ThinkProgress: The concentration of carbon dioxide, the greenhouse gas that drives climate change, hit 402 parts per million this week - the highest level recorded in at least 800,000 years.
The recordings came from the National Oceanic and Atmospheric Association`s Mauna Loa Observatory in Hawaii, which marked another ominous milestone last May when the 400 ppm threshold was crossed for the first time in recorded history.
Carbon dioxide (CO2) levels spike every spring but this year the threshold was crossed...