Straight from the sourceEarlier this month, our atmosphere crossed an important - and potentially dangerous - milestone: Based on readings from the Mauna Loa Observatory in Hawaii, we reached 400 ppm of CO2 for the first time in about 3 million years.
NASA has posted some reactions from 10 climate scientists on this historic event. As you can probably imagine, the mood is not very celebratory...
You can read all the quotes here.
Here's the two years chart for atmospheric CO2 concentration (notice the seasonal change caused by the fact that there's more landmass in the North half of the planet than the South):