Sonne

Die American Geophysical Union (AGU) gab kürzlich eine Pressemitteilung zu einer neuen interessanten Studie von Kobashi und Kollegen heraus, die sich mit den Auswirkungen solarer Aktivitätsschwankungen auf das Klima beschäftigt. Die Untersuchung fand, dass die Temperaturen in Grönland im Jahrzehnt-Maßstab signifikant von der Sonnenaktivität beeinflusst werden. Aufgrund von Ozeanzirkulationseffekten führt hier eine starke Sonnenaktivität zu einer lokalen Abkühlung. Dabei entdeckten die Forscher klimatische Verzögerungseffekte von 10-40 Jahren, die es dauerte, bis sich das solare Auslösersignal im trägen Klimasystem auswirkte. Bereits 2013 berichteten wir an dieser Stelle über eine Vorgängerarbeit derselben Arbeitsgruppe (siehe unseren Blogartikel „Grönlandische Temperaturen von Sonnenaktivität beeinflusst: Je stärker die Sonne, desto kälter war es in Grönland„).

Im folgenden die Pressemitteilung der AGU vom 16. Juli 2015 zur neuen Studie:

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Sun’s activity controls Greenland temperatures

The sun’s activity could be affecting a key ocean circulation mechanism that plays an important role in regulating Greenland’s climate, according to a new study. The phenomenon could be partially responsible for cool temperatures the island experienced in the late 20th century and potentially lead to increased melting of the Greenland ice sheet in the coming decades, the new research suggests. Scientists have sought to understand why Greenland cooled during the 1970s through the early 1990s while most of the Northern Hemisphere experienced rising temperatures as a result of greenhouse warming.

The new study suggests high solar activity starting in the 1950s and continuing through the 1980s played a role in slowing down ocean circulation between the South Atlantic and the North Atlantic oceans. Combined with an influx of fresh water from melting glaciers, this slow-down halted warm water and air from reaching Greenland and cooled the island while temperatures rose across the rest of the Northern Hemisphere, according to the new study accepted for publication in Geophysical Research Letters, a journal of the American Geophysical Union. The new research also suggests weak solar activity, like the sun is currently experiencing, could slowly fire up the ocean circulation mechanism, increasing the amount of warm water and air flowing to Greenland.

Starting around 2025, temperatures in Greenland could increase more than anticipated and the island’s ice sheet could melt faster than projected, according to Takuro Kobashi, a climate scientist with the Department of Climate and Environmental Physics at the University of Bern in Switzerland and lead author of the new study. This unexpected ice loss would compound projected sea-level rise expected to occur as a result of climate change, Kobashi said. The melting Greenland ice sheet accounted for one-third of the 3.2 millimeters (0.13 inches) rise in global sea level every year from 1992 to 2011. “We need to really consider how solar activity will change in the future,” said Kobashi. “If solar activity becomes really low, as scientists expect, the Greenland ice sheet will melt faster than we expected from the climate model with just greenhouse gas [warming].”

The new study compared past solar activity with historical temperature records to figure out if the cooling Greenland experienced during the late 20th century was part of a long-term pattern.The authors of a new paper placed ice from subsections of Greenland ice cores in glass flasks. Under a vacuum, the ice melted, releasing the air trapped within the ice. The scientists used the trapped air to calculate the island’s temperatures for the past 2,100 years and compare them to vacillations in solar activity.

The team used ice cores drilled from the Greenland ice sheet to reconstruct snow temperatures for the past 2,100 years. A relatively new technique, which measures argon and nitrogen gases trapped in the ice, allowed the scientists to measure small changes in temperature at 10- to 20-year increments. The ice cores showed that for the past 2,000 years changes in Greenland temperatures have generally followed any temperature shifts occurring in the Northern Hemisphere. The new research found that the change in Greenland temperatures vacillated up and down around the average change in Northern Hemisphere temperatures over time. The vacillations coincided with changes in the sun’s energy output that occurred over multiple decades, according to the new study.

When the sun’s energy output increased, there was a bigger drop in Greenland’s temperature compared to the change in average temperature across the Northern Hemisphere. When the sun’s energy output decreased, there was a larger increase in Greenland’s temperature compared to the change in average temperature that occurred across the Northern Hemisphere. Climate models showed that changes in solar activity could prompt shifts in ocean and air circulation in the North Atlantic that affect Greenland’s climate, according to the new study.

Shifting circulation patterns

Water circulation in the Atlantic follows a steady pattern of movement, called the Atlantic Meridional Overturning Circulation (AMOC). Warm water flows from the South Atlantic toward the North Atlantic, transferring heat toward Greenland. As the water cools, it sinks to the ocean floor and travels south toward the tropics, completing the circular pattern. During a period of high solar activity, more energy from the sun reaches Earth and is transferred to tropical waters. When this warmer-than-usual water reaches the North Atlantic, it is not dense enough to sink. With nowhere to go, the water causes a traffic jam and the water circulation pattern slows down. Changes in solar activity can also alter the atmospheric circulation pattern over the Atlantic, which in turn affects ocean circulation, but how this process works is still unknown, said Kobashi.

In the late 20th century, there also was a compounding problem. Large amounts of freshwater gushed into the North Atlantic as climate change caused increased melting of glaciers, icebergs, and the Greenland ice sheet. Freshwater, being more buoyant than salt water, entered the intersection where cool water drops to the ocean floor and travels south to the tropics. Climate models showed that the water in the intersection became less salty and less likely to sink. Models also showed that additional freshwater came from an increase in rainfall, according to the new study. The traffic jam worsened and the water circulation pattern that transfers heat from the South Atlantic to the North Atlantic slowed. This slow-down caused the air above Greenland to cool and temperatures there to drop, according to the new study.

Because the oceans take a long time to heat up or cool down, the temperature changes in Greenland lagged 10 to 40 years behind the high solar activity, showing up from the 1970s through the early 1990s, according to the new study. The new study suggests low solar activity could have the opposite effect and lead to warmer temperatures in Greenland in another decade. When there is less solar energy reaching the Earth, water reaching Greenland easily sinks and returns to the tropics along the ocean floor. The water circulation pattern speeds up, quickly funneling heat toward Greenland and warming the island.

Greenhouse gases versus solar activity

The new study makes a good case that the solar maximum in the 1950s through the 1980s may have played a role in the cooling Greenland saw in the late 20th century, said Michael Mann, a climate  scientist with the Department of Meteorology at Penn State University in University Park, Pennsylvania, who was not involved in the new study. Another recent study by Mann and his colleagues proposed that trapped greenhouse gases from fossil fuel burning caused warming across the Northern Hemisphere and triggered an increase in ice melt. This led to the slowdown in ocean circulation and a cooler Greenland.

Both studies suggest buoyant meltwater from melting glaciers would have interrupted the sinking of the AMOC and its return to the tropics along the bottom of the ocean. But the new research suggests solar activity is the main driver behind the changes to the ocean circulation pattern. “I’m open-minded that the real answer is more complicated, and it may be a combination of the two hypotheses,” said Mann. “This article paves the way for a more in-depth look at what is going on. The challenge now will be teasing apart the two effects and trying to assess the relative importance of both of them.” Kobashi contends that solar activity explains the change in ocean circulation and Greenland warming since 1995, which he says cannot be explained by increasing greenhouse gases alone.

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Hier der Abstract der beprochenen Arbeit von Kobashi et al., die im Juli 2015 in den Geophysical Research Letters erschien:

weiter lesenNeue Studie dokumentiert bedeutenden Zusammenhang zwischen Sonnenaktivität und Temperaturen in Grönland

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:

weiter lesenSonne macht Klima: Neues aus Indien

Schwankungen der Sonnenaktivität stellen einen bedeutenden Klimafaktor in China dar wie eine Vielzahl von Studien zeigt. Dies gilt auch für Tibet. Im November 2013 publizierte eine Forschergruppe um Yuxin He in den Quaternary Science Reviews eine Untersuchung zur Klimageschichte der letzten zweieinhalb Jahrtausende auf dem nördlichen Tibetplateau. He und Kollegen fanden starke natürliche Schwankungen, die gut mit der solaren Aktivitätsentwicklung korrelierten. Hier der Abstract:

Late Holocene coupled moisture and temperature changes on the northern Tibetan Plateau
The northern Tibetan Plateau involves complex interactions between the mid-latitude westerly circulation and the subtropical Asia monsoon circulation, acting as a bridge communicating high and low latitude climatic processes. Previous studies from the region suggest relatively wet conditions in cold periods during the late Holocene, for instance, the Little Ice Age (LIA). However, the inference of such temperature-moisture association is subject to the large uncertainty in lacustrine ¹⁴C chronology, due to the particularly large lake reservoir effect in the region. Here we take a different approach by reconstructing paired temperature and moisture records from the same sediment cores to assess the temperature-moisture association, independent of chronology uncertainty. We use alkenone indices U^K′₃₇ and %C_37:4 to reconstruct high resolution temperature and moisture changes simultaneously from two lakes in the Qaidam Basin, northern Tibetan Plateau, over the last ∼2500 years. Characterized by marked climatic variability, our paired records confirm the warm-dry and cold-wet association in arid northwestern China during the late Holocene, opposite to the warm–wet and cold–dry association in subtropical Asian monsoonal regions. Our moisture records further suggest substantially drier conditions during the Medieval Warm Period (MWP) than the current warm period. Lastly, the temperature and moisture changes inferred from our records can be well correlated with solar irradiance changes, suggesting a possible link between solar forcing and natural climate variability during the late Holocene on the northern Tibetan Plateau.

Im April 2014 fügte ein Wissenschaftlerteam um Hai Xu eine weitere Studie zum Tibetplateau hinzu, die in den Quaternary Science Reviews erschien. Mithilfe von Isotopenuntersuchungen an Muschelkrebsschalen rekonstruierten die Forscher die Temperaturgeschichte und fanden erneut einen starken Bezug zu Sonnenaktivitätsschwankungen. Hier die Kurzfassung:

Decadal/multi-decadal temperature discrepancies along the eastern margin of the Tibetan Plateau
Knowledge of the synchronicity and discrepancy of temperature variations along the Eastern margin of the Tibetan Plateau (ETP) is critical in understanding the driving forcing of regional temperature variations. In this study, we established δ¹⁵N timeseries in organic matter and δ¹³C timeseries in ostracod shells from sediments of Lake Lugu and attributed their variations to decadal/multi-decadal temperature variations. We compared temperature variations along the ETP transect during the past four centuries based on our presently developed and previously developed temperature proxy indices, as well as temperature variations reconstructed by other researchers. We found that: (1) Over the north ETP area (N-ETP), the decadal/multi-decadal variations in temperature correlate well with each other. (2) Over the south ETP area (S-ETP), temperature variations correlate not so well with each other; while those at south to west portion of the Tibetan Plateau are rather local. (3) The decadal variations in temperature are generally synchronous with those in precipitation over the N-ETP area, and they are broadly anti-phase/out-of-phase with the corresponding ones over the S-ETP area. (4) The long term temperature and precipitation trends are coupling over the N-ETP but decoupling over the S-ETP. We speculate that because the N-ETP is located at the frontier of the Asian summer monsoon (ASM) region, temperature variations there are not as strongly influenced by the ASM; they are most likely dominated by changes in solar activities, and show general similarity to the average of the Northern Hemisphere. Over the S-ETP area, decadal temperature variations are obviously influenced by precipitation. Because the decadal/multi-decadal precipitation variations are anti-phase and/or out-of-phase between the N-ETP and S-ETP, the decadal/multi-decadal temperature variations between these two regions are also anti-phase and/or out-of-phase.

Und noch einmal Tibetplateau. Ein Forscherteam um Xiumei Li beschrieb im Juni 2015 in The Holocene eine Klimarekonstruktion für die vergangenen 2000 Jahre mit ausgeprägter solarer Suess-de Vries 200-Jahres-Zyklizität:

Centennial-scale climate variability during the past 2000 years on the central Tibetan Plateau
It is currently suggested that climate change on the northeastern Tibetan Plateau (TP) was influenced alternately by the monsoon and the Westerlies. However, the mechanisms driving Holocene climate change on the TP remain unclear, since the extent of the influence of individual atmospheric circulation systems has not yet been clearly defined because of the shortage of high-quality paleoclimatic records. This is especially true in the central TP, where only a few ice core and paleolimnological records are available. Here, we present a decadal-resolution temperature record from Dagze Co in the central TP for the past 2000 years, based on the unsaturation index of long-chain alkenones, using an updated temperature calibration, and a record of precipitation isotopes from compound-specific isotope ratios of leaf waxes. The centennial-scale variation of the temperature and precipitation isotope records captures well-known climatic events over the past 1000 years, for example, the ‘Little Ice Age’, which was cooler and drier than the ‘Medieval Warm Period’. However, the relationship between temperature and the precipitation isotope records differed during the interval at 2000–1000 cal. yr BP compared to the past 1000 years, probably because of changes in precipitation seasonality and the additional influence of the Westerlies on the central TP. In addition, the temperature records exhibit a prominent 210-year cyclicity, suggesting a possible influence of solar radiation on temperature variability.

Gehen wir nun in den Nordwesten Chinas. Auch hier prägt der solare Suess-de Vries-Zyklus das Klima, wie Tiwari und Rajesh im Mai 2014 in den Geophysical Research Letters dokumentierten. Die Autoren präsentieren eine Rekonstruktion der Niederschläge für die vergangenen 700 Jahre, wobei das Grundwasser im Takte der Sonne schrumpfte und expandierte:

weiter lesenSonne macht Klima: Neues aus China