yes, a good point.

    Tom,
    Indeed, I agree that there are also other factors that are not being included. Interesting point about volcanoes. I hadn’t realised that volcanic emissions may be slighter higher than had been previously estimated. I’m not quite following your parenthetic comment, but maybe I should drink my coffee and read it again later 🙂

  5. Andrew dodds says:

    Jyyh-

    Do bear in mind that the ice albedo feedback would stop when we ran out of ice. That helps to limit the warming.

  6. However, it is possible to recover and burn as much as 5000GtC, so it’s not impossible that we would do so. I suspect, however, that it is highly unlikely, one reason for which being that it would be extremely stupid to actually do so.

    Going from past actions I don’t think that because an action is extremely stupid it will stop humanity trying it. Luckily it maybe quite hard as according to some stats, known fossil fuel reserves amount to less than 3000GtC. I am not saying 5000GtC is impossible but Governments have tended to be lazy and go down the most economical route rather than the least stupid path.

  7. The observations on linearity up to 5000GtC are very interesting. After a meeting I attended sometime back, I remember discussing CO2 rising concentrations (higher than for 800,000 years etc.) with someone I can only describe as a ‘climate scientist stalker’ (who will remain nameless) who knew enough to be dangerous, but not enough to be self-aware of his limited knowledge. He uttered the [for them, killer] argument “yes, but it’s logarithmic!”, mantra style.

    Nevertheless, it does feel rather academic to be discussing “between 6.6 and 11C” [average global temperature] warming. Given that 4C would be “catastrophic” … “a different world” … contemplating 6.6-11C is like catastrophe squared!

    I have heard of double murders, but double suicide … that is a novel concept!

  8. BBD says:

    Andrew

    Do bear in mind that the ice albedo feedback would stop when we ran out of ice. That helps to limit the warming.

    True, but interestingly, there was no cryosphere to speak of back in the Palaeocene, but we still got the PETM off the back of ~3000GTC.

  9. Andrew dodds says:

    Bbd.

    Yes, it was just my congenital optimism kicking in.

  10. Bernard J. says:

    …interestingly, there was no cryosphere to speak of back in the Palaeocene, but we still got the PETM off the back of ~3000GTC.

    And off the back of a younger, fainter sun…

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