Die Sonnenaktivität schwankt im Takt einer Reihe von charakteristischen Zyklen. Am bekanntesten ist der 11-jährige Schwabezyklus. Natürlich sind 11 Jahre aus klimatischer Sicht eine relativ kurze Zeit. Durch die Trägheit des Klimasystems ist hier nicht mit großen klimatischen Auswirkungen zu rechnen. Dennoch lohnt es sich hier genauer hinzuschauen. Eine Reihe von Studien hat sich in den vergangenen Jahren dem Schwabe-Zyklus angenommen und in historischen Datensätzen nach einer möglichen klimatischen Kopplung gesucht. Um es vorweg zu nehmen: Die Suche hat sich ausgezahlt: Der solare Schwabe-Zyklus hat durchaus eine messbare und nicht zu unterschätzende Klimawirkung.
Beginnen möchten wir in Deutschland. Eine Forschergruppe um Dominik Güttler von der ETH Zürich untersuchte in Süddeutschland tausendjährige Eichen aus der Mittelalterlichen Wärmeperiode. Mithilfe von C14-Alterdatierungen und Baumringzählugen konnten die Wissenschaftler einen deutlich pulsierenden solaren 11-Jahreszyklus nachweisen. Die Arbeit erschien im Januar 2013 in den Proceedings of the Twelfth International Conference on Accelerator Mass Spectrometry. Hier der Abstract:
Evidence of 11-year solar cycles in tree rings from 1010 to 1110 AD – Progress on high precision AMS measurements
Oak tree rings from Southern Germany covering the AD 1010–1110 years have been analyzed for radiocarbon with accelerator mass spectrometry (AMS) at the laboratory at ETH Zurich. High-precision measurements with a precision down to 12 years radiocarbon age and a time resolution of 2 years aimed to identify modulations of the 14C concentration in tree ring samples caused by the 11 years solar cycles, a feature that so far is not visible in the IntCal calibration curve. Our results are in good agreement with the current calibration curve IntCal09. However, we observed an offset in radiocarbon age of 25–40 years towards older values. An evaluation of our sample preparation, that included variations of e.g.: chemicals, test glasses and processing steps did not explain this offset. The numerous measurements using the AMS-MICADAS system validated its suitability for high precision measurements with high repeatability.
Weiter nach Italien in das Ionische Meer. Forscher fanden den solaren 11-Jahres-Zyklus dort in Klimaarchiven der letzten 2700 Jahre. Die Arbeit erschien im März 2015 in Climate of the Past:
A high-resolution δ18O record and Mediterranean climate variability
A high-resolution, well-dated foraminiferal δ18O record from a shallow-water core drilled from the Gallipoli Terrace in the Gulf of Taranto (Ionian Sea), previously measured over the last two millennia, has been extended to cover 707 BC–AD 1979. Spectral analysis of this series, performed using singular-spectrum analysis (SSA) and other classical and advanced methods, strengthens the results obtained analysing the shorter δ18O profile, detecting the same highly significant oscillations of about 600, 380, 170, 130 and 11 years, respectively explaining about 12, 7, 5, 2 and 2% of the time series total variance, plus a millennial trend (18% of the variance). The comparison with the results of multi-channel singular-spectrum analysis (MSSA) applied to a data set of 26 Northern Hemisphere (NH) temperature-proxy records shows that NH temperature anomalies share with our local record a~long-term trend and a bicentennial (170-year period) cycle. These two variability modes, previously identified as temperature-driven, are the most powerful modes in the NH temperature data set. Both the long-term trends and the bicentennial oscillations, when reconstructed locally and hemispherically, show coherent phases. Furthermore, the corresponding local and hemispheric amplitudes are comparable if changes in the precipitation–evaporation balance of the Ionian sea, presumably associated with temperature changes, are taken into account.
Im April 2014 berichteten Liang Zhao und Jing-Song Wang vom Pekinger National Center for Space Weather im Journal of Climate über einen anderen Schwabe-Fund. Die Autoren untersuchten Schwankungen des Ostasiatischen Monsuns und konnten auch hier einen deutlichen Einfluss des 11-Jahres-Sonnenzyklus nachweisen. Hier die Kurzfassung der Arbeit:
Robust Response of the East Asian Monsoon Rainband to Solar Variability
This study provides evidence of the robust response of the East Asian monsoon rainband to the 11-yr solar cycle and first identify the exact time period within the summer half-year (1958–2012) with the strongest correlation between the mean latitude of the rainband (MLRB) over China and the sunspot number (SSN). This period just corresponds to the climatological-mean East Asian mei-yu season, characterized by a large-scale quasi-zonal monsoon rainband (i.e., 22 May–13 July). Both the statistically significant correlation and the temporal coincidence indicate a robust response of the mei-yu rainband to solar variability during the last five solar cycles. During the high SSN years, the mei-yu MLRB lies 1.2° farther north, and the amplitude of its interannual variations increases when compared with low SSN years. The robust response of monsoon rainband to solar forcing is related to an anomalous general atmospheric pattern with an up–down seesaw and a north–south seesaw over East Asia.
Zwei Monate zuvor hatte eine Forschergruppe um Zhongfang Liu in den Environmental Research Letters eine Untersuchung zum nordamerikanischen Winterklima veröffentlicht. Überraschenderweise fanden die Wissenschaftler eine starke Beeinflussung durch den solaren 11-Jahreszyklus, der über pazifische Zirkulationssysteme das Klima der kalten Jahreszeit in den USA und Kanada mitbestimmt. Hier die Kurzfassung:
Solar cycle modulation of the Pacific–North American teleconnection influence on North American winter climate
We investigate the role of the 11-year solar cycle in modulating the Pacific–North American (PNA) influence on North American winter climate. The PNA appears to play an important conduit between solar forcing and surface climate. The low solar (LS) activity may induce an atmospheric circulation pattern that resembles the positive phase of the PNA, resulting in a significant warming over northwestern North America and significant dry conditions in the Pacific Northwest, Canadian Prairies and the Ohio-Tennessee-lower Mississippi River Valley. The solar-induced changes in surface climate share more than 67% and 14% of spatial variances in the PNA-induced temperature and precipitation changes for 1950–2010 and 1901–2010 periods, respectively. These distinct solar signatures in North American climate may contribute to deconvolving modern and past continental-scale climate changes and improve our ability to interpret paleoclimate records in the region.
In den Conclusions heißt es:
Our results have shown the influence of the 11year solar cycle on the PNA associated atmospheric circulation pattern and winter surface climate in North America.
Auch im Beringmeer hat eine Forschergruppe den solaren Schwabe-Zyklus nachgewiesen. Kota Katsuki und Kollegen fanden den Zyklus in Klimaarchiven von vor 13.000 Jahren. Die Studie erschien im April 2014 in den Geophysical Research Letters:
Response of the Bering Sea to 11-year solar irradiance cycles during the Bølling-Allerød
Previous studies find decadal climate variability possibly related to solar activity, although the details regarding the feedback with the ocean environment and ecosystem remain unknown. Here, we explore the feedback system of solar irradiance change during the Bølling-Allerød period, based on laminated sediments in the northern Bering Sea. During this period, well-ventilated water was restricted to the upper intermediate layer, and oxygen-poor lower intermediate water preserved the laminated sediment. An 11-year cycle of diatom and radiolarian flux peaks was identified from the laminated interval. Increased fresh meltwater input and early sea-ice retreat in spring under the solar irradiance maximum follow the positive phase of Arctic Oscillation which impacted the primary production and volume of upper intermediate water production in the following winter. Strength of this 11 year solar irradiance effect might be further regulated by the pressure patterns of Pacific decadal oscillation and/or El Niño-Southern Oscillation variability.
Last but not least sei noch auf ein Klassiker-Paper zum Schwabe-Zyklus einer Gruppe um Hiroko Miyahara aus dem Jahr 2009 in den Proceedings of the International Astronomical Union hingewiesen:
Influence of the Schwabe/Hale solar cycles on climate change during the Maunder Minimum
We have examined the variation of carbon-14 content in annual tree rings, and investigated the transitions of the characteristics of the Schwabe/Hale (11-year/22-year) solar and cosmic-ray cycles during the last 1200 years, focusing mainly on the Maunder and Spoerer minima and the early Medieval Maximum Period. It has been revealed that the mean length of the Schwabe/Hale cycles changes associated with the centennial-scale variation of solar activity level. The mean length of Schwabe cycle had been ~14 years during the Maunder Minimum, while it was ~9 years during the early Medieval Maximum Period. We have also found that climate proxy record shows cyclic variations similar to stretching/shortening Schwabe/Hale solar cycles in time, suggesting that both Schwabe and Hale solar cycles are playing important role in climate change. In this paper, we review the nature of Schwabe and Hale cycles of solar activity and cosmic-ray flux during the Maunder Minimum and their possible influence on climate change. We suggest that the Hale cycle of cosmic rays are amplified during the grand solar minima and thus the influence of cosmic rays on climate change is prominently recognizable during such periods.
