Last Saturday the Guardian started publishing the carbon dioxide (CO 2 ) measurement from Mauna Loa, Hawaii. Far from big sources, this is an indicator for the whole northern hemisphere. Each day you will see the numbers change a little. Over the last year they varied between 404 and 415 parts per million (ppm). They were lowest in late summer due to plant photosynthesis and greatest in spring from emissions that accumulated during the winter.

But compare year on year and you will see CO 2 rising as emissions from our fossil fuel burning exceed what natural systems absorb. Globally, about 70% of CO 2 emissions comes from cities. In leafy Egham, west of London, summer CO 2 decreases by day due to plant photosynthesis and then plant respiration releases some of this at night. Along with emissions from fossil fuels, typical summer days show a variation of about 25ppm.