Uncharted Territory

January 19, 2016

Two More Extreme UK Months: March 2013 and April 2011

Filed under: Effects, Global warming, Science, Sea ice, Snow cover, UK climate trends — Tim Joslin @ 7:17 pm

My previous post showed how December 2015 was not only the mildest on record in the Central England Temperature (CET) record, but also the mildest compared to recent and succeeding years, that is, compared to the 21 year running mean December temperature (though I had to extrapolate the 21-year running mean forward).

December 2010, though not quite the coldest UK December in the CET data, was the coldest compared to the running 21 year mean.

I speculated that global warming might lead to a greater range of temperatures, at least until the planet reaches thermal equilibrium, which could be some time – thousands of years, maybe.  The atmosphere over land responds rapidly to greenhouse gases. But there is a lag before the oceans warm because of the thermal inertia of all that water. One might even speculate that the seas will never warm as much as the land, but we’ll discuss that another time. So in UK summers we might expect the hottest months – when a continental influence dominates – to be much hotter than before, whereas the more usual changeable months – when maritime influences come into play – to be not much hotter than before.

The story in winter is somewhat different.  Even in a warmer world, frozen water (and land) will radiate away heat in winter until it reaches nearly as cold a temperature as before, because what eventually stops it radiating heat away is the insulation provided by ice, not the atmosphere.  So the coldest winter months – when UK weather is influenced by the Arctic and the Continent – will be nearly as cold as before global warming.   This will also slow the increase in monthly mean temperatures.  Months dominated by tropical influences on the UK will therefore be warmer, compared to the mean, than before global warming.

If this hypothesis is correct, then it would obviously affect other months as well as December.  So I looked for other recent extreme months in the CET record.  It turns out that the other recent extreme months have been in late winter or early spring.

Regular readers will recall that I wrote about March 2013, the coldest in more than a century, at the time, and noted that the month was colder than any previous March compared to the running mean.  I don’t know why I didn’t produce a graph to back then, but here it is:

160118 Extreme months in CET slide 1b

Just as December 2010 was not quite the coldest December on record, March 2013 was not the coldest March, just the coldest since 1892, as I reported at the time.  It was, though, the coldest in the CET record compared to the 21-year running mean, 3.89C below, compared to 3.85C in 1785.  And because I’ve had to extrapolate, the difference will increase if the average for Marches 2016-2023 (the ones I’ve had to assume) is greater than the current 21-year mean (for 1995-2015), which is more than half likely, since the planet is warming, on average.

We’re talking about freak years, so it’s surprising to find yet another one in the 2010s.  April 2011 was, by some margin, the warmest April on record, and the warmest compared to the 21-year running mean:

160119 Extreme months in CET slide 2

The mean temperature in April 2010 was 11.8C.  The next highest was only 4 years earlier, 11.2 in 2007.  The record for the previous 348 years of CET data was 142 years earlier, in 1865, at 10.6C.

On our measure of freakishness – deviation from the 21-year running mean – April 2011, at 2.82C, was comfortably more freakish than 1893 (2.58C), which was in a period of cooler Aprils than the warmest April before the global warming era, 1865.  The difference between 2.82C and 2.58C is unlikely to be eroded entirely when the data for 2016-2021 is included in place of my extrapolation.  It’s possible, but for that to happen April temperatures for the next 6 years would need to average around 10C to sufficiently affect the running mean – the warmth in the Aprils in the period including 2007 and 2011 would need to be repeated.

So, of the 12 months of the year, the most freakishly cold for two of them, December and March, have occurred in the last 6 years, and so have the most freakishly warm for two of them, December and April. The CET record is over 350 years long, so we’d expect a most freakishly warm or cold month to have occurred approximately once every 15 years (360 divided by 24 records).  In 6 years we’d have expected a less than 50% chance of a single freakishly extreme monthly temperature.

According to the CET record, we’ve had more than 8 times the number of freakishly extreme cold or warm months in the last 6 years than would have been expected had they occurred randomly since 1659.

And I bet we get more freakishly extreme cold or warm months over the next 6 years, too.

 

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January 14, 2016

Just How Exceptionally Mild Was December 2015 in the UK?

Filed under: Global warming, Science, Sea ice, UK climate trends — Tim Joslin @ 5:24 pm

“Very” is the answer, based on the 350+ year long Central England Temperature (CET) record.  Here’s a graph of all the CET December temperatures since 1659:

160114 Dec 2015 related CET analysis slide 1
As is readily apparent from the graph, the mean temperature of 9.7C in December 2015 was much higher than in any previous year.  In fact, only twice before had the average exceeded 8C.  Decembers 1934 and 1974 were previously tied as the mildest on 8.1C.

But how much was the recent mild weather due to global warming and how much to normal variability? Apart from anything else the mild spell has to coincide with a calendar month to show up in this particular dataset.  And it so happened that the weather turned cooler even as the champagne corks were in the air to celebrate the end of 2015.

To help untangle trends from freak events, I’ve included some running means on the graph above.  The green line shows the mean December temperature over 5 year periods.  For example, thanks in large part to December 2015, the 5 Decembers from 2011 to 2015 are the mildest in succession, though other periods have come close.

The red and black lines show 11 and 21 year running means, respectively.  The black line therefore represents the long-term trend of December temperatures.  These are close to the highest they’ve ever been, though in some periods, such as around the start of the 19th century, the average December has been as much as 2C colder than it is now.  Perhaps some exceptionally mild Decembers back then – such as 1806 – were as unusual for the period as December 2015 was compared to today’s Decembers.

I therefore had the idea to plot the deviation of each December from the 21 year mean centred on that year, represented by the black line on the graph above.  If you like, I’ve simply subtracted the black line from the blue line.

A health warning is necessary.  I’ve had to extrapolate the 21 year mean, since we don’t yet know what weather the next 10 Decembers (2016 to 2025) will bring.  We’ll have to wait until 2025 to see precisely how unusual December 2015 will prove to have been.  In the meantime, I’ve set the mean temperature for 2016 through 2025 to the last 21 year mean (i.e. the one for the years 1995 through 2015).

With that proviso, here’s what we get:

160114 Dec 2015 related CET analysis slide 2a
The green line now shows the difference between the mean December temperature for a given year and the mean December temperature for the 21 years including the 10 before and the 10 after the given year.

We can see that December 2015 was, at 4.91C much more mild than contemporary Decembers than was any other December, with the proviso that I’ve not been able to take Decembers after 2015 into account.

The next most freakish December was the aforementioned 1806 which was 3.86C warmer than the mean of Decembers 1796 through 1816.

What’s going on? Is it just weather – something to do with the ongoing El Nino, perhaps – or is something else afoot?

One hypothesis might be that, with the climate out of equilibrium due to global warming, greater variability is possible than before. Our weather in 2015 may have been driven by a heat buildup somewhere (presumably in the ocean) due to global warming. On average this perhaps doesn’t happen – we may suppose our weather to be often determined by regions of the planet where the temperature hasn’t changed much, at least at the relevant time of year. Specifically, the Greenland ice-sheet hasn’t had time to melt yet.

It won’t have escaped the notice of my eagle-eyed readers that the graph above also shows 2010 to be the most freakishly cold December in the entire CET record.

Perhaps, until the ice-sheets melt, the deep oceans warm and the planet reaches thermal equilibrium, we’ll find that when it’s cold it’s just as cold as it used to be, but when it’s warm it’s a lot warmer than it used to be.   Just a thought.

It might be worth mentioning a couple of other, not necessarily exclusive, possibilities:

  • Maybe the situation will continue even when the planet is in thermal equilibrium.  Maybe, for example, assuming there is some limit to global warming and the Arctic seas still freeze in winter, we’ll still get cold weather in winter just or nearly as cold as it ever was, but we’ll get much warmer weather when there’s a tropical influence.
  • It could be that weather patterns are affected by global warming, especially through the later freezing of Arctic ice.

Or December 2015 could just have been a freak weather event.

February 6, 2013

Actually Boris, This Winter’s Been Surprisingly Mild, Considering

Filed under: Global warming, Science, Sea ice, UK climate trends — Tim Joslin @ 5:11 pm

Kate Ravilious’s contribution to the Weatherwatch column in last Saturday’s Guardian mentioned a silly comment piece Boris Johnson wrote for the Telegraph a couple of weeks ago, reminding me to post my own response. Boris’s (or possibly a Telegraph editor’s) title says it all: “It’s snowing and it really feels like the start of a mini ice age”.

When Boris’s piece first appeared the Guardian Environment blog’s Leo Hickman was quick to post a rejoinder and the London Mayor’s contribution to climate science is still attracting (adverse) comment and not just at Uncharted Territory. Jenny Jones asks this morning whether “Boris Johnson’s climate flippancy translates into policy failure”.

When I first read Boris’s blather I was puzzled by his comment on the latest science:

“I see from the BBC website that there are scientists who say that ‘global warming’ is indeed the cause of the cold and snowy winters we seem to be having. A team of Americans and Chinese experts have postulated that the melting of the Arctic ice means that the whole North Atlantic is being chilled as the floes start to break off — like a Martini refrigerated by ice cubes.”

It seems Boris has conflated two research ideas:

(1) James Hansen suggests that the Greenland ice sheet could break up faster than expected because current estimates are based on historic “forcing” (extra energy gain by Earth) whereas we’re warming the planet up much faster. The main worry would be more rapid sea-level rise than currently estimated later this century, but a side-effect would be the “Martini effect” Boris refers to. But no-one (except Boris) is suggesting this is “the cause of the cold and snowy winters we seem to be having”. The North Atlantic is not full of ice-bergs. The suggestion is that this might occur a few decades hence.

(2) The theory Ravilious refers to in her Weatherwatch piece that loss of Arctic sea ice has disrupted Northern Hemisphere winter weather patterns:

“Xiangdong Zhang, a climate scientist at the University of Alaska Fairbanks, analysed the weather patterns over the last thirty years and has shown that the extreme winters are associated with a strong Siberian high pressure system, hovering over Eurasia, which sucks cold polar air down to mid latitudes.

In the journal Environmental Research Letters, Zhang and his colleagues explain how Siberian highs have intensified, possibly due to warmer Arctic temperatures and thinner sea ice. Temperature redistribution, and weakening mid-latitude westerlies, help to spin up stronger Siberian winter highs. So it seems colder winters will remain.”

The point is clear when we look at one the Arctic ice maps provided by the NSIDC (the US’s National Snow and Ice Data Center):

130205 Surprisingly Mild Winter Considering slide 7

This is the latest image, but any other day over the last few months would have told the same story.

The theory as I understand it is that the lack of insulating ice, particularly in the areas highlighted, results in warmer than usual temperatures. This disrupts weather patterns. One way of looking at it is that the warmer air results in lower pressure (warm air rises). Since everything is relative this tends to intensify the usual high pressure systems over Greenland and Eurasia, blocking the normal westerly air flow. Another way is to consider how close the warm water is to the pole. The Earth being spherical (sorry, I need to point this out for Boris!) air wants to circulate along lines of longitude. The warm area of sea disrupts the circulation, letting cold polar air escape.

The upshot for the UK is a greater likelihood of polar air masses being steered our way by a northerly air-stream circulating around the Greenland high (as in December 2010 and indeed just now – see the Met Office weather map below), or the Eurasian high extending over Scandinavia and sending easterlies over us (as in January this year).

Having set up a rather fragile straw man – the “Martini” theory that isn’t intended to explain our current winter weather anyway! – Boris dismisses it and, with his usual hyperbole suggests it’s all to do with the Sun:

“As a species, we human beings have become so blind with conceit and self-love that we genuinely believe that the fate of the planet is in our hands — when the reality is that everything, or almost everything, depends on the behaviour and caprice of the gigantic thermonuclear fireball around which we revolve.”

Sigh!

There are a couple of problems for Boris:

(1) Winter weather in the UK depends on the direction it is coming from. So any theory needs to explain how that can change and not simply assert that:

“When the solar acne diminishes, it seems that the Earth gets colder. No one contests that when the planet palpably cooled from 1645 to 1715 — the Maunder minimum, which saw the freezing of the Thames — there was a diminution of solar activity. The same point is made about the so-called Dalton minimum, from 1790 to 1830. And it is the view of Piers Corbyn that we are now seeing exactly the same phenomenon today.”

though he does go on to argue – not very convincingly – that:

“Lower solar activity means – broadly speaking – that there is less agitation of the warm currents of air from the tropical to the temperate zones, so that a place like Britain can expect to be colder and damper in summer, and colder and snowier in winter.”

Hold the next IPCC report! We need to include Boris’s chapter on “agitation”!

(2) It’s not actually been that cold this winter!

I like the Central England Temperature (CET) record because it provides a decent length series of data. The mean temperature figure for January 2013 has just been published. It was only 0.3 below the 1961-90 average. This was because whilst the middle of the month was reasonably cold – not bitter like December 2010, but enough to snow – both the start and end of the month were very mild. I recollect the average for the first 8 days was 4.5C above usual, and that the temperature in the London area (I usually look at the data for Heathrow) approached 14C one day near the end, which won’t have been far off the UK record for the date (these vary from 15-18C in late January).

To put it in a nutshell, we’re getting very mild spells as well as colder spells. Determining whether this behaviour is more pronounced than in the past would require considerable research effort, but everyone agrees that we have had more cold snaps and snow in the last few years than in the previous twenty or thirty. But both general global warming and disruption of the circulation drawing air from further south than usual over the UK may be giving us milder mild spells so that the average winter temperature is not that different to that in the past.

It might be worth pointing out that whilst Boris’s ramblings after some snow during a not particularly cold cold snap have been highly publicised and debated, I struggle to recall a single article in the Telegraph, Express or Mail exclaiming how astonishingly mild it was in the first week or so of January. Or indeed a great deal of comment about the freakishly warm month of April 2011. Perhaps a masters student could write a dissertation comparing the number of column inches and TV documentary minutes on the record April with those discussing December 2010.

As Ravilious suggests, though, the recent cold and cold-ish winters have not just been media hype – there is a real-world phenomenon to explain. So let’s adopt as a working hypothesis the proposition that the high latitude Northern Hemisphere atmospheric circulation has become disrupted, as suggested by Xiangdong Zhang and then look a little more at recent temperatures.

The changes in circulation affect much of the whole Northern Hemisphere (at least), of course, but we’re only really interested in the UK. In fact just the London area, to be blunt!

It seems to me a consequence of Zhang’s theory that the air would flow more north to south than west to east compared to previously. A huge research project could be set up to try to confirm this by analysing isobars on weather maps over the last few decades. Are the lines more N-S than W-E compared to before? Are N-S lines longer than they were (implying air is moving across a greater range of latitude)? I await someone taking up this suggestion and publishing the results. In the meantime, anecdotal evidence will have to do. I noticed how much the BBC weatherman on Monday evening was taken with how cold air was moving right down over the UK, through France and Spain to North Africa. He excitedly presented a chart rather like this one from the Met Office (I added the blue arrows indicating the cold air flow):

130205 Surprisingly Mild Winter Considering slide 1

The effect of this cold air will indeed be felt in North Africa, as shown on these maps from the Wetterzentrale site:

130206 Surprisingly Mild Winter Considering slide 3

The purple area in the map for Friday indicates where temperatures are expected to be 10C below usual for the time of year.

Increased disruption of the usual westerly airflow over the UK may not cause uniformly cold winters, because, whilst the air can flow west to east (at least at our latitude), any flow north to south must be accompanied by a (moreorless) equal and opposite flow from south to north. Somewhere is going to be very mild when we’re very cold and sometimes we’re going to be mild when somewhere else is cold. Whilst this and the general effect of global warming neatly explains why this winter’s been surprisingly mild, considering all the snow, the question is whether or not we can in general expect more or fewer than our fair share of cold spells in winter or at any other time of year. Again, proper research is required, but one suspects that most of the colder than average months over the last few years in the CET have occurred in winter.

Given that the planet is generally warmer, the mild spells we do get over the year do seem to generally outweigh the cold spells when we calculate an average temperature. Here, thanks to the Met Office again, are the monthly CET figures since the start of 2010:

130205 Surprisingly Mild Winter Considering slide 8

Sorry – I’ve made this image huge, as I’ve jammed a lot into it.

The only months over these three and a bit years that have been 1C or more colder than usual were January, February, November and December 2010 and October 2012. In contrast 12 months have been 1C or more warmer than usual, 8 of them during 2011! There have been 2 freak months – December 2010 at a staggering 5.3C colder than the mean and April 2011 at 3.9C warmer.

This small sample is intended to merely illustrate the point rather than prove anything statistically, of course. Still, one might tentatively suggest that recent weather patterns – perhaps connected to the disappearance of Arctic sea ice – favour colder winters in the UK than would otherwise be the case and milder springs and autumns.

—————–

I thought I’d finish up by speculating whether the winter 2012-13 temperatures are going to be at all notable. Compared to December 2010 and even winter 2009-10 it’s all fairly pathetic (despite the usual hyperbole in the tabs, the Express generally the most sensationalist).

The models are forecasting the cold weather to continue for a while yet:

130206 Surprisingly Mild Winter Considering slide 6

This is for London, which is likely to be warmer than the CET area. Mean temperatures therefore look like staying below the 3.8C average for February for a week or two.

Further out, I haven’t seem any model runs showing anything very mild. Many show cold easterlies developing over the UK, as in these snapshots from the ECMWF site (their model) and Weatheronline (the GFS model):

130206 Surprisingly Mild Winter Considering slide 4

130206 Surprisingly Mild Winter Considering slide 5

Be interesting to see how accurate these turn out to be.

The Met Office is currently suggesting colder than usual weather is most likely for the rest of February:

“UK Outlook for Monday 11 Feb 2013 to Wednesday 20 Feb 2013:

It is likely to be cloudy for many central, southern and western areas through Monday with outbreaks of rain, sleet and snow clearing slowly south as the day progresses. Elsewhere, it should be drier and brighter with a few wintry showers, especially towards North Sea coasts. Most places are likely to be mainly dry, bright and cold through Tuesday with overnight frost and ice an ongoing risk. Gradually turning more unsettled again from mid-week onwards with outbreaks of rain spreading southeastwards, with the chance of this turning to sleet and snow at times, especially over hills. Perhaps becoming a little less cold but staying rather unsettled for the remainder of the period, with an ongoing risk of sleet and snow, especially across higher ground.

UK Outlook for Thursday 21 Feb 2013 to Thursday 7 Mar 2013:

There are large uncertainties at this forecast range. Conditions will most likely start with temperatures close to or a little below average, with fairly typical winter weather across much of the UK. There are signs that a slightly colder spell may then prevail throughout much of the rest of the period, with more widespread overnight frosts.

Issued at: 0400 on Wed 6 Feb 2013”

So 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.

We might also consider how the colder winters are affecting annual CET means. The graph provided by the Met Office shows that years below the long-term mean are now rare, 2010 being the only one so far of the third millennium:

130206 HadCET_graph_ylybars_uptodate

2012 wasn’t far off, though, and a glance at the data (in the big picture, above), shows that it would have been below average but for March, which was 2.6C above the mean for the month. It therefore seems there is a good chance of the year April 2012 through March 2013 being cooler than the long-term or pre-global warming mean, which is worth at least a footnote in the interesting weather records of the 2010s. Watch this space!

My previous post on weather/climate called 2012 as not the warmest year in the CET. This was as early as last July, which was bold of me. The reason the record is significant is that climate change sceptics expect each year to be warmer than the last. Such an expectation is unwarranted, of course, but nevertheless a record year, albeit only in the CET, might shut them up for a while. Last year I remember thinking in mid-Feb that the record might not be on, since a cold snap had more than offset January (1.6C above the mean) so that the year as a whole was at the mean. If the rest of February had been even average, it would have meant the average for the rest of the year would have had to have been 1.6C above the mean in order to break the record, which seems less likely than the whole 12 months being 1.35C above. But one of those mild spells in the second half of February offset the cold spell in the first half. With that in mind, it’s too early to start saying the record is unlikely this year. This is what the Met Office are currently saying:

“The highest annual mean CET ever recorded was 10.82 which is 1.35 higher than normal. To beat this record the anomaly must be higher than 1.53 for the remainder of the year.”

Back to the day job…

April 8, 2010

Ice Sickle

I continue to fret about the emphasis on the Arctic sea-ice extent as an indicator of global warming (GW).

I have to chop down (got to justify my blog entry title somehow!) a Guardian story, “Arctic sea ice still low despite winter recovery” (p.20 in today’s print edition), the online version titled incoherently “Arctic winter ice recovers slightly despite record year low, scientists say” and cryptically subtitled “Figures from the National Snow and Ice Data Centre [the NSIDC] indicate six or seven-year low over past three decades”. (They mean 2010 has had the 6th or 7th lowest maximum ice extent – which occurs in March – on record, i.e. of the last 32 years).

The story itself is garbled as well:

“Last night [NSIDC] released the data for the winter of 2009-10 showing the maximum extent reached on 31 March was 5.89m square miles (15.25m sq km). This was 250,000 square miles (650,000 sq km) below the 1979 to 2000 average for March…”

What the NSIDC actually said was that the average for March (15.10m km2 or 5.83m square miles – btw, wouldn’t it be simpler if we all standardised on km2?) was 250,000 square miles below the 1979-2000 March average. In fact, NSIDC’s news posting was titled “Cold snap causes late-season growth spurt” and noted that the maximum sea-ice extent occurred later than usual at the end of March, when the ice extent was only marginally below the 1979-2000 average for that date, as can be seen in the graph illustrating this BBC story about the launch of a satellite to monitor the situation.

I would have thought the real story was the recovery in the maximum Arctic sea ice extent compared to the last few years. “Arctic sea ice still low” is arguably a little misleading.

It is really not helpful to keep spinning Arctic sea ice shrinkage as an indicator of GW. There will be a vicious backlash should nature conspire to undermine the Arctic ice melt narrative. It will then become even more difficult to muster the political will to deal with GW.

The Guardian story goes on to note that:

“Last month, Japanese scientists reported in the journal Geophysical Research Letters that winds rather than climate change had been responsible for around one-third of the steep downward trend in sea ice extent in the region since 1979. The study did not question global warming is also melting ice in the Arctic, but it could raise doubts about high-profile claims that the region has passed a climate “tipping point” that could see ice loss sharply accelerate in coming years.”

Maybe this is what the researchers did actually say – I may have to go the library to check – but, as I pointed out before, it makes no sense to try to distinguish “winds” from “climate change”. Winds are not caused by some arbitrary external force, they are determined by differences in temperature, albedo (reflectivity), moisture content and so on between different areas of the planet. Winds are part of the climate system that is changing, so it is simply meaningless to separate the cause of ice melt into “winds” and “climate change”.

Solving the GW problem is difficult enough without the constant drip-feed of confusing reporting of the issue.

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.

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 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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