Billions of phones fall out of use every year. Most disappear in a drawer.
Google Research's answer: rack them and run a cloud on them.
The research: 10 used Pixel 3As vs an AWS server, same workload, 3 years:
-up to 18.9x less carbon
-$1,027 vs $40,404
~80% of a phone's lifetime CO2 is emitted before you open the box. That carbon is already spent, and the phone retires at a quarter of the life its processor can do. Reusing it doesn't just avoid a server, it displaces a new device.
Acurast has run this at global scale for years: 287K onboarded phones, 175+ countries, almost 1B on-chain transactions.
Full breakdown: