It's a problem of fossil fuel combustion [1], not capitalism. The per-capita CO2 emissions of the USSR were also above the presently sustainable level for Earth

It's a problem of fossil fuel combustion [1], not capitalism. The per-capita CO2 emissions of the USSR were also above the presently sustainable level for Earth's 7.8 billion people. [1] And land use changes, larger herds of ruminant food animals, high altitude airplane contrails... But mostly fossil fuel combustion.