Since 2020 the Horn of Africa has experienced prolonged drought periods punctuated by short intense rainfall which can lead to flash flooding, causing acute food insecurity for more than 4mn people. A new report from the World Weather Attribution international academic collaboration finds that not only does human-induced climate change increase the likelihood of these events, it also increases the severity of their impact.
Much of the data that permits investigations such as this, and many others, has been supplied for decades by Earth observation satellites crucial to understanding the components of planet’s climate, their interaction with one another and the effect of factors such as human activity.
About 80 satellites operated by more than 30 agencies criss-cross the skies, largely unnoticed, gathering data on subjects ranging from sea level change to atmospheric carbon dioxide levels.
Combining brute force with precision, the satellites, some SUV-sized, are lofted into orbits hundreds of kilometres up from where they are designed, to provide constant data, often for years.
The Grace satellites, operating for 15 years from 2002 and their successors from 2018, the Grace-FO satellites, are a collaboration between Nasa and the German Aerospace Center.
Their role is to track changes in masses on Earth such as water and ice, exploiting a simple principle: mass exerts gravitational pull and measuring that pull allows the deduction of equivalent mass.
Most observation satellites’ instruments are pointed towards the Earth, but Grace-FO’s instruments are aimed at each other as they follow the same polar orbit, 200 kilometres apart.
A mass such as a glacier or water aquifer will pull the spacecraft as they overfly it, causing distance changes between them, as shown in the sequential diagram below.
The changes are tiny but the onboard microwave system can detect changes one-tenth the width of a human hair.
Grace-FO carries an additional experimental laser distancing system to supply even more accurate measurements. GPS signals then link the distance changes to location of the pull-inducing mass on Earth. Repeated observations build up a dynamic picture of Earth’s mass changes.
The pairs of satellites, nicknamed Tom and Jerry, have observed major developments triggered by climate change during their 21-year chase.
