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Met Office decadal forecasting explained: the reality

Posted on 11 January 2013 by John Mason

"Decadal forecasts provide essential information about ocean ‘weather’ and how it will evolve in the next few years in the context of a globally warming world, but they do not tell us anything about  long-term climate sensitivity  (i.e. how much the planet will warm for a specified increase in radiative forcing related to greenhouse gases)."

UK Met Office, January 2013

In 2012, the Hadley Centre (the climatology section of the UK Met Office) introduced its latest multi-year forecast model, HadGEM3, into the decadal forecasting system, replacing the earlier HadCM3 (figs. 2 and 3, below), developed in the earliest years of the 21st Century. HadGEM3 represents the product of many years of detailed research and involves a better understanding of the many variables that work together to bring us the climate we experience. Running the model involves a phenomenal amount of number-crunching, to such an extent that it is only run out to T+ 5 years — to run it out further would hog too many computing resources. It is the first of these five-year runs that has attracted so much media attention of late. But what kind of forecast is it, exactly? Here lies the source of much of the misunderstanding. Having spoken at length to the Met Office about this subject on January 10th, I'll try to explain.

Earth's climate system is made up of four basic elements. These are the atmosphere, the oceans, the land and the cryosphere, each of which have their own intrinsic properties in terms of heat capacity, energy balance and hydrological balance. Landmasses determine where ocean currents can or cannot flow; mountain ranges have marked effects on atmospheric currents and thereby exert a strong influence on weather, in places on continental scales. Ice-caps refect energy that would otherwise be absorbed by bare earth or rock. But on normal (non-geological) timescales, it is only the atmosphere and oceans that are obviously in free circulation. Even there we see a difference: because the density and heat capacity of the oceans are orders of magnitude greater than that of the atmosphere, their circulation patterns (currents) evolve at a much slower rate and are long-lived compared to those of the atmosphere (winds). And herein lies the difference between forecasts of weather, run out for a matter of days, and decadal forecasts, run out for years. Decadal forecasting is essentially a prediction of ocean 'weather', i.e. how the oceanic circulation is likely to evolve in the coming years, and its subsequent impact on what goes on in the atmosphere.

We know perfectly well that oceanic pattern-changes can have major impacts on weather and, indeed, on global temperatures, either upwards or downwards: ENSO, the El Nino/La Nina oceanic oscillation, can by itself swing global average temperatures in a single year more than the longer-term anthropogenic greenhouse warming signal can currently manage in a decade, as our recent animation clearly demonstrates. Those who, using the phenomenally warm 1997-8 El Nino as a starting-point, like to pretend that global warming has stopped, are merely (if perhaps unwittingly) demonstrating this very same point. The bottom line is that when making decadal predictions, we need to try to figure out how the oceans are going to behave.

HadGEM3: the ensembles

Fig.1: the forecast ensemble generated by HadGEM3. Graphic - Met Office.

Ensembles - what are they?

The forecast itself (fig.1) consists of an ensemble of ten runs. All models consist of ensembles - for example, weather forecast models such as the Global Forecasting System (GFS) produce an operational run but experienced forecasters will also examine the ensemble members of that run to see to what extent the operational run is favoured before coming to their conclusions. The reason why ensembles are used is in order to better reflect the chaotic nature of what we are dealing with on such climatologically short timeframes, bearing in mind that major, long-term climatological trends are multidecadal in nature. The testing of HadGEM3 is done by running it to hindcast previous five-year periods over the past fifty years, which do show that the model has demonstrable skill, although it does not catch every twist and turn in the evolution, as fig. 2 (below) reveals. This is not surprising, given that El Nino/La Nina predictability is limited to — at best — a year or so in advance.

Skeptical Science: decadal forecast explained

Fig. 2: the latest (December 2012) decadal forecast, produced using the new HadGEM3 model, with annotations by author explaining what's what.  The black lines are observational records from the HadCRUT3, NCDC and GISS datasets. Graphic: Met Office.

http://www.skepticalscience.com/pics/globa--forecast-1.jpg

Fig. 3: the previous decadal forecast, using HadCM3. Note that, because this used less computer resources, it was possible to run it out further, to well beyond 2017. Graphic: Met Office.

The reaction

So, onto the reaction. As Dana has already pointed out, the release of this new forecast has been spun to death by various activists and elements of the media. Some of the spinning is quite ironic: this, from David Whitehouse writing for the self-styled Global Warming Policy Foundation, for example:

"We were also told that the recent temperature standstill was unimportant and that the underlying rate of global temperature increase continued at a constant 0.2 deg C per decade. This, as we pointed out, is only true if one considered decadal averages. If one considers 5-year averages then one arrives at a very different conclusion."

Exactly, David! 5-year averages contain so much natural variability (remember that a single ENSO cool-warm transition in 12 months can outdo ten years of anthropogenic decadal trend) that this forecast is more about natural variation than the longer-term, multidecadal signal. However, irony aside, there has been a lot of stuff like the following (fig. 4), the very first comment posted at the start of a discussion thread following an article by George Monbiot on the extreme heat and bushfires in Australia, published in The Guardian on January 8th:

typical response to anything that mentions climate

Fig. 4: another day in the climate wars! Monbiot's article went online at 8pm and the irrelevant anti-Met Office talking points started cascading in within a matter of minutes.

Within just six minutes of the article appearing online, someone had time to find it, extract one sentence and bang in a completely inaccurate talking-point. That's impressive. And it is going on all over the place. It is the face of organised climate change denial: take something, twist it until it fits the political narrative then, as quickly as possible, echo it far and wide. Meanwhile, the Met Office themselves have taken the opinionated columnist James Delingpole to task on a risible screed of nonsense he churned out for the UK's Daily Mail, a newspaper that apparently takes the editorial stance that right-wing politics are above the laws of physics!

Back to reality

Back to the real story, the forecast itself. The maps in fig. 5 (below) detail the ensemble mean and lower and upper values for regional temperature anomalies, relative to the 1971-2000 global average, up to 2017. All three show greater warming over most land areas than over the oceans plus something we have become used to, enhanced warming over the Arctic, scene of 2012's record sea-ice meltdown. One thing of interest is that all the ensemble members show a change to much cooler ocean temperatures in the NW Atlantic: do these suggest that we may be seeing a change in the Atlantic Meridional Overturning Circulation? In the Pacific, there is a forecast cooling in the NE & SE, similar to that observed in the Pacific Decadal Oscillation, suggesting a continued cool phase here, similar to that of the last decade. But oceans, dense and with tremendous heat capacity, change only slowly and this, being an oceanic-based forecast, tends to reflect such characteristics. Of greater concern is the continued, unabated warming of the shallow seas off Canada and Siberia, with their vast stores of frozen methane trapped in vulnerable subsea permafrost.

Regional forecast, 2012-2017

Fig. 5: regional 2012-2017 temperature anomalies, relative to the 1971-2000 global average temperature (in degrees C) from the HadGEM3 model. Graphic: Met Office.

So finally: what's the take-home? Simply that this is a forecast of how things will likely develop in the world's oceans and the effects that will have on the atmosphere over a period of five years, a timeframe within which natural variability can outpace or cancel out a decade's worth of the multidecadal anthropogenic signal and that it is an experimental forecast based on ten ensemble-members that attempt to reflect the chaotic nature of these very short-term natural variations - or, to reiterate what the Met Office themselves say:

"Decadal forecasts provide essential information about ocean ‘weather’ and how it will evolve in the next few years in the context of a globally warming world, but they do not tell us anything about  long-term climate sensitivity  (i.e. how much the planet will warm for a specified increase in radiative forcing related to greenhouse gases)."

Acknowledgement: my thanks to Dan Williams of the UK Met Office for helpful discussions prior to the writing of this piece.

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Comments

Comments 1 to 22:

  1. I love this dependence we have on models. Here, for instance, is a summary of some dozen models predicting ice extent in the Arctic compared with observations.
    http://thinkprogress.org/climate/2012/12/06/1293011/noaa-climate-change-driving-arctic-into-a-new-state-with-rapid-ice-loss-and-record-permafrost-warming/
    I wonder if these met office models are any more reliable. You can only model what you know. Not what you haven't yet discovered.
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  2. Beats an uneducated guess or just stumbling about, living for the passing moment. What gets me is when people assume that model projections are actually predictions, as if scientists aren't testing the range of possibilities using the most likely range for each of the many variables.
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  3. This is speculation on my part, but I would imagine that modeling Arctic sea ice loss is an entirely different animal from surface temps. With surface temps you can at least do hindcasting to test the models. I image that hindcasting, or any other testing methods, would not be as accurate for sea ice loss, if possible at all.
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  4. I mean, try walking across the street without a model of how traffic moves on a street. The intuitive model will likely be accurate enough to get a person across the street without incident, though inaccuracies will occur from time to time (hopefully not fatal). Using no model . . . well, it keeps the EMTs busy anyway.
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  5. Fortunately we're not dependent on models. We've got past climate.

    This Knutti and Hegerl graphic from 2008 is getting dated, there's been so much work on this in the past 5 years which it doesn't cover.
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  6. DSL, precisely. We humans use different kinds of models all the time (usually informal ones). Without models, one could neither predict the future nor understand the past. All that would remain is instinct.

    And although our models usually are incomplete and often wrong in one way or another, thinking with models is a lot better than not thinking at all.
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  7. Aircraft design relies upon computer models. Jetting off somewhere, or just feeling lucky?
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  8. The Australian newspaper Australian article
    today has a story which, in part, claims that global warming stopped 17 years ago and this new analysis extends that to at least 20 years. This is despite also quoting from the Met Office response!
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  9. newairly @8, The Australian is a disgrace. Its repeated inaccuracies in reporting, not just on climate change, make it little more than a propaganda rag, IMO.

    With regard to that particular article, they write:

    "On one analysis, the forecast confirms what many people have been saying for some time. Global warming effectively stopped 17 years ago and, if the new forecast is accurate, that "pause" will be extended to 20 years."

    By my analysis, made not by simply eyeballing the chart, but by digitizing it and analyzing it, the new predictions while lower than the previous predictions, predict that 4 out of the next 5 years will break the current HadCRUT3v annual global temperature record. Not only that, but the temperature trend from December 1996 will increase by a factor of five relative to the current trend from that period, said trend being much ballyhooed as "no warming".

    The only possible way these figures can be treated as extending "the pause in global warming" to twenty years is by pure spin. The deniers are taking us for fools; and in the Australian's "Environment Editor" have found a fool ready to swallow any claptrap they put out without analysis and without thought.

    Year 1961-1990 1971-2000
    1998 0.529 0.411
    2013 0.503 0.385
    2014 0.544 0.426
    2015 0.578 0.46
    2016 0.587 0.469
    2017 0.541 0.423
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  10. Here is a link to the article in the Australian that doesn't require subscription.

    They heavily rely on the not very reliable David Whitehouse of the GWPF.
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  11. Lars Karlsson @ 10, I followed your link to the Australian article, but I have installed a Firefox add-on called 'Web Of Trust', that warns me if I navigate to a site with a poor reputation. Sure enough, I received a warning that the site had been rated 'untrustworthy' by other users. What's this? A mainstream media site untrustworthy? Well, I am surprised. (/sarc)

    Taking heed of good advice, I did not bother over-riding WOT to read the article. ROTFLMAO. "8-)
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  12. In the Australian article Davie Whitehouse of GWPF wrote:

    "the so-called sceptics who were derided for questioning them were actually on the right track"

    But back in 2008 the Australian was publishing articles predicting rapid cooling and an ice age. Given current weather events I would suggest that those particular 'sceptic' predictions were most definitely not the right track...

    The Australian will publish anything as long as it denies AGW.
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  13. Doug @11,

    Please provide the location to your addon, I'm very curious...

    As for 'the ausralian's article, I simply don't have time to waste for reading it. If I had to opine it, I'd violate the coent policy with my choice of words, so no, thanks.
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  14. The commonly seen denial meme that suggests anything involving models must be wrong/suspect (exemplified perfectly by William @#1) is based on a complete lack of understanding of what a 'model' is, in the scientific sense of the word. Here is a useful, simple primer which is a good starting point for anyone at a basic level of understanding (with a number of useful links at the end).
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  15. chriskoz @ 13, it is available from the Firefox extensions searchable when you navigate to Tools/Add-ons. I have had it for ages and it seems pretty reliable, although it relies upon users to add their own ratings when they encounter problems with a particular site.

    Mods: apologies for the off-topic post, but I don't see a way to email chriskoz privately - is that possible?
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    Moderator Response: [DB] Request sent on your behalf.
  16. The reality, of course, is very different from how those in denial about AGW and fake skeptics are trying to spin this story. As always, it is best to go to the source.

    Here is a BBC Radio 4 segment in which they try and set the record straight, but without actually bringing themselves to admit they messed up or apologizing to the Met Office.

    I would encourage people who are still convinced in the myth being promoted by those in denial to listen to the words of Prof. Julia Slingo, who is the chief scientist of the agency who actually made the experimental forecast.
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  17. Looking at Figures 2 and 3. the slope at the later portions of the white line on the two graphs are markedly different, a difference that I don't think has been discussed here. Exactly what is that white line showing and why the differences? Presumably these differences are due to the differences between HadGEM3 and HadCM3. Is that correct? If so and as HadCM3 has been used in earlier predictions, is it not possible that in the future, predictions from HadGEM3 will be shown to be different from those of the succeeding generation of climate models? Should the answer be in the affirmative this could indicate the science of climate change may be somewhat less settled than has been suggested. Whatever, I'd very much appreciate elightenment on the questions I've asked re the "white line"
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  18. Ray, I'd guess that's the case; HadGEM3 seems to do a better job in general though both fail to nail the 1997-8 super El Nino - as I said in the post such things are notoriously awkward in terms of predictability, despite the level of scientific knowledge today.

    In turn, that creates an interesting possibility. What if there was another super El Nino in a few years' time? Looking at the temperature records, the difference between 1996 and 1998 is around 0.3C: given the mean HadGEM3 forecast figure of 0.43C relative to 1971-2000 climatology, such an event in the next 5 years could produce a spike of getting on for 0.75C.

    At that point, of course, the temperature would be blamed by the usual suspects on natural variation, followed in the years to come by Daily Mail articles proclaiming how global warming stopped in 2015!!
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  19. If, as you say, El Niño has such a significant effect on the global annual temperature, what is the value of the UK's Met Office 5 year forecast?

    If in the year 2017 we have a super El Niño or for that matter a super El Niña what will comparing actual and forecast mean?

    It seems to me that decadal forecasts are really only useful when done as hindcasts, that is when the ENSO and possibly volcano effects are well known.
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  20. The Daily Mail and the Mail on Sunday will continue to deny climate change until long after the sea-level has risen above the roof of the British House of Commons and Big Ben is seen by shipping as an hazard to navigation.

    (The British House of Commons under water? Oh well, ‘it is an ill wind ...!’)
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  21. Martin #19, primarily this is all experimental work: the aim is to see what is possible in forecasting. If we ever get to the stage where reliable 5-year forecasts can be done in detail - and we may not - then think of the very obvious benefits e.g. to agriculture.
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  22. An informative article which asks: Does predicted cooling in the NW Atlantic (and NE Pacific) indicate change in ocean currents? Possible – but possibly this arises from predicted warming in the Arctic and consequent loss of land-based and sea ice producing cold water flowing into the northern Pacific and Atlantic Oceans before sinking?
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