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.

Ice Pie

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.

Why the AMO Overshoots

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)

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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.

The Earth is a Fridge

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.