We passed the milestone of 400 parts per million CO₂ in the air in May of:

Name: We passed the milestone of 400 parts per million CO 2 in the air in May of:
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The milestone of 400 parts per million of CO2 in the air was passed in May of 2013. This was a significant indicator of the ongoing increase in atmospheric carbon dioxide levels due to human activities such as the burning of fossil fuels.

We Passed the Milestone of 400 Parts Per Million CO2

We Passed the Milestone of 400 Parts Per Million CO₂