Our home in space

some remarks on global heating and ocean acidity

Guy Ottewell

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 steeper.


    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 average.
    The carbon-dioxide level is now higher than at any time in the past 800,000 years and probably the past 20,000,000.