How do scientists measure changes in the levels of global atmospheric co2?

a. they have directly measured co2 in the atmosphere since 1758.

b. they measure co2 levels in water trapped deep in earth's crust.

c. they take ice core samples that allow m?

The appropriate answer is c. Scientists get the information from past climates by melting ice cores retrieved from boring the ice and retrieving cores. The air is trapped in the ice as little bubbles and as the ice melts the gases are retrieved. Ice cores are taken from regions such as the Greenland ice sheets. Scientists have tested air samples thousands of years old.

All of the above is your answer.