The concentration of carbon dioxide in earth's atmosphere is
poised to pass 400 parts per million. That might not sound like a
lot, but it's nearly half as much again as pre-industrial levels,
and higher than it's been for hundreds of thousands of years.
We asked scientists what the number means, and what we should
take from the fact that this once distant prospect is now just
around the corner.
On the side of a Hawaiian volcano sits the Mauna Loa
Observatory, where scientists have been measuring levels of
carbon dioxide in the atmosphere since the 1950's.
Other observatories around the world monitor carbon dioxide too.
But the emblematic Keeling curve - named after C. David Keeling who
started the measurements in 1958 - is the longest standing
On May 1st, the instruments at Mauna Loa were recording 399.39
parts per million of carbon dioxide in the atmosphere -
and media reports have suggested that we're
about to pass the 400 ppm mark. Averaged over a
month, the values are a bit lower. The latest monthly average for
March is 397.34
Will it be this year?
Plants and trees absorb carbon dioxide as they grow. This means
concentrations of the gas rise and fall with the seasons, reaching
a peak in May just before summer in the northern hemisphere.