FAQ on Climate Models

I promised in a comment yesterday that I would post this today. It’s an FAQ put together by RealClimate, a blog definitely worth giving a regular perusal. Here is just one sample from the armchair geek’s paradise:

What is robust in a climate projection and how can I tell?

  • Since every wiggle is not necessarily significant, modellers need to assess how robust particular model results are. They do this by seeing whether the same result is seen in other simulations, with other models, whether it makes physical sense and whether there is some evidence of similar things in the observational or paleo record. If that result is seen in multiple models and multiple simulations, it is likely to be a robust consequence of the underlying assumptions, or in other words, it probably isn’t due to any of the relatively arbitrary choices that mark the differences between different models. If the magnitude of the effect makes theoretical sense independent of these kinds of model, then that adds to it’s credibility, and if in fact this effect matches what is seen in observations, then that adds more. Robust results are therefore those that quantitatively match in all three domains. Examples are the warming of planet as a function of increasing greenhouse gases, or the change in water vapour with temperature. All models show basically the same behaviour that is in line with basic theory and observations. Examples of non-robust results are the changes in El Niño as a result of climate forcings, or the impact on hurricanes. In both of these cases, models produce very disparate results, the theory is not yet fully developed and observations are ambiguous.

Clear as mud? They actually do a pretty good job of explaining what those pesky modellers are up to and why we should listen to them. I hope you will give it a read.

25 Comments Posted

  1. Has anyone emailed this to Rodney Hide. It might help him come up to speed with AGW and prevent further embarrassment for the poor chap , shortly.

  2. Erosion is a thing to keep in mind. The ability of the ocean to undermine a sea-cliff and wipe away tons of beach in a single storm means that simply being higher up the hill may not be enough. That’s why the simple 20 meters when 12 would probably do for both WAIS and GIS… which would be my worst case scenario for the next 1000 years.


  3. Hmmm… 20 meters seems a stretch for me. I actually have a couple of related considerations.

    The first is that I don’t believe that major infrastructure projects should be entertained less than 20 meters above MHW. That’s an engineering safety margin on top of what I actually expect is a worst case for the next 500 years.

    The other is that Transmission Gully has to be ready because the coastal road will be in trouble with a mere 2 meters of rise. Where it passes Porirua, through Mana and over the inlet (as well as SH-58) and also where it goes between Pukerua Bay and Raumati. That’s not as bad but is a good candidate for enhanced erosion problems if the tide is a couple of meters higher. Erosion less of a problem in Porirua.


  4. models are unanimous in their prediction of substantial climate warming under greenhouse gas increases,

    Just so you don’t MISS this bit


  5. What the IPCC says of the models…


    “Nevertheless, models still show significant errors. Although these are generally greater at smaller scales, important large-scale problems also remain. For example, deficiencies remain in the simulation of tropical precipitation, the El Niño – Southern Oscillation and the Madden-Julian Oscillation (an observed variation in tropical winds and rainfall with a time scale of 30 to 90 days). The ultimate source of most such errors is that many important small-scale processes cannot be represented explicitly in models, and so must be included in approximate form as they interact with larger-scale features. This is partly due to limitations in computing power, but also results from limitations in scientific understanding or in the availability of detailed observations of some physical processes. Significant uncertainties, in particular, are associated with the representation of clouds, and in the resulting cloud responses to climate change. Consequently, models continue to display a substantial range of global temperature change in response to specified greenhouse gas forcing (see Chapter 10). Despite such uncertainties, however, models are unanimous in their prediction of substantial climate warming under greenhouse gas increases, and this warming is of a magnitude consistent with independent estimates derived from other [non-model] sources, such as from observed climate changes and past climate reconstructions.”

    If you really want to get down and dirty, this chapter is a good place to start.


  6. BJ,

    Cant find the reference you made so am going from memory, but it had to do with your pro Transmission Gully stance. You being for the development as sea level rises would render the coast road underwater by some 20 metres.

    If my memory fails me (and the years are pilling on) then my apologies.

  7. Gerrit

    The situation in the pacific atolls is one in which the volcanic base is sinking, not crustal rebound… the rebound occurs on a timescale measured in tens of thousands of years.

    It won’t necessarily be effective mitigation of the the WAIS or GIS breaking loose, which would have effect in hundreds of years. It may cause local quakes however, and those seem to have had some effect in terms of raising the Wellington side of the harbour in the past. I don’t know all the answers. I don’t know the answer to this one.


  8. Gerrit

    I have never seen anyone who claimed to know what the effect of the rebound might be on the planet.

    The only guess I have that I am willing to venture is that faults will become more active and vulcanism more prominent… and even THAT could be wrong.

    This is just another reason why I dislike the experiment being done on the planet by its human population. Too many unknowns.



  9. Thanks Henry,

    While the GCM models are fine, I’m more interested in the “how” they are actual structure and constructed. Seems like we have GCM out thwere (like your example) but we dont know the drivers (that predict the outcome) contained within the models.

    Anyone can produce a model to produce numbers to suit an end argument.

    What I’m interested to see is the “guts” of a GCM and what the parameters are for getting to the numbers.

    What is becoming more obvious is that “how” the model is constructed has a bearing on the outcomes. Hence my interest.

    Reading some of the material on the BJ supplied links it is obvious that some models dont take into account what could be significant variations such as not taking into consideration the eliptical path the earth takes around the Sun and the effect this has in local temperature changes and their viability as an accurate recording.

    It also seems none take into account the earths magna creep around its core and the changes this will have in local temperature variations.

    The sea level model does not factor in the earths crust rising and falling as ice weight is removed and redistributed around the oceans as water.

    We have facts now on Greenland rising plus the sea floor falling (pacific ocean island disappearing below non rising sea levels). Factors to consider if BJ premise that sealevels will rise 20 metres are to be taken seriously.

  10. BJ,

    Reading the comments on the realclimate link regarding modelling science, your question regarding global climate models’ (GCM) accuracy is astute.

    It would seem there is much debate about accuracy, hence the sceptics.

    To quote one commentator

    The answer is that while the number of explicit or implicit input parameters into a GCM is in the hundreds at least, the number of degrees of freedom in the output model is enormous . What we see emerging is storms, fronts, the structure of the trposphere and stratosphere, precipitation patterns, soil moisture patterns, etc. etc. These are not just a few numbers but great patterns and patterns of patterns.

    That we get these approximately right is a measure of what we know. That we don’t get them exactly right is a measure of what we don’t know. However, it is true enough that the more controls we have over these models, the more we can fudge the results. There is, therefore, a danger that we can add too many “knobs”, and ultimately end up studying the model rather than nature.

    Further down is much discussion on what is left out of GCM.

    Well worth a read for both sceptics and believers.

    Question – Where can we find details of a GCM?

    Who created the models and what parameters are used?

    As there seems to be a massive debate going on about what to include and what to leave out.

    Are annual variations in forcings modelled? or just a yearly average number- eg CO2 in Hawaii goes up an average of 2-3ppm/yr, but it varies by ~5ppm throughout the year. What is modelled?

    [Response: Some of the answers are available in the glossary definition of GCM. But briefly, the models time step is usually around 20 or 30 minutes, so the annual numbers are averages over ~20,000 loops of the physics. Some forcings have important seasonal cycles (stratospheric ozone for instance) and that seasonality is inlcuded. The seasonality of CO2 is not very important radiatively, and currently is not included in the GISS model runs (but it may be at other institutes). – gavin]

    And speaking of modelling, it there a geological model of what will happen to the earths mantle as the weight of melted ice is removed from one concentrated spot (Greenland and Antartica) and distributed more evenly in the world oceans? Will the continental shelf that New Zealand sits on, be lifted (and Wellington not be 20 metres under water)?

  11. Kelpie

    I reckon that I could reversibly (albeit crudely) alter and control the climate of the planet for a mere 80-100 billion using current technology. More money, better control.

    The USA has already committed over 4 TRILLION dollars to various forms of financial rescues. It hasn’t put a dime into actually reaching the thermostat. It keeps working on changing how much heat escapes.


  12. Human beings, who are almost unique in having the ability to learn from the experience of others, are also remarkable for their apparent disinclination to do so.” Douglas Adams

    Has our time come to join the dinosaurs?

    Most will dance their way uncaring to oblivion; but you can’t stop the 2% who want to be as King Knute is portrayed.

    In those wee honest moments, do any of you really believe you can halt the evolution of the earth?

  13. As you continue to close your eyes and ears to the mounting evidence against your theory, you come across more and more as 9/11 conspiracists.

    As you descend further into the nether world of climate sceptic ideology, you become laughably shrill.

    Note for the interested: October was the second warmest in the record book, and the warmest ever over land.

    So much for cooling.

  14. Probably sceptics would be more impressed if these models had some predictive skill. Clearly they don’t. They are complete rubbish. We’re supposed to be experiencing continued and even accelerating warming according to these models, yet the world is cooling.

    So the real question is, just why do you believers continue to have such faith is these models even after they have been proven wrong. Seriously. How can such cognitive dissonance be maintained?

    But more than that, the alternative theories of climate change – ones which correctly ignore anthropogenic CO2 as a significant agent – have correctly predicted the recent cooling.

    In science, when predictions are made and then proven wrong, you have to discard the theory no matter how attached you are to it. And when predictions come about, as they have with the alternative theories, that must be taken as evidence in their favour.

    As you continue to close your eyes and ears to the mounting evidence against your theory, you come across more and more as 9/11 conspiracists.

  15. dickealy Says:
    Unfortunately, like 9/11 conspiracists, climate change denialists/skeptics will never be satisfied with the widely accepted science while there is even a miniscule degree of uncertainty — until there is NO uncertainty they will shout that we shouldn’t risk taking action while the science remains “inconclusive”.

    In any case, it doesn’t matter how many people you succeed in convincing. There are over 6 billion people in the world and by far the majority are NOT going to act on what climate change believers say.

    I see precious few real greenies in NZ.

    How many of you have given up the family car? Or given up hot showers heated by electricity?

    Until there is a significant body of true greenies FLOODING our roads on bikes and super-lightweight greenfleet vehicles, no progress will be made and the climate change argument will continue to be a fruitless distraction.

    As long as you use cars the argument is all just hot air and no-one is listening.

  16. dickealy:

    Of course, the only way there can be NO uncertainty is when big changes have already happened (and there will always be wriggle room somewhere arguing over the cause) and it will be _way_ too late to do much about it – even now the best we can do is mitigate it. (OK, I’ll stop preaching to the choir)

  17. Unfortunately, like 9/11 conspiracists, climate change denialists/skeptics will never be satisfied with the widely accepted science while there is even a miniscule degree of uncertainty — until there is NO uncertainty they will shout that we shouldn’t risk taking action while the science remains “inconclusive”.

  18. Really good FAQ.

    I like the section: “How do I write a paper that proves that models are wrong?”

    Looks like academia in action 🙂 (quite apart from how vested interests might do this)

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