By Elizabeth Landau, CNN
In some ways, it's just a number, but it's a big number with enormous implications.
For the first time, scientists measured an average concentration of atmospheric carbon dioxide of 400 parts per million in Mauna Loa, Hawaii, where the National Oceanic and Atmospheric Administration observatory is located, on Thursday.
"Most experts that really study CO2 amounts estimate that we haven't seen that amount of CO2 in our atmosphere in about 3 million years," said J. Marshall Shepherd, climate change expert and professor at the University of Georgia. In other words, modern humans have never seen carbon dioxide in these proportions before.