Climate myths: Human CO2 emissions are too tiny to matter

See all climate myths in our special feature.

Ice cores show that carbon dioxide levels in the atmosphere have remained between 180 and 300 parts per million for the past half-a-million years. In recent centuries, however, CO2 levels have risen sharply, to at least 380 ppm (see Greenhouse gases hit new high)

So what's going on? It is true that human emissions of CO2 are small compared with natural sources. But the fact that CO2 levels have remained steady until very recently shows that natural emissions are usually balanced by natural absorptions. Now slightly more CO2 must be entering the atmosphere than is being soaked up by carbon "sinks".

The consumption of terrestrial vegetation by animals and by microbes (rotting, in other words) emits about 220 gigatonnes of CO2 every year, while respiration by vegetation emits another 220 Gt. These huge amounts are balanced by the 440 Gt of carbon dioxide absorbed from the atmosphere each year as land plants photosynthesise.

Similarly, parts of the oceans release about 330 Gt of CO2 per year, depending on temperature and rates of photosynthesis by phytoplankton, but other parts usually soak up just as much - and are now soaking up slightly more.