Im September 2014 erschien im Fachblatt Palaeo3 unbemerkt von der ansonsten so klimainteressierten Presse ein wichtiges Paper einer Forschergruppe um Philip Menzel von der Universität Hamburg. Eine Pressemitteilung hierzu gab die Universität leider nicht heraus. Und auch die ebenfalls beteiligten deutschen Institute, darunter das Geoforschungszentrum Potsdam und das Senckenberg Institut schwiegen beredt. Dies ist umso bedauerlicher, da die Wissenschaftler um Philip Menzel spannende neue Erkenntnisse gewonnen hatten, die einen lang gehegten Verdacht bestätigten: Sonnenaktivitätsschwankungen spielen in der Klimahistorie der letzten 10.000 Jahre eine bedeutende Rolle.
In der Studie rekonstruierten die Forscher anhand eines Seesedimentbohrkerns das Monsungeschehen in Zentral-Indien für die vergangenen 11.000 Jahre. Dabei fanden Menzel und Kollegen eine starke Variabilität mit einem Wechsel von feuchten und trockenen Phasen. Die Dürrezeiten ereigneten sich dabei während solarer Schwächephasen. Wenn dann die Sonne wieder aufdrehte, kam auch der Regen zurück. Interessanterweise weist diese Niederschlagszyklik ein hohes Maß an Synchronität mit Temperatur-Zyklen im Nordatlantik auf, den sogenannten Bond-Zyklen, die laut Bond et al. 2001 ebenfalls einen solaren Ursprung haben. Kältephasen im Nordatlantik korrespondieren hierbei mit Dürrezeiten in Zentral-Indien. Diese sogenannte Millenniumszyklik ist aus zahlreichen Studien aus der ganzen Welt beschrieben (siehe Details in unserem Buch „Die kalte Sonne“). Die neue Studie bringt ein weiteres wichtiges Mosaiksteinchen in das wissenschaftliche Bild, in dessen Kontext auch die klimatische Millenniumszyklik der letzten 1000 Jahre mit Mittelalterlicher Wärmeperiode, Kleiner Eiszeit und Moderner Wärmephase zu sehen ist.
Im Folgenden die Zusammenfassung der Arbeit von Menzel und Kollegen:
Linking Holocene drying trends from Lonar Lake in monsoonal central India to North Atlantic cooling events
We present the results of biogeochemical and mineralogical analyses on a sediment core that covers the Holocene sedimentation history of the climatically sensitive, closed, saline, and alkaline Lonar Lake in the core monsoon zone in central India. We compare our results of C/N ratios, stable carbon and nitrogen isotopes, grain-size, as well as amino acid derived degradation proxies with climatically sensitive proxies of other records from South Asia and the North Atlantic region. The comparison reveals some more or less contemporaneous climate shifts. At Lonar Lake, a general long term climate transition from wet conditions during the early Holocene to drier conditions during the late Holocene, delineating the insolation curve, can be reconstructed. In addition to the previously identified periods of prolonged drought during 4.6–3.9 and 2.0–0.6 cal ka that have been attributed to temperature changes in the Indo Pacific Warm Pool, several additional phases of shorter term climate alteration superimposed upon the general climate trend can be identified. These correlate with cold phases in the North Atlantic region. The most pronounced climate deteriorations indicated by our data occurred during 6.2–5.2, 4.6–3.9, and 2.0–0.6 cal ka BP. The strong dry phase between 4.6 and 3.9 cal ka BP at Lonar Lake corroborates the hypothesis that severe climate deterioration contributed to the decline of the Indus Civilisation about 3.9 ka BP.
In den Highlights zum Paper schreiben die Autoren:
Changes in solar activity seem to cause the centennial climate shifts.
In den Conclusions gehen die Autoren etwas genauer auf die Zyklik und den nordatlantischen Vergleich ein:
The long term climate trend is superimposed by several shorter term climate fluctuations. Some of these fluctuations have also been observed in other high resolution climate records from Asia, and they can be correlated with the North Atlantic Bond events (Bond et al., 2001; Bond et al., 1997). The correlation is the same as observed for the long term trend with cold periods in the North Atlantic correlating with dry periods over South Asia and vice versa. All the 9 Bond events during the Holocene are isochronally (within dating uncertainties) reflected in the Lonar Lake record. This points to a connection between the two climate systems or to an identical trigger of climate variability. The fact that the Bioclastic Climate Index (BCI) quite well delineates the solar output proxy 14C production rate (Bond et al., 2001) corroborates the assumption that variations in solar activity triggered centennial scale variability of the Indian monsoon climate during the Holocene.
Einige Monate später, im April 2015, bestätigte eine Forschergruppe um Hai Xu im Fachblatt The Holocene das generelle Ergebnis: Der wiederholte Ausfall des Indischen Sommermonsuns während der letzten 10.000 Jahre geht hauptsächlich auf Schwankungen in der Sonnenaktivität zurück:
Abrupt Holocene Indian Summer Monsoon failures: A primary response to solar activity?
Knowledge of the millennial abrupt monsoon failures is critical to understanding the related causes. Here, we extracted proxy indices of Indian Summer Monsoon (ISM) intensity during the early to mid-Holocene, from peat deposits at Lake Xihu, in southwestern China. There are a series of abrupt, millennial-scale episodes of ISM weakening inferred from the Lake Xihu records, which are generally synchronous with those inferred from other archives over ISM areas. An important feature is that the ISM failures inferred from the Lake Xihu proxy indices synchronize well with abrupt changes in solar activity. We argue that changes in solar activity play a primary role in producing most of these millennial ISM failures, while some other causes, including freshwater outbursts into the North Atlantic Ocean and changes in sea surface temperatures of the eastern tropical Pacific Ocean, may have also exerted influences on parts of the millennial ISM failures.
Einen Sonnenbezug des Indischen Monsuns fanden auch Hiremath und Kollegen, nachzulesen in der Februar 2015-Ausgabe des Fachblatts New Astronomy:
Indian summer monsoon rainfall: Dancing with the tunes of the sun
There is strong statistical evidence that solar activity influences the Indian summer monsoon rainfall. To search for a physical link between the two, we consider the coupled cloud hydrodynamic equations, and derive an equation for the rate of precipitation that is similar to the equation of a forced harmonic oscillator, with cloud and rain water mixing ratios as forcing variables. Those internal forcing variables are parameterized in terms of the combined effect of external forcing as measured by sunspot and coronal hole activities with several well known solar periods (9, 13 and 27 days; 1.3, 5, 11 and 22 years). The equation is then numerically solved and the results show that the variability of the simulated rate of precipitation captures very well the actual variability of the Indian monsoon rainfall, yielding vital clues for a physical understanding that has so far eluded analyses based on statistical correlations alone. We also solved the precipitation equation by allowing for the effects of long-term variation of aerosols. We tentatively conclude that the net effects of aerosols variation are small, when compared to the solar factors, in terms of explaining the observed rainfall variability covering the full Indian monsoonal geographical domains.
Ganz frisch auch die Studie einer Forschergruppe um Manesh Tiwari, die im September 2015 in The Holocene herauskam. Die Autoren wiesen auf einen zeitlichen Verzug von wenigen hundert Jahren hin, der zwischen solarem Auslöser und Klimareaktion beobachtet wurde:
Multi-centennial scale SST and Indian summer monsoon precipitation variability since the mid-Holocene and its nonlinear response to solar activity
Indian Summer Monsoon (ISM) shows a weak correlation with solar variability in the 20th century. However, such climatological observations on solar activity–monsoon relationship are very short and hence uncertain. A few paleomonsoon records also exhibit prominent correspondence with solar activity during early Holocene and beyond. But despite the strong recent solar minima (e.g. Maunder, Spörer, Oort, Wolf), their correlation with monsoon precipitation is weak and inconclusive. Additionally, many of the earlier studies have been from the western Arabian Sea that provides records of the ISM wind intensity instead of the ISM precipitation. We present here mid-Holocene to recent sea surface temperature (SST) reconstructed from Mg/Ca measurements of planktic foraminifera (Globigerinoides ruber; white, sensu stricto) on a centennial-scale resolution from the southeastern Arabian. These measurements are used to correct the oxygen isotope ratios of G. ruber to reconstruct salinity related to monsoon runoff in this region more precisely than hitherto. The long-term trend indicates that the ISM precipitation has declined since the mid-Holocene similar to the solar activity. On shorter multi-centennial timescale, we show that the ISM precipitation declined concurrently with the recent periods of strong solar minima, but lagged by a couple of hundred years beyond 1300 yr BP toward the mid-Holocene – confirmed statistically using wavelet analysis. This nonstationary phase relationship between the ISM and the solar activity indicates the possible influence of the tropical coupled ocean–atmosphere phenomenon.
Auch im kürzeren Maßstab wurden zeitliche Verzögerungen beobachtet. Im Dezember 2014 berichteten Venkat Ratnam und Kollegen im Journal of Atmospheric and Solar-Terrestrial Physics über einen zeitlichen Verzug von 13 Jahren, den sie in der Sonne-Monsun-Dynamik bei Betrachtung der vergangenen 60 Jahre nachweisen konnten. Diese „time lag“-Effekte müssen bei Korrelationsbetrachtungen also unbedingt mitbetrachtet werden. Hier die Kurzfassung der Arbeit von Venkat Ratnam und Kollegen:
Solar cycle effects on Indian summer monsoon dynamics
Solar activity associated with sunspot number influences the atmospheric circulation on various time scales. As Indian summer monsoon (ISM) is the manifestation between warmer Asian continent and the cooler Indian Ocean, changes in the solar cycle are expected to influence the ISM characteristics. Among several elements of ISM, Tropical Easterly Jet (TEJ), Low Level Jet (LLJ), and rainfall are important features. As a part of CAWSES India Phase II theme 1 (solar influence on climate (0–100 km)) programme, we made an attempt to investigate the role of solar cycle variability on these ISM features using long-term data available from NECP/NCAR (1948–2010) and ERA-Interim (1979–2010) re-analysis products. To check the suitability of these data sets, ground based observations available over the Indian region are also considered. ISM characteristics are studied separately for the maximum and minimum as well as increasing and decreasing solar cycle conditions. Amplitudes corresponding to the solar cycle observed in TEJ, LLJ and rainfall are extracted using advanced statistical tool known as intrinsic mode function. Long-term trends in TEJ reveal decreasing trend at the rate of 0.13 m/s/yr (between 1948 and 2000) and no perceptible trend in LLJ. There exists inverse relation between TEJ strength and Central India rainfall. Large difference of 2 m/s (5 m/s) in the zonal winds of TEJ between solar maximum and minimum (increasing and decreasing trend) is noticed. There exists a difference of ~2 m/s in LLJ winds between solar maximum and minimum and increasing and decreasing trend of the solar cycle. However, no consistent relation between the ISM rainfall and solar cycle is noticed over Indian region unlike reported earlier but there exists a delayed effect around 13 years. We attribute the observed features as linear and non-linear relation between dynamics of ISM, rainfall and solar cycle, respectively.
Auch gegen Ende der letzten Eiszeit spielte die Sonne für den Indischen Monsun eine bedeutende Rolle, wie Mahjoor Ahmad Lone und Kollegen im Februar 2014 in Palaeo3 erläuterten:
Speleothem based 1000-year high resolution record of Indian monsoon variability during the last deglaciation
A high resolution record of the Indian summer monsoon (ISM) is generated using a δ18O time series from a stalagmite collected from the Valmiki cave in southern India. This record covers a time span of ~ 1000 years from 15,700 to 14,700 yr BP (before 1950 AD) with an average sampling resolution of ~ 5 years. High amplitude δ18O variation in this record reflects abrupt changes in ISM activity during the last deglaciation and suggest an age for the onset of Termination 1a (T1a) at ~ 14,800 yr BP in the Indian sub-continent. This record shows evidence for strong changes in tropical climate during the last deglaciation. Coincident variability in VSPM4 δ18O with speleothems from southern China during Termination 1a suggests that these caves reflect fluctuations in ISM activity. The variance in δ18O amplitude reveals significant multidecadal variability in ISM activity. Our record reveals intervals of strong monsoon activity during the later phase of Heinrich event 1 (H1) and shows synchronous multidecadal variability between ISM and East Asian monsoon (EAM). Spectral analysis of δ18O time series in VSPM4 reveals solar forcing and strong ocean–atmospheric circulation control on ISM dynamics during the studied time interval.
Ebenso bedeutsam ist die Erkenntnis, dass solare Aktivitätsschwankungen je nach Region auch gegenteilige Klimaeffekte hervorrufen können. Animesh Maitra und Kollegen zeigten dies im Dezember 2014 im Journal of Atmospheric and Solar-Terrestrial Physics im Rahmen einer Studie des Indischen Monsunniederschlags der vergangenen 40 Jahre:
Solar control on the cloud liquid water content and integrated water vapor associated with monsoon rainfall over India
A long-term observation over three solar cycles indicates a perceptible influence of solar activity on rainfall and associated parameters in the Indian region. This paper attempts to reveal the solar control on the cloud liquid water content (LWC) and integrated water vapor (IWV) along with Indian Summer Monsoon (ISM) rainfall during the period of 1977–2012 over nine different Indian stations. Cloud LWC and IWV are positively correlated with each other. An anti-correlation is observed between the Sunspot Number (SSN) and ISM rainfall for a majority of the stations and a poor positive correlation obtained for other locations. Cloud LWC and IWV possess positive correlations with Galactic Cosmic Rays (GCR) and SSN respectively for most of the stations. The wavelet analyses of SSN, ISM rainfall, cloud LWC and IWV have been performed to investigate the periodic characteristics of climatic parameters and also to indicate the varying relationship of solar activity with ISM rainfall, cloud LWC and IWV. SSN, ISM rainfall and IWV are found to have a peak at around 10.3 years whereas a dip is observed at that particular period for cloud LWC.
Der deutliche Zusammenhang zwischen solarer Aktivität und Monsun in Indien ist auch Thema von Aslam und Badruddin in einem Artikel aus dem Oktober 2014 in Advances in Space Research. Die Autoren schlussfolgern, dass Sonnenaktivitätsschwankungen auch im Modernen Klimageschehen keinesfalls unterschätzt werden dürfen und dringend weiterführende Untersuchunge benötigt werden.
Study of the influence of solar variability on a regional (Indian) climate: 1901–2007
We use Indian temperature data of more than 100 years to study the influence of solar activity on climate. We study the Sun–climate relationship by averaging solar and climate data at various time scales; decadal, solar activity and solar magnetic cycles. We also consider the minimum and maximum values of sunspot number (SSN) during each solar cycle. This parameter SSN is correlated better with Indian temperature when these data are averaged over solar magnetic polarity epochs (SSN maximum to maximum). Our results indicate that the solar variability may still be contributing to ongoing climate change and suggest for more investigations.
In den Highlights zum Paper heißt es:
–Indian climate appears to be influenced by solar variability.
–Mechanism for Sun–climate relationship may be related solar polarity also.
Abschließend sei noch auf ein Paper von Krusic und Kollegen hingewiesen, das im April 2015 in den Geophysical Research Letters herauskam. Die Wissenschaftler rekonstruierten die Temperaturgeschichte des Himalaya-Landes Bhutan für die vergangenen 640 Jahre und entdeckten, dass besonders kalte Phasen stets mit solaren Schwächeperioden einhergingen:
Six hundred thirty-eight years of summer temperature variability over the Bhutanese Himalaya
High-resolution tree ring reconstructions from the Himalaya provide essential context for assessing impacts of future climate change on regional water reserves and downstream agriculture. Here we evaluate a small network of tree ring chronologies from Bhutan to produce a 638 year summer temperature reconstruction, from 1376–2013 (Common Era) C.E. Relative to the 1950–2013 C.E. average summer temperature three prominent cold periods stand out, two in the midfifteenth century, and one in the late seventeenth century. The warmest period began in the first decade of the 21st century coinciding with the timing of general glacier recession in the eastern Himalaya that continues to the present. The Bhutan temperature reconstruction exhibits a significant correlation to known volcanic eruptions (p = 97%) and anomalously cold periods appear to align with solar irradiance minima in the fifteenth, late seventeenth, and early nineteenth centuries, implying a link between solar variability and decadal-scale temperature variability.