Earth to warm 4C by 2100 under business as usual

Since I’ve been keeping a low profile for the last few days, and since Rachel’s been keeping an eye on my blog, I hope she doesn’t mind me restarting my blogging by reblogging this post. It’s about a paper by Sherwood et al. suggesting that if one treats convection, and it’s influence on low-level clouds “correctly” then climate sensitivity is likely to be closer to the high-end of the range than the low. I haven’t had a chance to read the paper (hence my thought that reblogging this post would be better than writing my own). What I have heard are some who think this “alarmist” paper should be attacked as much as those that suggest climate sensitivity is low. Not having read the paper, I can’t really comment. All I can say is that just because the result are alarming, doesn’t make it an alarmist paper.

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30 Responses to Earth to warm 4C by 2100 under business as usual

  1. Rachel says:

    I’m glad you’ve reblogged this because I’d like to hear what people think about it. I did read the paper but for obvious reasons can’t pass judgement on it and I didn’t really understand it all anyway. I fail to see though how a paper which puts climate sensitivity within the IPCC range for climate sensitivity can be alarmist. They didn’t really even pitch it at the high end of the range, instead saying that a climate sensitivity of less than 3C does not fit with observations.

    I also thought the video of Sherwood explaining the paper was really good and should probably get a higher profile. It didn’t come through in your reblog so I’ll post it again in this comment.

  2. BBD says:

    But… but…TCR is really low, right at the bottom of the range because Nic Lewis says so and he’s better that AR5, because he says so. So it must be true.

  3. BBD says:

    Part of the reason there is such a large range is because of the uncertainty in how clouds will behave in a warmer world.

    Presumably they will behave exactly as they did in the past when it was warm… say about 55Ma, when something rather interesting happened at it got *lots* warmer relatively quickly.

  4. BBD, I haven’t had a chance to look at the paper yet and I don’t know enough to really know the details anyway, but it does seem that there is very little evidence to suggest that we’ll suddenly strong negative feedbacks and much more evidence to suggest that we might see stronger positive feedbacks than some models suggest.

    Rachel, I wonder why the video didn’t come through here. Thanks for posting it in the comments.

  5. Skeptikal says:

    Sherwood is speculating… he doesn’t know what will actually happen in a warmer world.

    The right answer is in your post…

    Current projections of climate sensitivity fall in the range of 1.5-4.5°C. Part of the reason there is such a large range is because of the uncertainty in how clouds will behave in a warmer world.

    The reality is that nobody can say for certain what will happen in a warmer world… we’ll only find out when, or if, it happens.

  6. Skeptical, as far as I’m aware, he’s doing what most of us would call science. Of course we can never know what will happen in the future. Science isn’t about certainty, it’s about understanding what might happen and assigning confidence to the various possibilities. Sherwood has not claimed that he knows and suggesting that he has is completely wrong.

  7. OPatrick says:

    As an aside I find it interesting and illuminating to compare responses to this paper and Nic Lewis’ sensitivity paper last year. There seems an imbalance in the responses from ‘sceptics’ and those on the consensus side of the debate that may be worth documenting in more detail.

  8. Skeptikal says:

    Science isn’t about certainty, it’s about understanding what might happen and assigning confidence to the various possibilities.

    I disagree. Science IS about certainty. It’s about having a theory which matches observations and being able to produce repeatable experiments. Speculating on what might happen and assigning confidence is in the realm of tea leaves and tarot cards.

  9. OPatrick, I was wondering the same thing myself. To be honest, I’ve never quite understood Nic Lewis’s paper. My concern (and I think this is what James Annan was trying to get at in an exchange with Nic Lewis) is that you can select a prior that gives what appears to be a statistically robust results but isn’t physically motivated – I may be wrong and may not have expressed that as clearly as I could. However, I don’t think anyone on the “consensus side” has particularly attacked that work. I think people are simply surprised that he’s getting much lower values than other work (including Otto et al., on which Nic Lewis is an author). Similarly, I don’t know if the Sherwood work will stand up to scrutiny. Time will tell, but it is a little alarming if the study has merit.

  10. I disagree. Science IS about certainty. It’s about having a theory which matches observations and being able to produce repeatable experiments. Speculating on what might happen and assigning confidence is in the realm of tea leaves and tarot cards.

    You really are talking nonsense now. You really should consider that you just don’t have a clue what you’re talking about.

  11. And all these years I thought the health professionals I and my family consulted were medical scientists when they gave us the probabilities of successful operations, life spans beyond terminal diagnoses, efficacies of drug regimens, etc.
    In future, we will be demanding CERTAINTY from these practitioners or they’ll be relegated to the level of psychics, mediums, witchdoctors and lawyers! Bloody speculators!

  12. Tom Curtis says:

    I would be cautious about Sherwood, Bony & Dufresne’s result.

    In making their model/observation comparison, they use radiosonde data in addition to reanalysis products as their “observations” for S, their index of change in humidity with altitude over the range of small scale lower tropospheric mixing (850-700hPa). However, only reanalysis products are used as “observations” of D, the ratio “of shallow to deep overturning”. “Observational” values of S fall in the midrange of model values, while those of D are extreme. Therefore their result depends very much on the reanalysis values of D, values which are model produced rather than explicitly observational.

    Although reanalysis products are observation constrained, it is not clear that the observations constrain D. Plausibly, using a different model but the same observational constraints, you may get a different, much lower value for D. That being the case, Sherwood et al have really compared models with the specific behaviour of just two models. Of course, it is possible that use of almost any reasonable model with the MERA or ERAi observational constraints would produce high values for D, in which case their conclusion would follow.

    Unfortunately, they do not discuss this issue explicitly, so I cannot judge from their paper to what extent this is a factor. Further, in this aspect of climate science I cannot claim even to be well informed, so I would be very interested to see discussion of this issue.

  13. BBD says:

    Skeptical

    I disagree. Science IS about certainty.

    We can be certain that the PETM and other Cenozoic hyperthermals happened.

  14. > Science IS about certainty.

    That would be the law bit. Science can be said to seek natural laws, after all. Does that mean the scientific endeavour should be restricted to the description of natural laws? I doubt it.

    Sometimes, all you can hope for are law-like regularities. There are also explanations, and observations, and theories, and hypotheses, and models, et cetera, none of which are certain.

    The “yes, but certainty” is certainly interesting, as the most lukewarm line is “yes, but uncertainty”.

  15. jsam says:

    I’m jsut guessing. But Skeptical may not be a seismologist. Maybe he keeps a cat in a box in Copenhagen?

  16. Tom, interesting, thanks.

  17. Rachel says:

    It’s about having a theory which matches observations and being able to produce repeatable experiments.

    From what I understand, this is what the authors have done. They’ve got an explanation for the mixing of layers in the lower and middle troposphere which is inferred from observations. Climate models that predict a lower climate sensitivity are not consistent with these observations.

  18. Rachel says:

    I just want to add that these observations about clouds are consistent with a couple of other studies (which are also linked to from a discussion of this paper at RealClimate) and they are:

    A less cloudy future: the role of subtropical subsidence in climate sensitivity
    and
    A determination of the cloud feedback from climate variations over the past decade

  19. I suppose none of will be around in 2100 to check how accurate this paper was, however we are one 6th of the way to 2100 from 2000 so if it is going to happen we should seeing some pretty big increases in temperature in the near further, increases which are a lot steeper and more devastating than anything we have so far seen. I believe the current series of storms suffered by the UK over the last month are related to climate change, however if we see increases in temperature as predicted in this study, the scenario for the frequency and intensity of storms hitting the UK is truly nightmarish.

  20. Rachel says:

    Gareth,
    The 4C projection is dependent on business as usual so if emissions do start to fall, and I think most of us are hopeful that they will, then we will not see the 4C rise. See the RCP85 projection in the image below:

    Global temperature projections
    source: http://www.climatechange2013.org/images/figures/WGI_AR5_FigSPM-7.jpg

    But I agree that 4C is a nightmarish scenario and hopefully evidence that this is increasingly likely under business as usual will provide some much needed impetus for concrete action.

  21. BBD says:

    Rachel

    See also Hansen et al. (2013) (full HTML).

  22. BBD says:

    Gareth

    You are not alone in that opinion:

    Sir John Beddington, former government chief scientific adviser, told BBC Radio 4’s Today programme that the UK was experiencing “an increasing frequency of extreme weather events”, such as droughts and flooding, which is related to climate change.

    Source: BBC News website:

  23. Are we really unsure about how much warming can be expected from a certain atmospheric CO2 level, or are we merely expressing uncertainty about the timetable? Because if it is just the timetable, then are scientists giving overly cautious and technical answers to the simple political question: “How bad is this going to be?”

    The discussion, it seems to me, is about the timetable of expected events, as these questions usually demand that the answer involve the year 2100. But do we really not know what warming we are likely to see?

    First of all, we have the historical record. x amount of CO2, y amount of temperature increase.
    Secondly, we could be using the ‘Spherical Cow’ analysis: treat the atmosphere and first 3 miles of Earth as a material with a uniform standardized heat content potential and plug in the numbers, as if the experiment was in a physics lab. Both of these methods would give as answer to the resulting equilibrium temperature – the long-term temperature increase.

    But scientists are trying to figure out the short-term temperature changes, using modeling, in order to figure out the *slope* of the temperature change, which can then be used to answer what we might expect on the x-axis, which is the year 2100.

    But is this the question to answer? Is this really the answer to the political question? I don’t think it is, and I am not sure we are doing anyone any favors by trying to answer the wrong question. Because the real question – how bad can this be – IS the important question, because it implies that our timetable of response should be based on a reasonable long-term analysis. And a reasonable answer to the political question, it seems to me, informs us that we don’t need to do any more modeling – we already have enough information – and what we need to be doing is building and deploying brand new renewable energy systems as fast as humanly possible.

  24. Roger,

    Are we really unsure about how much warming can be expected from a certain atmospheric CO2 level, or are we merely expressing uncertainty about the timetable? Because if it is just the timetable, then are scientists giving overly cautious and technical answers to the simple political question: “How bad is this going to be?

    I think we really don’t know. We are fairly confident that doubling CO2 will by itself lead to around 1.2oC of warming. This is the equilibrium value so is the eventual temperature rise. We know that there are feedbacks that can lead to additional warming, but that is what is uncertain, in particular clouds.

    If one considers recent observation estimates, then it appears that feedbacks are providing about as much warming as the anthropogenic GHGs. That might suggest an eventual warming of 2.4oC if CO2 were to double. However, these estimates ignore likely non-linearities and may be under-estimating the cooling influence of anthropogenic aerosols. Also, these estimates typically only consider fast feedbacks. There are slower feedbacks that can lead to additional warming. Hence most would think 3oC a reasonable estimate. Long-term, maybe even more.

    The point of this study is that they claim to have modelled low-level clouds more accurately than any other model and get higher ECS estimates. A rather disturbing result.

    Because the real question – how bad can this be – IS the important question, because it implies that our timetable of response should be based on a reasonable long-term analysis. And a reasonable answer to the political question, it seems to me, informs us that we don’t need to do any more modeling – we already have enough information – and what we need to be doing is building and deploying brand new renewable energy systems as fast as humanly possible.

    I don’t think many here would disagree will you in general. Of course there are others who seem to think that any economy based on something other than fossil fuels is doomed to failure and that the economic risk outweighs any environmental risks from climate change/global warming. It’s them that need to be convinced that the risks associated with climate change are too great to ignore.

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  27. dana1981 says:

    I think the paper takes a really interesting approach, because the cloud feedback is one of the biggest remaining uncertainties in models and hence climate sensitivity estimates. Rob Painting has a good just-published post at SkS describing the basics of the paper.
    http://www.skepticalscience.com/New-Study-Suggests-Future-Global-Warming-at-the-Higher-End-of-Estimates-4-C-Possible-by-2100.html

    “The authors of Sherwood (2014) looked at the way that the various climate models handled the cloud feedback and found models with a low climate sensitivity were inconsistent with observations. It turns out that these models were incorrectly simulating water vapour being drawn up to higher levels of the atmosphere to form clouds in a warmer world. In reality (based on observations) warming of the lower atmosphere pulls water vapour away from those higher cloud-forming levels of the atmosphere and the amount of cloud formation there actually decreases. The diminished cloud cover leads to greater warming (a positive feedback).”

    As Rachel notes, the Sherwood results are consistent with several other papers that take a similar approach to trying to pin down the cloud feedback. I’ll have a post on Thursday that summarizes them.

  28. Rachel says:

    Rob’s article is great thanks, Dana.

    Steve Sherwood has one in the Conversation today: How clouds can make climate change worse than we thought

  29. dana1981 says:

    Yep that’s a good one. And here’s mine, touching on Sherwood and a couple other recent papers.
    http://www.theguardian.com/environment/climate-consensus-97-per-cent/2014/jan/09/global-warming-humans-not-sun

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