Thursday, 31 October 2013

Evidence that the Little Ice Age in Europe was a Global Phenomenon

Climate scientists have traditionally held the view that the extended period of cooling experienced in Northern Europe, most severe around 1645-1715, was a regional phenomenon. It coincided with a marked decrease in solar activity (the so called Maunder Minimum), as did also the less severe European cooling associated with the later Dalton Minimum. These two extended sunspot minima, along with the late 20th Century Modern Maximum, are illustrated here:

Central England Temperature records going back to 1650 correlate closely with these periods of decreased solar activity, even the modest lapse in solar output which occurred in the 1960's/70's. It is clear that during the Little Ice Age associated with the Maunder Minimum, winters in Britain were more often than not very harsh indeed and summers were often wet and miserable, resulting in many crop failures. Historical records show that the cooling was simultaneous across much of Northern Europe, across mid-latitude Asia and into North America. So, in extent at least, there is firm evidence that the Little Ice Age was a mid-latitude Northern Hemisphere phenomenon.

The cooling appears to have been particularly severe in North Western Europe and may have been exacerbated here by a weakening of the North Atlantic Oscillation (NAO), precipitated by, or merely coincidental with the decrease in solar activity, with a consequent 'slowing' of the Jet Stream, causing it to track much further south. The NAO, via the Gulf Stream, via the North Atlantic Drift, is largely responsible for the pleasantly mild maritime conditions experienced in NW Europe, particularly during winter. During the LIA, it seems that our winters hailed largely from the continent, from the much colder east and north east, as was the case during the winters of 2009/10 and 2012/13. The Jet Stream during these most recent periods was noticeably far south, allowing much colder air from Siberia and the Arctic to penetrate our shores. The pattern of a south-tracking Jet Stream was established during the summer of 2012 when it sat over the UK, instead of being north of us, resulting in the now infamous washout summer of flooding. Come winter, it moved even further south, depriving us of our typical mild wet winter which was replaced by a much colder continental one, lasting well into late Spring.

So, why is all this so important right now? Well, for a start we are definitely looking like we are headed into Dalton Minimum territory in terms of solar activity during the present cycle (SC24) and on into the next. But here's where it gets interesting: Mike Lockwood of Reading University has increased his estimate of the probability of the Sun lapsing into Maunder Minimum type solar activity from 8% to 25-30% and he says that a repeat of a Dalton Minimum is 'more likely than not' (IPCC speak!). He related this information to Paul Hudson, Look North BBC Weather Presenter, and Paul wrote a blog post which appears to have stirred up a real hornet's nest, not least comprised of stinging comments from Lockwood himself who claims that the Hudson blog has misrepresented him.

The basic gist of Hudson's blog is that there is now a real possibility of Little Ice Age type conditions impacting upon us in the UK and in Northern Europe. Lockwood's research confirms this possibility of regional cooling, though he is careful to stress that it is only regional and that it must be viewed in the context of the wider effects of CO2 induced global warming, which he considers are still pre-eminent. So any UK cooling, though possibly severe in the short term, would presumably be eventually aced by anthropogenic CO2 and, on a global scale, Lockwood clearly states that even a repeat Maunder Minimum would have little, if any impact.

Hudson takes a slightly different view and raises the possibility in his blog that, according to research by Mann et al, 2001, global temperatures might also be expected to nose-dive by 0.3C to 0.4C, smaller than the regional change, but still significant, enough to wipe out the warming seen since the 1950's. How Lockwood comes to the conclusion that this addition by Hudson somehow misrepresents what he has to say about regional climate change, I don't quite figure. I suspect he is just a bit miffed that Hudson took the step further in his blog and is concerned now that others might therefore misquote him: Paul Hudson certainly did not and he quite rightly stands by his blog post. Just for the record, here is what Mike Lockwood has said on Facebook:

"It amazing how one can be misrepresented no matter how clear one tries to make it! One point I made to Hudson is that many of the so called bits of "evidence" for solar influence on global temperature actually come from Europe in winter (cf Eddy and all that) and so are not global at all. Depressing.

So there is absolutely no misunderstanding here - I too am 'vociferous advocate' of (the known science that anthropogenic greenhouse gases causes) global warming!

Since 1985. I've explained below I am only talking about a possible (stress possible) solar influence on European blocking events and not at all about global temperatures. But it is a fact that solar activity HAS declined since 1985, the "exceptional" long and low solar minimum of 2008/9 and the current very weak cycle 24 are both part of that trend and we are now back to conditions last seen around 1920.

Hi David, Bru - too right I am only talking of regional effects!!!!!!!!!!!!!!!!! On the global front I refer you to Jones, Stott and Lockwood (JGR 2012 doi: 10.1029/2011JD017013) in which we showed that even a Maunder minimum would have almost no effect on global temperatures - very similar to the result in Stefan's paper. I have always made it clear (and did make it absolutely clear to Paul Hudson - I couldn't have stressed it more!) that I am only talking about blocking events so for example if such an event brings cold Arctic air to central Europe in winter, it takes warm moist air up to Greenland. The offending web pages had disappeared when I looked so I have no idea how badly I am being misquoted and misrepresented here. I'd point you at a review I wrote last year Lockwood, M. (2012) Solar influence on global and regional climates. Surveys in Geophysics, doi: 10.1007/s10712-012-9181-3 which also makes it absolutely clear that although there is some (as yet not cast iron) evidence for European winters of some solar effects theres none on a global basis."

Note what Lockwood says about evidence for the effects of solar activity on winters globally - there is none according to him. I'm here to tell you that there is some! In point of fact, the evidence for a simultaneous global impact of the Maunder Minimum is quite extensive and growing, due to increased coverage of proxies from the Southern Hemisphere, in particular Antarctica. Scripps Institution of Oceanography, UC San Diego graduate Orsi et al analysed data from an ice core drilled into the West Antarctic Ice Sheet and found that, during the LIA Antarctic temperatures were an average 0.52 ± 0.28°C colder than the last 100-year average. It should be stressed that this was contemporaneous with the temperature drop in the Northern Hemisphere, therefore did not involve a simple redistribution of heat around the globe. I quote:

"The period between 1400 and 1850 was marked by an average temperature drop of just less than 1 degree C (1.8 degrees F), but not just in the Northern Hemisphere. Scripps graduate student Anais Orsi and colleagues found evidence of the same cooling trend in samples of ice from the West Antarctic Ice Sheet. That contradicts prevailing theories that the Little Ice Age was not globally synchronized, but a regional cooling possibly triggered by changes in ocean circulation that created a temperature see-saw effect between the hemispheres."

A good summary by CO2 Science of various papers pointing to the global aspect of the LIA can be found here.

The Simms et al paper mentioned in the above link can be found here. CO2 Science says (of this paper) that:

"Although Simms et al. write that initial studies on ice cores "suggested that the timing of the most recent Neoglacial advance in West Antarctica may have been out of phase with the Little Ice Age in the Northern Hemisphere (Mosley-Thompson and Thompson, 1990)," they report that other ice core records from East Antarctica and elsewhere in West Antarctica support an in-phase relationship between climate events in the two hemispheres, citing the work of Li et al. (2009) and Bertler et al. (2011). And they add that the marine record provides ample evidence "for cooler conditions around ~250-550 calBP (1400-1700 AD)," citing the studies of Domack and Mayewski (1999), Brachfeld et al. (2003), Yoo et al. (2009), Hass et al. (2010) and Shevenell et al. (2011)."

I let CO2 Science have the last word here as the author states very clearly and eloquently the following:

"Clearly, the greater weight of real-world evidence in this controversy resides with Simms et al. and the many other researchers who have identified and dated a Little Ice Age in various parts of Antarctica that coincides in time with the Little Ice Age of the Northern Hemisphere. And that dating of the Little Ice Age, plus the comparing of its temperature with the temperatures that both preceded and followed it, also pretty much confirms the existence of the Medieval and Current Warm Periods in Antarctica, which are thus found to have occurred contemporaneously with the Medieval and Current Warm Periods in the Northern Hemisphere. And when viewed in this global and oscillatory context (and when extended even further back in time through the Dark Ages Cold Period and the Roman Warm Period), it becomes ever more clear that there is nothing unusual, unnatural or unprecedented about the global warming of the 20th century. Nor is there any need to invoke atmospheric CO2 enrichment as the driver of 20th-century warming, as previous equivalent ups and downs in earth's surface temperature occurred during times of both low and relatively constant values of the air's CO2 content."

Tuesday, 1 October 2013

UK Regional Cooling - Predicted or Not by the IPCC?

On September 26th, the eve of the release of the IPCC's Fifth Assessment Report, the UK Telegraph ran this story. The next day, the Global Warming Policy Foundation tweeted a link to the article, mysteriously suggesting that the Telegraph might have just pulled it for some unexplained reason. As it happened, the story remained. I thought it slightly curious at the time.

What the Telegraph is basically saying is that the Atlantic Meridional Overturning Circulation, which includes the North Atlantic thermo-haline ocean circulation which drives the North Atlantic Drift, is predicted to weaken over time which will impact upon the climate of North-Western Europe. The other component of AMOC is an atmospheric phenomenon driven by wind patterns associated with the Coriolis Force and the pressure difference between tropical and higher latitudes (often wrongly referred to as The Gulf Stream - a major ocean current of which the North Atlantic Drift is but one extension), which transports mild air from the tropical regions to the cooler northern latitudes. AMOC is in turn mediated and influenced by the variable bipolar climate oscillation known as the Nort Atlantic Oscillation! Confusing, or what!? Almost inevitably, any mention at all of the Gulf Stream in the media is likely to be technically and scientifically inaccurate or misleading because of this confusion with ocean currents and atmospheric circulation.

The Telegraphs says:

"It will say that the circulation of warm and cold water in the Atlantic, which includes the Gulf Stream, will weaken by 20 to 44 per cent by the end of the century. Scientists claim that such a slow-down in the Gulf Stream will have a big impact on Britain, causing cooling of about 1.8F (1C) and disrupting weather patterns . . . . . . . . The report will say that the warming of the oceans will interfere with the currents in the Atlantic, also known as the Atlantic Meridional Overturning Circulation (AMOC). It will state: “It is very likely that the Atlantic Meridional Overturning Circulation will weaken over the 21st century. It is likely that there will be some decline in the AMOC by 2050, but there will be some decades when the AMOC increases.”"

I can as yet find no specific mention of predicted cooling in the UK or its magnitude in AR5. To search, I went naturally to Chapter 14: Climate Phenomena and their Relevance for Future Regional Climate Change. On page 49 in the section 14.8 Future Regional Climate Change (14.8.6 Europe and Mediterranean), the IPCC says:

"In summary, there is high confidence in model projections of mean temperature in this region. It is very likely that temperatures will continue to increase throughout the 21st century over all of Europe and the Mediterranean region. It is likely that winter mean temperature will rise more in NEU than in CEU or MED, whereas summer warming will likely be more intense in MED and CEU than in NEU. The length, frequency, and/or intensity of warm spells or heat waves are assessed to be very likely to increase throughout the whole region".

My guess is that the Telegraph has used the IPCC's figures and linked them with other research which predicts cooling in the UK if the Gulf Stream weakens.

Searching deeper with respect to the North Atlantic Oscillation and the closely related Arctic Oscillation, also the Atlantic Multi-Decadal Oscillation, one comes up with the following info, which appears to demonstrate an acceptance by the IPCC that natural changes in these bipolar climate modes have dominated, and probably will continue to dominate, in the short term, regional climate change in areas affected by them. Hence the possibility that natural variation may result in climate change for Europe, unrelated to anthropogenic global warming causing Arctic ice melt with consequent weakening of the Gulf Stream due to freshening polar waters.

"Model agreement in projections indicate that future boreal wintertime North Atlantic Oscillation is very likely to exhibit large natural variations and trends of similar magnitude to those observed in the past and is likely to become slightly more positive on average, with some, but not well documented, implications for winter conditions in the Arctic, North America and Eurasia."

"Multiple lines of evidence from paleo reconstructions and model simulations indicate that the Atlantic Multi-Decadal Oscillation is unlikely to change its behaviour in the future as the mean climate changes. However, natural fluctuations in the Atlantic Multi-Decadal Oscillation over the coming few decades are likely to influence regional climates at least as strongly as will human-induced changes with implications for Atlantic major hurricane frequency, the West African wet season, North American and European summer conditions."
[p7, Ch14]

"Model simulations have underestimated the magnitude of the large positive trend from 1960-2000 in winter NAO observations, which now appears to be more likely due to natural variability rather than anthropogenic influences"
[p26, Ch14]

"The North Atlantic Oscillation which exhibited a positive trend from the 1960s to the 1990s has since exhibited lower values, with exceptionally low anomalies in the winters of 2009/2010 and 2010/2011 (Section 2.7.8)."
[p37, Ch10]

For mention of anthropogenic influences causing regional climate change in Europe, we have the following:

"Sea ice in the Arctic has declined significantly in recent decades (Chapter 4.2), which might be expected to reduce the surface salinity and increase freshwater content as freshwater locked in multi-year sea ice is released. Generally, strong multidecadal variability, regional variability, and the lack of historical observations have made it difficult to assess long-term trends in ocean salinity and freshwater content for the Arctic as a whole (Rawlins et al., 2010). The signal that is now emerging, including salinity observations from 2005 to 2010, indicates increased freshwater content, with medium confidence". 
[p19, Ch3]

"Based on the assessment of the CMIP5 RCP simulations and on our understanding gleaned from analysis of CMIP3 models, observations and our understanding of physical mechanisms, it is very likely that the AMOC will weaken over the 21st century. The best estimate decrease in 2100 is about 20–30% for the RCP4.5 scenario and 36–44% for the RCP8.5 scenario."
[Ch12 p61]

This is obviously the source of the Telegraphs's headline claiming that [anthropogenic] warming will cause cooling in the UK by affecting Atlantic ocean currents. For there to be sustained and significant cooling in Northern Europe, there would presumably need to be a sustained weakening of AMOC, as confirmed by the Carbon Brief here:

"A recent study by McCarthy et al. describes how researchers observed a decline in the AMOC during the winter of 2009/2010 by around 30 per cent. They attribute the decline to an unusual wind event and changes in the deep branch of the AMOC (3000-5000 m). But the decline was followed by a swift recovery. For significant cooling to occur, such a decline would have to persist for a much longer period."

It is nevertheless somewhat puzzling that the IPCC appears not to mention the possibility of regional cooling in the UK, stating in fact the complete opposite, that regional warming is expected, when, by its own admission, it anticipates a weakening of AMOC which would probably mean cooling (as in 2009/10). Perhaps this is fortunate from the perspective of the UK government: already increasingly unpopular measures in the UK designed to mitigate global warming, in particular sky-high energy bills due to renewables subsidies, would become potential political dynamite if the world's authority on climate change was to say that global warming in the UK would mean actual cooling. Colder winters would not go down well at a time when fuel bills are rocketing because we're supposed to be 'all in it together' in the effort to avoid warmer, wetter winters and scorching summers!

In the final analysis however, all this may be somewhat irrelevant as sustained cooling in the UK looks set to proceed not as a result of Arctic ice melting due to CO2 emissions, but as a result of declining solar activity (see my previous post).