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Climate change commitments

Filed under: — gavin @ 3 March 2010 - (English)

Una traducción está disponible aquí.


212 Responses to “Climate change commitments”

  1. 1
    cervantes says:

    On the other hand this assumes no positive feedbacks are already in play, no?

  2. 2
    Angelo Lamana says:

    The authors’ very brief summary of the letter (access to the letter is limited to subscribers), at the link you provided, doesn’t seem to match your discussion above.

    “Committed climate warming” H. Damon Matthews & Andrew J. Weaver

    Abstract
    The perception that future climate warming is inevitable stands at the centre of current climate-policy discussions. We argue that the notion of unavoidable warming owing to inertia in the climate system is based on an incorrect interpretation of climate science.

    What gives?

  3. 3
    Nonlinear Guy says:

    How can you discuss the topic of future warming commitment
    based on AR4 models which totally neglect the real driver
    of future warming, namely ice-sheet responses?
    The Hansen et al study (2004) on target atmospheric CO2
    and climate sensitivity is quite clear on this topic:
    equilibrium responses would double the GCM-based estimates,
    with very little to be said about transient effects.

  4. 4

    I think this framing is still arbitrary, in that the CO2 emissions are set to zero, but aerosol emissions are (presumably) allowed to continue. It is thus the climate commitment of only CO2, not considering other anthropogenic climate forcings.

    The warming commitment if we stop all human emissions (GHG and aerosol) is probably very substantial: The cooling effect of the aerosol will very quickly disappear, thereby ‘unmasking’ the greenhouse warming, approximately half of which has been canceled by aerosol cooling up to now.

    Not considering the change in net uptake of carbon by the ocean, you can put the following numbers on that (based on Ramanathan and Feng, 2009):

    Global average surface temperatures have increased by about 0.75 degrees Celsius since the beginning of the industrial revolution, of which ~0.6 °C is attributable to human activities. The total radiative forcing by greenhouse gases is around 3 W/m2, with which we have ‘committed’ the planet to warm up by 2.4 °C (1.6-3.6 °C), according to a climate sensitivity of 3 °C (2-4.5 °C) for a doubling of CO2. The observed amount of warming thus far has been less than this, because part of the excess energy is stored in the oceans (amounting to ~0.5 °C), and the remainder (~1.3 °C) has been masked by the cooling effect of anthropogenic aerosols.

    This simple analysis shows that the ‘2 degree target’ of ‘dangerous anthropogenic interference’ is looming on the horizon, as the climate equilibrates and aerosol pollution is cleaned up. An increased uptake of carbon by the oceans (in the hypothetical situation of stopping all emissions immediately) is not likely to cancel the ‘unmasked’ greenhouse warming in addition to canceling the ‘committed’ ocean warming.

  5. 5
    Sean says:

    I agree with @cervantes. This should be taken with a grain of salt, given that our understanding of the small scale processes such as clouds and aerosols as feedbacks are fairly poor. I’d like the physical argument for why cutting back emissions to zero will affect these processes so quickly.

  6. 6

    One could also argue that the stratosphere isn’t being loaded with all sorts of aerosols right now because of “social interia” and that the “correct interpretation of climate science” is therefore that there’s a strong cooling commitment. As gavin euphemistically puts it, the practical implications of such reframing would be “small”. Yet another culture vs. nature quasi-theological debate which amounts to nothing…

    Until such time as meaningful mitigation is seriously considered and planned for, the real-world scenario remains BAU and the debate which actually matters remains the one about the size of the fossil fuel reserves.

  7. 7
    Sou says:

    Am I right in thinking that, what this paper is saying is that it is incorrect to say that even if there were zero emissions from today the earth would still warm for a bit? That instead, going cold turkey to net zero emissions, the surface would continue to absorb CO2 and the temperature would in fact fall or at least stay the same?

    And the reason for continued warming is simply because it is impractical or more properly not possible for us to stop emitting CO2 suddenly and totally, therefore CO2 will continue to rise and the temperature will rise.

    And that provided we significantly curb emissions over the next couple of decades, a new equilibrium will be reached.

    I’ve been assuming we will have to virtually stop CO2 emissions at some stage, but doubt even that is possible to the nth degree given the need to grow food, build houses etc., at least not without sequestration, afforestation or other offsets of some kind. Is there a level worked out that we can emit some and reach a new radiation balance? Or do we have to go to net zero, or even negative?

  8. 8
    Ray Ladbury says:

    Angelo Lamana,
    I don’t see the inconsistency. All Gavin has done is spell out the “why” behind the brief statement in the abstract.

    [edit]

  9. 9
    Alan Robock says:

    It seems that Matthews and Weaver ignore sulfates. If we stop emissions today, there will be warming of about 0.5C as the atmosphere clears out. I don’t understand how temperatures will not rise if we clean out the troposphere and remove the aerosols which are now cooling.

  10. 10
    Josie says:

    @ 2 Angelo Lamana says “The authors’ very brief summary of the letter (access to the letter is limited to subscribers), at the link you provided, doesn’t seem to match your discussion above.”

    Seems to match it perfectly to me. What are you talking about?

  11. 11
    John E. Pearson says:

    2: Angelo wrote “what gives?”

    I don’t see any contradiction between Gavin’s summary and the abstract. I don’t have access to the article right now either but the abstract certainly is not in obvious contradiction with Gavin’s summary. As Gavin clearly stated the meaning of “committed climate change” is ambiguous. Both the constant composition assumption and the zero CO2 emissions seem like entirely reasonable definitions to me. The constant composition assumption results in another quarter degree of warming while the zero CO2 emissions results in no further warming.

  12. 12
    Paul says:

    “Climate scientists are paid to do climate science,” said Gavin A. Schmidt, a senior climatologist with the National Aeronautics and Space Administration’s Goddard Institute of Space Studies. “Their job is not persuading the public.”

    Hey Gavin, if your job is to do ‘climate science’ and not ‘persuade’ the public, why do you spend so much time and effort with RealClimate.Org?

    [Response: Because I think it is worthwhile even if I don't get paid for it. - gavin]

    [edit]

  13. 13
    arch stanton says:

    Angelo Lamana-

    Mathews and Weaver address “inertia in the climate system”. Gavin addresses “societal inertia” – the likelihood that the strict criteria of “immediate zero CO2 emissions” will actually be met (as opposed to the easier but also unlikely possibility that we could hold current atmospheric CO2 concentrations steady through immediate 60-70+% emission reduction).

    If I understand correctly, Gavin makes 2 points with regards to this paper:

    - Realistically, continued warming is inevitable.

    - If however, the (almost inconceivable) abrupt global total cessation of (fossil) C emissions were to occur, then we could expect warming to stop without further need for mitigation.

  14. 14

    The first paragraph seems to go against what we know about Convection and Evaporation does it not?

    While an increase in the amount of radiation the Earth receives by trapping outgoing IR would increase surface temps, would the increased surface temp not increase the convection and evaporation rates introducing a negative feedback?

    We observe that the troposphere increases in altitude going from the poles to the tropics, and this behavior shows that as energy pressure (temperature) increases, the volume of the atmosphere also increases since it is elastic in nature. Without this elastic property of the atmosphere, the temperatures in tropical regions would be higher than we observe since the volume of the atmosphere would would be less as described by PV=nRT.

    If convection and evaporation were not present, I could see the argument being made that a slight increase in Radiation having some warming effect, however convection and evaporation do exist within the Troposphere and the rate of cooling the two exhibit increases as surface temps increase.

    We must also take into consideration that there are three ways to cool the planet within the Troposphere and account for them all. The Earth is only heated via Radiation, and cools via Radiation, Convection, and Evaporation in order of increasing magnitude.

    Am I missing something?

  15. 15
    Ian says:

    Angelo, the summary refers to what the authors see as a common misunderstanding (in their words, “an incorrect interpretation of climate science”): that we are committed to further warming from past emissions. Warming to this point is presumably from past emissions, but their results suggest that it’s future emissions, not past, that commit us to warm more from this point forward. Further down in the letter they say

    “Constant-composition commitment is often misinterpreted as the unavoidable warming that is yet to manifest in response to past greenhouse-gas emissions. However, the climate warming commitment from past greenhouse-gas emissions is more correctly defined as a ‘zero-emissions commitment’ — that is, the future climate change that would occur, should greenhouse-gas emissions be eliminated entirely.” (As in their Fig 1 that Gavin inlined above.)

    “…From this we conclude that the elimination of carbon dioxide emissions leads to little or no further climate warming; that is, future warming is defined by the extent of future emissions, rather than by past emissions.”

    Their results do NOT imply that we can continue emitting at current levels without experiencing warming – but that sure is easy to read into the abstract. Also, I would expect that their findings will not always hold – if we end up with a concentration too great for the biosphere to handle, say, then past emissions alone will commit us to future warming.

  16. 16
    Silver R. says:

    The point they make is that future climate change due to inertia has been assessed by keeping CO2 concentrations at some predetermined level, that is, that there will be no future increase in human input of CO2 into the atmosphere. That, they argue however, is somewhat arbitrary and actual climate inertia should be measured by assuming that human emissions are cut to zero. Basically, they say that let’s see what the climate will do when we remove humans entirely, whereas other studies have asked: what will the climate do if we continue to pollute as usual, but without increased emissions.

    As far as I can tell.

  17. 17
    Tom says:

    The last paragraph of this post is poorly written/argued.

    As a practical matter for what? As written it only makes sense to be read “as a practical matter for predicting the future”.

    The charter of climate science (and especially this subtopic) has always been contingent prediction. Since this kind of teasing out does seem relevant to discussions along those lines, you are undermining yourself in a muddy way. At the very least your language is confusing as to what you think the “practical” matter is.

    [Response: The practical matter is whether we need to be working on adaptation to further warming. And we do. - gavin]

  18. 18
    Tom Fiddaman says:

    I don’t have access to the original, but it seems odd that M&W would suggest that socioeconomic inertia matters, then reframe emissions commitment in a way that ignores that. From a human perspective, commitment (of resources) occurs at the time of construction of GHG-emitting capital. A more policy-relevant test of commitment might be a scenario with emissions decaying with some lifetime that reflects natural capital turnover (15 to 50 years). That path (no new emitting capital, but no abandonment) is a better reflection of what’s possible with great effort than the zero-emissions-now path suggested.

  19. 19
    thomas hine says:

    I don’t understand their figure. The IPCC AR4 scenario A1B(21) calls for warming between approx. 2-5 deg C by 2100. This is a “middle of the road” scenario which would look outrageous on their figure. It also is beginning to look outrageous to systemic eyes.
    I want an answer to this, please: In order for the AR4 scneraio I mention to come true, the decedal increase in temp observed from 2000-2010 would need to jump 100% to 700% per decade to reach the modeled change. Is this outrageous to a physicist at this point????????

    [Response: The AR4 results in the figure are from constant-2000 composition, not any of the scenarios, which of course warm up significantly more. - gavin]

  20. 20
    Elmar Veerman says:

    Did they really consider all human emissions? As I understand it (from the IPCC report and from Ramanathan en Feng, Sept 23 2008 in PNAS) stopping all emissions suddenly would cause about 1.6 degree Celsius of extra warming, because short-lived pollution would quickly be removed from the atmosphere. No more global cooling, while the greenhouse gases would stay.
    Did I misunderstand something?

    [Response: This is a very good point - and I'm pretty sure they did not assess this. - gavin]

    And indeed, what about positive feedbacks?

  21. 21
    thomas hine says:

    Sorry, it looks like I didn’t realize there figure was the “constant” model ensembles. But my second question still remains, as you’ve conceded to the reality of the issue towards the end of the post.

  22. 22

    Wait, that doesn’t sound right at all. If emissions dropped to zero immediately, the concentrations would start to fall, but relatively slowly (given what we know about the half-life of carbon in the atmosphere) – i.e. over decades. Throughout this time, the concentrations will be elevated over pre-industrial levels, meaning a stronger greenhouse effect, and hence give a continued warming effect. I don’t see how the blue line could be flat.

    Ramanathan and Feng gave a figure greater than 2 degrees for this zero emissions scenario, but they also zeroed out aerosol pollution too. Schellenhuber’s editorial on the R&F paper attempts to compare what happens if don’t zero out the aerosols (and gets something closer to 1 degree). But both of them show continued warming for decades.

  23. 23
    Nick says:

    Angelo Lamana:

    I think you’re missing something.

    The letter notes that “Stable atmospheric concentrations of greenhouse gases would lead to continued warming, but if carbon dioxide emissions could be eliminated entirely, temperatures would quickly stabilize or even decrease over time. Future warming is therefore driven by socio-economic inertia, and is only as inevitable as future emissions.”

    This isn’t in contrast to the discussion above, which notes that Matthew and Weaver make an interesting point – if we want to talk about ‘true’ climate inertia, we should set our future emissions to zero and see how the system responds. “From this we conclude that the elimination of carbon dioxide emissions leads to little or no further climate warming; that is, future warming is defined by the extent of future emissions, rather than by past emissions.” This notes that we aren’t really “committed” to future warming if we stop all GHG emissions now.

    But we know that isn’t realistic – there is a societal inertia to such a radical change in emissions. We know we won’t cut all emissions tomorrow, and as noted above, even cutting emissions by 60-70% and keeping atmospheric levels constant has considerable inertia.

    The point made above is that while the authors of the letter do point out something that is indeed true, it simply isn’t realistic – there may be little climate inertia, but there is a mountain of socio-economic inertia.

  24. 24
    Paul Levy says:

    Angelo #2 – Perhaps you could explain why the Matthews-Weaver’s abstract and Gavin’s discussion don’t match…? Because I don’t see any contradiction at all.

  25. 25
    Anne says:

    “….adaptation (over and above what is necessary to reduce vulnerabilities to current climate conditions)…”

    Adaptation is what we humans do best. Are the “vulnerabilities to current climate conditions” unusual? (other than the recent record snowfalls)

    I’m ready to adapt to a couple of extra degrees in the near future.

  26. 26
    Mukei says:

    Angelo @2: No, it does match, actually. The idea of climate inertia is that when you increase the CO2 concentration in the atmosphere it takes the climate system a good deal of time for all its components to fully adjust and reach a new equilibrium temperature. Glacial melt and ocean warming, etc., result from, but take longer than, the warming in the atmosphere caused by the increased CO2. There’s the additional fact that while some parts of the climate system can adjust on a timescale of a few decades, others adjust so slowly that they will still be out of equilibrium several centuries from now.

    The question is, how great is the inertia in the short-term, say, through the end of this century? What will the short-term adjustment look like, how much warming are we already committed to, based on the CO2 we’ve already emitted?

    To answer that we need to define what “based on the CO2 we’ve already emitted” means. Does it mean the level of CO2 in the atmosphere currently? If it does, then that level could be held constant if, hypothetically, we were to cut our emissions by 60-70%, to levels that that no longer exceed what the oceans and the biosphere can simultaneously absorb.

    But if we take “based on the CO2 we’ve already emitted” literally, then the relevant thing to ask is how would the climate system adjust, how would global temperature behave, if we immediately cut our emissions back to zero? COThat’s apparently what they’re doing here. But it’s really a question about how the climate system responds to what we’ve been doing to it, rather than what our policies should be.

  27. 27
    Mukei says:

    Angelo @2: No, it does match, actually. The idea of climate inertia is that when you increase the CO2 concentration in the atmosphere it takes the climate system a good deal of time for all its components to fully adjust and reach a new equilibrium temperature. Glacial melt and ocean warming, etc., result from, but take longer than, the warming in the atmosphere caused by the increased CO2. There’s the additional fact that while some parts of the climate system can adjust on a timescale of a few decades, others adjust so slowly that they will still be out of equilibrium several centuries from now.

    The question is, how great is the inertia in the short-term, say, through the end of this century? What will the short-term adjustment look like, how much warming are we already committed to, based on the CO2 we’ve already emitted?

    To answer that we need to define what “based on the CO2 we’ve already emitted” means. Does it mean the level of CO2 in the atmosphere currently? If it does, then that level would be held constant if, hypothetically, we were to cut our emissions by 60-70%, to levels that that no longer exceed what the oceans and the biosphere can simultaneously absorb.

    But if we take “based on the CO2 we’ve already emitted” literally, then the relevant thing to ask is how would the climate system adjust, how would global temperature behave, if we immediately cut our emissions back to zero, so that CO2 concentration in the atmosphere can begin to fall. What’s the effect of that on temperature? That’s apparently what they’re asking here. But it’s really a question regarding how the climate system responds to what we’ve been doing to it, rather than what our policies should be.

  28. 28
    RichardC says:

    Since temperatures are already raised and there is more warming in the pipeline, who is to say that CO2 will adjust downwards as opposed to temperatures adjusting upwards. CO2 has been much higher in the past. There is nothing magical about pre-industrial CO2 levels.

  29. 29
    Geoff Beacon says:

    Cervantes #1. That’s what worries me – Arctic ice is a positive feedback already happening.

    February’s Arctic Sea Ice News & Analysis, (http://nsidc.org/arcticseaicenews) says:

    A recent paper by Thorsten Markus at NASA Goddard Space Flight Center suggests that the later freeze-up is the dominant factor lengthening the melt season. The analysis shows that, on average, autumn freeze-up starts nearly four days later each decade. Extensive open water at the end of the summer melt season, combined with warmer autumns, delay the autumn freeze-up. The larger expanses of open water absorb more solar energy, and before ice can form again, that heat must be released back to the atmosphere.

  30. 30
    Jean says:

    Once again, the scientific study referenced, certainly paid by tax payers, is not available to the general public.

    If I cannot have access to the information, I cannot communicate around me, because I don’t know.

    We therefore cannot assess the claims and should not expect the general public to be well informed and make rational voting decisions.

    When will they get it?

  31. 31
    Dan Olner says:

    The paper took me a few reads, think I’m still confused. This sounds contradictory:

    “Climate change commitment is defined as the future warming to which we have committed ourselves by virtue of past human activities. Because of the slow response time of the climate system, the equilibrium climate consistent with current levels of greenhouse gases will not be reached for many centuries. This so-called constant-composition commitment results as temperatures gradually equilibrate with the current atmospheric radiation imbalance, and has been estimated at between 0.3 °C and 0.9 °C warming over the next century.”

    vs

    “Constant-composition commitment is often misinterpreted as the unavoidable warming that is yet to manifest in response to past greenhouse-gas emissions. However, the climate warming commitment from past greenhouse-gas emissions is more correctly defined as a ‘zero-emissions commitment’ — that is, the future climate change that would occur, should greenhouse-gas emissions be eliminated entirely”

    Eh? In that 1st para, they say past human activity has committed us to 0.3 to 0.9 degrees in the next century, as the climate system has a lag. They call this the constant-composition commitment… but then go on to say that it *isn’t* unavoidable warming yet to manifest. Am I being daft or is that contradictory?

    Or are they saying the “so-called constant composition” commitment shouldn’t be labelled thus, and should instead be called the “zero-emissions commitment?”

    So – they’re concluding that if we magically turned off all carbon emissions tomorrow, we’d almost immediately cease temperature increase?

  32. 32
    davey says:

    Angelo,

    The first two sentences don’t give enough context to the letter. Here’s the first paragraph:

    The perception that future climate warming is inevitable stands at the centre of current climate-policy discussions. We argue that the notion of unavoidable warming owing to inertia in the climate system is based on an incorrect interpretation of climate science. Stable atmospheric concentrations of greenhouse gases would lead to continued warming, but if carbon dioxide emissions could be eliminated entirely, temperatures would quickly stabilize or even decrease over time. Future warming is therefore driven by socio-economic inertia, and is only as inevitable as future emissions. As a consequence, mitigation efforts to minimize future greenhouse-gas emissions can successfully restrict future warming to a level that may avoid dangerous anthropogenic interference with the climate system. The challenge of climate mitigation, although daunting, is fully within the scope of human control.

  33. 33
    HotRod says:

    2 Angelo Lamana says:

    Angelo, that’s exactly what Gavin says?

    ‘ … subsequent temperatures (depending slightly on the model you are using) would either be flat or slightly decreasing. With this definition then, there is no climate change commitment because of climate inertia.’

  34. 34
    John E. Pearson says:

    After reading the full letter I’m curious about one thing. They claim that if human emissions were stopped tomorrow that CO2 would fall at a “rate similar to the mixing of heat into the deep ocean.” There ought to be pretty large error bars on the uptake of CO2. As of a few years ago the “missing sink” (where roughly 3 gigatons of carbon go every year) was still not fully understood. These guys discuss it. http://www.whrc.org/carbon/missingc.htm It is also worth noting that zeroing out CO2 emissions requires not only cessation of fossil fuel burning it also requires cessation of changes in land use which I believe account for about 20% of CO2 emissions (at least that’s my reading of the Woods Hole page).

  35. 35

    re #2 Angelo Lamana says:
    3 March 2010 at 9:10 AM
    “”"”The authors’ very brief summary of the letter (access to the letter is limited to subscribers), at the link you provided, doesn’t seem to match your discussion above.

    “Committed climate warming” H. Damon Matthews & Andrew J. Weaver

    Abstract
    The perception that future climate warming is inevitable stands at the centre of current climate-policy discussions. We argue that the notion of unavoidable warming owing to inertia in the climate system is based on an incorrect interpretation of climate science.

    What gives?”"”"

    Angelo, It sounds like you are cherry picking, purposefully trying to confuse people and putting up a straw man. Use science and not politics.

    I saw at the place I used to be, a certain shift when climate commitment was published by at least two of the senior scientists there. After that, I saw for the first time a certain feeling of quiet desperation emerge.

    One of the senior scientists there (extremely conservative senior scientist who got after me occasionally about being too bold), publicly used the phrase, “this [human caused climate change evidence] is alarming to me.”

    For an almost reactionary conservative senior scientist to say this was a paradigm shift to say the least.

  36. 36
    Ron Taylor says:

    Angelo, read Gavin’s post again: “Matthews and Weaver make the point that this is a little arbitrary and that the true impact of climate inertia would be seen only with emissions cut to zero. That is, if we define the commitment as the consequence only of past emissions, then you should set future emissions to zero before you calculate it. This is a valid point, and the consequence of that is seen in the lower lines in the figure.”

    “CO2 concentrations would start to fall immediately since the ocean and terrestrial biosphere would continue to absorb more carbon than they release as long as the CO2 level in the atmosphere is higher than pre-industrial levels (approximately). And subsequent temperatures (depending slightly on the model you are using) would either be flat or slightly decreasing.”

    Gavin has simply expanded the summary based on the full contents of the letter.

    Sko sswhere

  37. 37
    Hank Roberts says:

    Is this referring to Charney or longterm climate response?
    I think of Charney factors as changing over decades, and longterm as changing over a few thousand years. This chart is for a couple hundred years.

  38. 38
    Frank says:

    If the radiative forcing due to GHGs were stabilized, you say the planet would still have a radiative imbalance, and “warming will continue until the oceans have warmed sufficiently to equalise the situation – giving an additional 0.3 to 0.8ºC warming over the 21st Century”. However, isn’t it true that most energy is removed from the earth’s surface by convection and evaporation, not radiation (because the lower troposphere contains so much GHGs, especially water vapor)? Isn’t net radiative equilibrium for the whole planet therefore restored when the upper troposphere, not the ocean, warms? Can this happen in a short period of time (weeks?) rather than decades because there is relatively little mass that needs to warm?

    After radiative cooling, air subsiding from a warmer upper troposphere may eventually slowly warm the oceans. However, this probably shouldn’t be characterized as a return to an equilibrium situation, since the earth’s atmosphere and oceans are far from equilibrium at all times. As can be seen your graph, our climate models make a wide range of predictions (perhaps 0.5-5 degC, a 10-fold uncertainty) about how much “committed warming” will occur in the future under any stabilization scenario, so we don’t seem to have a decent understanding of these processes.

    [Response: The issue is the balance at the top of the atmosphere, not the surface. - gavin]

  39. 39
    NoPreview NoName says:

    “On the other hand this assumes no positive feedbacks are already in play, no?”

    No, just no runaway positive feedback – which we pretty clearly don’t have yet.

  40. 40
    Gail says:

    “…terrestrial biosphere would continue to absorb more carbon than they release…”

    I would imagine this is a false assumption. The terrestrial biosphere is shrinking. Trees and the understory shrubs are in decline, thus they must be emitting more carbon than they absorb. They are dying, not growing.

  41. 41

    It would have been nice to see scenarios on the other side of the “social inertia” parameter included, too, given that emissions are likely to (continue to) increase in the short term as the recession ends.

    And absent meaningful agreements on mitigation, of course.

  42. 42
    Lynn Vincentnathan says:

    Well, if we’re really committed to not mitigating and to spending $$kazillions on adaptation, which is looks like we are, then where’s the money going to come from?

    I have an idea, we could save perhaps even in the $$kazillions from mitigating global warming and reducing GHGs down by 80% (at least down to 70% without any sacrifice or lowering of productivity or living standards), and use that for adaptation :)

  43. 43
    Chris Dudley says:

    Your point that it does not matter depends on what we call mitigation. If we embark on a path that is equivalent to setting emissions to zero now (say by having a period of negative emissions in the 2035 to 2050 time frame), and call the sequestration we accomplish mitigation then mitigation can arrest climate change, make adaptation unneeded and bring us to a safe concentration of carbon dioxide in the atmosphere as Hansen has pointed out. If we instead consider our clean up efforts to be a form of adaptation to prior mess making, then we have a choice between effective adaptation in the clean up and panicked and unseemly adaptation in the response to sea level rise etc….

  44. 44
    Lee Norton says:

    This does not correlate with Dr. Dan Lunt’s observations at Bristol University that 3 million years ago, the CO2 was 340 – 400 ppm and the temperature was 2 – 3 deg warmer. Why?

    [Response: In the paper we discuss the impact of longer term processes - particularly vegetation change and ice sheet change. These are both amplifying factors . - gavin]

  45. 45
    Bruce M. says:

    Slightly OT question.

    My understanding of climate sensitivity is that it is dynamic in nature. In other words, the present day climate system with its current configuration of ice sheets, biota, contents, etc. has an S value of about 2.5 degrees. How does this value compare with what what we think S was in earlier periods in Earth’s history (e.g. do we think it was closer to 1 when there was little ice and/or carbon locked up in vegetation)?

    Thanks,

    Bruce M.

  46. 46
    Lynn Vincentnathan says:

    Well, I even hate to bring up some minor disprecancy among climate scientists. You know where the denialists would go with it, and that’s what would REALLY be damaging to science. A chilling effect on even scientists’ usual disagreements that lead to better and better science, because they’re always having to look over their shoulders at the blood-dripping fangs of the denialists close on their heels, looking for some climate scientist to break from the pack so they can attack.

    But, what about the suggestion by Ramanathan and Feng (2008) that even if we go down to zero GHG emissions, we are committed to 2.4 C warming. What accounts for the difference between the Wigley figure of .3 to .8 C (which would mean ? up to 1.6 C warming) and the 2.4 C figure. And which one seems closer to reality, as perceived by today’s science? See:

    Ramanathan, V., and Y. Feng. 2008. “On Avoiding Dangerous Anthropogenic Interference with the Climate System: Formidable Challenges Ahead.” Proceedings of the National Academy of Sciences 105.38: 14245-14250.

  47. 47
    dhogaza says:

    The authors’ very brief summary of the letter (access to the letter is limited to subscribers), at the link you provided, doesn’t seem to match your discussion above.

    It seems consistent to me. What about it seems inconsistent to you?

  48. 48
    Black says:

    Angelo:

    The summary seems to me like a very good match to the discussion. The point being that it isn’t inertia in the climate system that makes warming inevitable — it is inertia in the social system.

  49. 49
    Len Ornstein says:

    Gavin:

    You state:

    “the planet still has a radiative imbalance, and the warming will continue until the oceans have warmed sufficiently to equalise the situation”.

    Isn’t the main problem that, even if we stopped adding any fossil-fuel-derived CO2 to the atmosphere, the ocean circulations haven’t yet reached ‘steady state’ – i.e., a stable thermocline and deep ocean temperature – and therefore THAT is the source of the Hansen et al. “heat in the pipeline”? That only after the atmosphere has ‘equilibrated’ with THAT ocean, will the atmospheric temperature have peaked?

    And (because of the unavailability of the full text of their paper), from their abstract:

    “We argue that the notion of unavoidable warming owing to inertia in the climate system is based on an incorrect interpretation of climate science.”

    isn’t THAT what they’re addressing?

  50. 50
    Edward Greisch says:

    A Previous article: “David Frank and his Swiss coworkers at WSL have just published an interesting new approach to answering it. They empirically estimate the distribution of gamma, the temperature-induced carbon dioxide feedback” appears to directly contradict the above. “Climate change commitments” also contradicts the other things I have read and the fact that we have already crossed some thresholds such as Arctic ocean ice melting, tundra peat bog thawing and clathrate melting.

    An additional 0.8 degrees C of warming is a doubling of the present warming.

    Since I don’t have access to Nature Geoscience either, please explain the incorrect interpretation and tell us who really got it wrong. If Mathews and Weaver are correct, that is a little less bad news. I find it hard to believe, but even if correct not a reason for celebration. No doubt the denialists will find reason to celebrate. The Republicans will also find reason to further reduce the Climate Bill and the Clean Air Act. They will read only the “60% cut” and say that President Obama’s goals are too “severe.”

    I hope Mathews and Weaver are correct. I am awaiting further research and clarification.


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