For years there has been a discussion about whether global climate change could be slowed down by technical measures - so-called geoengineering. As early as 2006, the Nobel Prize winner Paul Crutzen suggested letting balloons rise into the stratosphere and releasing sulfur dioxide there. That should then intercept some of the sunlight and stop global warming.
The researcher was inspired by the eruption of the Pinatubo volcano in the Philippines in 1991. According to calculations by scientists, around 17 million tons of sulfur dioxide were thrown up to 19 kilometers high into the atmosphere. This reduced solar radiation by five percent, and global temperatures fell by 0.4 degrees. So why not actively use this effect to stop the further rise in temperatures?
Critics point out that the side effects of such a massive intervention in the Earth system cannot be estimated and that the whole thing is a gamble. Among other things, it is to be feared that aerosols introduced into the stratosphere could reopen the ozone hole, which is just closing again.
There have only been small experiments with aerosols here and there, but a comprehensive strategy and the necessary international agreements have not yet been established. With reference to this, a start-up from the USA was recently banned from introducing sulfur dioxide into the atmosphere over Mexico by balloon.
Unexpected effects on the complex physics and chemistry of the atmosphere are not to be feared, however, if the light coming from the sun is not only intercepted there, but beforehand in space, i.e. if you open a cosmic umbrella, so to speak.
Scientists have developed scenarios of how solar radiation on Earth could be reduced by one to two percent with huge, thin foils stretched out in space. It makes sense to station the shade donors in the so-called Lagrange point 1.
This is the point in space on the line between the earth and the sun where the gravitational forces of these two celestial bodies exactly cancel each other out. That means: An object positioned at this point would stay there stably for a long time.
However, the area of such a parasol would have to be huge - many times the area of Germany. Such a project seems theoretically feasible. However, transporting the shielding foils to the Langrange point would incur astronomically high costs. According to the current state of technology, the cosmic parasol does not appear to be a realistic solution.
But now scientists from the Harvard-Smithsonian Center for Astrophysics and the University of Utah are presenting a new idea in the journal "Plos Climate" on how the shading of sunlight can also be achieved without large man-made structures - namely simply with dust.
The researchers are building on knowledge gained from observing cosmic dust clouds. How they deflect and absorb light from stars is widely understood. They present model calculations according to which a dust cloud stationed at Lagrange point 1 could absorb one to two percent of the sunlight on its way to Earth.
The researchers suggest using dust from the lunar surface for this, as transporting it from there to the place of use would be much cheaper in terms of energy and money. "It's fascinating to think about how lunar dust -- formed over the course of four billion years -- could solve a problem on Earth that we created in less than 300 years," says Scott Kenyon of the Center for Astrophysics, who is an author of the study is.
The decisive advantage of the dust concept is that significantly less mass is required to achieve a certain degree of shading than for the stationing of foil screens. "The model calculations show that we can actually block a lot of sunlight with quite a bit of dust," says physicist and lead author of the study Ben Bromley of the University of Utah.
However, there is a catch. The dust does not remain as stable at the Lagrange point as a large, material object. The solar wind - small energetic particles emitted from our central star into space - blows the dust away over time.
So you would need a kind of space station at the Lagrange point that keeps releasing new dust on site. The constantly required replenishment would cause immense costs, even with comparatively small quantities. The researchers therefore present another scenario in which the dust is shot directly from the lunar surface onto a suitable path, on which it arrives precisely at the Lagrange point according to the laws of celestial mechanics.
This option would probably be cheaper to implement. "It's amazing that the Sun, Earth, and Moon are in just the right configuration for this scenario to be possible," says Kenyon.
But that still doesn't mean that this vision can be financed. "We're not experts in the technology that would be needed to transport dust from one place to another," says Bromley, "but we didn't want to fail to present a potential game changer for a critical human problem."
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