Which of the following is true regarding atmospheric carbon dioxide levels?

A) They are currently well above the average of the past 600,000 years.
B) They are currently well below the average of the past 600,000 years.
C) Similar deviations from the average have occurred over the last 600,000 years.
D) a and c

Atmospheric carbon dioxide levels are currently well above the average of the past 600,000 years. This is largely due to human activities such as the burning of fossil fuels and deforestation, which have significantly increased CO2 concentrations in the atmosphere. Historical data from ice cores and other sources show that current levels are unprecedented in the recent geological past.