Nature balanced CO² levels under 300 (ppm) for thousans of years. Therefore CO2free society considers 300 the safety level.
Since 1950 CO² levels crossed the 300 threshold and started to rise gradually, at equal pace as the amount of fossil fuels burnt. Put simply, nature can't clean much more than the CO² output of 1950, all additional output accumulates in the atmosphere, like a bucket filling up.
In 2014 the 400 ppm threshold was registered all over the world, in 2019 the 415 threshold is crossed.
The conclusion is simple, this change of CO² levels is caused by humanity burning fossil fuels. And we are doing worse every year, adding extra CO² faster and faster, which is leading to dramatic climate changes.