(CNN) -   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.   Carbon dioxide changes climate and drives
acidification of the ocean.    The amount of carbon dioxide
varies daily somewhat and has cycled historically in accordance
with changes in the Earth's orbit, a phenomenon known as
Milankovitch cycles. But the exponential rise in carbon dioxide
levels since the Industrial Revolution is far out of the
ordinary, experts say.  The number 400 parts per million is
symbolic of what many scientists believe to be th