Carbon dioxide amounts
The natural, or pre-industrial, amount of carbon
dioxide in our atmosphere is about 280 ppm (parts per million).
The amount now is about 390 and is growing at an accelerating rate.
Scientists find, mainly from bubbles
of air trapped in deep levels of the Greenland and Antarctic icecaps,
that the amount was steady at 260-280 ppm over the preceding 10,000
years. Hundreds of thousands of years earlier there were variations
down to about 180 during ice ages and up nearly to 300 between them.
Since the industrial revolution (roughly
1750-1850), atmospheric carbon dioxide has been increasing. It reached
320 in the 1960s; 387 in 2009. And it is now increasing exponentially:
that is, not by an equal addition each year but by a percentage
(like compound interest). The upward slope of the graph is growing
There is a yearly oscillation in the graph,
of a few ppm, with blips in May and dips in October. This is because
there is more land, therefore more trees and other vegetation, in
earth's northern hemisphere; so the removal of carbon dioxide from
the air by leaves reaches a peak when the northern hemisphere's
foliage is thickest, then declines as that foliage withers and the
removal is carried on by the sothern hemisphere' lesser vegetation.
This annual fluctuation is insignificant compared with
the graph's overall upward climb. Though all the world's trees are
doing their best to absorb carbon dioxide, they cannot keep up with
our emission of it.
Atmospheric carbon dioxide isn't evenly
spread: there is more of it near the ground, and in industrial countries,
and in cities, and indoors, where there can be 10 times more than
The carbon-dioxide level is now higher than
at any time in the past 800,000 years and probably the past 20,000,000.