Currently more than 8,000 satellites are orbiting at altitudes of between 300 and 1000 km in the Earth's upper atmosphere – also called the thermosphere. While changes in space weather, such as coronal mass ejections and solar flares, can cause temporary changes in the density of this region, scientists at the University of Birmingham suggest the effects caused by global warming are likely to be much longer term.
This is because of the effects caused by greenhouse gases (GHGs) in the Earth's upper atmosphere. In the lower atmosphere, GHGs are trapped by the atmosphere, and they retain infrared radiation from the Earth's surface, leading to an overall warming effect. But this effect also pulls heat, via conduction, from the upper atmosphere. And as this region cools down, it also contracts, leaving satellites orbiting in areas where the atmosphere is less dense.
This change in density means less of an atmospheric drag on satellites – and that has important implications for their life expectancy in space. Currently, this friction is useful because it reduces the orbital energy of satellites, causing them to sink back down to Earth after several years.
Without it, satellites have the potential to stay in space for longer, increasing the chances of collision, and producing more space debris – as well as leaving less space for new satellites to be launched.
The findings, published today (10 Mar) in Nature Sustainability, provide a clear link between ground-level sustainability and space sustainability.
Lead researcher Matthew Brown, from the SERENE research group, said: "Often we think only about the ground-, or sea-level impacts of climate change, but this research demonstrates that the impacts will reach as high as space.
"The numbers of satellites in low Earth orbit is rapidly expanding and we rely heavily on them for communications, Earth observation, weather forecasting and navigation. For this reason, we need to take the long-term sustainability of space very seriously."
Dr Brown added: "Discussions are already underway about how many satellites can be put into space at any one time, as more and more instruments are being launched into low Earth orbit. Without checking this proliferation, we are in danger of entering a 'Kessler syndrome', where a chain reaction of collisions causes space to become unusable.
"While technology can help these objects avoid collision, recognising the influence that the Earth's natural environment has on our ability to operate in space is becoming increasingly important. A coordinated approach to reducing greenhouse gas emissions would ensure we can prevent the exploitation of the thermosphere and protect it for future generations."
