Uncharted Territory

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”!)

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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.

April 26, 2010

On Climate, and Causes in Complex Systems

Filed under: Complex decisions, Credit crisis, Economics, Global warming, Reflections, Science — Tim Joslin @ 4:06 pm

Why do so many travellers, such as those marooned by the Eyjafjallalokull ash cloud, invoke a panic response on finding they can’t leave a foreign land? I remember that when I was on a memorable trip to Albania in, if I recollect correctly, 1996, our group was playing leapfrog, as it were, with another minibus full of tourists, along a road to the coast. When we stopped – often – for a passenger to relieve the symptoms of one or other of the local stomach-bugs, the other bus passed us, only for us to see them stopped by the roadside a few minutes later. Eventually we pulled up beside them to chat. It turned out that Albania’s borders were shut, in the hope of trapping whoever had blown up the Tirana police-chief. The other group were cutting their trip short. So illogical. We simply carried on with our holiday. [I drafted this a few days ago, but, since then, I’ve heard, Radio 4’s Today programme is discussing the psychology – and even genetics – of the have-to-get-home phenomenon, right now!].

It beats me why so many people spent thousands of euros hiring cars to drive across Europe. Surely staying until the ash alert blew over would have been both cheaper and less stressful.

Whatever they did, though, even those who have spent the last week hitch-hiking from Athens to Calais abroad cannot fail to have heard that, apparently, global warming will lead to more volcanic eruptions.

Is this something we should worry about?

In short, no.

Volcanoes and ice ages

The scare seems to be based on a study of the end of the last ice age:

“Huybers and Langmuir spliced two databases of volcanic eruptions worldwide over the last 40,000 years.

Eruption levels stayed low until around 12,000 years ago, then suddenly they suddenly shot up. The melting ice released so much pressure that the newly liberated volcanoes erupted at up to six times their normal rate, the researchers estimated.

The inferno lasted for 5,000 years and could have pumped enough CO2 into the atmosphere to raise concentrations between 40 and 50 parts per million, the researchers estimate. Changes in ocean chemistry probably released the rest.”

I love the eruption levels “suddenly” shooting up “suddenly”!

Now, a couple of kilometres of ice over a volcano is one thing. It’s reasonable to suppose that would prevent the pressure in a magma chamber that would otherwise have caused an eruption from doing so.

But melting a kilometre or so of ice takes quite some time. And besides, since the last ice age, there aren’t so many ice-sheets left. Worst case, in a few centuries, perhaps, we could feel the effects of some pent-up volcanic activity.

In the meantime, the worst that could happen is that some eruptions are brought forward by a few years.

The hype around the volcano scare exploits our innate difficulty in conceiving long periods of time. It also resonates with research a while back which noted more eruptions at certain times of year. The suggestion was that particular weather conditions – changes in pressure – around volcanoes, could set them off.

This triggering is an entirely different kettle of fish.

Volcanoes and the weather or short-term climate change

Consider a simple model of volcanic eruptions as the sudden release of something we might call “pressure” that builds up over time. Let’s suppose that the main cause of the build-up of “pressure” is geological. Let’s also assume that the weather can cause seasonal variations in pressure. In this model, eruptions will occur when the total pressure crosses some threshold, as in the following diagram:

Because I’m lazy, and Powerpoint is a step back from pen and paper (and reverting to that and scanning is a hassle right now), I’ve shown the total pressure (dotted line) as the sum of the geological pressure and seasonal variations for the first eruption only, but hopefully you get the idea.

It’s not very usual for volcanoes to erupt every 3-5 years, of course – 50 years or so might be more usual – and in real life every eruption is different.

Hopefully, though, it’s fairly easy to see that eruptions are much more likely in this system during the period when the seasonal effect tends to increase pressure. Over this period the total pressure (dotted line) increases much faster than when the seasonal effect is to decrease pressure.

In fact – and hold this thought – if the rate of increase in pressure due to short-term variability is faster than the long slow build-up of pressure, then eruptions, according to this simple model, will always occur during the short-term upswing in pressure.

My proposition is that it is very easy to exaggerate the effect of the seasonal cycle as a “cause” of eruptions. It is merely a trigger.

You can also see that, if, say, the seasonal pressure changes – a gradual trend on top of the annual fluctuations, perhaps, or an increase in amplitude of the cycle – it will not have a large effect on the frequency of eruptions over a long period. The periodicity of the system will still be driven by geological processes. The weather is a secondary driver in this system.

Now, if you didn’t know that volcanic eruptions are caused by a build-up of “pressure” underground, you might hypothesise that they’re caused by weather conditions. You might collect a lot of data and calculate correlation coefficients to “prove” your theory. You might even argue convincingly that, because we know what causes the weather, the weather must cause volcanic eruptions rather than vice versa and furthermore, it is not the case that both the weather and volcanic eruptions are caused by a third factor.

But you’d be wrong.

Could this mistake happen in other circumstances, though?

Solar cycles and the AMO

That old chestnut, solar cycles, surfaced yet again in New scientist a week or two ago. The claim is that there’s a “compelling link between solar activity and winter temperatures in northern Europe.”

Well, maybe there is.

But anyone who’s stayed awake this far will realise that it’s not enough to determine a correlation between solar cycles and weather patterns. Maybe the solar cycle does trigger a change from one state of the AMO (Atlantic Multidecadal Oscillation) to another. But that doesn’t make it the sole or even the main cause of the variability.

To recap, I first explored the idea of the AMO when I became concerned that the emphasis being put on shrinking Arctic ice as an indicator of global warming (GW) could backfire if the shrinkage reverses. My first post on the topic was therefore titled: Spin Snow, Not Sea Ice, the AMO Is Real!. Back then, I noted that the AMO cycle – likely to be variable in length, especially now we have the extra GW complication – tends to be of the order of 60 years or so, with the previous cooling phase lasting from the 1940s to the 1970s. Maybe we’re entering another one.

That first post suggested a mechanism for the AMO, which I discussed a little more in my second post on the topic, Why the AMO Overshoots. So I won’t repeat myself today.

Later, in 1740 And All That I looked at a historical example of a sudden switch from mild to cold winters in NW Europe. The weather pattern that leads to cold winters might be termed an “anti-monsoon”, as I first discussed back in January in Snow Madness and the North-West European Anti-Monsoon.

Two other posts Ice Pie and Ice Sickle explore aspects of the AMO.

The basic argument in all these posts is that the natural cycle – the AMO – is characterised by a set of feedbacks. Positive feedbacks – perhaps including the effect of lying snow, as considered in That Snow Calculation – produce distinct warming (Arctic ice melt) and cooling (Arctic ice recovery) phases. Negative feedbacks cause one phase to flip to the other. But the exact timing of the tipping-point may be caused by external triggers.

My proposition is that by the end of warming phase of the AMO, the seas (especially the Arctic and the North Atlantic) are relatively warm compared to the land. Any sudden cooling event could trigger a flip to the cooling phase, because the land cools quicker than the ocean, so would become relatively even colder.

Possible sudden cooling events are volcanic eruptions or the change to a cooling phase of the solar cycle, as discussed previously for the case of 1740.

A critical point is that the sunspot cycle is much shorter than the AMO (see AMO discussion and graphs in my first post on the subject):

NASA graph of yearly sunspot numbers

The sunspot cycle indicates the total irradiance from the sun, and the rate of variation is comparable to that of other causes such as GW:

IPCC Fig 2.16 recent changes in solar irradiance

Further Implications

Not too many scientists claim volcanic eruptions are “caused”, as opposed to triggered, by variation in the weather or by climate change. Most understand that only over the sort of long timescale that is needed to melt an ice-sheet would the frequency of eruptions change.

But far more common is the explanation of apparent climate cycles – such as the AMO – by variations in solar output. In cases such as this, it is necessary to do more than just prove a correlation. The causal mechanism needs to be clear and must be shown to be quantitatively sufficient to explain the observed phenomena.

Considerable care is required whenever attempting to explain the “causes” of complex system behaviour.

The need to distinguish between triggers and underlying causes of cyclic behaviour also applies elsewhere in the climate system. Furthermore, the distinction between triggers and underlying causes may become blurred – both may be of similar magnitude, creating a resonant system. In particular, over longer timescales than so far discussed, the Milankovitch cycles are not enough alone to explain the ice age cycle. Perhaps they resonate with another cycle internal to the climate system.

In other domains too, it is not possible to assume that the “cause” in a complex system is just that which is evident on the surface. The lax lending practices and cheap money that are held to have caused the credit crisis may just be one part of a deeper, more complex cycle of optimism, deregulation, increased trade and globalisation on one hand and retrenchment and nationalism on the other.

April 8, 2013

March 2013 WAS equal with 1892 as coldest in the CET record since 1883!

10 days or so ago I discussed the possibility that March 2013 would turn out to be the coldest in the Central England Temperature (CET) record since the 19th century.

Well, it did it!

Here’s a list of the coldest Marches since 1800 in the CET:

1.   1883  1.9C
2.   1845  2.0C
3.   1837  2.3C
4= 1892  2.7C
4= 2013  2.7C
5.   1962  2.8C

A few questions and not quite so many answers occur to me:

1. Why hasn’t the Met Office trumpeted March 2013 as the coldest since the 19th century?
What I’m alluding to here is, first, that the Met Office records for the UK and England only go back to 1910, but also that, as detailed on the Met Office’s blog, it turns out that March 2013 was only the joint 2nd coldest for the UK as a whole:

“March – top five coldest in the UK
1 1962 1.9 °C
2 2013 2.2 °C
2 1947 2.2 °C
4 1937 2.4 °C
5 1916 2.5 °C”

and second coldest for England as a whole:

“Looking at individual countries, the mean temperature for England for March was 2.6 °C – making it the second coldest on record, with only 1962 being colder (2.3 °C). In Wales, the mean temperature was 2.4 °C which also ranks it as the second coldest recorded – with only 1962 registering a lower temperature (2.1 °C). Scotland saw a mean temperature of 1.3 °C, which is joint fifth alongside 1916 and 1958. The coldest March on record for Scotland was set in 1947 (0.2 °C). For Northern Ireland, this March saw a mean temperature of 2.8 °C, which is joint second alongside 1919, 1937, and 1962. The record was set in 1947 (2.5 °C).”

The figures all tally suggesting that the parts of England not included in the CET were less exceptionally cold than those included, as I suggested before.

2. Why hasn’t the Met Office trumpeted March 2013 as the second coldest on record?
What I’m alluding to here is that the Met Office only made their “second coldest” announcement on their blog, not with a press release. The press release they did issue on 26th March was merely for “the coldest March since 1962”, and included somewhat different data to that (above) which appeared on their blog for the whole month:

“This March is set to be the coldest since 1962 in the UK in the national record dating back to 1910, according to provisional Met Office statistic [sic].

From 1 to 26 March the UK mean temperature was 2.5 °C, which is three degrees below the long term average. This also makes it joint 4th coldest on record in the UK.

Looking at individual countries, March 2013 is likely to be the 4th coldest on record for England, joint third coldest for Wales, joint 8th coldest for Scotland and 6th coldest for Northern Ireland.” (my stress)

and a “top 5” ranking that doesn’t even include March 2013, which eventually leapt into 2nd place!:

“March – Top five coldest in the UK
1 1962 1.9 °C
2 1947 2.2 °C
3 1937 2.4 °C
4 1916 2.5 °C
5 1917 2.5 °C.”

As I’ve also mentioned before, it’s odd to say the least that the Met Office have formally released provisional data (and not the actual data!) to the media.

So I’ve asked them why they do this, by way of a comment on their blog:

“The Met Office’s [sic – oops] announced a few days ago that March 2013 was only the ‘joint 4th coldest on record’ (i.e. since 1910) rather than the joint 2nd coldest. This was based on a comparison of data to 26th in 2013 with the whole month in earlier years, which seems to me a tad unscientific.

Maybe it’s just me, but it seems that there was more media coverage of the earlier, misleading, announcement.

Why did the Met Office make its early announcement and not wait until complete data became available at the end of the month?”

I’ll let you know when I receive a response – my comment has been awaiting moderation for 4 days now.

3. Why was it not clearer from the daily CET updates that March 2013 would be as cold as 2.7C?
And what I’m alluding to here is the end of month adjustment that seems to occur in the daily updated monthly mean CET data. I’ve noticed this and so has the commenter on my blog, “John Smith”.

I didn’t make a record of the daily mean CET for March to date, unfortunately, but having made predictions of the final mean temperature for March 2013 on this blog, I checked progress. From memory the mean ticked down to 2.9C up to and including the 30th, but was 2.7C for the whole month, i.e. after one more day. At that stage in the month, it didn’t seem to me possible for the mean CET for the month as a whole to drop more than 0.1C in a day (and it had been falling by less, i.e. by 0.1C less often than every day). Anyway, I’ve emailed the Met Office CET guy to ask about the adjustment. Watch this space.

4. Does all this matter?
Yes, I think it does.

Here’s the graph for March mean CET I produced for the previous post, updated with 2.7C for March 2013:

130408 Latest weather slide 1 CET graph

A curiosity is that never before has a March been so much colder – more than 5C – than the one the previous year. But the main point to note is the one I pointed out last time, that March 2013 has been colder than recent Marches – as indicated by the 3 running means I’ve provided – by more than has occurred before (except after the Laki eruption in 1773).

I stress the difference with recent Marches rather than just March 2012, because what matters most in many areas is what we’re used to. For example, farmers will gradually adjust the varieties of crops and breeds of livestock to the prevailing conditions. A March equaling the severity of the worst in earlier periods, when the average was lower, will then be more exceptional and destructive in its effects.

The same applies to the natural world and to other aspects of the human world. For example, species that have spread north over the period of warmer springs will not be adapted to this year’s conditions.  And we gradually adjust energy provision – such as gas storage – on the basis of what we expect based on recent experience, not possible theoretical extremes.

OK, this has just been a cold March, but it seems to me we’re ill-prepared for an exceptional entire winter, like 1962-3 or 1740. And it seems such events have more to do with weather-patterns than with the global mean temperature, so are not ruled out by global warming.

March 28, 2013

2013 UK Weather: Coldest First Quarter Since 1987

Filed under: Global warming, Science, UK climate trends — Tim Joslin @ 5:06 pm

Indulge me in one more post on this month’s weather. After all, we’re surely seeing the most noteworthy cold-weather episode since December 2010.

Besides, I’ve already prepared a chart. I’ve already noted that the meteorological winter 2012-13 (December, January and February) was no colder than 3 of the previous 4, although there has been an abrupt change from the milder winters seen since 1990-1, the winter of “the wrong kind of snow”.

Nor, as we also saw, has the “long winter” (December through March) 2012-13 been as cold as 2009-10.

But if we disregard December and instead take the “late winter”, or the first quarter of the calendar year, then 2013 is colder than 2010, in fact the coldest since 1987:

130327 Coldest since 1987 slide 1 v2

There are a few points to note from this graph:

1.The roll-call of Great Winters from the Dec-Feb and Dec-Mar graphs in previous posts is not affected too much.  1684 and 1740 are still the stand-outs.  1963 drops to 5th, with 1795 being a shade more notable than in the other analyses.

2. The abrupt change over the last few years, with two first quarters averaging well under 4C following over a decade of milder starts to the year.  The 5-year running mean (green line) is down 1.3C or so from its peak around the turn of the millennium.

3. The fact that the cold start to this year makes it very unlikely that 2013 will be the warmest in the CET – I’ll endeavour to make this the topic of another post.

4. The fact that even in this analysis there have been lots of winters colder than 2013.   Indeed, the 5 year running mean temperature for the first quarter is still higher than it’s been most of the time, even during the 20th century!  This year as a whole has so far been significantly milder than those in the period 1985-7 (when I seem to recollect spending a lot of time walking across frozen car-parks) and 1979, the Winter of Discontent, let alone 1963, 1947, 1917, 1895…  Perhaps that’s something that ought to be borne in mind by those whose responsibility it is to secure the UK’s gas and other energy supplies.  Despite the experience of the last few winters I suspect we’re still woefully under-prepared for what Nature could throw at us.

February 21, 2013

The Severe Winter of 2012-13: 4th Coldest in 5 Years Shock!

I’m sure it would be possible to spend all one’s spare time on a blog devoted solely to exposing exaggerations about the weather in the British press. I therefore ignore most hyperbole I see. Today I’m making an exception. This is what the Express wrote yesterday in its latest attempt to grab the attention of the elderly demographic which no doubt makes up a large proportion of its readership:

“A bitter cold blast from Scandinavia will see the mercury plummet to -15C (5F) over the next few days with more snow on the way.

The freezing conditions are expected to hold out until the beginning of next month bringing almost a fortnight of harsh frosts and icy roads.

And it is set to make this February the coldest for 22 years.”

Most of their readers will not see overnight frosts more than 5C below zero and the snow is expected to be light – a dusting at most.

But what stood out for me was the claim that February “is set to” be “the coldest for 22 years”, that is, since the year of “the wrong kind of snow” in 1991. This seemed to me to be quite a strong claim, at least for the Central England Temperature (CET), which can be taken as representative of the UK as a whole, since, up to the 20th, February is so far averaging 3.8C, 0.1C above the mean for 1961-90. The outlook is for a few cold, but not extreme days, warming a bit towards the end of the month, as shown in this screen grab from the useful Weathercast site:

130221 Severe Winter 2012 13 4th coldest in 5 years shock slide 1

It seems unlikely to me that a cold snap could knock much more than 0.5C, 1C tops, off the mean for February, given there’s only a week or so until the end of the month. Surely that couldn’t get us down to a 20 year low? The 20 year low, I reasoned, must be a lot less than 3.8C since the mean CET for February can actually be negative. I still remember February 1986 when an easterly brought cold air from the Continent (but little snow) for practically the entire grim month.

So I dusted off my spreadsheets and looked at the CET data.

The mean CET for February 1986 was indeed seriously cold: -1.1C. Remember we’re at 3.8C so far this year!

The coldest since 1986 was indeed 1991 at 1.5C. I’d agree there’s no chance we’ll beat that.

But February 2010, during the coldest winter in the CET since 1978-9, averaged 2.8C and 1996 was a tad colder at 2.5C.

I’d say it’s possible but unlikely that February 2013 will be colder than in 2010 and even more unlikely that it will beat 1996.

No, the Express is not really justified in writing that:

“[The current cold snap] is set to make this February the coldest for 22 years.”

“Might” or, even better, “could just possibly” rather than “is set to” would have avoided overstating the case. But let’s see how the temperature data shapes up over the rest of the month.

————-

Temperatures for the winter as a whole are also worthy of discussion. I wrote last time that:

“…it seems fairly likely that February 2013 will at least be less than 0.2C warmer than average. With December 2012 0.1C warmer and January 0.3C cooler that would make winter 2012-13 colder than the historical average (over 1961-90). Even this is by no means certain, even though everyone, especially Boris, is saying what a severe winter it’s been. It just goes to show how much we got used to milder winters through most of the 1990s and 2000s.”

It’s now clear that February 2013 will not be more than 0.2C above average, so we might say it’s “officially been a cold winter.”

The thing is, this is the 4th “cold winter” of the last 5! And it looks sure to be the mildest of those!

One might imagine that 4 cold winters out of 5 is somehow freakish. But if we imagined a 50% chance each of cold and mild winters we’d expect even 5 cold winters in a row one in 32 (2^5, or 2 to the power 5, or 2x2x2x2x2) sets of 5 winters.

Furthermore, the 5 year running winter CET mean is not that notable, as shown on one of my CET graphs:

130221 Severe Winter 2012 13 4th coldest in 5 years shock slide 2

I’ve brought this right up to date by estimating that the mean CET for February 2013 will end up at 3.3C.

Although the 5 year running mean (the green line) has dipped significantly in the last few years, it’s not only done so just as dramatically many times before – most notably around 1740 – but also remains higher than at many times in the past, most recently in the early 1980s.

The 11 year running mean (the red line) has also dipped, but from its all-time peak of 5.28C for the 11 years centred on 2003. It remains at historically very high levels.

And mean winter temperatures averaged over the last 21 years (the thick black line) remain higher than at any time before the 21 years centred on 1995.

Whilst it does seem that the pattern of the UK’s winter weather has shifted over the last 5 or so years, the hyperbole surrounding the phenomenon is counterproductive. Not only does it help sustain misleading claims global warming has gone away or never existed in the first place, it also makes us entirely unprepared for a really severe winter of the likes of 1962-3. The apparent lack of contingency in UK power supply should be of greater concern to government. Winter power-cuts now would be even more of a shock than in 1963, when the deprivations of the War and its aftermath were much more recent memories.

September 23, 2011

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

Filed under: Global climate trends, Global warming, Science, UK climate trends, Volcanoes — Tim Joslin @ 4:03 pm

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:

March 22, 2010

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.

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