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.

 

March 26, 2013

March 2013 in UK: Coldest in CET since 1892 or 1883?

I see the Daily Mail is now suggesting that 2013’s “could be Britain’s coldest March since 1892.”

The nation-wide statistics published by the Met Office only go back to 1910, so the Central England Temperature (CET) record is needed to put current weather in a long-term context.

1892 is an odd year for the Daily Mail to choose, since the CET for March that year was 2.7C, whereas 1962, the year we have to “beat” for it to be the coldest since 1892, saw a mean March CET of 2.8C. We’re unlikely to say this March is “the coldest since 1883”, since if it comes in at 2.8C we’d probably say it’s the “equal coldest since 1892″ and if it comes in at 2.7C we might say it’s the “equal coldest since 1883″.

Furthermore, given the possibility of rounding, the difference between 1892 and 1962 could be much less than 0.1C, for all I know.

In addition, the difficulties of calibrating temperature readings between 1892 and 1962 make a difference of 0.1C in a monthly mean fairly insignificant (and probably statistically insignificant). To put it another way, the error bars on the temperatures are probably greater than 0.1C. Perhaps we shouldn’t really be quibbling over the difference between monthly means of 2.7C and 2.8C. But then again, we do like our weather records!

If this March is colder even than that in 1892, the next mark is 1883, when March saw a CET of 1.9C. It’s no longer on the cards for it to be as cold as 1.9C this March.

But what are the chances of the CET this March being colder than 2.8C or even 2.7C?

Here’s the state of play at the moment:

130326 Latest ensemble forecasts slide 1

This is moreorless in line with my projection of a few days ago. But that was based on ensemble forecasts on 22nd March, and, as I noted yesterday, the forecast for the rest of March has just kept getting colder since 22nd:

130326 Latest ensemble forecasts slide 3

Based on the forecast for 22nd March I wrote:

“Ignoring today (22nd) as transitional, it now looks likely that the 5 days 23rd through 27th March will be seriously cold, so let’s knock 0.1C off the monthly average for each of them. That gets us down to 3.1C.

The 28th will most likely be around the new average (3.1C), so it all depends on when the mild air comes in from the Atlantic. The computer model runs (grey lines) differ, and the average (yellow line) for 30th and 31st are for it to be relatively mild. If that’s the case, then we’d need to add on 0.1C for each day, so would roughly equal 1969.”

It’s certainly now not the case that 30th and 31st will be “relatively mild”, so we won’t have to add on 0.1C for each of those days. This March is therefore very likely to be colder than 1969 (3.3C) and therefore the coldest since at least 1962.

But could it be even colder than the 2.8C in 1962?

Here’s a larger image of the current ensemble forecast from the Weathercast site:

130326 Latest ensemble forecasts slide 2

The CET mean for March so far is 3.2C. To depress this average the mean for the rest of March would have to be lower than 3.2C, obviously. And since there’s 31 days in the month, each degree it is lower than 3.2C over one day (what we might call a degree-day) will depress the monthly mean by 1/31st of a degree.

The ensemble chart suggests the mean temperature for London for the rest of March will be about 1.5C – your judgement is as good as mine – over 6 days, so that’s very roughly 12 degree days lower than 3.2C (about 2C each day), so dividing by 30 (rounding 31), we might expect the mean for the month to come out about 0.4C lower than it is now, at 2.8C.

This estimate is very rough and ready since I’ve assumed in particular that London is representative of the CET region. It’s quite possible the region as a whole will be colder than London. Not only might this be the case generally, but there’s a lot of lying snow in more northerly areas, which tends to depress temperature readings (because it resists warming by reflecting sunlight and because its latent heat buffers warming of the ground surface at about 0C, both preventing warming of the air above it, and it is the near-ground air temperature that’s being measured).

Additionally, I’ve noticed the CET is sometimes adjusted downwards before the final figure for the month is published, a few days into the next month. I don’t know why this is. Maybe the data for more remote (and colder) weather-stations is slow to come in. Or maybe it’s to counter for the urban heat island effect, to ensure figures are calibrated over the entire duration of the CET.

By way of a sanity-check, here’s another view of much the same ensemble data as in the previous image, from the Wetterzentrale site:

130326 Latest actual weather

Note that to depress the average for the month so far the temperature would need to be around 4C less than usual, since the mean CET mean (!) for the whole of March is about 5.7C and it’s near the end of the month when mean daily temperatures around 7C would be typical. On that basis the Wetterzentrale maps suggest that 12 degree-days lower than the mean for the month so far is a reasonably estimate for the outlook over the next 6 days.

If a best guess is that the mean CET for March 2013 is 2.8C (“equal coldest since 1892”), with some uncertainty, it certainly seems possible that it could instead come in at 2.7C (“equal coldest since 1883”). In either case, though, it might be more accurate to simply say it has been one of the coldest 3 Marches since March 1883. I like to be fairly conservative, but I suppose there’s just an outside chance the mean CET this month could be even lower, at 2.6C, say, in which case we’d probably claim it has been the coldest since 1883.

Of course, this is all just estimation: the mean CET for March 2013 might end up “only” as cold as say 2.9C, the coldest for 51 years!

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.

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* 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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