Harsche Kritik an Klimamodellen: Kühlwirkung von Schwefeldioxid in Wirklichkeit geringer als in den Simulationen angenommen

Vor fünf Jahren kritisierten wir in unserem Buch “Die kalte Sonne” den sogenannten Aerosol-Joker. Klimamodellierer hatten in ihren Simulationen die Kühlwirkung von Schwebstoffeilchen (Aerosolen) nach Belieben verändert, immer gerade so, wie es gerade gebraucht wurde, um das gewünschte Endresultat in den Modellen zu erzielen.Im Prinzp wurde mit den Aerosolen eine übersteigert angenommene CO2-Erwärmungswirkung neutralisiert. Wenn die Aerosole aber gar nicht so stark kühlen, kann das CO2 auch gar nicht so stark erwärmend wirken. Letztendlich muss dann die CO2-Klimasensitivität entsprechend herunterkorrigiert werden. Das wäre ein großer Schritt, der politisch jedoch große Bauchschmerzen verursachen würde. Beim letzten IPCC-Bericht (AR5) griff man daher zu einem Trick und gab außer einer sehr weiten Spanne für die Klimasensitivität einfach keinen Mittelwert an. Dadurch vertuschte man elegant, dass der beste Schätzwert schon lange nicht mehr bei 3°C Erwärmung pro CO2-Verdopplung liegt, sondern bereits Richtung 2°C abgesackt ist.

Nachdem die Aerosol-Forschung von den Modellierern lange missbraucht wurde, schlagen die Aerosol-Spezialisten nun mit geballter Kraft zurück. Zunächst wagte sich vor 2 Jahren nur das Hamburger Max-Planck-Institut nach vorne, ein mutiger Schritt, Chapeau! Siehe unseren Blogartikel “Direktor des Hamburger Max-Planck-Instituts für Meteorologie: Aerosole kühlen weniger stark als vormals angenommen“. Nun hat sich der Rest der Schwebstoff-Experten dem Protest angeschlossen. Am 22. Juni 2017 veröffentlichte eine 35-köpfige Forschergruppe um Florent Malavelle eine neue Arbeit in Nature. Darin zeigen sie an einem vulkanischen Beispiel, dass die Kühlwirkung von Schwefeldioxid-Aerosolen viel geringer ist, als in den gängigen Klimamodellen angenommen. Hier der Abstract der wichtigen Arbeit:

Strong constraints on aerosol–cloud interactions from volcanic eruptions
Aerosols have a potentially large effect on climate, particularly through their interactions with clouds, but the magnitude of this effect is highly uncertain. Large volcanic eruptions produce sulfur dioxide, which in turn produces aerosols; these eruptions thus represent a natural experiment through which to quantify aerosol–cloud interactions. Here we show that the massive 2014–2015 fissure eruption in Holuhraun, Iceland, reduced the size of liquid cloud droplets—consistent with expectations—but had no discernible effect on other cloud properties. The reduction in droplet size led to cloud brightening and global-mean radiative forcing of around −0.2 watts per square metre for September to October 2014. Changes in cloud amount or cloud liquid water path, however, were undetectable, indicating that these indirect effects, and cloud systems in general, are well buffered against aerosol changes. This result will reduce uncertainties in future climate projections, because we are now able to reject results from climate models with an excessive liquid-water-path response.

In einem populärwissenschaftlichen Begleitbeitrag in Nature erläuterte Bjorn Stevens die Bedeutung der Ergebnisse:

The extent to which aerosols affect climate is highly uncertain. Observations of clouds interacting with aerosols from a volcanic eruption suggest that the effect is much smaller than was once feared.

Der Ball ist nun im Feld der Klimamodellierer. Sie müssen jetzt die Kritik der Aerosol-Experten schlucken und reagieren. Als Folge wird die CO2-Klimasensitivität sinken müssen, soviel steht fest.

Hier noch die dazugehörige Pressemiteilung der University of Exeter vom 21.6.2017:

Role aerosols play in climate change unlocked by spectacular Icelandic volcanic eruption

Cloud systems “well buffered” against aerosol changes in the atmosphere, research shows.

A spectacular six-month Icelandic lava field eruption could provide the crucial key for scientists to unlock the role aerosols play in climate change, through their interactions with clouds. An international team of climate scientists, led by the University of Exeter, have meticulously studied the effects that the 2014-15 eruption at Holuhraun, in Iceland had on cloud formations in the surrounding region. They found that the 2014-15 Holuhraun fissure eruption, the largest since Laki which erupted for eight months in 1783-4, emitted sulphur dioxide at a higher rate than all 28 European countries added together causing a massive plume of sulphate aerosol particles over the North Atlantic.

As would be expected, these aerosols reduced the size of cloud droplets, but contrary to expectations did not increase the amount of water in the clouds. The researchers believe these startling results could significantly reduce uncertainties in future climate projections by outlining the impact of sulphate aerosols formed from human industrial emissions on climate change. The pioneering study is published in leading scientific journal, Nature, on Thursday 22 June. Dr Florent Malavelle, lead author of the study and from the Mathematics department at the University of Exeter said: “The huge volcanic eruption provided the perfect natural experiment in which to calculate the interaction between aerosols and clouds. “We know that aerosols potentially have a large effect on climate, and particularly through their interactions with clouds. However the magnitude of this effect has been uncertain. This study not only gives us the prospect of ending this uncertainty but, more crucially, offers us the chance to reject a number of existing climate models, which means we can predict future climate change far more accurately than ever before.”

Aerosols play a pivotal role in determining the properties of clouds as they act as the nuclei on which water vapour in the atmosphere condenses to form clouds. Sulphate aerosol has long been recognised as the most significant atmospheric aerosol from industrial sources, but other natural sources of sulphate aerosol also exist, including that formed from sulphur dioxide release as a result of volcanic eruptions. The 2014-15 Holuhraun eruption is thought to have emitted between 40,000-100,000 tons of sulphur dioxide every single day during its eruptive phase. Using state-of-the-art climate system models, combined with detailed satellite retrievals supplied by NASA and the Université libre de Bruxelles, the research team were able to study the complex nature of the cloud cover formed as a result of the eruption.

They found that the size of the water droplets produced was reduced, which in turn led to cloud brightening – which results in an increased fraction of incoming sunlight being reflected back into space and, ultimately, providing a cooling effect on the climate. Crucially however, these aerosols had no discernible effect on many other cloud properties, including the amount of liquid water that the clouds hold and the cloud amount. The team believe the research shows that cloud systems are “well buffered” against aerosol changes in the atmosphere.

Professor Jim Haywood, co-author of the paper and from the Met Office and the University of Exeter added: “Explosive and effusive volcanic eruptions are very different. The massive explosive eruption of Pinatubo in 1991, which injected aerosol to altitudes of 25km or more into the stratosphere, has been the go-to event for improving our model simulations of the impact of explosive volcanic eruptions on climate. Now volcanoes have provided a new clue in the climate problem: how aerosols emitted at altitudes similar to those from human emissions impact the climate. Without a doubt, the effusive eruption at Holuhraun will become the go-to study in this regard.” Strong constraints on aerosol-cloud interactions from volcanic eruptions is published in Nature, and received funding from the Natural Environment Research Council.

 

Weitere Blogartikel zum Thema SO2-Aersole hier.