Uncharted Territory

December 27, 2010

Call this a Cold Winter? Maybe…

Filed under: AMO, Global warming, Science, UK climate trends — Tim Joslin @ 6:29 pm

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!

 

December 6, 2010

Hey, Who Moved Greenland?

Filed under: AMO, Global warming, Science, Uncategorized — Tim Joslin @ 4:53 pm

The temperature in London is forecast to be below freezing all day today, which is fairly unusual, especially as the last week-long cold snap only ended a couple of days ago. It’s not quite there yet, but we’re well on our way to a memorable winter, possibly even a Great Winter. Regular readers will be aware of my interest in the question as to whether winters like those of yore could occur today. I’m not expecting a Frost Fair (because we’re keeping the Thames warm) but am puzzled why – since 1947 and 1962-3 – we don’t nowadays see conditions similar to those which caused such events.

Anyway, right now it’s a northerly airstream that’s causing the bracing conditions. You can see it in this chart for midnight last Saturday/Sunday night from the Met Office:

I really like this particular map because it illustrates the effect of one of the great meteorological forces on the planet. Greenland is a massive chunk of ice, the only respectable ice-sheet in the northern hemisphere and the coldest thing this side of Siberia. Just nearby the North Atlantic (or Norwegian) current flows past the fjords. The current generates the largest temperature anomaly on the planet, that is, it makes the sea much warmer than anywhere else at that latitude. The contrast between cold Greenland ice and the warm sea has created the pressure difference that’s funnelling Arctic air down over Scotland and eventually here to London. Of course, the cold air over Scandinavia plays a part too.

But why is this year different?

Perhaps this map from the National Snow and Ice Data Center (NSIDC) gives a clue:

The salient feature is the ice on the east coast of Greenland, which in some places extends further out to sea than the median for the period 1979-2000, despite there generally being less ice because of global warming. Contrast this with the situation to the west of Greenland, in the Labrador sea, Hudson Bay and Canadian Archipelago, as well as that in the Bering Sea.

In meteorological terms, sea ice surely behaves more like land than open water. It’s cold, there’s little evaporation, so high pressure will tend to build. It’s as if Greenland has been moved east compared to years when there is less ice to the east and/or more to the west. Hence this week’s particular weather pattern.

Of course, all this is a result of the Atlantic Multi-decadal Oscillation (or AMO), about which I have previously written.

Historical records show an oscillation between periods when there is more ice to the west and more to the east of Greenland. But what could be driving this? Simple physics, that’s what. When the North Atlantic is warmer than usual this will cause a higher pressure difference between Greenland (which is always just as cold) and the ocean. The Greenland high will be more intense than usual, pumping air (and sea ice) down its east coast and warmer air up the west coast, as we’ve seen for the last couple of years. Hence more chance of cold weather in the UK. Who needs those supercomputers, eh?

The reason for the oscillation, I suggest, is that this weather pattern cools the North Atlantic. Of course, with global warming, this cooling phase may take longer and/or cause stronger winds round Greenland, affecting those of us in its vicinity.

———–
As an addendum I show the Met Office forecast for the same time (midnight yesterday morning, Sunday 5th December) from 3 days before:

Note how much less tight the isobars are between Greenland and Norway than in the actual chart (above). Could it be that the Met Office is underestimating the forces creating the pressure difference? The current forecast is for temperatures to climb slightly to a toasty 2 or 3 degrees above freezing towards the end of this week. But if that north wind is a bit stronger than expected maybe it’ll pump down more Arctic air and we’ll get more of a freeze. Watch this space.

March 22, 2010

Ice Pie

Filed under: AMO, Global warming, Science, Sea ice — Tim Joslin @ 9:22 pm

I was prompted earlier to complete my previous post by an article in today’s Guardian which reported on “[n]ew research [that] does not question climate change is also melting ice in the Arctic, but finds wind patterns explain steep decline”.  The word “also” is confusing – you can hardly consider “climate change” to be entirely separate phenomenon from (changing) “wind patterns”.  I’m also a little confused as the paper by “Masayo Ogi, a scientist with the Japan Agency for Marine-Earth Science and Technology in Yokohama, and… colleagues… to be published in the journal Geophysical Research Letters” (presumably around now, 22/3/10) sounds strikingly similar to the one “led by Son Nghiem at NASA’s Jet Propulsion Laboratory” mentioned in January on a NYT blog, also “appearing this week [i.e. that of 13/1/10 when the blog entry was published] in Geophysical Research Letters”.

Anyway, the findings provide even more food for thought. The point is that:

“…winds have blown large amounts of Arctic ice south through the Fram Strait, which passes between Greenland and the Norwegian islands of Svalbard, and leads to the warmer waters of the north Atlantic. These winds have increased recently, which could help explain the apparent acceleration in ice loss.

‘Wind-induced, year-to-year differences in the rate of flow of ice toward and through Fram Strait play an important role in modulating September sea ice extent on a year-to-year basis,’ the scientists say. ‘A trend toward an increased wind-induced rate of flow has contributed to the decline in the areal coverage of Arctic summer sea ice.’

Ogi said this was the first time the Arctic winds have been analysed in such a way.

‘Both winter and summer winds could blow ice out of the Arctic [through] the Fram Strait during 1979-2009,’ she said.”

First, the idea is compatible with a natural Arctic sea-ice cycle.  In cold Northern Hemisphere (NH) winters – which, to recap, I suggest are more likely to occur when the Arctic sea-ice extent is less than usual at the end of summer – air pressure over Greenland (and other northern land areas) is relatively higher than usual.  The resulting anticyclonic winds would tend to drive ice down the east Greenland coast.  Once the trend reverses, not only would more ice form in the Arctic, the weather-patterns would also change and less ice would be blown out of the Arctic through the Fram Strait (east of N Greenland).  So the Atlantic Multi-decadal Oscillation (AMO) would be expected to include a see-saw in sea-ice to the west (Labrador Sea) and east of Greenland.  Maybe someone should check the history books.

Second, all this ice flowing (or should that be “floeing”?!) into the North Atlantic (NA) is a negative feedback.  It will contribute towards NA cooling, cutting off the flow of warm water into the Arctic, reducing ice melt.

Third, it might be worth noting that the mechanism involves the removal from the Arctic of fresh water (in the form of ice).   It’s conceivable that this could be important, as the saltier the surface waters in the Arctic, the colder the water will get before it freezes.  That is, the sea can lose more heat to the atmosphere, or by radiating it away, before freezing over and insulating the waters below from the atmosphere. Likely, more cold deep saline water will form too, driving the thermo-haline circulation (THC).  Maybe someone should do some maths to see how significant this effect is.

That Snow Calculation

Filed under: AMO, Global warming, Science, Snow cover — Tim Joslin @ 4:55 pm

I remain perturbed about the possibility that the recent rapid rate of Arctic sea ice melt is at least partly due to a natural cycle.  My hypothesis is that warming causes sea ice melt which causes cooling which restores the sea ice and so on.

Rather alarmingly, you can’t just subtract the natural cycle to obtain the global warming trend.  Instead, global warming interacts with the mechanisms driving the natural cycle, with uncertain but quite likely destabilising consequences.

If the hypothesis is correct, then there would (obviously) have to be mechanisms for the Earth to lose more heat when the Arctic sea ice extent is reduced.  This could happen in several ways.  One is that the Arctic may simply be warmer in the autumn and winter than it “needs” to be for the Earth to be in thermal equilibrium.  That is, without the insulating effect of the sea-ice at the end of summer, enough heat may simply be radiated away from the ocean waters into space to make a difference.

But it may also be the case that the absence of sea-ice changes weather patterns, in particular by causing cold winters in Europe and North America.  Essentially, instead of cold air remaining in the Arctic all winter, the circumpolar circulation breaks down and cold Arctic air cools the Northern mid-latitudes.  Obviously the cold air can’t cool everywhere at once – maybe one way of looking at the effect is to imagine air masses being cycled through the Arctic “fridge” – but it does tend to produce colder winters in the east of North America and in Europe.

During a cold Northern Hemisphere (NH) winter, the southerly winds which are the counterpart of northerlies tend to pass to the west of Greenland, and of North America, so Alaska for instance is warmer.  Or, to put it another way, the continental highs over Greenland, North America and Europe have more effect on the winter weather than usual.

The result is a lot more snow.  For example, the US eastern seaboard is affected by “nor-easters” – depression systems moving up the coast – dragging cold air down from the north inland, the resultant mixing leading to heavy snowfalls.  Heavy snow can also occur in Europe and indeed Asia.

How much effect could this extra snow have, compared to a normal winter?  The purpose of the following calculation is not to quantify the effect with any accuracy, merely to determine whether it could be significant.  It seems it could.

I asked in a previous post:

“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.”

I now wonder if this is the right way to look at the problem.   The thing is, sunlight is reflected from snow whether it falls in London in December or Barcelona in March.  It might be possible to calculate the effect of all that snow without having to estimate how much longer snow cover remains in a cold winter.  All we have to assume is that the energy to melt the snow comes from sunlight falling on it.  This will be true for a large area of snow – only the border of such an area will be melted (or sublimed) by heat imported from surrounding land or especially sea (since the sea stores far more heat than the land).

Let’s take our extra 10 million km2 of snow in a cold winter and assume there’s an average of an extra 1m of snow over this area.  Warmer parts – London and Barcelona – will only receive an extra 10cm or so, but further north far more than an extra metre is conceivable.  I’ve had anecdotal accounts of snow depths of more than that, but the point is that this is the total over the winter – some will melt (or sublime) before spring and the snow will be replenished.

The “sublime”s I’ve put in brackets are important:

To melt 10m km2 of snow 1m deep takes: 10*10^6*1000*1000 (for km2 to m2)*100*100 (to cm2) *10 (estimating snow as 10% water)*334J = 3.34*10^20J.

But to sublime the same amount of snow takes ~2.6*10^21J because the latent heat of vaporisation of water is 2270J/g whereas the latent heat of fusion is only 334J/g (I’ve added the two latent heats to find the number for sublimation).

Now, a lot of snow sublimes, e.g. as a result of Chinook winds.  In general snow will sublime rather than melt if the air temperature is below 0C.  Let’s assume that half our snow sublimes as a result of incident sunlight during winter and spring.  This will absorb ~1.3*10^21J directly.

But, as I said in my previous post, this is not the major effect.  The big deal is the sunlight reflected while this process is going on.  The albedo of snow is 80-90% – call it 85%.  So only ~15% of the energy of sunlight is available to melt or sublime the snow.  The albedo of the ground absent snow is around 20% on average.  So even rounding down, 4x as much energy is reflected (85%-20% rounded down to 60%, divided by 15%) as goes into melting the snow.  This calculation is independent of the snow depth in any given location as well as how often lying snow disappears only to return over the course of the winter and spring.

The total energy cost to the planet of 5m km2 of on average 1m total snow cover is therefore about 5*1.3 – call it 6*10^21J, assuming all the energy to sublime it comes from incident sunlight.   This is equivalent to a continuous forcing over the ~250m km2 of the Northern Hemisphere (NH) of 6*10^21 / (250*10^6*10^6 to get metres squared*33*10^6 seconds in the year) = 6^10^21/8*10^21, i.e. about 0.75W/m2.

And we haven’t yet allowed for the other 5m km2 of snow that merely melts!

Since the forcing of greenhouse gases (GHGs) totals around 2.5W/m2, a 0.75W/m2 negative forcing is significant.  In fact, given that the Earth will have warmed to compensate for the GHG forcing, the albedo feedback of a cold NH winter may be enough to slow warming* and could even be enough to produce cooling against the warming trend. And this is in addition to the additional heat loss from the Arctic because of reduced sea-ice cover, which I discussed in one of my earlier posts on this topic.

—–

* Note that 2010 is an El Nino year, so the global average surface temperature may be warmer than in previous years despite the cold NH winter.

March 6, 2010

1740 And All That

Filed under: AMO, Global warming, Media, Science, Science and the media, UK climate trends — Tim Joslin @ 6:42 pm

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”!)

March 1, 2010

The AMO in the CET?

Filed under: AMO, Global warming, Science — Tim Joslin @ 10:26 am

A couple of weeks ago a commenter suggested I “personally check” the “statistical significance” of “temperature trends”.  Well, I’m not sure that’s really my role in life, but as it happens I did take a peek at the Central England Temperature (CET) record last week to see if I could see any evidence for an AMO (Atlantic Multi-Decadal Oscillation – see previous posts), which hypothetically alternately masks (e.g. ~1940-70) and reinforces (e.g. ~1970-2000) global warming and may even overshoot, producing some cooling.

The CET record is claimed by the UK’s Met. Office to be “the longest available instrumental record of temperature in the world.”  You can download the data here.

My hypothesis is that the AMO drives the Arctic Oscillation (AO) (measured by the NAO or the NAM) which determines the nature of UK winters.  Cold winters, influenced by the high pressure over Greenland and Scandinavia occur during the negative phase of the AO when there is low pressure in the Arctic which itself occurs, I propose, when the Arctic is relatively warmer, as we might expect around the time when sea ice cover is at a minimum (2007 in the current cycle).

A testable prediction might therefore be of cyclic severity of UK winters, as measured by the CET.

The graphs on the Met. Office site only cover the period from 1772, yet the record extends back to 1659.  I therefore found myself importing the raw data into Excel.  Here’s the result of my first plot:

[Btw I just found that, to avoid corrupting its appearance, I had to export the chart from Excel to Word to Powerpoint to a JPG and then import here using WordPress facilities.   Damn you Bill Gates!]

First, a couple of comments about the plot:

  • I’ve included running means calculated for the years either side of a given year (i.e. not trailing).  So, the 21 year mean for 1900, for example, is the mean of the temperatures for 1890 to 1910, inclusive.
  • 2010 data is not yet included – the down-leg at the extreme right is 2009 (average temperature 3.53C).

Clearly statistical analysis will be necessary to prove anything, and I’m sure lifetimes have been spent sifting through this data.  Nevertheless, it is possible to make a few initial observations:

  • There’s no clear evidence of a short-term link between volcanic activity and extreme winters in the CET temperature record (maybe I’ll look at annual temperatures and other seasons another time), with the exception of  a run of cold winters following the eruption of Laki in Iceland in 1783-4.  But there are other similar series of cold winters when there was no volcanic forcing, for example from 1939-42.  Krakatoa (1883) caused measurable global cooling, but occurred in the middle of a run of relatively mild winters in the CET.  Tambora occurred after the exceptionally cold winter of 1813-14, although there is evidence of another major eruption in 1810.  More recently Agung occurred after the famously cold winter of 1962-3 and other eruptions have similarly had little apparent effect on succeeding average winter temperatures in the CET.  This supports the hypothesis that the winter CET record is primarily influenced by weather patterns – central England is mild in winter when the Atlantic influence on the weather dominates, and cold when Continental air flows over the Britich Isles.   Winter temperatures in the CET may therefore be a valid proxy for the Arctic Oscillation, as I proposed at the outset.
  • There appears to be an underlying warming trend in the data.  During the 20th century the 21 year mean always remained around a degree above the low-point in the 17th century.
  • There have been two or three previous periods (e.g. ~1680 – 1730s, 1890s – ~1920) of increasingly milder on average winters as well as that leading up to the first decade of the 21st century (the 2000s).
  • There have been runs of exceptionally mild winters prior to that in the 2000s, though the 2000s were slightly warmer than the antecedents.  The 1730s stand out, but there were also runs of mild winters in the 1910s and 1970s.   In fact, there are so many runs of mild winters that I feel obliged to point out that – if variation in winter temperature were distributed randomly – we would expect 8 milder than average winters in a row to occur only once in 256 years (strictly 256 sets of 8 years which would need 263 years of data – we have 350), 9 once in half a millennium and 10 once a millennium.  Without wishing to commit myself, a closer look at the data may be in order (note that the presentation here slightly underestimates the length of warm and cold sequences, since a 21 year running mean is too short not to be dragged upwards by the series – 8 to 10 mild years – of data we’re looking for).
  • Long sequences of colder than average winters seem to be rarer than those of mild winters, but maybe this is because cold winters tend to represent a larger deviation from the mean than do warm winters.
  • There are cases (e.g. 1739-40, 1962-3) of exceptionally cold winters occurring soon after milder periods and of mild winters (e.g. 1685-6, 1795-6) following cold ones.  Perhaps the first question to ask is whether the data is random.  If it’s not, then we can start to try to work out what sort of oscillations there are in the system.

February 24, 2010

Why the AMO Overshoots

Filed under: AMO, Complex decisions, Global warming, Reflections, Science, Sea ice — Tim Joslin @ 8:00 pm

I’ve had a bit of off-line feedback on my previous post Spin Snow, not Sea Ice, the AMO is Real!, so I thought I’d try to correct any misconceptions arising from my clumsy presentation.

1. I am only attempting to explain general climate trends, not annual variation in the weather. In particular, I am assuming that the SST (e.g. as measured by satellite) correlates with the heat stored in ocean surface waters (to 100-200m depth, say).  Hence I don’t model “heat” and “temperature” separately.  Over periods of less than a decade, the SST may be determined more by atmospheric variability (including cloud cover) than heat loss from the ocean.   Additionally, there will be different patterns of Atlantic SST variability at different latitudes.  (Since the underlying cycle is of more and less heat lost at high latitudes over decadal timescales, the idealised model would be of an alternately steeper and shallower temperature gradient from (steadily warming) low latitudes to high latitudes – though we can’t rule out heat transfer between the hemispheres as well).

2. Although I have used the term “AMO” (Atlantic Multi-Decadal Oscillation), this (i.e. apparently cyclic variation in the Atlantic sea surface temperature (SST)) is just one measure (another affected is the NAO/NAM, see previous post).  Since the Arctic exchanges water with the Pacific via the Bering Strait as well with the Atlantic via the Fram Strait and Barents Sea, the mechanism itself requires another name, so perhaps “AMO” should be read as the Arctic Multi-decadal Oscillation!  I only modelled the Arctic and the Atlantic, but the Pacific waters cooled by flow of surface currents to the Arctic would be affected much the same as the Atlantic, so I don’t think the extra complexity is required for a proof of principle.

3. Which brings me onto the final point: I’m only attempting a proof of principle, in particular in my graphics.  All I was setting out to do was represent what I perceive to be the logical consequence of coupling between the temperatures of the Arctic and the North Atlantic and Pacific.

In actual fact, I suspect the heat exported to the Arctic varies with a higher power of the Atlantic temperature and not linearly.  The point is that less and thinner Arctic sea ice at the start of winter allows more cold deep water formation which is accompanied by the dispersal of more heat because there’s more of it and also because the surface water was initially warmer.  Introducing a square function leads (as well as to a more chaotic system) to a shortening of the AMO cycle in a warming world.  E.g.:

Any fool can produce an oscillation in a spreadsheet, so why do I think the AMO mechanism is real and important?

1. We keep being told that the Arctic is warming faster than predicted by the climate models. This means it is dissipating more heat than predicted – by radiation into space, by evaporating water that falls as snow or rain and so on.  The climate involves net heat gain at low latitudes, heat transport in the atmosphere and oceans and heat loss at high latitudes.  If the Arctic is warmer than would be expected for steady global warming, then what we’re going to get is unsteady warming (as in the 1930s-40s, see previous post).

2. The criteria exist for an oscillating system – the temperature of the Arctic depends on that of the North Atlantic (and the North Pacific) and vice versa (i.e. there is a negative feedback loop) and there are delays in the system.  These arise because the rate of surface water flow to the Arctic (and deep water flow back) is variable and adjusts only slowly.  There needs to be a relatively large temperature difference between the North Atlantic (NA) (please read North Pacific too) and the Arctic to generate a sufficiently strong current to cool the NA. As every MBA student knows (e.g. from the Beer Game) any negative feedback loop with delays results in an oscillating system.

The system round Antarctica is somewhat different – the coldest area is land and water can flow freely from warmer areas to colder ones (i.e. those with seasonal sea-ice and hence deep cold water formation). This is not to say there aren’t oscillations down there, just that they’re not the same (or, probably, as extreme).

3. The AMO mechanism is that, as the NA warms, the Arctic warms too (because there’s always a current from the NA), reducing the amount of insulating sea ice (and multi-year ice is thicker and a better insulator than first year ice) and therefore increasing its capacity to drain heat (by creating new ice and cold deep water) from the NA.  The critical point – the delay in the system – is that warming and cooling takes some years, so the Arctic will continue to warm even as it starts to cool the NA, and will cool (forming more multi-year ice) even as the NA starts to warm.

4. It seems to me – and my incredibly simplified modelling supports this – that the Arctic will keep warming until it cools the NA, however warm the NA gets (of course, the NA can also lose heat in different ways).  Until, that is, first, the capacity of the Arctic to dissipate heat is reached, and then the system breaks – when the Arctic gets so warm it can no longer generate an overturning circulation.

And once the Arctic has warmed enough to cool the NA, it will overshoot (this could already have happened in the current cycle if the summer sea ice minimum has already been reached), because the NA will still be warm enough to warm the Arctic even while it (the NA) is cooling, albeit at a slower and slower rate until the process reverses.  For similar reasons, the Arctic will also overshoot in the reverse phase, i.e. it will continue to cool even after the NA has started warming again.

5. A rough calculation suggests a net oceanic transfer of heat to the Arctic of 60TW or ~2*10^21J/yr [1], which luckily is compatible with the figures I calculated in my previous post The Earth is a Fridge.  Now, the IPCC estimates that the oceans have gained on average ~14*10^22J between 1961 and 2003 (including ~8*10^22J from 1993-2003) because of global warming (the blue bars are 1961-2003, the burgundy bars 1993-2003):

Heat gain by global warming (IPCC Fig TS.15)

That is, the oceans have been gaining heat at a rate of around 3*10^21J/yr on average (and around 8*10^21J/yr from 1993-2003).  Let’s attribute 1 or 2*10^21J/yr to the NH which after all is mostly land.

It seems to me at least plausible that an overshooting strengthening of the AMO by more than 50% from its 2*10^21J/yr average – and remember it will be strongest when there is no sea-ice at all in summer, which is still some way from the case – could pump heat out of the northern oceans at a faster rate than they are gaining it by GW (this is all very approximate, proof of principle stuff, but note that a 50% volume increase oceanic circulation in the positive phase of the AMO would be 50% more water containing more heat – conceivably 4*10^21J/yr, perhaps, rather than 2*10^21J/yr).  That is, the AMO could create some cooling for a period.  Of course, this would be followed up by even faster warming, then an even stronger reaction, until the system reaches its capacity as I mentioned earlier, after which we’d just see steady warming.

I conclude with a final figure from the IPCC (panel (a) is mislabelled, the graph shows just the minimum sea-ice extent each year, not the anomalies in it):

Arctic and Antarctic sea-ice anomalies (IPCC Fig. TS.13)

———-

Reference

[1] “Modelling Arctic Ocean heat transport and warming episodes in the 20th century caused by intruding Atlantic Water”, Wang Jia et al, Chinese Journal of Polar Science, Dec 2008.

February 23, 2010

Spin Snow, not Sea Ice: the AMO is Real!

How unfortunate. Back in 2000, yes, that’s not a typo, in 2000, the Independent wrote that:

“According to Dr David Viner, a senior research scientist at the climatic research unit (CRU) of the University of East Anglia, within a few years winter snowfall will become ‘a very rare and exciting event’.

‘Children just aren’t going to know what snow is,’ he said.”

In Viner’s defence, he did go on to say that rare snow events would cause chaos.

It’s No Joke

For a long time it’s seemed to me that one problem global warming (GW) is likely to throw up is that snow events, like other forms of precipitation, will become more extreme. That is, when it does snow, it’ll be heavier.

A commenter on one of my recent posts suggested I go and do some statistical analysis on temperature measurement data to see if trends are significant. In actual fact, about 5 years ago, I did exactly this with data on snowfall. If I recollect correctly, I found that there was a statistically significant trend in the number of heavy snow days (above a particular depth) in the middle of winter (i.e. not in months when, due to GW, some pf what would otherwise have been snow might fall as rain) in the data I found on the net for a particular Rocky Mountain ski resort. If I come across my notes I’ll bring the analysis up to date.

Here’s the real concern. A few decades down the line, the planet will be a lot warmer and we’ll be seeing much heavier precipitation in some regions. Some of this will be snow. Furthermore, there’s always the chance of a cold snap, for example, when a volcano goes off (and we really should be worrying more about this climate risk, IMHOP – more another time, maybe). Or after a geo-engineering accident (sorry, couldn’t resist). At the start of the cold event at least, the oceans will still be warm, because of stored cumulative GW heat, and they will therefore continue to pump moisture into the atmosphere. But the dust shroud will rapidly cool land areas, so that some places used to dealing with just heavier rain suddenly find themselves trying to cope with a foot or two of the white stuff.

It’s a shame climate scientists haven’t been warning people about the vulnerability of flat roofs to heavy snow.

Skating on Thin Ice

On the other hand, there’s been a worrying tendency over the last few years to treat the continually diminishing amount of Arctic sea ice each year (at the minimum extent in September) as a GW canary in the coal-mine, like glaciers.

It would have been better to stick to glaciers. Because changes in Arctic sea ice may well be part of a natural cycle. Of course, there’s an underlying warming trend tending to reduce the amount of Arctic sea ice. But if and when the natural cycle starts to dominate, sceptics will have another field day.

It’s worse than this. The cycle – which is called the Atlantic Multi-Decadal Oscillation, or AMO for short – could affect the temperature of the entire Northern Hemisphere (NH). [See previous post Musings of the Hemispheres – there may be similar processes in the SH, but I’m not going to discuss those just now].

Before I go on, there was a fuss a while back – serious stuff: letters to the Guardian editor, that kind of thing – when a Professor Latif was accused of explaining GW with AMO. His position, like mine, is that both GW and AMO affect the climate. I just want to make it clear that I’m with the Professor on this, even if simplistic sceptic brains find this position a logical contortion.

Evidence for the AMO (1): IPCC Data

Consider the following graph from the IPCC (AR4, the most recent report):

Global mean surface temperature relative to 1901-50, compared to climate models (IPCC Fig TS.23)

What gets me about the IPCC data is the anomaly around 1940. The average temperature was simply too high, and this is not adequately explained (if it was, I guess the models would be corrected).

We can drill down a little further:

Continental-scale breakdown of actual and modelled temperatures compared to 1901-50 (IPCC Fig TS.22)

Here we see that by and large the models represent land temperature fairly well, but that ocean temperatures were outside what are presumably intended to be some kind of confidence limits – for what looks like around an entire decade (just before mid-century).

This is not a very satisfactory state of affairs.

Note from Fig TS.22 above that the land temperature range over the past century has been around 1C, and that of the oceans perhaps 0.7C.

Consider what’s happened in the North Atlantic:

AMO from 1850-2005 (temperature relative to 1961-90 (IPCC Fig. 3.33)

The North Atlantic sea surface temperature (SST) (top graph) has increased by nearly 1C since its lowest point soon after the turn of the 20th century.

A 1C increase in ocean temperature is unsustainable. Land has a lower heat capacity (i.e. you have to put in less heat for a 1C temperature rise) than ocean, so must warm faster. The North Atlantic heat will have to dissipate.

Evidence for the AMO (2): The Historical Record

If I were a climate specialist about to make a song and dance over a particular piece of evidence for GW, I think I’d make pretty sure the phenomenon in question hadn’t happened before.

It just so happens that the area of Arctic sea ice has shrunk dramatically before, and not so long ago.

Yeap, you’ve guessed it, the Arctic warmed from around 1920 to 1940. Here’s the Abstract of a paper The early twentieth century warm period in the European Arctic that looks kosher – it must be, it costs $42! A site, www.arctic-warming.com seems to be devoted to the issue (particularly of warming around Spitsbergen in 1918-22) and cites some other papers discussing the 1920-40 episode, “one of the most spectacular climate events of the 20th century”. There’s even a book about the event.

None of these sites offer a clear explanation for the Arctic warming, so I’m going to have a bash.

Explaining the AMO

The point is that loss of Arctic sea ice – absence in summer and thinning year round – is not just a symptom of warming. It is part of a cyclic causal mechanism.

As I pointed out in a previous post, The Earth is a Fridge, the less sea ice there is at the start of winter (the Arctic ice extent is at a minimum around mid-September!), the more heat the Arctic waters can lose to the atmosphere and hence into space during the winter. Water covered by ice can’t lose heat because ice is an insulator, and the process of freezing is itself an important mechanism for losing heat.

Clearly the Arctic waters will lose most heat in winter when there is no summer ice. In a steadily warming world, you might expect first the summer ice to disappear, at which point the Arctic would have reached it’s maximum effectiveness in getting rid of heat (imported in currents from lower latitudes) and gradually the maximum extent of ice each year would reduce.

But there is an oscillation in the system.

Modelling the AMO

At first I was going to simply draw a curve on a piece of paper and scan it in, but my better half is a bit of an Excel whizz and persuaded me to do something a bit more sophisticated.

It was astonishingly easy.  Here’s the result, first without taking account of global warming (GW):

I can’t emphasise enough how easy it was to produce this graph. It’s hugely simplified, including as it does just two ocean masses and nothing else and making no attempt to distinguish between heat and temperature, and between temperatures at different times of year.  But I don’t see why it isn’t qualitatively valid – it produces the asynchronous sinusoidal temperature curves I’d deduced anyway, but with the added theoretical basis of generating them by heat exchange between the Arctic and the NA.  And since I’ve tied the temperature curves very roughly to historic data, the timescale of future temperature changes could conceivably be roughly correct.  The fact that what I wanted to show drops so easily out of the spreadsheet suggests some underlying veracity – I claim no more than that – at least to me.  End of disclaimer.

All I’ve done is calculate the temperature of the Arctic (purple line) in a given year as its temperature the previous year (times a cooling factor) plus the North Atlantic (NA) temperature the previous year times a factor (15% in this instance).  All I’m assuming is that the warmer the NA is, the warmer the Arctic will be.  After all, we know surface water flows from the NA to the Arctic.

So far, so simple.  The next bit is the critical point.

I’ve calculated the temperature of the NA (green line) similarly, but included a negative feedback.  In the model, the NA temperature is equal to its temperature the previous year (times a cooling factor) minus the Arctic temperature the previous year times a factor (6% in this instance, less than the 15% for the reverse case because the NA is bigger than the Arctic).

The minus in this calculation says that the warmer the Arctic is, the more NA heat it can absorb and disperse ultimately into space.    Remember, my argument is that the thinner and less extensive the Arctic ice, i.e. the warmer it is on average over the year, and in particular at the start of winter, the more NA heat it can disperse over the year, but in particular in winter.  [A more complex model could try to model the Arctic temperature at different times of year].

Obviously I’ve adjusted the numbers and starting conditions to fit the graph roughly to the historical record.  (The anomaly on the vertical axis is arbitrary, 0 is intended to be the long-term equilibrium – if you start with 0 for both anomalies, the graph is flat).

As well as the Arctic and NA temperatures I’ve included in my schematic an indication of the Northern Hemisphere (NH) temperature, produced by simply adding the NA and Arctic values (yellow line).  This shows a peak in 1940, which is what we’re trying to explain, as well as a peak around 2005 and, as predicted by Professor Latif, subsequent cooling for quite some time.

The good news is that we won’t have to wait too long to find out whether the AMO is real.  The bad news is, that, if it is, it’ll be like putting rocket fuel in the sceptic bandwagon.

I thought I’d go a little further and see if my model predicts anything else.  I’ve therefore included an “Arctic Oscillation” (AO) (blue line) which I’ve calculated by subtracting the NA temperature from the Arctic temperature.  The AO – represented by real-world indicators such as the North Atlantic Oscillation (NAO) and the Northern Annular Mode (NAM) – is an atmospheric phenomenon which correlates with the nature of NH winters.  My logic is that the higher the temperature of the Arctic compared to the NA, the lower the air pressure will be over the Arctic in comparison with the NA, which is in principle what the NAO and NAM measure.

Anyway, here, again from the IPCC, is the actual historical record of the NAO/NAM:

NAO/NAM indices (IPCC Fig 3.31)

Compare these real-world measurements with my model which (blue line) predicted a positive AO from 1900 to the 1930s and again from the 1960s to around 2000.  Could they possibly fit together?

Future temperatures, Global Dimming and Global Warming

I have to say I’m rather alarmed that, based on the timescales of the historic 20th century AMO cycle, my model shows temperatures falling for another 15 years.  I thought I’d better factor in a bit of global warming, so I played around in Excel a bit more:

This time I’ve allowed for GW by adding an arithmetically progressively larger term into the NA and Arctic temperatures each year.  As in the previous figure, the vertical anomaly scale is entirely arbitrary and not intended to map to real temperature deviations.

I’ve also extended the model to 2050 and calculated the NH temperature (yellow line) by adding the NA temperature (green line) to a halved, rather than the whole, Arctic temperature (purple line), since the NA is bigger than the Arctic.  Clearly the temperature cycles still exist, it’s just that the AMO is imposed on an underlying trend, so both peaks and troughs in the temperature curves are higher.

In this very rough calculation, we still see NH temperatures declining for a couple of decades.  Worrying.

I should add that the usual explanation for the cooling period from around 1940 to 1970 is “global dimming”, i.e. the blocking of sunlight by industrial pollution.   The AMO hypothesis suggests that at least some of this cooling was caused by a natural cycle.

Next Steps

A perfect computer model would accurately represent sea ice melting and freezing and the resultant exchanges of heat between the sea and the atmosphere and effect on oceanic circulation.  It would therefore predict long-term natural climate variability such as may – and I stress “may” – be caused by the AMO.

Current climate models do not correctly retrodict (i.e. predict known data) the warming up to 1940 and they have under-estimated the Arctic warming that has occurred over the last decade or so.

It seems to me that – prior to the IPCC’s next report on the science, AR5 – serious effort needs to be made to evaluate the evidence and theoretical basis for an AMO, and take account of it in projections of the future climate.

I used to be highly sceptical of long-term natural climate variability, but now I’ve realised there could be feedbacks between Arctic ice-melt and NA temperatures, I’m suddenly convinced.  I’d like to see some serious modelling of the AMO and similar decadal variability that logically should also occur in the SH.

Maybe the effect of GW will be to completely swamp the natural AMO.  But I’d like to see proof of that.

A failure to explain the AMO would lead to increased climate scepticism and a loss of political will to deal with GW.  We could be left totally unprepared for a steep rise in temperatures starting in a decade or two’s time.

February 1, 2010

The Earth is a Fridge

Filed under: AMO, Global warming, Science, Sea ice — Tim Joslin @ 3:25 pm

No, I’m not a teapot. I’m serious. The way the climate system works is that, over a year, there is a net gain of heat in low latitudes and a net loss at high latitudes. Heat is transported from more tropical regions and radiated away at the poles.

Now, I’ve been mulling over the mystery of why Northern Hemisphere warming (as measured by the mean surface temperature) appears to have slowed over the past decade or so. I suggested a while back that, in view of the rapid industrialisation of China in particular, perhaps renewed global dimming has a role to play.

I recently felt some encouragement to persist from Sue Solomon’s comments in the Guardian recently that:

“…there are climate scientists round the world who are trying very hard to understand and to explain to people openly and honestly what has happened over the last decade.”

And so they should.

Realclimate was a little sniffy about the Guardian’s reporting of the science aspect, with a curious exchange at comment 47, but the (tentative) conclusion seems to be that Solomon’s findings relate to some kind of poorly understood feedback mechanism rather than a climate driver (i.e. an external effect on the climate system).

Back to the story. As I said at the start, the Earth is a giant fridge.

Now, it has suddenly occurred to me that the efficiency of the fridge could be different when the whole system is in a warmer (or cooler) state. If this effect is significant you’d therefore expect periods of more and less rapid warming as the Earth’s ability to radiate away heat changed.

Cutting to the chase, it seems to me that sea ice cover reduces the ability of the planet to radiate heat away; more to the point, loss of sea ice increases its ability to radiate heat away. Ice is a good insulator.

What’s been happening up in the Arctic is that “multiyear” ice has disappeared rapidly over recent years.

Now, if some relatively warm water ends up under some ice that’s already there, at best it can slowly cool to around -2C (when it is in equilibrium with the ice) – because of insulation the ice will not get much thicker. But if, come winter, the sea is not already covered in a layer of ice, the water can cool relatively more and can turn to ice and lose a lot more energy in doing so. Simples. [Actually, it’s not: what may be critical is the amount of surface water that, as it cools, becomes more dense and sinks, allowing heat to be lost from a greater volume of water than at a lower initial surface temperature. The amount of “ventilation” of the water column (by wind) may also be an important factor in determining how much heat can be lost before the insulating ice layer is formed at the surface. Furthermore, Wikipedia notes the process of “brine rejection” whereby water just under the freezing layer becomes more dense (because ice doesn’t incorporate salt) and sinks may also be important – obviously the amount of brine rejection depends on how much freezing occurs each year.].

What I’m suggesting is that the Earth’s refrigeration mechanism will be more efficient the less – in extent and thickness – sea ice there is at the start of winter. This doesn’t mean the planet will start cooling, of course, but it could slow the warming.

I thought I should do a rough calculation to see how much energy it takes to melt the Arctic sea ice each year. The interesting Stoat blog links to some data showing that very roughly 10 million km2 of ice freeze and melt each year.

I’ve seen the nature documentaries, so let’s guess that this ice is on average 1 metre thick.

To melt this ice alone takes 10^7 (the area) *10^6 (to metres cubed) *10^3 (to litres ~= kg) *334*10^3 J (latent heat of fusion of water) = ~3.34*10^21J.

I also happen to know that doubling CO2 will lead to a forcing of around 4W/m2 over the whole planet. 1W/m2 is therefore quite a significant number. How much is 1W/m2 over 1 hemisphere over a year?

The area of the Earth’s surface is ~500m km2, so 1W/m2 of the northern hemisphere is, over 1 year, 250*10^6 *10^6 (converting to m2) *365*24*3600 (a year’s worth of seconds = ~30*10^6) = ~7.5*10^21J.

So, just freezing the Arctic sea ice every year, never mind cooling the water or ice down implies that the Earth radiates away heat equivalent to a continuous forcing of around 0.4W/m2 of the entire surface of the northern hemisphere.

In fact, if we assume the water has to be cooled down as well, that 0.4W/m2 becomes a little bigger (the specific heat of water is around 4J/g/C – i.e. 4J heats 1g by 1C).

Of course, the extra heat loss in winter while the water is cooling and freezing when the ice extent is low needs to be weighed against the extra heat gain in summer by the albedo change due to the absent ice sheet. Looking at it another way, when there’s no permanent sea-ice, the albedo-feedback-assisted summer melting and winter freezing exactly cancel out. Obviously. My point, though, is that there is a circulation and the Arctic cools water that ends up flowing back south as a cold deep current (so it’s the 4J/g/C released when water cools rather than the 334J/g when it freezes that’s important). This mechanism is cut off by the insulating effect of a layer of sea ice. A corollary is therefore that improved Arctic fridge efficiency should strengthen the thermal oceanic circulation. In total, over a year, once it’s warm enough for the sea-ice to disappear in summer, more cold water should sink and flow south than before, thereby allowing more warm surface water to drift north.

There could be an optimum Arctic cooling efficiency when it’s still cold enough for the ice to freeze by the end of the winter (to reduce heat uptake during the early summer) but warm enough to mostly thaw by the end of summer.

In conclusion, I present, in the hope of encouraging progress towards an explanation of the lack of 21st century warming in the northern hemisphere, and to supplement the Renewed Global Dimming Hypothesis, the possibly even more tentative Strengthened Earth Refrigeration Mechanism Hypothesis.

I should repeat what I may term the Warming Warning, that is, that, if underlying warming is being masked, or postponed, by either of these mechanisms and/or others, we could be in for a real shock in later decades.

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