Hockey sticks: Round 27

Two more teams in the seemingly endless jousting over the ‘hockey-stick’ have just made their entry onto the field. In the first two (of four) comments on the original McIntyre and McKitrick (2005) (MM05) paper in GRL, von Storch and Zorita, and Huybers have presented two distinct critiques of the work of M&M.

The two comments focus on the ‘PC normalisation’ issue raised in MM05 which we discussed previously. Specifically, von Storch and Zorita show that in a GCM model emulation of the Mann, Bradley and Hughes (MBH) method, changing the PC normalisation technique makes no difference to the eventual reconstruction (i.e. it is not the normalisation that creates the ‘hockeystick’), consistent with earlier conclusions. Huybers comments that neither of the two suggested normalisations are actually optimal, and proposes a third method which looks like it gives results halfway between MBH and MM05. However, given the von Storch result, this too is unlikely to matter in the final reconstruction.

Huybers additionally makes an interesting point regarding the calculation of significance levels in MM05 and shows that a crucial step (the rescaling of variance of the proxies to match the variance in the instrumental calibration period) was missed out. Including it produces results identical to MBH.

For each comment comes a reply, and in the M&M responses, they introduce a number of further complications and focus on the quality of some of the proxies that were input data into the MBH methodology. We note as an aside that this is quite a different criticism than claiming that MBH’s methodology contains ‘coding errors’ (to quote one of the Ms). Indeed, the quality of paleo-climatic data and its relationship to climate variables has been discussed all along (see for instance MBH99).

Their further calculations will take time to assess, but of the original claims in MM05, the first (the PC normalisation issue) demonstrably makes no difference to the reconstruction, and the second (the calculation of the significance of the RE statistic) was just wrong. So for this round at least, it looks like ‘Hockey Team: 2, MM: 0’.

Look out for the next bout coming to a journal near you…

133 comments on this post.
  1. Lynn Vincentnathan:

    RE Gavin’s response in #98. So where are we on the logarithmic curve re IR absorption? Would the tangent line be more toward the flat side, or steep side? Or rounding the bend with a moderate slope?

    Even if more toward the flat side, we’d still want to reduce GHGs, Nanny, because every little bit of harm hurts. But instead of banning SUVs, why not make them electric, or hybrid (I think some are), or hydrogen? Or move closer to work/school. Or run multiple errands. Or at least keep them tuned and tires properly inflated.

    [Response: It’s self-similar, there is no flat bit. Instead of forcing increasing by +Y (w/m2) for every +X in CO2 (ppmv), it increases by +Z for every *(1+X) in CO2. Ie, the forcing increase from 1*CO2 to 2*CO2 is the same as from 2*Co2 to 4*CO2. Etc – William]

  2. nanny_govt_sucks:

    #101 – “Even if more toward the flat side, we’d still want to reduce GHGs, Nanny, because every little bit of harm hurts. ”

    Harm? What harm? The connection between GHGs and extreme weather has not been shown. Droughts are likely cyclical. Other than a slight rise in sea levels over the course of 100-200 years, I see virtually all positive aspects to warming. A warmer, wetter CO2-fertilized planet will be and has been a boon to plants and the animals (including humans) that feed on them. Longer growing seasons mean more farm production. Expanded fauna habitats mean enhanced biodiversity. Green is good. Cold, freezy planets are bad. More info:

    Global Warming Sparks Increased Plant Production in Arctic Lakes
    “Biological activity in some Arctic lakes has ratcheted up dramatically over the past 150 years as a result of global warming, according to a new study.”

    Earth is becoming a greener greenhouse
    “Our results … indicate that the April to October average greenness level increased by about 8% in North America and 12% in Eurasia during the period 1981 to 1999.”
    “the growing season is now about 12 ± 5 days longer in North America and 18 ± 4 days in Eurasia”

    Climate driven increases in global terrestrial net primary production from 1982 to 1999
    “Most of the observed climatic changes have been in the direction of reducing climatic constraints to plant growth.”

    Greening of arctic Alaska, 1981-2001
    “Here we analyzed a time series of 21-yr satellite data for three bioclimate subzones in northern Alaska and confirmed a long-term trend of increase in vegetation greenness for the Alaskan tundra that has been detected globally for the northern latitudes.”

  3. Thomas Lee Elifritz:

    Sorry, there is nothing mythical about the Paleocene – Eocene Thermal Maximum.

    Just as there is nothing mythical about the new geological era, more commonly referred to as the ‘present’, soon to be the ‘future’.

  4. Lynn Vincentnathan:

    Sorry you misunderstood the anthropological concept “myth,” which refers to truth, under which natural history, history, and science would fall – that’s our truth, thus our myth. Unfortunately in common parlance “myth” means falsehood or fiction. Just like some people might use “theory” to mean something unproven (as in “it’s just a theory, it’s a myth”). Falsehood or fiction is the stuff of folktales, novels, science fiction….

    I’ve been alerting my students to AGW and more recently to past extincitons from GW as reality, not fiction. For the first time in 15 years since I’ve been talking about AGW, they are beginning to take it seriously, even though I’m the only professor I know of on campus talking about it. So, if I can squeeze it in somewhere in my courses, I do. Too bad I’m not teaching science — that would be good place to teach it….and I heard 3 years ago sci profs here were not teaching it; maybe that’s changed since.

  5. Steve Bloom:

    Nanny, where did you get those cites? If you found them yourself rather than picking them off one of “those” sites (which I hope is not the case, as that would be a form of trolling), then you ignored a bunch of recent work like (quoting from the conclusions):

    “Overall, the future trends are likely to be further increases in stem mortality and recruitment rates, with mortality overtaking recruitment, leading to declines in stem density at an increasing number of sites. Simultaneously stand-level growth is expected to asymptote or decrease and biomass losses from mortality continue to rise. As a result stand biomass may well decrease in surviving old-growth forests well within the current century. While there is considerable uncertainty on the future trajectory of the drivers and the responses of tropical forests, the expected changes in the drivers plausibly predict that the current C sink contribution of mature tropical forests to buffering the rate of climate change is very likely to diminish and quite possibly reverse in the future (e.g. Cox et al. 2000).

    “The drivers of change we document can be grouped into four categories: those caused by
    “(i) decadal-scale natural climatic oscillations;
    “(ii) fossil fuel emissions and resulting climate change;
    “(iii) increasing industrialization in the tropics; and
    “(iv) the further integration of forest products and land into expanding national and international market economies.

    “To slow or halt widespread changes in remaining tropical forests will require large reductions in fossil fuel emissions, a different form of development in tropical countries than has occurred in temperate nations, and a minimum of carefully regulated markets. Under â??business as usualâ?? conditions, rapid global changes will continue to alter the worldâ??s remaining tropical forests with global consequences for biodiversity, climate and human welfare.”

    Remember also that studies of this sort may not consider large-scale climatic shifts such as the Amazon basin switching from rain forest to savannah, which would involve a very large scale loss of carbon to the atmosphere (and would take place by means of severe droughts just like the one happening now).

    If you sincerely want to learn more about this aspect of the science, use Google Scholar and note in particular the “cited by” link that allows you find the most recent work. Survey articles in the major pubs within the last two years or so are probably the best bet for getting the state of the science, although unfortunately the article titles don’t always make it obvious which articles are the surveys.

    I notice that the third article you linked is a survey, but it appears to a little long in the tooth. I checked on the Science site (where the article originally appeared) and found :

    Abstract: “Drier summers cancel out the CO2 uptake enhancement induced by warmer springs

    “An increase in photosynthetic activity of the northern hemisphere terrestrial vegetation, as derived from satellite observations, has been reported in previous studies. The amplitude of the seasonal cycle of the annually detrended atmospheric CO2 in the northern hemisphere (an indicator of biospheric activity) also increased during that period. We found, by analyzing the annually detrended CO2 record by season, that early summer (June) CO2 concentrations indeed decreased from 1985 to 1991, and they have continued to decrease from 1994 up to 2002. This decrease indicates accelerating springtime net CO2 uptake. However, the CO2 minimum concentration in late summer (an indicator of net growing-season uptake) showed no positive trend since 1994, indicating that lower net CO2 uptake during summer cancelled out the enhanced uptake during spring. Using a recent satellite normalized difference vegetation index data set and climate data, we show that this lower summer uptake is probably the result of hotter and drier summers in both mid and high latitudes, demonstrating that a warming climate does not necessarily lead to higher CO2 growing-season uptake, even in high-latitude ecosystems that are considered to be temperature limited.”

    So, perhaps not quite an improved planet.

    (RC team: I don’t recall and couldn’t locate an RC post on this subject, but it seems like a good candidate for one.)

  6. Stephen Berg:

    Re: #102,

    Your head must be in the sand. No negative results of GW…

    How about sea level rise which will force millions of people from their homes? Mass desertification of areas normally agriculturally fertile. Mass extinctions (brought upon by US!).

    Come on, Nanny. Haven’t you heard what’s going on and what’s predicted?

  7. TCO:

    William (response to my comment, which was posted, then commented on, then deleted…btw, huh?):

    I’m not asking for an unmoderated board. I know that ad hominems and trolling are forbidden…here. I am asking to be allowed to make points and arguments on the hockeystick issue. To be allowed to debate and make converse arguments in keeping with the blog policy.

    [Response: If your comment was deleted, then you stepped over the invisible line. Have another go, but more carefully… – William]

  8. Joseph O'Sullivan:

    Re #102 (Nanny)

    You are repeating the CO2 is a fertilizer and warmer is better line.

    The one area of climate change science that I have more then a basic understanding is the ecological effects of climate change.

    CO2 in the oceans is not acting as a fertilizer, instead it is a pollutant. CO2 is not enhancing growth or primary productivity because in the oceans because CO2 is not the limiting factor. Read the Acid Oceans post or this recent paper in Nature
    In terrestrial ecosystems increased CO2 has only had a limited effect on increased primary production.

    Warmer doesn’t mean everything will grow more. All ecosystems are adapted to specific environmental conditions. Changes in climate cause force ecosystems to adapt mostly by shifting geographically, and if they are not able to shift individual species and ecosystems become locally or globally extinct.
    A good summary of the scientific literature on climate change and ecosystems is here

    Climate change in Alaska is creating problems for the communities there. Melting permafrost is causing erosion and major structural damage to buildings, roads and oil drilling facilities. Warmer temperatures have caused trees to die and forest fires have become serious problems as a result.

  9. Janne Sinkkonen:

    Re #102 (on greening the planet by CO2)

    The “greening” effect of GW depends on what time scale one is looking.

    If you look at the effect of raising temperature at the time scale of millions of years, then warming might indeed lead to increased biomass, extended tropical forests and higher biodiversity.

    GW as it happens now, however, is an abrupt shock into which ecosystems and species are not likely to be able to respond by adaptation. Moderate warming is likely to lead to moderate biodiversity loss, and excessive warming is likely to cause mass extinction. See for example a recent Nature article by Thomas et al., according to which “[…] 15â��37% of a sample of 1,103 land plants and animals would eventually become extinct as a result of climate changes expected by 2050.”

    In the absence of truly catastrophic warming, biodiversity is likely to return, but that will take 1-10 million years. So if you care about species richness, not about “greenness” in the sense of palm trees in your backyard, this is serious stuff.

  10. Dano:

    To add slightly to JO’S’s excellent comment,

    the Beeb today points out that the rate of ecological change is faster than the rate of many species’ adaptation. Also, NPP as measured has a limited benefit to humans, as there is evidence that graminaceous crops are less nutritious in an increased CO2 environment, and it is well-known that rising temps decrease yields, as most row crops are at the upper limit of their heat tolerance [as the recent cool season/hi yields in US and heat wave/lo yields in Europe attest].

    We don’t know when or where ecosystems will flip. This, in itself, is enough reason for serious discussion of policy action – this requires energy, energy that is wasted on picking nits off of an old paper.



  11. Hans Erren:

    winter temperatures are rising faster than summer temperatures

    Why is that?

  12. Ferdinand Engelbeen:

    Re #84 (comment)

    Mike, thanks for the comment, have reread several of the studies mentioned, but it still seems to me that the effect of solar is underestimated in the models. As can be seen in Fig.6 of the poster by Briffa e.a., the impact of large volcanic eruptions is in average some 0.1 C over the whole period 1400-2000, with a few quieter periods (1490-1570 and 1750-1790). This is based on tree rings (but I wonder if the fact that there is less incoming sunlight after an eruption, also has an impact on tree rings, besides temperature).

    That means that the rest of the temperature decrease during the LIA was caused by less solar activity (and some from a CO2 feedback on the resulting ocean temperature decrease). For the MBH98 reconstruction, the residual LIA low is app. -0.2 C, for Moberg, this is -0.7 C, a factor 3.5…

    As can be seen in Fig. 6 of what Gavin and you wrote, current models seem to overestimate the influence of volcanic, thus probably underestimate solar influences. Anyway, the past fit of the models point to a higher impact of solar, depending of the chosen reconstruction, and consequently a projection to a lower side of the IPCC range…

  13. Stephen Berg:

    Re: #111, “winter temperatures are rising faster than summer temperatures

    Why is that?”

    Just as nighttime temperatures are rising faster than daytime temperatures, the planet is retaining more heat than losing it in traditionally cooler periods.

    I’m sure the IPCC reports talk about this issue.

  14. Mark A. York:

    Here you go, more nutcases deriding your fine work here. Please rip them to shreds.

  15. Mark Rostron:

    CO2 concentrations in the earths atmosphere hover around the 300 ppm (parts per million), 0.0003 or 0.03%.

    The human contribution to this is around 2%, or 2 parts in a hundred, or 0.02.

    Can anyone explain to me how 0.000006 (0.0003*0.02) or 0.0006% is in any way a material number in the context of the earths atmosphere?

    And how a reduction of the human contribution by say, 10% of the total annual contribution, or 0.000006 (0.0006%) would make any meaningful difference to anything?

    [Response: CO2 levels have been well above 300 ppmv for quite some time now. will provide you a nice graph and a link to the data sources. As you can pretty clearly see from that graph, and other measurements also show, humans have increased CO2 levels from pre-industrial (280 ish) to now (380 ish). Also covered by RC. So all your numbers are wrong. As to the basic science, try this – William]

  16. Ferdinand Engelbeen:

    Re #113,

    Stephen, there is a notable exception: the Arctic. While the influence of CO2 must be visible more than in other latitudes (less absorption overlap by water vapour, due to much dryer air),

    [Response: This is a common assumption, but I’ve never seen any evidence for it. Greater warming in the Arctic is usually put down to icesnow albedo feedback – William]

    winter ice is declining far less than summer ice, which means that winter refreezing is near as high as summer melting, and increases over the last decade (see [Response: Arctic ice is decreasing in all months, though the summer trends are largest: see – W]

    This is caused by a decrease of cloud cover in winter (opposite to an increase in summer) over the Arctic. Cloud cover seems to have the largest influence on climate, be it that it is itself a feedback on drivers like solar and GHGs and related/unrelated oscillations. In this case it looks like a negative feedback on Arctic (or global?) warming.
    The full article by Wang and Key in Science mentions the cloud radiation changes as follows:
    “The wintertime net cloud forcing, which is primarily longwave forcing, has decreased at a decadal rate of -4.50 W/m2 (decreasing warming effect) in response to the decreasing cloud amount and surface temperature” and
    “During summer the large solar flux, increasing cloud amount, and decreasing albedo result in a trend toward increasing cloud cooling by -5.17 W/m2 per decade. There is also an increasing cloud cooling trend in the fall, largely a result of decreasing albedo, because there is only a weak trend in cloud amount. The annual trend in net cloud forcing is -3.17 W/m2, indicating an increasing cooling effect.”
    Compare that to the change in radiation balance by GHGs, which is less than 1 W/m2 in the past decade(s)…

    [Response: If your figures made sense, the decrease in Arctic sea ice wouldn’t be happenning – W]

  17. Janne Sinkkonen:

    Re #115

    Where have you been the last 50 years? :) The CO2 concentration is not howering around 300ppm but is around 380ppm now, and raising 1.5-2ppm per year. Accordingly, human contribution to the current atmospheric CO2 concentration is about 36%, not 2%. To reassure youself of this, use the Google search engine for ‘co2 atmospheric concentration’ and read the short RealClimate discussion on the topic.

    The bottom line on the greenhouse effect is that without it the earth would be about 33 Celcius degrees cooler than it is now, and that CO2 plays a significant part in this difference. Small concentrations of CO2 are significant for energy balance just like small concentrations of CFC’s are significant for the ozone layer, or, to use an even more extreme example, small concentrations of crack in the bloodstream are significant for cognition. Large effects do not always require high concentrations!

    In the absence of a better model you could assume the effect of CO2 concentration on temperature linear. Then, as a first approximation, doubling the CO2 would double its current warming effect. If the current contribution of CO2 to the 33K warming is 20%, doubling the original CO2 concentration would raise global temperature by 6.6K. About the relative importance of CO2 and other gases, google for “climate forcings”.

    In reality things are much more complex, of course, but I hope this simple line of thought helps you to understand the significance of CO2.

  18. Mark A. York:

    Here’s another fallacy from a comment on that same site.

    “However, melting of the arctic ice sheet would have virtually no effect on sea levels since most of this ice essentially is already floating in water (remember Archimedes jumping out of his bath tub and running through the streets shouting Eureka!). The arctic danger to sea levels would be if the Greenland ice sheet started to melt in a major fashion, since that ice (like Antarctica) is over land.”

    [Response: This looks like the usual confusion over words. The Arctic *sea ice* is indeed floating, and its melting would only affect sea level a tiny bit. There is no Arctic ice sheet, unless they have renamed the Greenland ice sheet – William]

    This same commenter was easily convinced that “realclimate was partisan” mind you. This is the complete up-is-downism being used against sound science today. It’s very discouraging.

    I’m not a physical scientist like you guys, a fish biologist, but this sounds completely offbase like the rest of the propagandists’ arguments. Where would the water go if not adding to the volume of water in the connected oceans?

  19. Ferdinand Engelbeen:

    Re #116 (comment):

    William, the influence of more CO2 is working everywhere, including (and even more at) the Arctic.

    [Response: Repeating the same evidenceless assertion won’t make it any more true. I asked you for some sources for it – do you have any? – William]

    This should lead to more retention of IR radiation in winter, thus warming, but the trend is more cooling (-0.34 C/decade at >60N, -2.2 C/decade at >80N, see Science), due to less clouds in winter. This is more than compensated by a temperature increase in all other seasons, the yearly trend is +0.57 C/decade in the past two decades (but the trend is tempered by more clouds in summer).
    The cooling in winter is large enough to refreeze most of the ice that was lost in summer, but not all: from 5 million km2 in 1940 to 7 million km2 in 2005. As summer ice loss is increasing more rapid than winter refreezing, the overall trend for all months is negative.
    Albedo changes are ambiguous in this case: less albedo means more warming in summer, but also more cooling in winter.

    [Response: Why more cooling in winter? – William]

    But again the yearly trend is negative in albedo and heat balance.
    Thus while clouds have an impressive influence on Arctic ice extent – compared to GHGs – the overall balance remains negative, probably as result of increased albedo, which in its turn may be caused by increased advection of heat from lower latitudes.
    All together a challenge for current GCM’s which have problems with cloud feedbacks…

    [Response: You are (as so many skeptics seem to be) obsessed by the idea that almost anything other than CO2 must be causing changes… As to the challenge: indeed: GCMs simulate the retreat of Arctic ice – William]

  20. wayne davidson:

    #116- Ferninand’s theories don’t match reality, it is very cloudy now in darkness – no cloud albedo to worry about. The clouds now do the opposite as when present during the summer, they keep a lid on heat trying to escape to space, therefore October was very warm in the Arctic (won’t be surprised if it is #1 warmest Arctic temperatures in history), these early winter clouds are caused by mainly open water, and of course cyclonic activity, it is not uncommon to see a cloudy “High” pressure system from space, mainly because of open water interacting with cold air. It is customary for the clouds to clear by late December to beginning of February, this is the time when the atmosphere gets really cold up here, fog and clouds return in full force usually by mid-April (spring ice break up). Arctic summer times are usually cloudy and that was correctly stated.

  21. Mark A. York:

    In the second part of the refutaion the writer cites this report:

    According to to this the ice isn’t melting and is stable. Amazing when the maps show the opposite is true.

  22. Don Baccus:

    Apparently envirotruth is another industry-funded shill site:

    The National Centre for Public Policy Research (NCPPR) is a US conservative think tank which advocates “free market solutions to today’s environmental challenges” on the basis that “private owners are the best stewards of the environment.” The NCPPR has set up a website,, which has the expressed aim of “injecting badly needed truth into the debate about our environment.”… However, the NCPPR, and specifically the EnviroTruth website, have received funding from oil company ExxonMobil, which must appreciate the research denying climate change. ExxonMobil has also given funding to sources used by Envirotruth: the Science and Environmental Policy Project (SEPP), and Dr Fred Singer, as “an active contributor in the battle against the ‘politicization’ of science” through the Hoover Institution and Atlas Economic Research.

  23. Stephen Berg:

    Re: #119, “The cooling in winter is large enough to refreeze most of the ice that was lost in summer, but not all: from 5 million km2 in 1940 to 7 million km2 in 2005.”

    Where does the graph show 5 million km^2 for 1940? Nowhere!

    As for using 1940 as an example year, this is intellectually dishonest, since 1940 must have been an exceedingly warm year (climate variability at its best).

    As you can see plainly in the graph you provided (, in the post-1950 period, there is a definite decreasing trend of “Northern Hemisphere Sea Ice Extent.” Thanks for the extra evidence, Ferdinand!

  24. Hans Erren:

    re 117

    In the absence of a better model you could assume the effect of CO2 concentration on temperature linear. Then, as a first approximation, doubling the CO2 would double its current warming effect. If the current contribution of CO2 to the 33K warming is 20%, doubling the original CO2 concentration would raise global temperature by 6.6K. About the relative importance of CO2 and other gases, google for “climate forcings”.

    Moderators where is your comment, ever since Arrhenius we know the relationship is logarithmic, recently confirmed by the simplified expression:
    dE=[alpha]ln([CO2]/[CO2}orig), where alpha is 5.35 (Myhre et al.)

    The same law that tells we have a warming of 33K by greenhouse gases (Stefan-Boltzmann) also tells us that a doubling of CO2 leads to less than 1 K.

  25. Ferdinand Engelbeen:

    Re #116 (comment):

    – From the calculation in Modtran (unfortunately only for the subpolar regions, not the poles): winter difference 330-375 ppmv CO2 (roughly 1970-2005): 0.32 W/m2, summer difference: 0.49 W/m2. A substantial change, but almost a magnitude less than what the change in cloud cover introduces.

    – Sorry, it was in fall, not in winter. From Science:
    “There is also an increasing cloud cooling trend in the fall, largely a result of decreasing albedo, because there is only a weak trend in cloud amount.”
    I don’t know how that works, maybe they refer to the fact that clouds have more effect over open sea than over ice for SW reflection?
    For winter, there is a decrease in albedo too (for the sun-lit parts 60-76N), but the effect that clouds have may be masked by the fact that there is a decreasing trend in cloud cover…

    – I am sceptic about computer models in general (own experience) and the applied magnitude of the different actors (including feedbacks), not about the basic effect of (human induced) greenhouse gases themselves. That the GCM’s can simulate the Arctic ice retreat is nice, IF they also simulate the observed change in cloud cover (and the resulting difference in summer/winter trends). If not, then they just have been lucky, but not reliable for any future projection…

    Re #120:

    Wayne, you are right about the role that clouds have, but the Science article mentioned before says that the observed trends in cloud cover show a decadal increase of 3.1% and 1.5% in spring and summer, little trend in autumn and a decrease of 5.7% in winter. The spring and summer increase leads to less warming than without cloud cover changes and the winter decrease leads to lower temperatures and more refreezing. Fall cloud cover may retain more (summer) heat, but at the other side reflect more (decreasing) sunlight. I have no idea where the balance is going if fall cloud cover is larger this year…

  26. Mark A. York:

    Thanks Don. I hadn’t had time to research it, but suspected as much.

  27. Wayne Davidson:

    #125 Ferdinand, from experience the yearly seasonal increase in clouds (at mid april) were phenomenal and cloud coverage persisted from that time till end of October. What happened in 2005 was equally interesting, an apparent net decrease in March/April clouds played a contributing role in warming things up. While fall of 2005 was mostly cloudy, but a little less then 2003-2004. Things are not quite ‘normal’ this year.

  28. Ferdinand Engelbeen:

    Re #123,

    Stephen, it is not about absolute figures, but about the difference in trends between summer and winter. Until 1945, there was no trend in summer or winter ice cover (as far as the data are reliable in the pre-satellite era) and the average difference between summer ice and winter ice cover is some 4.5 million km2 over the whole period 1900-1945. After a small increase and a sudden downward shift in 1953, the summer ice trend 1955-2005 is -2 million km2, while the winter ice trend is -0.5 million km2. In 2004/2005, this leads to more summer melting and more winter refreezing for in total some 7 million km2. Thus while there is more summer ice melting, in winter most of the Arctic summer melt refreezes. Of course, the larger summer ice melting lowers the yearly averages, even if all water would refreeze in winter.

    The interesting part is that both summer warming and winter refreezing are largely influenced by cloud cover changes, which need more investigation (probably AO induced now, but what will happen if there is a shift in AO index?).

  29. Barton Paul Levenson:

    Re #85 on natural limits to warming…

    There is indeed such a mechanism, the carbonate-silicate-weathering cycle first recognized by Walker et al. in a famous paper in 1981. Hart (1978) had suggested that continuously habitable zones around Solar-type stars were extremely narrow and thus habitable planets might be rare. Walker et al. pointed out that when CO2 and temperature go up, so does evaporation, thus weathering, thus CO2 deposition in the ocean, so it acts as a brake. When CO2 goes down and the Earth cools, ice covers land and evaporation decreases and less weathering takes place, so volcanism builds CO2 back up again. Applying this, Kasting (1993) was able to show that habitable zones around Solar-type stars were actually quite wide. Earth-type planets far from their primary star (not ridiculously far, but say to 1.7 times Earth’s distance for a primary with 1.0 x Solar luminosity — Forget and Pierrehumbert 1997 — will not have runaway glaciation as in the old energy balance models of Budyko (1969) and Sellers (1969). They will be warm because of lots of CO2 in their atmospheres. (Humans won’t be able to breathe such air, by the way — the high CO2 will induce narcosis. But life that evolved there will do just fine.)

    But this doesn’t mean that such natural feedbacks will constrain an artificially rapid rise in CO2 like we’re seeing now. The difference in time scale is crucial. By overusing fossil fuels we can bring the temperature way up fairly quickly. In the long run things will stabilize, but as FDR said, people don’t live in the long run.

  30. civil truth:

    As the unidentified commenter mentioned in comment #118, I’ve finally got a handle on how to communicate with you. I mistakenly used “arctic ice sheet” instead of “arctic sea ice”; I think I can clear up that matter with Mark.

    Let me get to the point of this query. I posted several questions on the comment thread of the GM Corner post in question, and Mark suggested I contact you folks directly. I hope this is the appropriate thread for it.

    1) Do other climate models which predict future temperature changes use the same proxy data set and methods that you used in the MBH model? Or to put it another way, are there other truly independent published calculations of future global temperatures using different data sets and/or methodology? If so, this would make the whole MBH versu M&M controversy of much less importance than skeptics are attaching to this controversy — in which skeptics seem to assert that the scientific consensus in favor of a genuine warming trend of historical concern rests squarely upon the validity of the MBH calculations.

    (If you discussed this on previous postings that included specific citations, feel free to refer me to those rather than wasting your time rehashing the same points here.)

    2) What are the signal to noise ratios regarding CO2 (and CH4 and other GHGs) versus other inputs? That is, how sensitive are climate models to detecting the magnitude of global temperature change in response to GHGs compared to the variability of other sources of warming (e.g. solar cycles, air-sea interactions, albedo changes)?

    3) What are the relative magnitudes of anthropogenic GWG effects versus the error bars?

    Again, if you have previously dealt with these questions, please point the way.


    [Response: You seem to be a little confused. MBH is not a computer model in the sense you mean at all, it was a methodology for reconstructing temperatures in the past. All of the attribution studies to explain the 20th Century warming, and to extrapolate how warm it is likely to get are based on GCM models. (see here or here for more info). If MBH disappeared, very little about the current consensus would change (see here). The relative magnitudes of the effects of well-mixed greenhouse gases (CO2, CH4, CFCs, N2O) are very large compared to intrinsic variability, and all other forcing factors (except aerosols, and volcanos) – at least in the global mean temperature diagnostic (it’s not so clear regionally). You can assess that for yourself by looking at some recent modelling studies such as described here. -gavin]

  31. Mike:

    I’m a layperson, who has read the relevant articles and responses in this debate extensively, and I still don’t quite understand why tree-ring time series that differ significantly from a mean centered on the 20th century should be more significant than others (hopefully I’ve stated that coherently.) Why is the mean for the 20th century more relevant than a mean averaged over the entire period? Is this a part of the stepwise analysis–that is, 20th century data is more accurate than data prior to that period? Is the idea, then, that series which more faithfully replicate the other 20th century indicators are then taken to be more accurate for past periods than other series?

  32. Michael Jankowski:

    Re#118: “Where would the water go if not adding to the volume of water in the connected oceans?”

    In regard to the ocean-based ice: put some ice in a glass and fill the rest of the glass with water until it’s close to full. Keep an eye on the water level as the ice melts.

    When the ice melts, the volume of water it “adds” to the surrounding water is counteracted by the volume of ice that is lost during the melting process.

    The wording on that site may be poor, but it is true that the melting of ice currently floating in the ocean would little-to-no effect on sea levels while the melting of land-based ice such as that in Greenland and Antarctica could have a major impact.


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