Jülich Researchers Warn Against Geoengineering
Cooling sulphates in the atmosphere enhance ozone depletion
[24. April 2008]
Jülich / Boulder, 24 April 2008 - Global warming could be slowed down by millions of tonnes of sulphates in the atmosphere at an altitude of 10 to 25 kilometres. This measure to counteract the impacts of the greenhouse effect is by no means science fiction, but has been seriously discussed by the Nobel laureate Paul Crutzen. However, together with American colleagues, atmospheric researchers from Jülich now raise a warning voice against the possible consequences of so-called geoengineering since the sulphates would cause serious damage to the ozone layer that provides protection against UV radiation at the poles. Their findings are published in the current issue of Science.
"Our results show that this approach to artificially reduce global warming would involve great risks", says Simone Tilmes, lead author of the study and a climate researcher at the National Center for Atmospheric Research in Boulder (NCAR). Together with her colleagues Rolf Müller from Forschungszentrum Jülich, a member of the Helmholtz Association of National Research Centres, and Ross Salawitch from the University of Maryland, she calculated the extent to which sulphate particles would deplete the ozone in the polar stratosphere at altitudes between 10 and 25 kilometres.
The sulphate particles would chemically change the stratospheric chlorine leading to rapid ozone depletion. This could destroy between one third and one half of the ozone above the Arctic.
Further ozone depletion is hardly possible above the Antarctic since the ozone in the stratosphere has already been completely destroyed there. However, the proposed measure would delay the recovery of the ozone hole that is now slowly beginning by another 30 to 70 years.
In their publication, the scientists compare the artificial injection of sulphur into the atmosphere with the natural injection of sulphates by volcanic eruptions. Basic data were provided by the eruption of Mount Pinatubo on the island of Luzon in the Philippines on 15 June 1991 in which about 10 million tonnes of sulphates were released into the atmosphere. These sulphates were then distributed around the globe and in subsequent years led to perceptible atmospheric cooling, and also enhanced the destruction of polar ozone. "Without the data from the Pinatubo eruption we would not have been able to make our estimates", says the Jülich atmospheric researcher Rolf Müller.
If as proposed by Paul Crutzen in his 2006 scenario a continuous supply of sulphur were to keep 5.3 million tonnes of sulphates in the atmosphere this would effectively block part of the incident sunlight. If the sulphate particles were of the same size as those originating from a volcanic eruption then the ozone above the Arctic would in parallel be depleted by roughly one third. If smaller particles were used with a larger chemically active surface then up to half of the ozone could be lost.
It would be especially hazardous if after an artificial injection of sulphur a major volcanic eruption were to take place. "We would then have to expect even greater, very severe ozone depletion in the stratosphere", says Müller. However, Müller would not completely rule out cooling of the Earth′s atmosphere by sulphate particles. "The impacts on ozone would be significantly lower in the second half of this century." The concentration of chlorofluorocarbons (CFCs) would then be perceptibly lower and the sulphate aerosols would contribute less to enhanced ozone depletion. "Geoengineering could help us to gain time so that the necessary measures can be taken to reduce greenhouse gases", adds Tilmes.
Nevertheless, at the moment geoengineering is not an option for action on global warming. "The possible impacts of geoengineering on ozone and climate must be studied more thoroughly - we are still just at the start of our investigations", says Müller in summary.
The article in Science:
"The sensitivity of polar ozone depletion to proposed geoengineering schemes", Simone Tilmes, Rolf Müller, and Ross Salawitch, Science Express, 10.1126/science.1153966, Science 24 April 2008
Ozone depletion above the Antarctic in October 2002: Section through the ozone layer at an altitude of approx. 24 km. The greatly reduced ozone values in the ozone hole are coloured blue. "Geoengineering" could cause a delay of many decades in the recovery of the ozone hole. Photo: Forschungszentrum Jülich
Strong ozone depletion in the polar vortex above the Arctic in March 1997. The figure shows a section through the stratosphere at an altitude of approx. 18 km. The high (natural) ozone values at the edge of the polar vortex are shown in red, and the strong chemical ozone depletion inside the vortex in green and blue. (Simulation calculations by the Jülich CLaMS model for simulating the stratosphere). Even greater ozone depletion could result from the application of geoengineering. Photo: Forschungszentrum Jülich
The most vigorous eruption of Mt. Etna on the island of Sicily in years in October 2002. Photograph taken from the International Space Station on 30 October 2002. Photo: NASA
A red sunset over Northern Sweden in winter 1992. The red colour of the sky is caused by the sulphate aerosol particle injected into the stratosphere by Mt. Pinatubo in June 1991. Also visible are mother-of-pearl clouds that consist of ice particles. Photo: Fred Podlak
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