Klimamodelle unter Druck: Klimasimulationen errechnen eine viel zu hohe Abkühlung durch Vulkanausbrüche, die mit der realen Entwicklung kaum in Einklang zu bringen ist

Große Vulkanausbrüche kühlen für ein paar Jahre das Erdklima, soviel ist klar und durch empirische Daten gesichert. Aber wie funktioniert der Kühlmechanismus eigentlich? Die University of Washington berichtete am 9. Dezember 2015 über neue Erkenntnisse zum Ablauf der klimatischen Wirkungsweise von Schwefeldioxid:

Iceland volcano’s eruption shows how sulfur particles influence clouds

It has long been suspected that sulfur emissions can brighten clouds. Water droplets tend to clump around particles of sulfuric acid, causing smaller droplets that form brighter, more reflective clouds. But while humans have pumped sulfur into Earth’s atmosphere since the Industrial Revolution, it’s been hard to measure how this affects the clouds above. New University of Washington research uses a huge volcanic eruption in Iceland to measure the change. The new study, to be published in Geophysical Research Letters, a journal of the American Geophysical Union, shows that sulfur emissions do indeed result in smaller cloud droplet size, leading to brighter clouds that reflect significantly more sunlight.„This eruption is a chance to nail down one of the big uncertainties in climate models,“ said first author Daniel McCoy, a UW doctoral student in atmospheric sciences.

The study takes advantage of a unique geologic event. During six months from summer 2014 until early 2015, a crack in the Bardarbunga volcano seeped lava and sulfur gas. This was not one of Iceland’s huge explosive eruptions that fill the skies with ash and shut down airplane routes. Instead it was a long, slow, low-elevation seep of sulfur emissions that produced an amount of lava second only to Laki in the recent history of Iceland eruptions. The UW researchers looked at data for that region recorded by NASA’s MODIS, or Moderate Resolution Imaging Spectroradiometer, instrument to measure the size of droplets in the marine cloud layer. While the volcano was spewing sulfur, the droplets were the smallest in the 14-year record of observations. „You can see the effect over an entire ocean for a two-month period,“ McCoy said. „It was a pretty unique geophysical event within the satellite record.“

The results confirm that volcanoes cool the planet not just by emitting particles high in the atmosphere, but also by releasing low-level sulfur to influence cloud formation. When the air contains aerosol particles, the same amount of water vapor condenses into many small drops, whose larger surface area reflects more sunlight. The difference in reflected solar radiation for September and October 2014 was 2 watts per square meter in the region over Iceland. „The effect of volcanic emissions on clouds has been a difficult one to quantify because of the ephemeral nature of most events,“ said co-author Dennis Hartmann, a UW professor of atmospheric sciences. „This eruption provides a natural laboratory that lets us test how clouds respond to aerosols.“

The results may help understand humans‘ impact on clouds. Human pollution since the Industrial Revolution is believed to have altered skies in the Northern Hemisphere. One uncertainty in climate models is how much human pollution has brightened the clouds, shielding the planet from the effects of the simultaneous rise in carbon dioxide. „One of the big uncertainties regarding climate change is how much human-produced aerosols have offset the warming until now,“ Hartmann said. „We hope the data from this eruption will improve the model simulations of cloud effects, and narrow the uncertainties in projections of the future.“

The most recent Intergovernmental Panel on Climate Change report was the first to include a chapter on clouds and aerosols, one of the biggest uncertainties in global climate models. This study will provide a benchmark for modelers to check their simulations of clouds and aerosols and improve their algorithms for the next generation of climate models. „The same way that the Mount Pinatubo eruption in 1991 was a big on-off signal that allowed us to evaluate models‘ response to volcanic forcings, I think this Iceland eruption is a unique event that will help us to better understand the interaction between aerosols and clouds,“ McCoy said.

Paper: Daniel T. McCoy, Dennis L. Hartmann. Observations of a substantial cloud aerosol indirect effect during the 2014-2015 Bárðarbunga-Veiðivötn fissure eruption in Iceland. Geophysical Research Letters, 2015; DOI: 10.1002/2015GL067070

Eine weitere Fallstudie wurde im Juni 2016 in Climate of the Past von einem Team um Rudolf Brazdil publiziert. Studienobjekt ist der Tambora-Ausbruch im April 1815 und die klimatischen Folgen für die Tschechische Republik. Die Forscher fanden in den historischen Quellen Berichte über einen sehr feuchten Sommer 1815 und einen extrem kalten Sommer 1816. Hier der Abstract:

Climatic effects and impacts of the 1815 eruption of Mount Tambora in the Czech Lands
The eruption of Mount Tambora in Indonesia in 1815 was one of the most powerful of its kind in recorded history. This contribution addresses climatic responses to it, the post-eruption weather, and its impacts on human life in the Czech Lands. The climatic effects are evaluated in terms of air temperature and precipitation on the basis of long-term homogenised series from the Prague-Klementinum and Brno meteorological stations, and mean Czech series in the short term (1810–1820) and long term (1800–2010). This analysis is complemented by other climatic and environmental data derived from rich documentary evidence. Czech documentary sources make no direct mention of the Tambora eruption, neither do they relate any particular weather phenomena to it, but they record an extremely wet summer for 1815 and an extremely cold summer for 1816 (the „Year Without a Summer“) that contributed to bad grain harvests and widespread grain price increases in 1817. Possible reasons for the cold summers in the first decade of the 19th century reflected in the contemporary press included comets, sunspot activity, long-term cooling and finally – as late as 1817 – earthquakes with volcanic eruptions.

In einer Studie zum vulkanischen Einfluss auf das Klima im tropischen Südamerika berichteten Colose et al. 2016 über jahreszeitliche Unterschiede. Es mache durchaus einen klimatischen Unterschied, ob ein Vulkan in der Monsun-Saison ausbricht oder während der Trockensaison.

Eine Gruppe um Markus Stoffel machte sich am 31. August 2015 in Nature Geoscience Gedanken über ein großes Rätsel in den Klimawissenschaften. Klimamodelle simulieren den vulkanischen Kühleffekt viel stärker als er in der Realität ist. In den historischen Rekonstruktionen über Baumringdaten sucht man vergeblich nach den massiven und langanhaltenden Temperaturstürzen, die die Klimarechenkästen postulieren. Zwischen Realität und virtueller Klimawelt klafft eine enorme Schere. Nun ja, Stoffel und Kollegen machten sich auf die Suche, um die beiden Pole anzunähern.

Dabei „massierten“ sie zum einen die Baumringrekonstruktionen ein wenig, um ein bisschen mehr Abkühlung zu erhalten. Zum anderen modifizierten sie die Klimamodelle, so dass die stark übertriebene Kühlwirkung etwas abgemildert wurde. Ein diplomatisches Vorgehen. Man muss abwarten, ob sich die Klimamodellierer die Kritik zu herzen nehmen und endlich realistischer in ihren Klimaparametern werden. Die Zeiten, in denen die gesamte Kleine Eiszeit irrigerweise durch eine handvoll Vulkanausbrüche erklärt wurde, scheinen jedenfalls endgültig vorbei zu sein. Im Folgenden die Kurzfassung der Arbeit von Stoffel et al. 2015:

Estimates of volcanic-induced cooling in the Northern Hemisphere over the past 1,500 years
Explosive volcanism can alter global climate, and hence trigger economic, political and demographic change1, 2. The climatic impact of the largest volcanic events has been assessed in numerous modelling studies and tree-ring-based hemispheric temperature reconstructions3, 4, 5, 6. However, volcanic surface cooling derived from climate model simulations is systematically much stronger than the cooling seen in tree-ring-based proxies, suggesting that the proxies underestimate cooling7, 8; and/or the modelled forcing is unrealistically high9. Here, we present summer temperature reconstructions for the Northern Hemisphere from tree-ring width and maximum latewood density over the past 1,500 years. We also simulate the climate effects of two large eruptions, in AD 1257 and 1815, using a climate model that accounts explicitly for self-limiting aerosol microphysical processes3, 10. Our tree-ring reconstructions show greater cooling than reconstructions with lower spatial coverage and based on tree-ring width alone, whereas our simulations show less cooling than previous simulations relying on poorly constrained eruption seasons and excluding nonlinear aerosol microphysics. Our tree-ring reconstructions and climate simulations are in agreement, with a mean Northern Hemisphere extra-tropical summer cooling over land of 0.8 to 1.3 °C for these eruptions. This reconciliation of proxy and model evidence paves the way to improved assessment of the role of both past and future volcanism in climate forcing.

Zum Abschluß noch eine Studie von Swindles et al. 2011 zu vulkanischen Aschelagen im nördlichen Europa während der vergangenen 7000 Jahre. Während der letzten 1000 Jahre lagerte sich etwa alle 56 Jahre eine Ascheschicht in Nordeuropa ab. Hier der Abstract:

A 7000 yr perspective on volcanic ash clouds affecting northern Europe
The ash cloud resulting from the A.D. 2010 eruption of Eyjafjallajökull in Iceland caused severe disruption to air travel across Europe, but as a geological event it is not unprecedented. Analysis of peats and lake sediments from northern Europe has revealed the presence of microscopic layers of Icelandic volcanic ash (tephra). These sedimentary records, together with historical records of Holocene ash falls, demonstrate that Icelandic volcanoes have generated substantial ash clouds that reached northern Europe many times. Here we present the first comprehensive compilation of sedimentary and historical records of ash-fall events in northern Europe, spanning the past 7000 yr. Ash-fall events appear to have been more frequent in the past 1500 yr. It is unclear whether this reflects a true increase in eruption frequency or dispersal, or is an artifact of the records or the way in which they have been generated. In the past 1000 yr, volcanic ash clouds reached northern Europe with a mean return interval of 56 ± 9 yr (the range of return intervals is between 6 and 115 yr). Probabilistic modeling using the ash records for the last millennium indicates that for any 10 yr period there is a 16% probability of a tephra fallout event in northern Europe. These values must be considered as conservative estimates due to the nature of tephra capture and preservation in the sedimentary record.