Andrew Montford on precipitation

Andrew Montford, who runs the Bishop Hill blog has produced a report for the Global Warming Policy Foundation/Forum (GWPF) on precipitation, deluge and flood. It’s the standard kind of message; climate models have failed, we can’t find a trend in some data, cherry-pick a few papers that support this position, argue that we shouldn’t do anything. I can’t really face going into it in much detail. However, if anyone does wish to read the report, it might be worth reading this RealClimate post that discusses one of the papers that Andrew Montford’s report relies on (H/T Gavin Cawley), and having a look at this Met Office webpage that suggests that 1 in 100 precipitation events may have become more frequent.

What really bugs me about these type of reports is that they fail to acknowledge our understanding of the underlying physics/science. There’s a reason why climate scientists are quite confident about the increase in precipitation in a warmer world : we understand the underlying physics quite well. Firstly, there’s a water cycle. Water evaporates from oceans, sea, and lakes, is released by plants and sometimes sublimates from ice and snow. This produces atmospheric water vapour. The water vapour then condenses to form clouds, and eventually precipates as rain or snow. The figure below, from NOAA, illustrates the basic processes.

credit : NOAA

credit : NOAA

Something else we understand quite well is the relationship between the water vapour content of the atmosphere and atmospheric temperature. As it warms, the atmosphere can hold more water vapour. This in itself may suggest that we’d expect warming to produce more precipitation. The only way it wouldn’t is if somehow the rate at which it precipitated stayed the same, despite the increase in atmospheric water vapour. The problem with this is that we expect the rate of evaporation to also increase with increasing temperature. If we’re increasing the rate at which we add water vapour to the atmosphere, then we’d expect the rate at which it precipitates to also increase. Therefore, we not only have a good understanding of the water cycle, but also of the relationship between atmospheric water vapour and temperature and the relationship between evaporation rate and temperature. If we manage to warm without precipitation increasing, it will be remarkably surprising.

Of course, you could argue that even if the above is true, it doesn’t necessarily tell us where precipitation will increase. However, our planet has climate zones, and these are largely set by the distribution of Solar insolation and the Coriolis effect. Therefore, we have a good understanding of where evaporation dominates, how the water vapour will be transported in the atmosphere, and where it will most likely precipitate. Unless our warming produces some extremely unexpected changes, we expect precipitation to increase in regions where it is already quite high. Of course, there is a chance that warming could produce completely unexpected outcomes, but assuming that this is likely and that these unexpected outcomes will be beneficial would seem remarkably optimistic (foolish?). If anything, it’s these high-risk, low-probability outcomes that are most concerning.

So, the GWPF commissioning reports from someone who appears not to understand the underlying science is, one might think, a little surprising. Surely they could commission these reports from an actual expert (say, someone from the Met Office) rather than someone who has written a couple of books and mainly concentrates on writing blog posts that appear to either mock mainstream climate scientists or simplistically criticise mainstream climate science. It might make you think that they don’t actually want people to realise just how well we understand the basic science/physics.

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39 Responses to Andrew Montford on precipitation

  1. William Ingram has some nice stuff on P-E and so on; perhaps. There’s a 2002 thing in Nature, too.

  2. William,
    Thanks, very useful. I shall have to search the 2002 edition of Nature 🙂

  3. BBD says:

    Denying that the hydrological cycle will speed up as the climate system warms is as stupid as denying that increased atmospheric CO2 alters ocean pH. But there’s no stopping them trying.

  4. The global mean rain rate is simply determined by the global mean evaporation. As there is more energy at the surface due to global warming, the evaporation and thus rain rate will increase. The only thing you could do about that is change how much of the additional energy goes into evaporation and how much goes into warming the surface. This partitioning can change if the relative humidity would change. If the GW Policy Foundation argues that precipitation will not increase (did I understand that right from the above post?), then they are arguing that the temperature and the relative humidity will increase more than currently thought. Both would increase the heat stress (with a higher relative humidity it becomes harder to cool down by sweating). I am not sure they would like to argue that, it does not seem to fit to their political line.

    It is expected that the relative humidity will stay about the same and that both temperature and rain rate will increase. The increase in absolute humidity does not change the mean rain rate directly. However, Kevin Trenberth argues that the higher absolute humidity will increase the rain rate when it is raining and that thus the variability of precipitation will increase. That sounds like a reasonable first order expectation by me, but colleagues I respect highly are not convinced by that argument. While they do not find the argument that convincing, I do think that most expect that strong precipitation will increase more than mean precipitation.

  5. Victor,

    then they are arguing that the temperature and the relative humidity will increase more than currently thought.

    Maybe you can explain this to me. Is what you’re suggesting that if more energy goes into warming and less into evaporation, then the hydrological cycle slows done and the relative humidity goes up?

  6. John Hartz says:

    ATTP: You keep forgetting one important fact — Montford and his minions live in a an altervative reality with its own unique set of physics and chemsitry.

  7. John H,
    Yes, that is certainly something I’ve considered.

  8. People in Southern France are starting to believe that climate change results in more incidents of very heavy rain

    They had first 25 cm in three hours and then a weak later 95 mm in one hour.

    A bit more than 10 years ago some places in Massive Central had about half a meter in 24 hours, which is a lot for any place in Europe.

  9. I think its the other way round. What VV is saying is that the only way to avoid increased evap (if the sfc warms) is if the RH goes up. This seems unlikely, as RH being approximately constant is what everything (models) say, but that’s an emergent property not (AFAIK) any kind of law of nature.

  10. Yes, William is right. Sorry for not being clear.

    Pekka Pirilä, I would expect that that is a reasonable assumption. You see that in regions where the temperature is higher and thus the absolute humidity, that also the showers are stronger.

  11. Okay, I think I see. If the atmosphere is closer to saturation, then the evaporation rate goes down.

  12. jsam says:

    Where is the auditor when he is needed? /sarc

  13. BBD says:

    John Hartz

    ATTP: You keep forgetting one important fact — Montford and his minions live in a an altervative reality with its own unique set of physics and chemsitry.

    It’s called Libertarian Physics™ and is published in that spectral journal Acta Manus Invisibilis.. And don’t be fooled by that ‘Acta’, kids. This one has nothing to do with the Invisible College.

  14. Michael says:

    C’mon ATTP,

    If you want an accounting of the likely effects on rainfall, then you obviously need an accountant.

    Go blog science!

  15. Montford, Monckton, Monfort over at CE — I used to have trouble keeping track of who’s who in the denialosphere.

  16. Michael 2 says:

    “Unless our warming produces some extremely unexpected changes, we expect precipitation to increase in regions where it is already quite high.”

    A different we believes precipitation will *decline* as the atmosphere warms because it will choose to keep its water in vapor state.

    As it happens, I share your belief but I recognize that many factors are involved, in particular the availability of water droplet nucleation substances.

  17. Michael 2 says:

    Having read some comments, those that discuss precipitation rather than personality, I suggest that two phenomena are being conflated here.

    The Olympic mountains of Washington State in the USA are an example of adiabatic condensation and nearly constant rainfall year-round. It is of course extremely influenced by the sea surface temperature of the nearby Pacific Ocean. Warmer air blowing over the sea will pick up more vapor and as it blows over the mountains will drop rather a lot of its moisture. But since it will be warmer, more of that moisture will make it over the top and the absolute rainfall on the western side rainforest would probably be relatively unchanged. The most conspicuous difference then would be more rain reaching the dry eastern part of Washington.

    Conversely, in the northern plains of North America you can expect storm events where warm moist air, made warmer and moister by global warming, collides with cold arctic air. However, the proponents of global warming suggest (or insist) that since the arctic is warming faster than anything else, the “cold” of arctic air will actually have less of a differential and thus less precipitating power. But since there’s more vapor to work on, in the end it may well turn out to be a “wash” with the most conspicious change being the AREA over which dramatic rainstorms happen should increase (ie, monsoon weather covering larger areas, moving north).

  18. dikranmarsupial says:

    Obvious cherry picking in the quote from the IPCC report, only a little further down we get to

    Regional downscaling methods are used to provide climate information at the smaller scales needed for many climate impact studies, and there is high confidence that downscaling adds value both in regions with highly variable topography and for various small-scale phenomena.

    The idea that single runs of a GCM can predict daily precipitation on a station by station basis is not one that ought occur to anybody that understood how climate models worked or were used (or were aware of downscaling methods). For a start a GCM would operate on a scale defined by a grid box that might easily encompass the whole of the UK, but the UK does not have equal rainfall everywhere, for example Manchester is much wetter than Norwich, so a single figure for a grid box containing both is hardly likely to be really representative of either!

  19. Gavin,
    A good point. This paragraph is also remarkable

    The IPCC is routinely accused of overstating the abilities of computer climate simulations, so to see the match between observations and simulations described as ‘modest’ suggests just how poor global climate models are when it comes to precipitation

    So, people accuse the IPCC of overstating model abilities, therefore if they say that the model has some problems, then these problems must be quite severe. One way to generate your own narrative.

  20. BBD says:

    Okay Michael 2

    A paleoclimate question for you: were warm climates of the past also wet climates? Were cold climates of the past also dry climates?

    Find out and report back.

  21. Joshua says:


  22. GregH says:

    Is there general agreement about the trend in Total Column Water Vapour? I really don’t know anything about how this data gets measured or the thermodynamics involved, other than warm air holds more water, but I see that there is consistent supporting evidence:

    But I have met people who argue that no, total water vapor is not increasing. These claims appear to be based on variants of this one, which comes from one of the Pielkes. How uncomfortable should they be with these bedfellows?

  23. Eli Rabett says:

    If total water vapor is constant and evaporation increasing because of the warming surface then it is raining more. End of story. See also TIGR for water vapor profiles and the mad Hungarian Miskolczi

  24. dikranmarsupial says:

    Joshua, not THE Gavin, just a Gavin ;o)

  25. KR says:

    Relative humidity appears to be holding fairly steady with warming, as per Dai 2006, stating “During 1976–2004, global changes in surface RH are small (within 0.6% for absolute values),”, although with statistically significant regional variations.

    It doesn’t look like the Clausius–Clapeyron relation will be invalidated any time soon.

  26. Just a new review article appeared: Future changes to the intensity and frequency of short-duration extreme rainfall

    GregH, interesting, that Pielke Sr. shows a figure from Forest Mims III, with data from one station and is much more salient as the abstract text that reports positive trends in humidity for all of the data.

    Humidity is hard to measure, is not given much priority by the meteorologists that are in charge of the measurements and the instruments change often leading to non-climatic changes.

  27. Joshua says:

    BBD –


    Dikran –

    Nice ‘stache!

  28. Michael 2 says:

    GregH says “How uncomfortable should they be with these bedfellows?”

    VERY uncomfortable. One picks a side and then defends it. Crossing over to the other side is treason. Science is your weapon. It’s theirs, too, so make sure yours is sharper.

  29. anoilman says:

    What is a Dikran Marsupial, besides a ‘Gavin’?

  30. dikranmarsupial says:

    A very minor character from a couple of P.G. Wodehouse novels (I just liked the name, so I appropriated it for my alter ego ;o})>

  31. BBD says:


    So *that’s* where you got it from. I too have wondered 😉

  32. anoilman says:

    I’m kinda partial to Dik Diks. They are cute little cat sized deer. (Hmm… Kinda doubt they’d like Canadian Winters.)

  33. Michael 2 says:

    BBD says: “M2: A paleoclimate question for you: were warm climates of the past also wet climates? Were cold climates of the past also dry climates?”

    The short answer is *sometimes*. Geology seems to play into it more than other factors.

    Along the Utah Arizona border are some impressive canyons and cliffs made of varying kinds of sandstone. What I have learned of that geology is that some of the layers, the thickest of the (Navajo sandsone if I remember right) was windblown sand across about half of North America piling up against the western mountains. That was a very hot and dry era. But other layers are sedimentary, the leftover from vast shallow seas, obviously wet. Geology plays into it and very likely so does the shifting boundary of the Hadley cell. These sandstone formations are north, but not by much, of the present day boundary of the Hadley cell and if I include Sedona Arizona then that’s right about where it is (monsoons to the south, none to the north).

    Some of the above commentary, the first three for instance, seem not to have thought out the water cycle completely.

    Increased air temperature can hold more water vapor, no question, but what makes it precipitate? No one has explained why there has to be more precipitation. Water vapor could stay in vapor state forever and not a drop of rain anywhere without the necessary precursors for precipitation.

    Think of air as a bucket. At any given temperature and pressure it can hold “X” amount of water per cubic meter. The bucket is never empty and never emptied; what happens is that the bucket shrinks going over a mountain or hitting a cold front. When it shrinks a bit of liquid is squeezed out of the bucket and falls to the ground — but certainly not all of the bucket or even very much! Only so much as will saturate the bucket at its new temperature and pressure.

    Since you never empty the bucket completely, how much of it is emptied depends entirely on its starting conditions and ending conditions; global warming proposes that the differential between hot and cold will be reduced and that means less precipitation on average — except of course, more absolute vapor. In the end I suspect your local geography will dictate what exactly happens but globally I expect very little change.

    During the *transition* phase where the equator is warming but the arctic hasn’t caught up, then you will have large temperature differential working on warm moist air. So when the SLOPE of global warming is high, that is when I expect more storms. When the slope of global warming is flat, I expect fewer storms, and that appears to be the case. What the actual temperature is when “flat” doesn’t matter a whole lot, but that is my nonexpert opinion speaking.

  34. Michael 2 says:

    Addendum — I can see where the equatorial rain belt could have more rain with global warming. Rising air is squeezed rather dry by the time it comes down in Arizona, a function of adiabatic cooling and less air pressure. So if there’s more to squeeze, then places such as Amazon rain forest or the Congo would probably expect more rain. Of course, that is good for rain forests.

  35. John Hartz says:

    Speaking of precipitation…

    New study maps countries most at risk from El Niño flooding by Roz Pidcock, The Carbon Brief, Oct 20, 2014

  36. Pingback: Attribution | …and Then There's Physics

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