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Richard Lindzen’s HoL testimony

Filed under: — gavin @ 14 February 2006

Prof. Richard Lindzen (MIT) is often described as the most respectable of the climate ‘sceptics’ and is frequently cited in discussions here and elsewhere. Lindzen clearly has many fundamentally important papers under his belt (work on the QBO and basic atmospheric dynamics), and a number of papers that have been much less well received by the community (the ‘Iris’ effect etc.). Last year, he gave evidence to and answered questions from, a UK House of Lords Committee investigating the economics of climate change, in which he discoursed freely on the science. I’ll try here to sort out what he said.

Firstly, it is clear that Lindzen only signs up to the first point of the basic ‘consensus’ as outlined here previously, that the planet has indeed warmed significantly over the 20th century. While he accepts that CO2 and other greenhouse gases have increased due to human activities, and that this should warm the planet, he does not accept that it is necessarily an important component in the 20th century rise. His preferred option (by process of elimination) appears to be intrinsic variability, but he provides no support for this contention.

In terms of scientific content, his testimony covers a few basic topics: the greenhouse effect, climate sensitivity, aerosol forcing and water vapour feedbacks. We have discussed these topics previously (here, here and here), and so my critique of Lindzen’s comments will come as no surprise. He intersperses his comments with references to ‘alarmism’ which I will get to at the end.

Greenhouse Effect

Lindzen accepts the main principle of the greenhouse effect, that increasing greenhouse gases (like CO2) will cause a radiative forcing that, all other things being equal, will cause the surface to warm. He uses an odd measure of its effectiveness though, claiming that a doubling of CO2 will lead to a ’2%’ increase in the greenhouse effect. How has he defined the greenhouse effect here? Well, a doubling of CO2 is about a 4 W/m2 forcing at the tropopause, which is roughly 2% of the total upward longwave (LW) (~240 W/m2). But does that even make sense as a definition of the greenhouse effect? Not really. On a planet with no greenhouse effect (but similar albedo) the upward LW would also be 240 W/m2, but the absorbed LW in the atmosphere would be zero, so it would make much more sense to define the greenhouse effect as the amount of LW absorbed (~150 W/m2). In which case, doubling of CO2 is initially slightly more*, but as soon as any feedbacks (particularly water vapour or ice albedo changes) kick in, that would increase. Due to the non-linearities in the system, you certainly can’t multiply the total greenhouse effect of ~33 C by 2% to get any sensible estimate of the climate sensitivity. So it’s not clear what relevance the ’2%’ number has except to make the human additions to the greenhouse effect seem negligible.

*Update: The initial post had an arithmetic error which I have excised (see comment 76 below).

Climate sensitivity

That leads in to Lindzen’s main theme in his evidence – how sensitive climate will be to increasing CO2. He starts off by giving the standard Stefan-Boltzmann no-feedback value for the climate sensitivity: “A doubling of CO2 should lead (if the major greenhouse substances, water vapour and clouds remain fixed), on the basis of straightforward physics, to a globally averaged warming of about 1°C”. But he couples this with an extremely misleading statement: “The current increase in forcing relative to the late 19th Century due to man’s activities [by which he means greenhouse gases alone] should lead to a warming of about 0.76°C, which is already more than has been observed, but is nonetheless much less than current climate models predict.” He repeats this point in the Q&A session as well. However, Lindzen is undoubtedly well aware (having written papers on the subject i.e. Lindzen, GRL, 2002) that lags in the surface temperature due to ocean thermal inertia imply that the transient response is always smaller than the equilibrium response, and that additionally, there are other forcings in the system (specifically land-use change and aerosols) that counteract the forcing from greenhouse gases alone. Since he does not mention these two factors in connection with this statement, a listener could be left with a rather misleading impression. He combines this with a (deliberate?) overstatement (Q130) of the ‘consensus’ value for the sensitivity as being 4 to 5°C (while the actual consensus is between 1.5 to 4.5°C, best guess around 3°C), misleadingly giving the impression that the mainstream is way off. Similarly, his claim that models overpredict the 20th Century temperature rise is easily shown to be false.

Later he states: “Attempts to assess climate sensitivity by direct observation of cloud processes, and other means, which avoid dependence on models, support the conclusion that the sensitivity is low. More precisely, what is known points to the conclusion that a doubling of CO2 would lead to about 0.5°C warming”. One wonders which attempts he is referring to, since it can’t be Lorius et al (1991), or Forrest et al (2004) or Andronova and Schlesinger (2002), given that they give ranges that are all significantly higher than this, and indeed, Gregory et al (2002) specifically rules out anything less than 1.6°C. A more recent estimate (Annan and Hargreaves, in press) using multiple lines of observational constraints places the sensitivity well within the value estimated by the models (i.e. around 2 to 4°C).

Actually, I think it is quite easy to rule out a sensitivity as low as 0.5°C by considering the last glacial period about 20,000 years ago. At that time the temperatures were globally around 5 or 6°C colder than the pre-industrial, and the forcings (from ice sheets, vegetation, greenhouse gases and dust) are estimated to be around 6 to 11 W/m2 (a slightly broader range than I previously quoted, updated from some of the PMIP2 results). This implies a sensitivity of between 1.8 and 4°C for a doubling of CO2, with a most likely value of around 3°C. If however, the sensitivity really was as low as 0.5°C, that would imply that either the forcings estimates are 3 to 8 times too low, or the temperature changes are 3 to 8 times too high. Since around 3 W/m2 of the ice age forcing is directly related to greenhouse gases and is well accepted (even by Lindzen), it would require the ice sheets to impart an enormous forcing even to get anywhere near a level consistent with his sensitivity estimate. That does not appear even remotely plasuible. On the other hand, it is unlikely that we have mis-interpreted the proxy evidence for temperature since it comes from very many different sources – snow lines, foraminefera, alkenones, Mg/Ca, pollen records, ice core isotopes, speleothems, faunal assemblages etc. To be sure, some of these data do not completely agree, but none would imply that global temperatures were only 1.5°C cooler (which is the minimum that would be required).

In summary, Lindzen’s testimony regarding on climate sensitivity is idiosyncratic at best, and certainly not supported by the literature.


He goes on to describe the attribution study of Stott et al. (2000) who showed that both natural (solar and volcanic) and human-related forcings (GHGs and aerosols) were necessary for a climate model to match the 20th Century temperature changes. This is seen in every model (for instance) and so is not the result of some individual model quirk. Lindzen goes on to claim that uncertainty in the forcings (particular solar and aerosols) imply that the result is somehow ‘fixed’ to give the observed result. Since we all agree that there are uncertainties in the forcings (which preclude strong statements about climate sensitivity being derived from the 20th century records for instance), is this criticism valid?

In the absence of any other constraints on either of these forcings and if their value was being defined a posteriori then he may have had a point. However, timeseries for solar forcing have been produced by groups unaffiliated with any modelling group, and the modellers have simply taken the values from the literature (Lean et al, 1995;2000;2005, Hoyt and Schatten, 1998) – any ‘fudging’ to produce the ‘correct’ answer would be immediately obvious. For aerosols, models are needed to produce the 3-dimensional distribution based on independently-derived emission data sets, but the validation is based not on the transient studies over the last 100 years, but on the satellite data and observations over the last 25 years. Once the various unknown parameters have been constrained as much as possible, they are fixed before the transient runs are started. However, aerosol modelling is indeed fraught with uncertainty, and so no group can claim that their resultant transient forcing is the unique best representation of the value found in the real world. Thus in papers such as Hansen et al (2005), it is clearly stated that the results are merely consistent simulations that match the surface temperature response and ocean heat content changes (as well as many other observations) but that this does not rule out a different combination of climate sensitivity and aerosol forcing having as good a match (see this post for more details). The point that needs to be emphasised is that all of these forcings are all very close to the ‘best guesses’ of the aerosol and solar communities.

Water Vapour

In the question session (Q143), Lindzen goes into more detail on the reason why he feels that climate sensitivity is so low – specifically, he believes that water vapour feedbacks are not only less positive than models suggest, but actually negative. That is he feels that the amount of longwave aborbtion by water vapour will go down as the planet warms due to increasing GHGs. This implies that actual water vapour amounts will decrease with increasing temperature. On the face of it this is a rather odd claim to make in general – the amount of water vapour that can exist in the atmosphere depends on the Clausius-Clapyeron equation that goes up with temperature. However, it is conceivable that convective processes might cause more extensive drying due to increased areas of subsidence (the basis of the so-called Iris effect), but this applies mainly to the upper troposphere and in the tropics only. As a general effect, reductions in water vapour as temperature increases in general seem rather unlikely.

But we can do better than simply speculating on the issue – we can look at the data and compare that to the models. The best examples to test this idea come from large and relatively rapid changes in the climate such as El Nino events, the eruption of Mt Pinatubo and the trends over the last few decades. In each case (Soden 1997; Soden et al 2002; Soden et al 2005), water vapour increases with warming, and decreases with cooling. There is some uncertainty about exactly how much it increases in the very uppermost troposphere (Misnchwaner and Dessler, 2004), but even those results show a positive feedback. So in summary, the data and the models both agree that not only is the water vapour feedback positive, it is quite close to the value suggested by the models – Lindzen’s insistence on the converse (while it has generated increased attention on the subject) seems increasingly perverse.

In general, I think it is incumbent on scientists when speaking to non-specialists to clearly deliniate what one’s personal opinion is, and what is generally accepted. That is not to say one should not state one’s opinion, but when a panelist specifically asks ‘how far your view of the role of water vapour is shared by other scientists?’ (Q144), one cannot honestly answer ‘That is shared universally’ when no other scientist in the field has made a case for a negative water vapour feedback. This is probably the most egregious mis-statement in the whole testimony and is deeply misleading.


Throughout his testimony, Lindzen refers to the global warming ‘alarmists’. In my dictionary an ‘alarmist’ is defined as ‘a person who alarms others needlessly’. However, Lindzen appears to define as ‘alarmism’ anything that links human activities to climate change. For instance, when discussing the statement from the NRC (2001) report (which he co-authored): The changes observed over the last several decades are likely mostly due to human activities, but we cannot rule out that some significant part of these changes is also a reflection of natural variability., he states that “To be sure, this statement is leaning over backwards to encourage the alarmists”. To my mind, this statement is actually a fair assessment of both the NRC report, and IPCC report to which it was referring. To claim that this is ‘alarmist’ is such a gross overuse of the term as to make it useless except as a rhetorical device.

Lindzen has frequently claimed that within the scientific community “alarm is felt to be essential to the maintenance of funding”. I have yet to see any empirical evidence of this and a brief perusal of active NSF grants related to climate change reveals a lot of interesting projects but none that jump out as being ‘alarmist’. Having sat on panels that decide on funding allocations and as a reviewer of proposals for both US and international agencies, my experience has been that these panels actually do a very good job at deciding which proposals are interesting, tractable and achievable. I have not seen even one example of where the degree of ‘alarmism’ was ever a criteria in whether funding was given. (NB. I don’t regard my own grants (viewable here) as remotely ‘alarmist’ and I don’t have too much trouble getting funding (fingers crossed!)).


In some ways Lindzen’s thinking on the climate change issue has not changed much since 1999, as can be seen in an older rebuttal of his position by Jim Hansen (scroll down to Table 1). However, he does seem to have become convinced that the 20th Century warming is real. What is interesting about the comparison between then and now, is that Hansen made two appeals to the data gathering community to test a) whether water vapour feedbacks can be observed, and b) whether the ocean heat content is increasing in line with the model predictions. It is quite telling that both of these data analyses have since been made and they confirm Hansen’s contentions, not Lindzen’s.

85 Responses to “Richard Lindzen’s HoL testimony”

  1. 51
    Hank Roberts says:

    Dr. Lindzen was on the air today, Michael Krasny’s “Forum” interview program — one of several people participating, although he had to leave to teach a class before listeners were able to call in with questions.

    I don’t have a transcript, just memory, but he didn’t speak very long.
    He said, if I have it right, that

    – there’s been much less warming so far than was predicted,
    – he sees no reason to be alarmed about changes observed,
    – he believes that most of the people doing research in the field had only gotten involved in it because of large amounts of research money becoming available in the past decade
    – only the GISS people, but not the other groups (he listed several) are claiming that 2005 was the warmest year yet.
    – temperature for the past ten years or so has “been flat” not increasing (no trend in the past ten years)
    – we are at a “high point” where any tenth of a degree can be a new record without being of any significance plus or minus.

  2. 52


    We have had frequent contributions to the climate sensitivities debate on this blog, where several fundamental questions remain unanswered. In the case of glacial-interglacial-glacial cycles, the direct forcing by CO2 changes is some 2 W/m2 (see Hansen, slide 12).
    But the effect on temperature is the sum of the changes caused by the forcings and their (different) sensitivities. In the case of CO2, in most cycles there is a (huge) overlap between temperature change (induced by Milankovitch cycles) and lagging CO2 changes. With one exception, the onset of the last glaciation. The lag of CO2 (in the Vostok ice core) is such large, that the temperature drop is near complete and ice sheet formation is at maximum, before CO2 levels start to drop (see here for the Eemian and post-Eemian plots).
    Ice sheet growth (including -semi- global sea surface/air temperature changes) is measured as delta18oxygen in the gas phase (from N2O) of the ice core, thus any dating error in lags between ice sheet formation, CH4 and CO2 levels is excluded.

    As there was no change of aerosols in the period of interest, and CO2 seems to have had little influence on (ice phase derived) temperature, near the whole temperature change at the onset of the last glaciation was caused by insolation shifts to other latitudes and ice sheet/vegetation feedbacks, with some help of CH4 and N2O. CH4 follows more closely temperature changes, while CO2 lags much longer, with in several periods of rapid temperature change no or little (and even opposite) changes.

    This points to a low sensitivity of climate to CO2 changes…

    [Response: No it doesn't. It points to a) the importance of other forcings in driving glacial-interglacial transitions (specifically the insolation changes), and b) a low sensitivity of CO2 to climate (the other way round from your conclusion). Remember that equilibrium sensitivity is only easily derivable from equilibrium situations, not transients. I have explained why I think that the LGM implies a mid range sensitivity - tell me where I am wrong in your opinion. -gavin]

  3. 53

    Re #52 (Gavin’s comment):

    That the transient sensitivity is lower than equilibrium sensitivity is of course true, but that is also the true for insolation, CH4, ice sheet, vegetation, aerosol,… shifts. The main problem is that in most cases it is impossible to know if there is a difference in sensitivities for different forcings, as in large transitions like glacials-interglacials-glacials, all (feedback) forcing changes are overlapping each other during the transition (that includes the LGM-Holocene transition). But in the exception described, most of the transition occurs in app. 10,000 years, I suppose long enough to reach a large part of the equilibrium for most forcing changes (including deep sea temperature changes), except for CO2, which lags all others. The CO2 decrease of 40 ppmv occured over a period of ~6,000 years, also long enough to reach much of the equilibrium, but the temperature cooled a little further and increased again in the same period (as ice sheets were melting again).
    I have not plotted the Milankovitch cycle induced insolation shifts (yet) as that may give some clue of the timing of insolation changes at different latitudes. It is difficult to obtain the relevant data…

  4. 54

    Re #50,

    Raypierre, there are some less nice examples were an outsider was conflicting with the scientific establishment of that time: Louis Pasteur. It took a lot of years, before the establishment accepted his work…

  5. 55
    Joel Shore says:

    Re #40 and 47: I also just recently read “Limits of Growth”. Having heard so much more about it from the point-of-view of the Wall Street Journal editorial page than from the environmental side, I expected to see a lot of embarrassing predictions for around the year 2000. So, I was also surprised to see that indeed such predictions are not there. I don’t think one can say that the future has yet proved LoG right at this point but it certainly hasn’t yet proven it wrong either.

    One of the few specific quantitative predictions one can find in the book is a prediction of the CO2 levels in the year 2000. When this prediction was made (~1970), CO2 levels were at 320 ppm, or about 40 ppm above the pre-industrial baseline. It was predicted that they would be at 380 ppm, or 100ppm above baseline, in 2000. In fact, that 380ppm is just being reached around now…so, what they predicted would happen in 30 years took about 36 years. That doesn’t seem like too bad a forecast to me, particularly given that this was made before the energy crisis of the mid-late 70s that resulted in substantial efficiency improvements in the use of energy in the U.S. (and presumably much of the industrialized world).

  6. 56
    Leonard Evens says:

    “They laughed at Galileo, they laughed at Pasteur, but they also laughed at Bozo the Clown.” I think that says it all as far as examples of scientists who went against the consensus is concerned. We don’t pay much attention to the many such scientists who were just plain wrong.

  7. 57
    Steve Bloom says:

    Re #51: There’s a free audio archive of this program (and all Forum programs going back several years) at . KQED is the main San Francisco NPR station. They had another GW program in January for which the skeptic of the day was Jim O’Brien (who made an utterly ridiculous claim – unrefuted perhaps because none of the other participants knew enough of the details to say anything – that he had gotten Kerry Emanuel to make a major change in his conclusions regarding increasing hurricane energy).

    Re #53: This exchange highlights the difficulty in establishing the extent of some of these forcings going back over millions of years. That said, an important part of the background that often doesn’t get mentioned is that the current cycle of intense glaciation (the Pleistocene) is a rare extreme when contrasted with the last few hundred million years, and that the obvious difference between this period and the much more extensive prior warm times is that CO2 levels now are much lower than they were then; i.e., normal conditions are little or no ice combined with high CO2 levels. One can postulate (as I think Ferdinand does) that the prior warm times are mainly due to their having been much greater insolation, with the increased CO2 levels being primarily a feedback, but as I understand it there is no evidence for the sun being that type of short-term variable star. What is known is that main sequence stars go fairly evenly from dim to bright over the course of their lifetimes. If I recall correctly the sun will increase its brightness by about 75% over its lifetime, the difference in brightness over the last couple hundred million years then being on the order of 1%. For insolation change to be the main forcing over that time, we would need to imagine either that there was extensive short-term variability to overcome the overall brightening, or that some combination of other forcings did the job. Considering all of this, it’s perhaps not impossible to come up with a scenario where insolation change is more likely than CO2 to be the key forcing in the present climate, but to try to do so begins to seem a little strained.

  8. 58
    Hank Roberts says:

    Is there any trace in the ice cores of changes in meteor dust, if that’s visible at all? I know comets don’t leave traces – I mean something changing the level of dust the earth’s atmosphere picks up for millenia but without a lot of chunks large enough to make craters. Maybe a time when the sun was moving through a thick stretch of interstellar dust — which would lower insolation independent of any change in the sun’s behavior and perhaps for a short time.

    Do the ice cores resolve anything that’s not attributable to terrestrial sources by composition?

  9. 59
    Steve Sadlov says:

    RE: “However, it is conceivable that convective processes might cause more extensive drying due to increased areas of subsidence (the basis of the so-called Iris effect), but this applies mainly to the upper troposphere and in the tropics only. As a general effect, reductions in water vapour as temperature increases in general seem rather unlikely.”

    A possibility: Increased energy content in the atmophere results in dynamics which cause the subtropical high pressure centers to grow in size and radial velocity. We go into something resembling a permanent slight La Nina mode (or at least La Nina dominates). That certainly would have effects beyond the tropics. Of course, this assumes that all of the added energy balanced out into actual dynamics, as opposed to simply being lost out into space.

  10. 60
    Steve Sadlov says:

    RE: “That said, it’s also clear that glaciations (which have not been present throughout much of history, with ice-free conditions existing as recently as 30M years ago and the current severe glaciations – the Pleistocene – having kicked in just a couple of million years ago) have not been able to exist in the past whenever CO2 has been above a certain level in the atmosphere.”

    It was a long time ago. I attended a talk by I believe Arawmick (again, it may have been someone else, but I think it was he) that proposed something a bit different from this. Namely, that the Pleistocene may have started with the closure of the Isthmus of Panama during the previous orogeny. Again, this was long ago and no doubt “discredited” by numerous “climate scientists.” Just thought I’d share my own experience here.

    [Response: Not sure what you're trying to imply, but the reason why the Isthmus of Panama idea is no longer subscribed to is simply that it appears to have occurred much earlier that the Pleistocene - more like 4.6 million years ago (Ravelo et al, 2004; Haug et al, 1998), which is too early for the effect you claim. -gavin]

  11. 61
    Steve Sadlov says:

    RE: “Not sure what you’re trying to imply, but the reason why the Isthmus of Panama idea is no longer subscribed to is simply that it appears to have occurred much earlier that the Pleistocene – more like 4.6 million years ago (Ravelo et al, 2004; Haug et al, 1998), which is too early for the effect you claim. -gavin]”

    I claim no specific effect. That said, 4.6M BP is not all that far in advance of the inception of the Pleistocene, in geological time terms. Let us imagine the orogeny closed the gap and that a number of domino like effects might have then proceded over some stretch of time. How long might it have taken the ocean conveyer to fully settle into its new regime? How long might corresponding changes in atmopheric circulation, after that closure, have taken to fully have their own impacts on the ice balance and on the Arctic Oscillation? Application of a “waveform” (e.g. the transient effects on ocean currents among other impacts) to the Earth’s complex “network” might have resulted in “ringing” and other harmonic effects(to borrow from the language used in field such as seismology and electronics).

    [Response: Not likely. The timescales for the atmosphere-ocean system to equilibriate to a change in the basin geometry are on the order of centuries to millennia at most. For resonances on million year timescales you need something tectonic. That isn't to say that the closing of the Isthmus wasn't necessary before the Pleistocene glaciation could occur, but it was not the proximate cause. You might want to read Haug et al (2005, Nature) for some other ideas of what happened at 2.7 Ma. - gavin]

  12. 62
    Hank Roberts says:

    I didn’t find the Nature article, but did find this; is it a fair abstract, would you say?

  13. 63

    In addition to #59,

    The Iris hypotheses of Lindzen was restricted to the “warm pool” Pacific, where deep convective clouds occur. The amount of cirrus clouds may be reduced if the SST is increasing, leading to faster convection and drying out of the upper troposphere. This would help with the escape of extra IR radiation to space, thus a negative feedback.

    The iris hypotheses was quite regional (within the warm pool) and difficult to detect (later work by Lin ea. found an opposite effect: a small warming over the warm pool, but there still is discussion. One of the arguments by Lindzen is where to put the border for cirrus cloud detection).

    Later work of Wielicki ea. and Chen ea. discovered that the Walker (between warm and cold parts of the tropics) and Hadley Cell circulation (between tropic regions and the subtropics) increased in strength. The total radiation budget over the whole tropics (30N-30S) changed (~5 W/m2 more IR to space and ~2 W/m2 more insolation, a net loss of ~3 W/m2), less upper troposphere humidity, less clouds and higher vertical air velocities, together with an increase of ~0.085 K/decade of sea air temperatures in the period 1985-1994.

    While it is rather difficult to know what is cause and effect in this case (higher SAT leading to faster circulation and less clouds, or less clouds, leading to higher SAT, or both), the net result is an increased loss to space of ~3 W/m2 in only 15 years. That is higher than the total forcing of greenhouse gases since the start of the industrial revolution, for halve of the earth’s surface.

    The origin of the changes is suggested to be from natural causes (internal variability, solar cycle(s) – the latter seems plausible, as -low- cloud cover follows closely the solar cycle) by the authors. Thus while Lindzen may be wrong with his Iris hypotheses, due to too small scale of the effect, on the whole tropics, there seems to be a direct connection between SAT and changes in radiation budget, which points to a negative feedback.

    Some similar changes in cloud cover happen at higher latitudes too, but the radiation budget by more or less clouds cancels out there. In the Arctic, again cloud trends are reducing the summer warming (more clouds) and increase winter cooling (less clouds).

  14. 64

    ad #11:
    Stefan’s last sentence suggests that if you care for the future of your children, you MUST agree to (his) views on climate change and AGW. This is quite an unscientific “procès d’intention”: I am the father of 4 children, and their future life environment surely matters for me. But this is not reason enough to embrace without restriction every theory or opinion held by Stefan, Gavin et al.

  15. 65
    Coby says:

    But given you concern for your children, why in the world would you not act given the list of scientific institutions that have all concluded there is a real danger? It is hardly just Gavin and Stephen’s “opinion”.

    UK RS

    Every major scientific institute dealing with climate, ocean, atmosphere agrees that the evidence says the climate is warming rapidly and the primary cause is human CO2.

    See also this joint statement endorsing the conclusions of the IPCC issued by the Australian Academy of Sciences, Royal Flemish Academy of Belgium for Sciences and the Arts, Brazilian Academy of Sciences, Royal Society of Canada, Caribbean Academy of Sciences, Chinese Academy of Sciences, French Academy of Sciences, German Academy of Natural Scientists Leopoldina, Indian National Science Academy, Indonesian Academy of Sciences, Royal Irish Academy, Accademia Nazionale dei Lincei (Italy), Academy of Sciences Malaysia, Academy Council of the Royal Society of New Zealand, Royal Swedish Academy of Sciences, and Royal Society (UK).

    How much more warning do you need?

  16. 66
    User2082 says:

    Being a non-specialist, I think it is important to be able to discuss in layman’s terms what we know and don’t know about climate change. What do you say to this comment, which came up with a discussion that I was having with someone on another forum?

    “Given the fact that the earth is coming out of an ice age that ended about 12,000 years ago, and that temperatures normally rise after periods of glaciation end, what is the net addition that mankind has added to this overall warming? and can you also account for temperature rises such that which occurred during the medieval optimum that are clearly not driven by human derived green house gasses?”

    Is there a source for this . . . and with respect to the rise in temperatures during the medevial period, does my friend bring up a valid point?

  17. 67
    Joel Shore says:

    Re #65: The joint national academies statement that you linked to is now a few years old and there is a more recent one that states its conclusions in even somewhat stronger terms…and also includes the U.S. National Academy of Sciences as one of the signers:

  18. 68
    Coby says:

    Thanks! I will update my talking points ;)

  19. 69
    Chris says:

    Regarding observations of water vapor, an important study came out this summer supporting a positive water vapor feedback that was not metioned in the (excellent) piece by Gavin. Trenberth et al. (2005) find that column-integrated water vapor path has increased globally since 1987, based on observations from satellite-based passive microwave observations. The increases are roughly in line with Clausius-Clapeyron and a constant relative humidity assumption (i.e., increasing SSTs have enhanced oceanic evaporation and led to increases in water vapor).

  20. 70
    Coby says:

    Re #66:

    The rise out of the last glaciation ended ~10,000 years ago and since then it has been relatively stable with a slight cooling until the anthropogenic disturbance we kicked off 100+ years ago. You can refer to this nice graph of various temperature reconstructions of the Holocene (the period since coming out of the last glaciation)

    As for the MWP, it seems according to the best available evidence that it was not very pronounced as a global effect, though it may have been so regionally some places.

  21. 71

    Re #65:
    Actually, I do not need more warnings, I am drowning in warnings! I think most intellectual honest people agree that there is a warming, and that human emissions is one amidst other factors contributing. The difficulty is estimating the importance and danger of this warming, and compare climate change danger to other perils that exist (like terrorism, pandemics etc…). When Hansen tells us that the tipping point will be reached in 7 to 10 years, he leaves not much hope for effective action. It is absolutly impossible to change completely to non fossil fuels in such short a time span; even with the best efforts, this switch-over (that most of us applaude!) will take time. So as a good teacher, will you discourage everybody by giving them impossible targets? And there remains the nagging doubt: what if Hansen is wrong? what if global warming will not accelerate, but slow down…. Is this such an impossible scenario?

  22. 72
    Coby says:

    I disagree completely with your profession of impossibility. There are literally hundreds of steps, small and large that can be enforced almost immediately with technology currently available. I don’t necessarily disagree that it is not going to happen.

    But without going into the details I would just like to say: whether you believe you can, or you believe you can’t, you’re probably right.

  23. 73
    llewelly says:

    Re 71, Francis Massen:

    # And there remains the nagging doubt: what if Hansen is wrong? what if
    # global warming will not accelerate, but slow down…. Is this such an
    # impossible scenario?

    A slow down of global warming is exactly the scenario folks like Jim
    Hansen are trying to cause. If we reduce or eliminate human
    emitted GHGs, global warming will slow down long before it stops.

  24. 74
    Roger Smith says:

    Re: 65, please review exactly what Hansen is saying. He talks about the need to stabilize and then start to reduce emissions levels (not atmospheric concentration) of greenhouse gases over the next 10 years. This is consistent with a recent post about whether avoiding 2 degrees of warming is feasible. Hansen is basically giving a reasonable deadline for starting serious efforts to curtail GHG emissions, suggesting no-regrets policies like reducing “black carbon” soot, and pointing out that *when* we act is significant- we can’t wait forever.

  25. 75
    C. W. Magee says:

    Re: #36:

    Is there any observational method which can determine which currently glaciated areas were ice-free during the last interglacial? Obviously the total ice mass must have been about 6% smaller, but where did it melt from?

  26. 76
    John Finn says:

    From your post

    so it would make much more sense to define the greenhouse effect as the amount of LW absorbed (~150 W/m2). In which case, doubling of CO2 is initially more like a 4% effect, but as soon as any …

    If we’re going to be pedantic the total greenhouse effect is nearer to 155 W/m2 and the forcing for a doubling of carbon dioxide is given by alpha x ln(2) where alpha=5.35 (why it’s as high as that I’m not sure) therefore the forcing for 2xCO2 would be 3.7 W/m2 which is around 2.4% of the total gh effect – so Lindzen’s 2% is closer.

    [Response: You are correct. I have amended the post. Thanks. - gavin]

    As for any feedback effect, I’d like to see the evidence that this is definitely positive.

    [Response: The Soden papers, Trenberth et al, Del Genio etc. -gavin]

  27. 77
    Steve Sadlov says:

    RE: Response to #61. So, if I read your reply correctly, you are willing to hang your hat on the assertion that there are no possible harmonics that could have an unexpected impact outside of a time scale of thousands of years? Do I understand your assertion correctly?

    [Response:Pretty much. I would state that I am >99% sure that there are no significant power at million year timescales in the coupled ocean/atmosphere/sea ice system. The only source of variability on those timescales is tectonic (including volcanic) and (conceivably) orbital, plus random stuff like asteroid impacts and the like. -gavin]

  28. 78
    Hans Erren says:

    How about multidecadal timescales, are all observed harmonics caused by external forcing? I don’t think so…

    [Response:Nor do I. Which is why you need to be very careful about attributing decadal variability to (for instance) solar cycle forcing.... - gavin]

  29. 79
    Hank Roberts says:

    I’ve wondered about tectonic time scale — has anyone mapped the kind of material being subducted over deep time?

    I understand we get new material up that’s spreading from rift zones, being covered with sediment, then all that pushed back down under the next continent to come along.

    Do we get layers of deep ocean sediment forming different kinds of material by the time it experiences subduction — so what’s being subducted in one geological period may be much higher in say carbon or calcium or iron or oxygen, because worldwide there was a long episode of a particular climate, long enough that all of its seafloor sediments are characteristic of that climate, then subduction’s chewing on that particular sort of content worldwide for a long while (and so the volcanos pushing up that material would also have a signature)?

    Or is the time scale such that the continents ‘average out’ what’s being subducted at any given, er, aeon?

    Perhaps an easier question (still one I can’t guess at) — is there any modeling done on this scale? I know the ‘snowball Earth’ idea that we might get ice shutting down primary production worldwide, followed after a long time by melting as volcanic CO2 built up.

  30. 80
    Steve Sadlov says:

    RE: #79. This would indeed be a fascinating area of further extensive study. Very old ocean crust would tend to be less mafic than newer crust. Consider crust being subducted by the Aleutian Trench vs. that being subducted by the Cocos Trench. Clearly, the former has a much greater amount of sediment on top of it than the latter. The chance that the former would contain significant variations in composition is greater than the latter. The former would be more rife, at the bulk level, with lighter phases. Etc.

  31. 81
    David B. Benson says:

    Re #75 and then #36 –

    The references I read all say the oceans were 6 m higher during the Eemian interglacial. So ‘about 5 m’ is in agreement. There is no reason to believe any of the largest ice sheets completely melted to produce this rise in sea stand. For example, some of Greenland and some of the West Antarctic ice sheets melting could produce the needed meltwater.

    By now there are many ice cores from Greenland. I don’t know that any of these offer evidence of partial melting during the Eemian, but then I find interpreting the ice core data interesting and difficult.

    Lastly, there are many fine books on Quaternary geology which may assist one in understnading the complexities. While somewhat older, I profited from studying

    A.G. Dawson
    “Ice Age Earth: Late Quaternary geology and climate”
    Routledge, 1992

    which by the way, certainly owes much to Shackleton.

    Might be our wonderful RealClimate hosts may care to suggest a more recent volume. If so, I’ll read it as well.

  32. 82
    David B. Benson says:

    Following up on #81 –

    Web trawling quickly located a gvnmt site regarding Greenland ice cores. The two long cores stop being readable, according to a summary there, at 110 kya. A possible interpreation, my own, not theirs, is that the Greenland ice sheet was losing mass until that date.

    There is ice below, but it appears that the drilling stopped about 200 m below the boundary of noninterpretable. This is
    consistent with a partial melting of the Greenland ice sheet during the Eemian interglacial. However, other causes may have
    been in play.

    The only safe one can say, which is what the web site said, was
    that there is no interpretable data to be recovered below the depth corresponding to 110 kya.

  33. 83
    Alan says:

    Three words: google Lindzen funding

  34. 84
    Neil Fisher says:

    You say that more water vapour will increase temperatures (or at least you imply it) – have you considered that more water vapour leads to more clouds, which increases albedo? Can anyone tell me which is more significant? (ie, does the GH effect out-weigh the higher albedo or visa versa)

    [Response: The effect of water vapour itself is a postive feedback. The changes in clouds aren't simply tied to water vapour changes (although they are related). However, the net effect of the cloud feedback is made up of both an albedo effect and a greenhouse effect and the net sign is currently unknown. - gavin]

  35. 85

    [...] and verified evidence. Quite simply, his words are false, and well known to be. See for example; Lindzen’s House of Lords testimony and Lindzen point by [...]

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