Grand Solar Minima do bring cooling to Europe Holocene record shows

solar minimum climate cooling EuropeBut now it’s not dry and icy five-month winters, but wet and windy springs instead. Or would you say these combine?

Unlike normal solar minima, which occur in the 11-year solar cycle, Grand Solar Minima last many decades to over a century. When climate people think of such prolonged ‘silent Sun’ events the Maunder Minimum is always the first to come to mind.

Accompanied by the less known Spörer Minimum (1450-1550 AD) and Dalton Minimum (1790-1820), the Maunder Minimum (1645-1725) is linked to the Little Ice Age – a well-documented somewhat colder period with long frosty winters in Europe and parts of North America.

Critics however say the overall difference between the dips and peaks in the Holocene solar cycles are simply too small to explain large climatic changes. The total solar irradiance (TSI) during the Maunder Minimum was not smaller than the TSI in 2007-2010 [the latest normal solar minimum], a publication in Geophysical Research Letters stated last year.

Then of course there are competing or perhaps complementing hypotheses to explain the Little Ice Age – like medieval volcanoes.

UV-radiation in stratosphere may determine tropospheric air pressure patterns

Of special interest was a Nature Geoscience publication by scientists of the UK Met Office Hadley Centre in October last year. It concluded the direct TSI differences in the normal solar cycle may not be large enough to let solar minima cause significant planetary cooling events [as many climate skeptics state], but locally significant climate effects can be induced.

The amount of incoming ultraviolet radiation influences stratospheric wave patterns, so it is thought – and this in turn determines the route and dominance of depressions and the position and likelihood of high pressure blockades – in the troposphere below it, the lower atmosphere, in which we live and breath, and enjoy and endure weather.

During solar minima the North Atlantic Oscillation would be tempted to move to a more negative phase, the British said. In winter this increases the chance of cold easterlies over Europe – and could therefore help explain the exceptionally cold winters of the Little Ice Age. To a lesser extent also the Arctic Oscillation would be more inclined towards a negative state during solar minima, favouring cold air plunges to North America and Eurasia.

Therefore it is safe to say there is still some controversy surrounding the causes of the Little Ice Age and the possible climatic effects of Grand Solar Minima in general.

Homeric Question: do cold winters and wet springs go hand in hand?

Fortunately science never stops. Yesterday another solar climate study appeared in Nature Geoscience, conducted by a group of scientists of the German Research Center for Geosciences (Helmholtz Centre Potsdam), Freie Universität Berlin, Lund University, Uppsala University and the University of Amsterdam.

They have however not focused their study of Grand Solar Minima on the Maunder period, but on an older one, the Homeric Minimum, of around 2800 years ago – the last large minimum before the three subsequent minima between 1450 and 1725 AD, mentioned above.

The Homeric Minimum was of comparable strength to the Spörer, Maunder and Dalton Minima, but it came more as a single sudden negative spike – occurring in a time with a higher TSI baseline than our planet had in for instance the Middle Ages.

Like their colleagues from the Hadley Centre these scientists image large-scale weather pattern disturbances are possible, as a result of stratospheric ultraviolet radiation fluctuations – possible resulting as local climate changes – of which indeed they have found evidence in German lake sediments.

Problem is these only presented data for windiness [possibly only for springtime] and isotope measurements to correlate solar activity. From these the scientists conclude the decreasing solar activity caused abrupt climate change, in an approximately two centuries long event:

“We find a sharp increase in windiness and cosmogenic 10Be deposition 2,759  ±  39 varve years before present and a reduction in both entities 199  ±  9 annual layers later. We infer that the atmospheric circulation reacted abruptly and in phase with the solar minimum.”

Question now is would (wet and) windy springs [makes best sense to interpret this as a sign of positive NAO – increased dominance of Atlantic depressions] logically follow a negative NAO winter? The authors have also tried to incorporate their results in a climate model:

“A shift in atmospheric circulation in response to changes in solar activity is broadly consistent with atmospheric circulation patterns in long-term climate model simulations, and in reanalysis data that assimilate observations from recent solar minima into a climate model. We conclude that changes in atmospheric circulation amplified the solar signal and caused abrupt climate change about 2,800 years ago, coincident with a grand solar minimum.”

That model seems to answer our above question with ‘yes’ – as it shows a weaker stratospheric vortex during solar minima during the winter months and an increase of easterly winds, especially north of 40 degrees latitude.

Overall this adds some paint to a picture of European climate cooling in response to Grand Solar Minima.

Not that chances are high we’ll have one any time soon. we’ll have a new Grand Minimum in the next couple of years, the NASA solar cycle model tells us. [update: But there are some speculations going on the current maximum 'could be the last for some time' – something to hope for if you live in Holland and you like iceskating. As under such a scenario especially the European and North American climates could be influenced for several decades, we'll get back to you about this.]

© Rolf Schuttenhelm | www.bitsofscience.org

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