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).