Fossil fuels also contain less carbon-13 than carbon-12, compared with the atmosphere, because the fuels derive from plants, which preferentially take up the more common carbon-12. The ratio of carbon-13 to carbon-12 in the atmosphere and ocean surface waters is steadily falling, showing that
more carbon-12 is entering the atmosphere.
Finally, claims that volcanoes emit more CO2than human activities are simply not true. In the very distant past, there have been volcanic eruptions so massive that they covered vast areas in lava more than a kilometre thick and appear to have released enough CO2 to warm the planet after the initial cooling caused by the dust (see
Wipeout). But even with such gigantic eruptions, most of subsequent warming may have been due to methane released when lava heated coal deposits, rather than from CO2 from the volcanoes (see also
Did the North Atlantic’s ‘birth’ warm the world?).