Current climate change is slowly reducing the amount of oxygen in the air.
Billions of tonnes of carbon have been released from fossil fuels - coal, oil, and gas.
The carbon joins up with atmospheric oxygen when it burns.
Each carbon atom joins with two oxygen atoms to make carbon dioxide.
Several scientific organisations measure the gases in the air.
One major set of measurements are from a laboratory in Hawaii.
This is all extra evidence that extra carbon dioxide in the air comes from burning fossil fuels.