The Wettest Drought in History

One of my responsibilities as a teenager was to keep the lawn under control. Flymos had presumably not yet been invented, and petrol-driven mowers were perhaps too much hassle, so ours was manual. If the grass got too long it was hard work and it could even become necessary to resort to shears, which was back-breaking work. But mowing was also difficult if the grass was damp. There was therefore a trade-off each spring. The first mow had to be done when it was mild enough for the grass to be reasonably dry, but couldn’t be put off until it was too long. And as the grass grew it dried out more slowly each day. So it was essential to make use of any opportunity to mow in case the weather turned wet again. It probably only happened once or twice, but it seems I was always caught out. I’d wait for one more dry day to make the job easier, but the skies would open and a week later the job would be twice as difficult.

Nowadays the internet and improved forecasting allows me to monitor the weather far more effectively. Thus it was I’d already been out with the mower in March, and, seeing the long-range forecast, made sure I got a mow in just before it started raining early in April.

The point is that the 5-10 day forecast is now fairly reliable.

Why, then, was the UK drought – declared in a few regions in March, with hosepipe bans from 5th April – officially extended in mid April?

Yes, that’d be in the middle of the wettest April on record!

We’re now in the farcical situation of the “wettest drought in history”, with a succession of “experts” (and junior ministers) popping up on TV claiming the rain in April somehow doesn’t count. Apparently it’ll run off compacted ground. Yes, maybe for the first day or two, but not after a month. With the wettest April on record followed by significant rain already in May, and more forecast in a day or two, the drought risk is simply receding. We’re in one of those surreal situations where reasons are being invented not to contradict previous claims, in this case that the drought would last into next year.

What baffles me is why the drought was extended when wet weather was forecast. Surely – since most of the time it’s dry – the drought risk is receding as long as there’s significant rain in the forecast. And, as the 5-10 day forecast is fairly reliable and everything after that isn’t, you simply run the risk of looking stupid if you don’t wait until the forecast is for dry weather.

I wonder whether there’s a tendency to believe long-term forecasts more than short-term ones. But long-term forecasts only indicate a small bias one way or another, as Met Office modelling indicates:

“New three-month forecasts by the Met office suggest little respite with April, May and June expected to be drier than average. ‘With this forecast, the water resources situation in southern, eastern and central England is likely to deteriorate further during the period. The probability that UK precipitation for April-May-June will fall into the driest of our five categories is 20-25% while the probability that it will fall into the wettest of our five categories is 10-15%, it says.’ ” [my emphasis]

So 20-25% dry plays 10-15% wet plays (presumably) 60-70% around average. Not sure I’d have put a lot of money on the “expectation” of a dry spring this year (certainly wouldn’t now!). Even less after I’d looked at the Met Office report (scroll down to find PDFs) because the model runs are all over the place.

And are these “probabilities”, anyway? Isn’t the modelling signal swamped by the noise of uncertainty? It seems to me likelihoods based on model-runs are not the same as probabilities in the real world.

I’d say the Met Office and the media (the quote marks indicate the introductory sentence was written by the Guardian’s John Vidal) need to mind their language. How about “slightly more likely than not to be” rather than “expected to be”? And perhaps “indication” rather than “forecast”? And “x% of model runs gave…” rather than “the probability that…”? And definitely “might” rather than “is likely to”!

Drill Ice, Baby, Drill Ice – Reflections on Clive Oppenheimer’s Eruptions that Shook the World

Clive Oppenheimer notes in his Acknowledgements that he “planned to finish writing this book in 1999!”. Whilst I found Eruptions that Shook the World very informative and readable, it would have benefited from just a bit more effort. For example, the date of the El Chichon eruption is referred to in several places as 1985, though in others, correctly, as 1982 (as I’m sure I read in some other review, though even Google can’t help me out here). More substantively, there is some repetition and an immense amount of cross-referencing. I would also have preferred the inclusion of a comprehensive list of eruptions rather than (or as well as) the superficial details that are included between the Preface and the Introduction and as Appendix A, which excludes the large category of Unknowns and many other events discussed in the book (some of which are in the earlier table). As well as being an incomplete reference source, the book has the feel of being a final draft rather than the finished article.

Most annoyingly, of recent eruptions of which we obviously have the best data, Pinatubo (1991) is discussed in detail (p.54-69), but El Chichon (1982) is referenced only in passing and Agung (1963) hardly at all. In particular, there seem to be important differences between the climatic effects of the El Chichon and Pinatubo eruptions, which would have been worthy of discussion.

Nevertheless, Eruptions fills a gap between school-level and academic material and anyone interested in the subject will find it a stimulating read. Some other reviews are listed here, though how carefully Kate Ravilious read it for New Scientist is in some doubt as she seems to think Oppenheimer discusses “thick layers of ash in Greenland ice cores” rather than the varying sulphuric acid fallout in the cores.

I should say that whilst I read Eruptions to better understand the effects of volcanoes on the climate, the book does discuss the other nasty things volcanoes can do to you, and a great deal more besides.

Minor gripes aside, I presume Oppenheimer’s account reflects the current state of academic thinking about the effects of eruptions on climate. It is this about which I have concerns, that is, the science itself, rather than Oppenheimer’s account of it.

Let me outline what appear to be the central tenets of the current paradigm, and comment as I go along:

1. The climatic effects of eruptions are entirely due to sulphuric acid aerosols.
Volcanoes eject varying amounts of sulphur in the form of sulphur dioxide and hydrogen sulphide into the atmosphere at varying heights and in varying proportions to the total amount of ash, lava and other material. The sulphur reacts to form sulphuric acid aerosols which can remain in the stratosphere for months to years, where they reflect light (and absorb heat, which helps keep them aloft). There is therefore a “recipe for a climate-forcing eruption” (Eruptions, p.69ff).

Eventually the sulphuric acid aerosols descend, and a historic record of sulpuric acid loading can be derived from ice cores, principally from Greenland and Antarctica. Oppenheimer draws on work at Rutgers University by Chaochao Gao, Alan Robock and Caspar Amman (presumably et al – this must have ben a lot of work) to produce an ice-core volcanic index (IVI). He reproduces Gao et al’s graph (as Fig. 4.6, p.98), which I kept referring back to. Here’s my copy-paste from Rutger’s site (for some reason there are spurious double lines on my version – check back at Rutgers if confused):

The IVI replaces H.H. Lamb’s famous Dust Veil Index (DVI). The idea that particles of dust as opposed to sulphuric acid could reflect light away is rejected entirely, or at least the effect of dust is considered insignificant. I find this assumption dubious. For example, the eruption of Huanyaputina in 1600 apparently had catastrophic effects on the climate – causing the Great Russian Famine – yet was, according to the IVI, only about twice as severe as Pinatubo, which really didn’t have a huge effect. Its sulphur emissions are dwarfed by those of Tambora in 1815 and Kuwae in 1452, yet it seems to have had at least as much of a cooling effect. Unfortunately, instrumental temperature records don’t go back to 1600, so we have to rely on anecdotal evidence. Here’s what Brian Fagan says in The Little Ice Age (p.104):

“The volcano discharged at least 19.2 cubic kilometres of fine sediment into the upper atmosphere. The discharge darkened the sun and moon for months and fell to earth as far away as Greenland and the South Pole. Fortunately for climatologists, the fine volcanic glass-powder from Huanyaputina is highly distinctive and easily identified in ice cores.

Huanyaputina played havoc with global climate. The summer of 1601 was the coldest since 1400 throughout the northern hemisphere… Summer sunlight was so dim in Iceland that there were no shadows.”

It seems to me at least plausible that the effect of eruptions on climate is due to dust particles as well as sulphuric acid aerosols. Indeed, my main problem with the IVI (see the paper Gao et al, 2008, which is available to download as a PDF from the Rutgers site) is that not enough has been done to establish how closely ice core sulphate levels correlate with climate impacts of volcanoes. As well as the possibility that there are significant effects due to other kinds of particle, there are other potential complicating factors:

• varying proportions of stratospheric sulphuric acid aerosol may end up in the ice, so that the IVI only gives an indication of the severity of the climate impacts of the eruption;
• the amount of sulphate in the ice gives no indication of how long it remained as sulphuric acid aerosol in the atmosphere – obviously the amount of sunlight reflected away is a function of time as well as aerosol density;
• some of the sulphate in the ice may not have reached as high as the stratosphere to cause significant climate effects (this must surely distort the figures for Icelandic, such as Laki, 1783, and Alaskan eruptions).

We have a lot of data on recent eruptions, which would seem to provide a means of establishing the usefulness of the IVI, which might be a good idea before translating it into a dataset to be plugged into climate models, as Gao et al have done. I can see the appeal of such a mechanistic approach, but it seems to me that the effects of different eruptions vary more than a single variable (OK in most recent cases we also have a date, or at least a season) would seem to suggest.

One problem with the IVI is that although it includes Pinatubo (1991), it does not include El Chichon (1982) because not enough of the Arctic ice cores were old enough, and there is no signal for El Chichon in the Antarctic (I’m unclear why a full signal for Pinatubo is apparently included). This misses a golden opportunity to validate the data. Clearly we need to get out to Greenland and drill more ice cores before the whole lot melts.

A further problem is that the IVI does not explain all of the data. For example, the cold period in the 1690s, including the exceptionally cold summer in 1695, as well as the record cold summer of 1725 (see my recent post on the cold summer of 2011) are completely unexplained. Note that the 1690s has long been a problem. Lamb interrupts his DVI list to discuss it (note the criticism of subjectivity which may affect the whole DVI – the eruption data may be deduced from the weather data rather than independent of it). Here’s a screen grab of part of the DVI which is accessible on Google Books:

Excerpt from Lamb, Climate: Past, Present and Future

More recently, I’d understood* that the dip in temperatures at the start of the 20th century was due to the Santa Maria eruption of 1902 (not to be confused with the famous Mount Pelee eruption of the same year, which is notable for causing a large number of fatalities). But there is only a small signal in the Gao et al data for 1902 (3.77 compared to 30.09 for 1991, the year of Pinatubo, limiting Gao et al’s spurious accuracy perhaps less than I should!).

* e.g. in the IPCC figure produced in a previous post.

2. Eruptions may affect only one hemisphere.
Tropical eruptions can have effects on both hemispheres, depending on (apart from the characteristics of the eruption and the weather at the time) latitude and time of year (and hence the position of the inter-tropical convergence zone, ITCZ). In their paper, Gao et al in fact separate out the hemispheric sulphate records:

Pinatubo affected both hemispheres, but El Chichon only the northern hemisphere (NH). El Chichon, though, seems to have reflected away at least as much heat, having produced a volcanic cloud “extending from the Equator to 30 deg N for more than 6 months, and then gradually spreading more widely” (Alan Robock, 2002, PDF). We can see this first in the atmospheric transmission of solar radiation record from Hawaii:

and, more to the point, in the record of oceanic heat content, where the dip in the early 1980s seems to have been greater than that in the early 1990s (though perhaps already underway by the time of the eruption):

So, if El Chichon removed more heat from the oceans than Pinatubo, and removed the bulk of it from the NH, you might expect some kind of effect on the Arctic ice. Here’s the annual ice extent for August 1979-2011, from the (US) National Snow and Ice Data Centre (NSIDC):

1983 and 1991 both seem to be above the annoying blue trend line (I always feel you need a better reason for drawing lines through data than that you feel like it!), but one might expect the effect to take longer than one year to play out. Indeed, if you imagine replacing the annoying blue line with one from around the turn of the millennium when one might suppose the effects of the two eruptions to have played out, the trend would seem to be a lot steeper. Maybe this tells us nothing more than that the eruptions cause a bit of an ice melt backlog, but I just thought I’d throw that point in.

Perhaps resolving the puzzle a tad, Realclimate have helpfully drawn my attention to ice volume data from PIOMAS, which I copy here purely for convenience:

This perhaps shows more clearly the greater effect of El Chichon (1982) than Pinatubo (1991) on the Arctic ice, though, again, we have trend-lines that confuse the issue, and, again, the eruption occurred somewhat after the temporary ice volume minimum at the start of 1982, and could not have influenced ice volume until at least mid-1982. Notwithstanding, if, here, one ignores the blue line and confidence-interval shading, one might postulate that the combined effect of the two eruptions was to negate any ice-melt that would have otherwise occurred – due to global warming and the fact that if the ice builds after eruptions, logic suggests that it must melt in their absence – for almost two decades, from 1982 to the turn of the millennium, and tentatively conclude that we’re now playing catch-up.

3. Tropical eruptions are climatologically more important.
The theory (Eruptions, p.72-3) seems to be that high latitude eruptions have less effect on climate, though time of year is obviously critical. Although Laki (1783) had dramatic effects on the climate, at least for a year or two, it was a very large eruption.

Oppenheimer briefly mentions the case of Kasatochi (August 2008), a moderate sized sulphur-releasing eruption in Alaska, and the most significant climatologically since Pinatubo (1991). Sure enough, you can see the signal in the Mauna Loa, Hawaii record, above (now I realise I should have numbered the figures). And here’s the possibility of an effect in another ice extent representation from NSIDC:

Not very conclusive**, but maybe the ice did start to re-form a bit quicker than usual in 2008.

** See also the Postscript to this post.

4. The climatic effects of eruptions last only for a few years.
There seems to be an emphasis in the literature on the short-term effects of eruptions. Presumably this is because an event, such as the eruption of Pinatubo, attracts a burst of interest – and generates a flurry of publications – for a few years, before everyone moves on to other projects. Oppenheimer (p.76), suggests forcing lasts around 3 years, after which aerosols disperse, temperature is affected for around 7 years, and sea-ice “perhaps for a decade”. But, he says, oceanic circulation “can be perturbed for up to a century”. Surely this in turn would affect climate? The emphasis on transient effects seems to conflict with the reconstructions of historic temperature records, when, I understood, the main explanation for century-scale variability (the Little Ice Age and all that) is the pattern of natural forcings, principally volcanic eruptions. The story doesn’t appear to be entirely straight, and perhaps this is due to an emphasis on debunking the idea that supervolcanoes (such as Toba 73kya) could have plunged the Earth “back into the ice age” (Oppenheimer, p.190ff).

5. The climatic effect of eruptions scales less than linearly – larger eruptions do not have a proportionately greater effect.
The theory (Oppenheimer, p.191-2) seems to be that larger eruptions produce so much sulphur that larger sulphuric acid particles form, which descend through the atmosphere quicker, so that larger eruptions (as indicated by the sulphuric acid loading in ice cores) do not have proportionately greater effects on the climate.

This all seems a bit speculative. I would have thought a sufficient explanation was that, assuming larger eruptions don’t affect the atmosphere for longer than less extreme events (you’d expect similar sized particles to descend at a similar rate however many of them there are), it seems impossible for effects to scale, given the amount of sunlight reflected away by even relatively small eruptions like Pinatubo and El Chichon (see the Mauna Loa diagram, above, again!). After all, there’s only so much sunlight to reflect away, so (as for greenhouse gases) the energy gain (negative in the case of volcanic aerosols) will be a log function of concentration.

6. The effect of eruptions is to produce cool summers and mild winters.
Except when they don’t.

This is a very confusing aspect, perhaps complicated by the small sample size of recent eruptions. There’s also a need to clarify what is meant.

It’s certainly true that it’s rare for the year of an eruption to experience a cold NH winter. This is what I naively expected when I first started looking at the Central England Temperature (CET) record – eruptions cool the planet, so winter should be colder, right? But in fact cold winters do not immediately follow eruptions, with one notable exception – 1784 after Laki, which also produced a hot summer (Oppenheimer devotes his chapter 12, The haze famine, p.269ff to this event, a repetition of which would, even, or maybe especially, in the 21st century, present serious challenges to health, transport – especially air – and agricultural services in Europe and maybe the entire Northern Hemisphere).

The general story seems to be that eruptions produce more zonal weather at least in the short-term, by heating the stratosphere and disrupting poleward heat transport by the large-scale atmospheric circulation. This leads to mild winters in western Europe (i.e. the zonal pattern of westerly airstreams dominates).

It seems to me there must also be an immediate effect on patterns of oceanic temperature and heat content. I’ve noted before that it appears volcanoes can trigger or exacerbate El Nino events, although this seems to be an area of controversy. Among other effects this may tend to produce mild NH winters.

But perhaps there are also persistent effects on patterns of oceanic heat content, thought to determine NH winter weather in particular. For example, there were generally mild winters in the UK at least for more than a decade after Pinatubo. Yet cold winters – and often runs of colder than usual winters – followed a few years after Huanyaputina (1600 – 1607 was extremely cold); the unknown 1809 eruption (General Winter defeated Napoleon in 1812 and 1814 was the last Thames Frost Fair); Katmai (1912 – 1917 was particularly cold); an eruption in 1925 which has a similar ice-core sulphur signature to Katmai (1929 was cold); Agung (1963); and El Chichon (1982). It’s a confusing picture, and it’s possible that these eruptions simply occurred during series of cold winters (e.g. the famously cold winter of 1962-3 was over by the time of the Agung eruption). Nevertheless, a hypothesis might be framed to relate the location (and season) of eruptions and hence their differential effect on ocean heat content in different regions (or just latitudes) to their effect on climate over a decade or more, through intensifying or weakening (or, in the case of the largest eruptions, completely overriding) the underlying multi-decadal cycles, such as the Atlantic Multi-decadal Oscillation (AMO).

Scientists often give the impression that they’ve answered all the questions. It’s often seemed to me that this puts off those most inclined to produce radical new ideas from specialising in the disciplines that seem to be “solved”. That is certainly not the case with the effect of volcanic eruptions on climate. There are more questions than answers. And, if the historic record is not enough, new events to investigate occur every few years. I’ll certainly be keeping an eye out for new developments in the field.

Postscript (2/10/11): Amended post to tidy up section on cold winters following eruptions, adding a reference to the 1809 event (location unknown) and to scale down some of the diagrams so they’re less in your face. Also, the figure below (from JAXA via Realclimate), perhaps shows the more than usually rapid ice build in 2008 more clearly than the NSIDC figure above, though you have top look closely at the spaghetti to see that the 2008 dark green line shows one of the lowest September ice extents in the period covered turning into one of the highest extents by November:

How unusual was the cool UK summer of 2011?

Why do so many in the media feel they have to get their story in before the final whistle? It’s always a risk. Towards the end of August, a number of articles, typified by this one in the Guardian, trumpeted 2011 as “the coldest summer since 1993″. Political correctness is the order of the day – judging from the figures quoted, the record refers to the whole of the UK. I prefer to use the Central England Temperature (CET) record, which goes back further, to 1659. And I waited until the final data was in (there’s always a delay at the end of the month before the Met Office provide the final figure) and updated my spreadsheet. Here’s my latest summer temperature graph:

CET for Summers 1660-2011 (smoothing shown at central point of date range)

Note that my running means (smoothing) are shown centred, i.e. for the central of the 5, 11 or 21 years averaged. I tried the possible alternatives (i.e. trailing and forward – the latter to try to see the effect of events, such as eruptions), but this representation seems clearest to me. This way, you can most easily see the effect of, for example, the mystery eruption of 1809 and the Tambora eruption of 1815, with all curves dipping at about the same time.

I was expecting to be writing that a comparison with 1993 is not a level playing field, since the eruption of Pinatubo in 1991 cooled the whole planet for a few years (see the graphs from James Hansen that I posted in 2010), making 2011 more freakish, since there hasn’t been a recent eruption. But, in the CET record at least, summer 2011 was in fact colder than 1993.

As the graph shows, there were some colder summers in the mid 1980s, but, again, the whole planet was cooled a tad at that time by the eruption of El Chichon in 1982.

So you have to go back to the 1970s to find a summer cooler than 2011 that wasn’t induced by a volcanic eruption.

Still, you can expect the coldest summer in 40 years every 40 years, so on this reckoning 2011 was not that exceptional – compared to, say, December 2010.

But let’s go a little bit further and take global warming into account. Because of global warming we’d expect warmer summers. Indeed, as the graph shows, prior to 1933, the CET summer mean had only exceeded 17C twice (in 1826 and 1846). The mean CET touched 17C in 1933 and edged past it in 1947. But in the last 40 years it has passed that mark on 5 occasions: 1976, 1983, 1995, 2003 and 2006. The 5, 11 and 21 year running means have all broken new ground.

We should really judge the freakishness of 2011 against the prevailing summer temperature. The trouble is, we don’t know whether temperatures will continue to increase, level off for a decade or two, or even dip – that’s why the 11 and 21 year running mean curves stop before they get to the present day. If summers over the next few years are as warm as from 2003-6, then 2011 will look very unusual – perhaps the most atypical summer since 1860, which was more than 1.5C cooler than might have been expected.

On the other hand, if it turns out that the atypical summers were 2003-6, and temperatures level off for a while, then summer 2011 will just represent the sort of anomaly that might be expected every few decades, rather than a once a century or two event.

Regardless, 2011 is a long way from matching 1725 as the most disappointing summer in the CET record. 1725 was even cooler than 1816, the “year without a summer” following the Tambora eruption!

So, 2011 was surprisingly cool, but not unprecedented.

———
Incidentally, anyone who followed the link to my previous post which looked at global temperature data might have noticed that the graph of the mean summer UK CET record is uncannily similar in shape to that of (annual, not just summer) Northern Hemisphere (NH) temperatures as a whole.

For more convenient comparison here’s a more recent graph (i.e. including 2010) from the GISS graph site (we’re primarily interested in the solid red line representing the 5 year running mean NH temperature):

The hemispheric temperature record from GISS

Note how, in both graphs, the temperature peaks around 1900, then dips (usually attributed to the 1902 Santa Maria eruption), rises from around 1920 to a peak around 1940, dips again to 1970 or so, then rises into the new millennium. Overall, the magnitude of UK summer temperature changes is about the same as that for the NH as a whole, though the 1930s to 1940s peak is a little more pronounced. So it’s not just UK summer temperatures that vary – as I said, in comparing summer temperatures for freakishness (rather than trends), we need to take account of global warming.

Note also the effects of the eruptions of Pinatubo (1991) and El Chichon (1982) on the NH temperature record (or at least the dips in NH temperature following the dates of the eruptions!). This justifies my decision to exclude 1993 and the mid 1980s summers from the comparison with 2011.

I recently visited my storage unit and discovered that some boxes had fallen and damaged a fan I bought in response to the heat in, I think, 2005. The fan had been gathering dust for a few years – I haven’t needed it. The fact that I’ve paid to store the thing surely shows, though, that I certainly didn’t expect such a change from 2003-6, when all summers exceeded a mean CET of 16C, to 2007-11 when none have (the sudden dip in summer temperatures is clearly shown by the green 5 year running mean in the first figure, above). This weather/climate business is sure full of surprises!

Call this a Cold Winter? Maybe…

If you want publicity for a scientific paper, global warming is definitely the topic to go for. Especially if you manage to feed our collective snow madness at the same time!  The Independent’s baby brother newspaper the 20p “i” even used the recent findings of Petoukhov and Semenov as the basis for its Christmas Eve front-page lead.  Basically, as far as I can glean without actually seeing their paper – it’s shameful that we’re expected to make policy on the basis of data that’s not open access – P&S have done a bit of very specific computer modelling showing that less sea-ice in the Russian Arctic can change weather patterns in such a way as to bring cold weather to Western Europe.  Pretty much what I’ve been wittering on about for quite some time, as have others, with more specific academic credentials, such as a Dr Overland.

Essentially, the lack of ice allows heat to escape, lowering the air pressure over the relevant part of the Arctic and therefore strengthening the continental highs, those over Greenland and Scandinavia being most relevant to the phenomenon of interest, namely those cold European winters as manifested in the UK in particular.  Strangely, the Independent writes that:

“Their [P&S's] models found that, as the ice cap over the ocean disappeared, this allowed the heat of the relatively warm seawater to escape into the much colder atmosphere above, creating an area of high pressure surrounded by clockwise-moving winds that sweep down from the polar region over Europe and the British Isles.” [my stress]

which is a bit confused to say the least, and doesn’t appear to have come from P&S themselves, at least judging by their press release.  The heat would create low pressure in the first instance.

A more reflective (obscure pun intended) source is a piece by George Monbiot who explained the effects on atmospheric pressure thus:

“Sea ice in the Arctic has two main effects on the weather. Because it’s white, it bounces back heat from the sun, preventing it from entering the sea. It also creates a barrier between the water and the atmosphere, reducing the amount of heat that escapes from the sea into the air. In the autumns of 2009 and 2010 the coverage of Arctic sea ice was much lower than the long-term average: the second smallest, last month, of any recorded November. The open sea, being darker, absorbed more heat from the sun in the warmer, light months. As it remained clear for longer than usual it also bled more heat into the Arctic atmosphere. This caused higher air pressures, reducing the gradient between the Iceland low and the Azores high.” [my stress again]

Maybe the Indy cribbed from George.  As every schoolboy knows, its always a giveaway when you copy your classmate’s errors.

What was George’s source?  Well it may have been Realclimate, where Rasmus wrote:

“One interesting question is how the Barents-Kara sea-ice affects the winter temperatures over the northern continents. By removing the sea-ice, the atmosphere above feels a stronger heating from the ocean, resulting in anomalous warm conditions over the Barent-Kara seas. The local warming gives rise to altered temperature profiles (temperature gradients) along the vertical and horizontal dimensions.

Changes in the temperature profiles, in turn, affect the circulation, triggering a development of a local blocking structure when the sea-ice extent is reduced from 80% to 40%. But Petoukhov and Semenov also found that it brings a different response when the sea-ice is reduced from 100% to 80% or from 40% to1%, and hence a non-linear response. The most intriguing side to this study was the changing character of the atmospheric response to the sea-ice reduction: from a local cyclonic to anti-cyclonic, and back to cyclonic pattern again. These cyclonic and anti-cyclonic patterns bear some resemblance to the positive and negative NAO phases.”

which doesn’t actually say that high pressure is caused by warmer air.  What Rasmus means by “local cyclonic” and “anti-cyclonic” patterns is anyone’s guess – I venture that he may not have been referring specifically to the air pressure over the Barents and Kara Seas.  Rather, he seems to be referring to the well-known positive (“cyclonic”) and negative (“anticyclonic”) NAO “patterns”. I can see a trip ahead to the British Library to access P&S’s original paper…

All I actually want to establish in this post – it’s Chrimbo after all, not a time to do anything resembling work – is that there is indeed a phenomenon to explain.

I’m prompted by a comment Rasmus made in his piece:

“I admit, last winter felt quite cold, but still it wasn’t so cold when put into longer historical perspective. This is because I remember the most recent winters more vividly than those of my childhood – which would be considered to be really frosty by today’s standards. But such recollections can be very subjective, and more objective measurements show that the winters in Europe have in general become warmer in the long run…”

I’m tempted to start with my own contrary anecdotal evidence, but let’s consider the data first.

The Beeb were reporting on all media (lead on News 24 and radio bulletins) on Christmas morning that this December is set to be the coldest since records began in 1890, sorry 1910 (from mid-morning – the online article presumably reflects this correction).  Totally confused and can’t be trusted.  In fact, earlier in the week they had me wondering what happened in 1910 – there’s a big difference between “since 1910″ and “since records began in 1910″.

Why there’s a Year Zero in 1910 is beyond me.  I’ll let you know when I find out.  Presumably someone has decided that records are unreliable before that point, despite the tens of thousands of hours of effort that have gone into constructing the Central England Temperature (CET) record which goes back to 1659.  I can believe that the monthly averages are off by 0.1 or 0.2C, but they’re going to be good enough for the purposes of comparison. Regular readers will be aware that I have imported the CET data into Excel.

The facts are as follows:

Mean December temperature

1. No “records began” in 1890 either.  December that year is the coldest in the entire CET at -0.8C.  There are only 5 other Decembers with mean temperatures below zero: 1676 at -0.5C; 1788, 1796 and 1878 at -0.3C  and 1874 with a pathetic -0.2C.

2. Only one December since 1890 has averaged below 1C – 1981 at 0.3C.

3. The CET for December 2010 up to and including 26th is -1.0C!  OK there are 6 days to go when the weather is expected to be a little milder.  Each of these could knock 0.1 or so off the monthly average.  Even so, it’s odds on that December 2010 will be only the 7th in the entire CET since 1659 averaging a temperature below 0C.

4. It might be worth pointing out that the first cold snap began in November, so the 30 or 31 days up to Boxing Day may be even more exceptional – although this may have happened in previous years as well.

5. December isn’t usually the coldest month.  In fact the last month averaging below 0C in the CET was nearly a quarter of a century ago (though it seems like yesterday, sigh!) – January 1986 at -1.1C.  Before that, not surprisingly was January 1979, the Winter of Discontent at -0.4.   Before that, we have to look to January and February 1963 at -2.1C and -0.7C respectively.  Postwar that only leaves February 1956 at -0.2C and February 1947 at -1.9C.

6. 2010 as a whole will average no more than 8.9C in the CET, so will be the coldest since 1986 at 8.74C (though there’s no chance of it being the coldest since 1963 as suggested at Real Science).

Record daily minima

Another way of assessing a spell of severe weather is by the number of exceptional days, in this case exceptionally cold days.  Ideally we’d ask how many days this year have been in (say) the 10 coldest on record, but I only have data as to the very coldest days, courtesy of The Wrong Kind of Snow, by Antony Woodward and Robert Penn (“W&P”).  This limitation introduces a little more randomness into the exercise than I’d ideally like.  You could have an exceptionally cold day corresponding just by chance to another one on the same date in the past.  In fact, this has happened several times this year:

- 2nd December 2010 was -20.9C at Altnaharra, but failed to beat the -21.1C at Kelso during the Great Frost of 1879.

- similarly the -20.4C recorded at Braemar on 3rd December 2010 is trounced by the -26.7C at Kelso in 1879.

- and there’s a bit of a pattern here as the same thing also happened on 6th and 7th December, when the cold didn’t quite match 1879.

- later in the month, the exceptionally cold Christmas and Boxing Days this year didn’t quite match those in 1878 and 1981 respectively.

The records in W&P go back well over a century, so on average you’d expect no more than 3 over December to be broken per decade.  Let’s make it tough for ourselves and set a benchmark of 5 over November and December per decade.

Now that I’ve built up the suspense how many record low daily minima have occurred so far this winter?

The following list isn’t necessarily complete, I could have missed some (I’ve jotted them in the margins of my copy of :

- 28th November: -18.0C in Llysdinam (Powys).

- 1st December: -21.1C at Altnaharra.

- 8th December: -18.3C at Tyndrum (finally beating one of those 1879 records).

- 19th December: -19.6C at Shawbury (removing one of those 1981 records).

- 20th December: -18.7C at Pershore.

- 21st December: -17.8C at Katesbridge.

- 22nd December: -20.2C at Altnaharra.

- 23rd December: -18.6C at Castlederg.

- and 24th December: -17.4C, also at Castlederg.

I make that 9 daily records.  On this basis, not just Decembers, but early winters (November and December) in the 2010s are after just one year notably cold!

There are a few comparable cold years, of course.  1919 has the coldest days from 13th-16th November (4 daily records), including -23.3 at Braemar on 14th.

1879 has lost 8th December to 2010, but still holds the records for the six days 2nd-7th December inclusive.  It must have been more intensely cold back then than in 2010 as trees were reportedly killed.  This happens somewhere below -20C when the sap can freeze and the tree splits with a loud crack.  W&P’s entry for 4th December reports the same phenomenon in 1855.  That hasn’t happened this year.  Yet.

More recently, 1981 has lost 19th to 2010 but still holds the daily records for 7 days: 11th-14th, 17th-18th and 26th December.  And the four 1995 daily records for 27th-30th December, including -27.2C at Altnaharra on 30th, don’t look under threat this time round.

So putting our global warming expectations to one side for a minute, on the basis of daily minima extremes, 2010 is up there with the 4 or 5 other most notable early winter cold snaps in the last century and a third.

Anecdotal Evidence

I mentioned earlier the very few postwar months averaging below 0C.  These occurred in 5 winters, three of which – 1947, 1962-3 and 1978-9 – feature in Frozen in Time by Ian McCaskill and Paul Hudson (“M&H”).  Of these, I only remember 1978-9.  And that is mostly for a single snow event between Christmas and New Year.  The dry powder snow was more severe than anything this year, but I’d say the 2010 winter weather has already been more sustained in Southern England, at least.

The exceptionally cold January 1986 isn’t covered in M&H.  I suppose it wasn’t photogenic and provides little to write about because there was very little snow.  It was a thoroughly miserable month.  I remember day after day of an unceasing easterly wind bringing grey, bitterly cold, but dry weather.  I was working onsite in a poorly heated office.  If my memory isn’t playing tricks, we eventually used a thermometer to back up our complaints about the conditions.   I also remember the cold 1985 well as the Year of Crossing Frozen Car-parks.  These winters do seem to occur in runs (though I haven’t yet been able to demonstrate any persuasive statistics).

1956, with its cold February, is occasionally mentioned as a severe winter, but largely forgotten.

Will the winter of 2010(-11) be one of those that is remembered decades hence?   Much depends on the social significance.  The industrial action during 1978-9 has become the stuff of legend.  It hardly deserves its place in the Big Three on meteorological grounds alone.  The impact of 1947 was exacerbated by the continuance of wartime rationing – 1940 was also severe, but not reported to the same degree apparently because of government restrictions enacted for reasons of morale and propaganda.   1962-3 was simply exceptionally severe and prolonged.

Conclusions

Can we draw any conclusions?  Whilst I certainly can’t remember a December as persistently cold, and the records suggest there hasn’t been one since the 19th century – I haven’t even discussed the 10 or so days of lying snow we’ve had in Southern England this December, compared to an average here of only one or two – objectivity and a look at the history books is called for.  Taking my various measures in the round, in terms of the early winter, I’d judge 2010 to be around a once in 30 years event, perhaps 50 if we’re feeling generous, and only 100 if we weigh duration a lot more than severity.  But, and it’s a big “but”, given global warming (and the absence of recent cooling volcanic events), and the perhaps unwise predictions of less frequent cold winters that have frequently been made, there is indeed a phenomenon requiring scientific explanation.

My feeling, though, is that we haven’t yet seen enough for 2010-11 to be ranked amongst the overall Great Winters.  The worst Januaries and Februaries are significantly colder than the worst Decembers.  And although there’s been disruption, it’s not really been unprecedented.  Both the snow events and the nights have been severe, but have not in themselves exceeded others in living memory.  There’s been nothing, for example, that you’d really describe as a blizzard and we’ve been a little way off recording the very coldest nights.  The most notable feature has been how long the cold and snow has gone on for, as evidenced by the number of record daily minima and the low mean temperature for December.

We can’t yet expect people to say in one breath, “1947, 1962-3 and 2010-11″.  But the show goes on – we’ll just have to see what happens over the next couple of months!

 

1740 And All That

The pain goes on.  The Met Office announced yesterday that they are giving up seasonal forecasts.  This is going to seem to most people – and I have to go along with the majority view on this – as if there’s something seriously wrong.  I don’t believe we’re dealing with butterflies’ wings here.  I simply don’t understand why it’s not possible to provide a broad brush indication of the weather in a coming winter or summer.  Presumably the right data is not available, and, from a cursory reading of the literature, what’s needed is a better picture of ocean temperatures at different depths.  I suggest that’s where resources must be focussed (and I gather plans are indeed afoot – codename Argo).  Because climate science needs to get out of the dog-house.

Managing the message

What we certainly don’t need is another PR disaster.

If Professor Latif’s prediction of a period of a decade or more of cooling either imminently or over the next decade or two is correct, then “we’ll have to eat crow” as one comment on a New Scientist article put it.  The expectation of what Latif terms “monotonic” – presumably meaning “steady” or “linear” – global warming has been set.

Furthermore, as I stressed before, the reliance on Arctic sea-ice as an indicator is unwise, to say the least.  The Guardian’s report of the Met Office’s latest assessment of the evidence gave prominence to the Arctic sea-ice graph yet again yesterday.

The Guardian also included a commentary by a Dr Chris Huntingford, the online title “How public trust in climate scientists can be restored” making a lot more sense than “We need to look beyond temperature” in the print version. Huntingford makes the point that:

“To preserve public confidence, we must ‘buy out’ the copyright from research journals of key papers so that these can be freely available to all for inspection. Datasets must also become more available for general scrutiny.”

Too right. I found myself this week in the British Library accessing a paper by Drs Phil Jones and Ken Briffa, yes that Phil Jones from the CRU at UAE, Dr Emailgate himself.

What I was interested in was what Jones and Briffa term the “Unusual climate in Northwest Europe during the period 1730 to 1745″. Before I report their findings, I’ll explain why I was interested in 1740 in the first place.

The 1740 Anomaly

In my last post I presented a graph of the Central England Temperature (CET) record from 1659 to 2009. I noted the cold winter of 1739-40 which occurred after the famously warm decade of the 1730s, with a run of winters as mild as anything that occurred before the globally warmed world of the last decade (though the 1920s is also comparable).

I wanted to see how anomalous 1739-40 was, so I replotted my graph with a longer running mean. In fact, I did several plots, but let’s consider the one with a 75-year running mean, which smooths out all but long-term temperature trends:

I then calculated the Standard Deviation (SD) of the winter 1739-40 temperature against the 75-year running mean. The 1739-40 winter was 3.14 SDs colder at -0.4C than the running mean (5.59C). A statistical table tells us that we should only expect such an anomalously cold winter about once every 1,100 years.  Yet a couple of centuries later 1962-3 came along and, although marginally milder, this was against a higher 75-year running mean, so was a once in nearly 5,000 years event.  It seems something non-random is going on.

Curiously, the 9-year running mean of winters from 1730-1 to 1738-9 was, at 4.81C, even more anomalous than the 1739-40 winter. It was 3.27 SDs warmer than the 75-year running mean centred on 1735 (3.58C). (Obviously, there is less deviation in 9-year means than of single year temperatures from the long-term mean so the SD is lower). If temperature fluctuations were random and normally distributed, you would only expect a run of 9 winters as mild as 1730-1 to 1738-9 about once in nearly 2,000 years.

So we had a once in 2,000 year series of mild winters followed by a once in 1,100 years cold winter. Curiouser and curiouser…

Curiousest, the annual deviation of the meteorological year Dec 1739 to Nov 1740 is even more significant (and the calendar year 1740 even more so!):

The annual mean temperature for 1740, at 6.93C was 3.72 SDs below the 75-year running mean of 9.21C. That is, a year as cold as 1740 would be expected to occur only once in 10,000 years!

The Jones and Briffa paper

Of course, winter 1740 has not escaped the attention of climate researchers.  It was a catastrophe for Ireland, as J&B note.  But J&B can only scratch their heads, noting in their Abstract that:

“Apart from evidence of a reduction in the number of explosive volcanic eruptions following the 1690s, it is difficult to explain the changes in terms of our knowledge of the possible factors that have influenced this region during the 19th and 20th centuries. The study, therefore, highlights how estimates of natural climatic variability in this region based on more recent data may not fully encompass the possible known range.”  [My stress]

Fascinating though their paper is, J&B merely describe the meteorological conditions that occurred around 1740.  The authors barely speculate on the underlying cause.

It turns out that winter 1739-40 was merely the second in a series of 6 winters when a strong high pressure developed over Scandinavia.  In several of these years this high extended far enough west to block the usual westerlies over the UK.  In the UK and Ireland, the period was generally dry as well as cold.

Lasting Effects of Cold Winters?

The dramatic winter of 1739-40 was just one in a series of 6 atypical winters.  This set me thinking.  We don’t have full instrumental records for 1740, but we do for less dramatic later examples, such as the cooling from around 1940, the start of another series of cold winters.  Here’s a hypothesis: could it be that the entire Northern Hemisphere (NH) could naturally gain heat (over and above underlying global warming) for a few years, which is then dispersed in cold years?

In a cold winter, compared to the normal circulation in the Arctic, air mixes with that from lower latitudes.  High pressure over continental land-masses (Canada, Greenland, Eurasia) pumps warm air further into the Arctic region than usual – Vancouver on the US west coast had a record mild winter for its Olympics this year – cools it and sends it south again – to northern China, the US East coast, and to the East of Greenland.  The Arctic this winter was 7C warmer than usual.

The net effect must be that more heat is radiated away than in a usual winter.   Maybe the climate modellers can calculate how much more.

One thing I can calculate reasonably easily, though, is one of the indirect effects.  I’m taken by the persistence of cold winters.  It follows that – as well as there being more of it – the snow will melt later in the spring.  My weather book (Barry & Chorley) reveals that the NH regions with 4 to 8 months snow cover extend over 10s of millions of square kilometers of NH land areas.  What if 10m km2 snow cover persists for just one extra week?  Besides taking extra energy to melt (which turns out to be relatively insignificant), such a surface would reflect around 50% of incident sunlight relative to a year when the snow cover melted earlier.  At the latitudes (between about 60N and 40N) we’re talking about, a rough, order of magnitude, estimate is that at least 100W/m2 extra energy could be reflected (or used just in melting the snow) for a week.  10m km2 is about 1/25th of the total NH surface, so the snow effect alone is of the order of a negative forcing of around 4W/m2 over the entire NH surface, that is, more than the additional forcing of greenhouse gases, but only for one week of the year.   But if my calculation is too conservative, and in fact it’s several weeks over 20m km2 then we could be talking about a serious feedback.  One cold winter might make it more likely that the next winter is also cold.

Triggers and Feedbacks

I suggested in my previous posts on the topic of the AMO (Atlantic Multi-decadal Oscillation) that the cycle is intrinsic to the system.

Indeed, cyclic behaviour is a feature of ice-sheets.  During the last ice age (and previous ones) there were a number of Heinrich events – discharges of ice-bergs from the Laurentide ice-sheet over Canada.  Brian Fagan in The Long Summer (p.47) gives this description:

“… the ice became thick enough to trap some of the earth’s heat, which thawed the base.  Mud, stones, and water resulting from the thaw allowed the ice to skate, as it were, across the underlying bedrock.  In a matter of a few centuries, Hudson Bay purged itself of the accumulated ice.  Eventually, the ice thinned enough for the cold surface layers to freeze again…  A Heinrich event, then, is a feed-back loop – a quick warming that causes its own end in a quick cooling.” [My stress]

I suggest a much quicker – decades rather than millennia – cycle could take place for Arctic sea-ice, with the common characteristic of “warming causing its own end”.

But it’s not quite as simple as that.

First, cooling events, such as volcanic eruptions which put a sunscreen into the stratosphere, or increased warming – fewer than normal eruptions, or increased greenhouse gas levels – will affect the wavelength of the cycle.  For example, cooling during a warming phase, when the Arctic ice is thinning, will extend the time until the cooling phase.

Second, there will come a point when the system is close to tipping and a sudden cooling event (warming events are more gradual) could trigger the transition from a warming to a cooling phase.

The paper by Jones and Briffa I discussed earlier mentioned an absence of volcanoes around 1740, but my textbook, Barry & Chorley, does include a graphic (Fig 2.11, p.21) showing an unidentified eruption in around 1739 (as well as a couple in the late 1720s and nothing else after 1700).  Perhaps an eruption triggered the 1739-1745 cooling phase.

Alternatively, the turn of the sunspot cycle – i.e. from increasing to decreasing insolation – might provide a trigger.  Barry & Chorley (Fig. 3.2, p.35) show a sunspot cycle peaking in around 1738.  Triggering by a combination of events is also possible, of course.

Once established, a cooling event will be self-sustaining as long as the cooling proceeds faster than underlying warming.  I suspect the thermostat is the Arctic sea-ice.  If warm North Atlantic water melts enough of it again the summer after a cold winter in Europe, then the conditions exist for another cold winter – more cooling is needed to restore equilibrium.  On the other hand, if the ice cover increases, this may be enough to tip the balance back.  Warm water will start to melt the ice from below, starting the cycle again.

I finish with a fairly ad hoc graphic, showing winter temperatures in the CET record against annual and summer temperatures (values adjusted so that the plots appear on the same graph):

Note the wide fluctuation in the difference between winter and summer temperatures (blue line) which, at 3C, exceeds that of annual, summer or (excepting the period before 1700) winter temperatures which have varied by only 2C.  When the difference is small (i.e. the winter is mild, shown by a larger value in the Figure), as in 1740 and especially the 1930s, and vice versa, this represents an imbalance that must correct itself.  As can be seen in the Figure, the difference at present is small, but the disequilibrium is not as great as in the 1930s.  On the other hand, global warming is expected to moderate winters more than it warms summers…

Because there are so many variables in the system, every cooling event will be different.  I wouldn’t rule out another cold winter next year, though!

———

9/3/10: Corrected serious typo (“even more anomalous than the 1739-40 winter” not “the 1939-40 winter”!)