Laut den IPCC-Berichten spielen Aktivitätsschwankungen der Sonne im Klimageschehen keine große Rolle. Diese Schlussfolgerung löst jedoch Verwunderung aus, denn der Blick zurück in die Klimageschichte der letzten 10.000 Jahre zeigt eine Vielzahl von eindrucksvollen Beispielen für eine bedeutende solare Klimabeeinflussung. Es ist unwahrscheinlich, dass der Mensch diese Wechselwirkung zwischen unserem Mutterstern und dem Erdklima plötzlich aufgehoben haben soll. Ein schönes Fallbeispiel stammt aus den Quaternary Science Reviews. Ojala und Kollegen veröffentlichten dort im März 2015 eine Studie zur Klimageschichte Skandinaviens auf Basis von Seesedimentuntersuchungen. Die Wissenschaftler fanden charakteristische Zyklizitäten, darunter die bekannten solaren Eddy (1000 Jahre), Suess-de Vries (200 Jahre) und Gleissberg (90 Jahre) Zyklen:
Effects of solar forcing and North Atlantic oscillation on the climate of continental Scandinavia during the Holocene
10,000-year-long varved sediment records from lakes Nautajärvi and Korttajärvi, Finland provide evidence of climate and environment oscillations at multi-decadal to millennial timescales. We used two independent methods to extract periodic features from these time series of clastic laminae and assess their statistical reliability. Analyses revealed that seasonal sediment fluxes correspond to environmental changes with statistically significant periodicities of 1500–1800, 1000, 600–800, nearly 300, nearly 200, 150–170, nearly 90 and 47 years, showing variable coherency with different climate forcing factors and other palaeoproxy records in the Northern Hemisphere. Results indicate that the Holocene winter climate in continental Scandinavia was forced by a combination of several factors, at least by solar variability and the North Atlantic ocean–atmosphere circulation-patterns, with varying influences through time.
Insbesondere der 1000-Jahres-Zyklus tritt hervor. In der Discussion schreiben die Autoren:
“Identification of the 1000-year cycle in the NJ record suggests that solar forcing has probably effected on millennial-scale climatic and environmental fluctuations in continental Scandinavia during the Holocene.”
Auch im Bereich der Nordsee gibt es neue Hinweise auf eine solare Klimabeeinflussung. Im Juli 2014 veröffentlichte eine Forschergruppe der Universität Mainz bestehend aus Hilmar Holland, Bernd Schöne, Constanze Lipowsky und Jan Esper im Fachblatt The Holocene eine Klimastudie auf Basis von Anwachsstreifen in Muschelschalen. Dabei deckten die Wissenschaftler die vergangenen 1000 Jahre ab. Holland und Kollegen fanden, dass das Klima immer dann besonders stark schwankte, wenn die Sonnenaktivität auf Minimalwerte absank. Dies gilt insbesondere für die solaren Maunder- und Spörer-Minima während der Kleinen Eiszeit. Hier die Kurzfassung der Arbeit:
Decadal climate variability of the North Sea during the last millennium reconstructed from bivalve shells (Arctica islandica)
Uninterrupted, annually resolved paleoclimate records are crucial to contextualize the current global change. Such information is particularly relevant for the Europe realm for which weather and climate projections are still very challenging if not virtually impossible. This study presents the first precisely dated, annually resolved, multiregional Arctica islandica chronologies from the North Sea which cover the time interval ad 1040–2010 and contain important information on supra-regional climatic conditions (sea surface temperature (SST), ocean productivity, wind stress). Shell growth varied periodically on timescales of 3–8, 12–16, 28–36, 50–80, and 120–240 years, possibly indicating a close association with the North Atlantic Oscillation, ocean-internal cycles of the North Atlantic controlled by ocean–atmosphere couplings, and the Atlantic Multi-Decadal Oscillation. Increased climatic instability, that is, stronger quasi-decadal variability, seems to be linked to the predominance of atmospheric forcings and some significantly decreased insolation phases (e.g. Spörer and Maunder Minima). Increased climatic variability of shorter timescales was also observed during some particularly warm phases or regime shifts (e.g. during the ‘Medieval Climate Anomaly’ and since c. 1970). More stable climatic conditions, that is, extended warm or cold periods (‘Medieval Climate Anomaly’, ‘Little Ice Age’), however, fell together with a predominance of multi-decadal oceanic cycles. Whether the sunspot number and the higher frequency climate variability are causally linked and which processes and mechanisms are required lie beyond this study.
Gehen wir nun einige hundert Kilometer nach Osten, nach Polen. Ein Team um Ivan Hernández-Almeida nahm sich im Nordosten des Landes ebenfalls die Klimageschichte des letzten Jahrtausends vor. In einem Artikel, der Mitte August 2015 in den Quaternary Science Reviews erschien, berichteten die Wissenschaftler von starken natürlichen Klimaschwankungen und einer deutlichen solaren Beeinflussung. Hernández-Almeida fanden eine klare Gliederung in Mittelalterliche Wärmeperiode, Kleine Eiszeit und Moderne Wärmeperiode. Dabei fielen die Winter vor 1000 Jahren während der Mittelalterlichen Wärmeperiode sogar milder aus als heute (Abbildung 1). In den letzten 50 Jahren ist im Datensatz zudem eine Verschärfung der polnischen Winter zu erkennen. Im Folgenden die Kurzfassung der Arbeit:
A chrysophyte-based quantitative reconstruction of winter severity from varved lake sediments in NE Poland during the past millennium and its relationship to natural climate variability
Chrysophyte cysts are recognized as powerful proxies of cold-season temperatures. In this paper we use the relationship between chrysophyte assemblages and the number of days below 4 °C (DB4 °C) in the epilimnion of a lake in northern Poland to develop a transfer function and to reconstruct winter severity in Poland for the last millennium. DB4 °C is a climate variable related to the length of the winter. Multivariate ordination techniques were used to study the distribution of chrysophytes from sediment traps of 37 low-land lakes distributed along a variety of environmental and climatic gradients in northern Poland. Of all the environmental variables measured, stepwise variable selection and individual Redundancy analyses (RDA) identified DB4 °C as the most important variable for chrysophytes, explaining a portion of variance independent of variables related to water chemistry (conductivity, chlorides, K, sulfates), which were also important. A quantitative transfer function was created to estimate DB4 °C from sedimentary assemblages using partial least square regression (PLS). The two-component model (PLS-2) had a coefficient of determination of Rcross2 = 0.58, with root mean squared error of prediction (RMSEP, based on leave-one-out) of 3.41 days. The resulting transfer function was applied to an annually-varved sediment core from Lake Żabińskie, providing a new sub-decadal quantitative reconstruction of DB4 °C with high chronological accuracy for the period AD 1000–2010. During Medieval Times (AD 1180–1440) winters were generally shorter (warmer) except for a decade with very long and severe winters around AD 1260–1270 (following the AD 1258 volcanic eruption). The 16th and 17th centuries and the beginning of the 19th century experienced very long severe winters. Comparison with other European cold-season reconstructions and atmospheric indices for this region indicates that large parts of the winter variability (reconstructed DB4 °C) is due to the interplay between the oscillations of the zonal flow controlled by the North Atlantic Oscillation (NAO) and the influence of continental anticyclonic systems (Siberian High, East Atlantic/Western Russia pattern). Differences with other European records are attributed to geographic climatological differences between Poland and Western Europe (Low Countries, Alps). Striking correspondence between the combined volcanic and solar forcing and the DB4 °C reconstruction prior to the 20th century suggests that winter climate in Poland responds mostly to natural forced variability (volcanic and solar) and the influence of unforced variability is low.
Abbildung 1: Schwankungen in der Härte der polnischen Winter während der letzten 1000 Jahre. Aufgetragen ist die Anzahl der Tage mit Temperaturen unter 4°C. Ausschlag nach unten zeigte strenge Winter, Ausschlag nach oben milde Winter an. Aus: Hernández-Almeida et al. 2015.
Auf unserem europäischen Streifzug durch die aktuelle Literatur zur Klimawirkung der Sonne geht es jetzt an den Südwestzipfel des Kontinents. In Portugal untersuchte eine Forschergruppe um Santos et al. die Temperaturgeschichte der letzten 400 Jahre. Zum Wissenschaftlerteam gehört unter anderem auch Eduardo Zorita vom Helmholtz-Zentrum in Geesthacht. Im Fachblatt Climate of the Past berichten Santos und Kollegen über klare klimatische Auswirkungen der solaren Maunder und Dalton Minima auf das Temperaturgeschehen:
New insights into the reconstructed temperature in Portugal over the last 400 years
The consistency of an existing reconstructed annual (December–November) temperature series for the Lisbon region (Portugal) from 1600 onwards, based on a European-wide reconstruction, with (1) five local borehole temperature–depth profiles; (2) synthetic temperature–depth profiles, generated from both reconstructed temperatures and two regional paleoclimate simulations in Portugal; (3) instrumental data sources over the twentieth century; and (4) temperature indices from documentary sources during the late Maunder Minimum (1675–1715) is assessed. The low-frequency variability in the reconstructed temperature in Portugal is not entirely consistent with local borehole temperature–depth profiles and with the simulated response of temperature in two regional paleoclimate simulations driven by reconstructions of various climate forcings. Therefore, the existing reconstructed series is calibrated by adjusting its low-frequency variability to the simulations (first-stage adjustment). The annual reconstructed series is then calibrated in its location and scale parameters, using the instrumental series and a linear regression between them (second-stage adjustment). This calibrated series shows clear footprints of the Maunder and Dalton minima, commonly related to changes in solar activity and explosive volcanic eruptions, and a strong recent-past warming, commonly related to human-driven forcing. Lastly, it is also in overall agreement with annual temperature indices over the late Maunder Minimum in Portugal. The series resulting from this post-reconstruction adjustment can be of foremost relevance to improve the current understanding of the driving mechanisms of climate variability in Portugal.
Schließlich sei noch auf einen Artikel aus Österreich von Auer und Kollegen hingewiesen, der im Februar 2015 in Climate of the Past erschien. Die Forscher untersuchten eine 17 Millionen Jahre alte Sedimentabfolge in den Alpen. Dabei stießen sie im Rahmen einer hochauflösenden paläoklimatischen Untersuchung auf eine stark ausgeprägte natürliche Klimavariabilität und charakteristische solare Zyklen des Gleissberg und Suess-de Vries Typs. Auer und Kollegen schlussfolgern, dass das Klima des Miozäns maßgeblich durch solare Aktivitätsschwankungen gesteuert wurde. Im Folgenden die Kurzfassung der Studie:
Two distinct decadal and centennial cyclicities forced marine upwelling intensity and precipitation during the late Early Miocene in central Europe
Within a 5.5 m thick succession of Upper Burdigalian (Karpatian) sediments in the North Alpine Foreland Basin (NAFB; Austria), dated to CNP-zone NN4, a high-resolution section was logged continuously. One hundred samples were taken with a resolution of ~10 mm (approximating ~17 years) per layer and analyzed using an integrated multi-proxy approach. Earlier analyses of geochemistry and calcareous nannoplankton assemblages hint at small-scale, short-term variations in paleoenvironmental conditions, such as water-column stratification, primary productivity, organic matter flux, bottom-water oxygenation, freshwater influx, and changes in relative sea level. The results indicate a highly dynamic shallow marine setting that was subject to high-frequency environmental changes on a decadal to centennial scale. Time-series analyses on nine different proxy data sets using REDFIT analysis and wavelet spectra were applied to resolve a possible cyclic nature of these variations. Analyses revealed that different proxies for precipitation, upwelling intensity, and overall productivity were likely controlled by different cyclicities. A best-fit adjustment of the likely sedimentation rates within the high-resolution section resulted in periodicities fitting well with the Lower (~65 years) and Upper (~113 years) Gleissberg cycle as well as the Suess/de Vries cycle (~211 years). The section covers a time span of ~1190 years based on the correlation with solar cycles, which resulted in an estimated sedimentation rate of 575 mm kyr−1. For the first time, short-term climate variability on a decadal to centennial scale is resolved in Lower Miocene shallow marine laminated sediments in a land-based section. The results hint at a close relationship between climate variability and solar forcing during the Late Burdigalian. Moreover, accepting that these cyclicities are indeed of solar origin, this would indicate that precipitation was driven by the two Gleissberg cycles, while upwelling was driven by the Suess cycle. Furthermore, proxies for primary productivity were influenced by both cycles, although the Suess cycle exerts dominant control, reflecting a stronger influence of upwelling on primary productivity.
