Luckily, nature helps mankind. During the past century, man injected billions of tons of carbon into the atmosphere but only some of it – 43% – really contributed to increasing the greenhouse effect. The rest was absorbed by oceans, forests and soil. Carbon is said to be trapped, stocked or even held as it does not – provisionally – return into the atmosphere. It therefore does not influence the climate. This is why forests and oceans are also referred to as “carbon sinks”.
Trees and vegetation usually absorb CO2 from the atmosphere through photosynthesis. They turn this gas into living matter and release oxygen: this is the carbon cycle. But oceans are the main natural carbon sinks. Here again, the chain starts with photosynthesis. Algae use it in the same way as terrestrial vegetation and some species of phytoplankton turn it into limestone to build their skeletons.
It is still difficult to estimate the amount of CO2 these natural sinks store. Taking them into account in measures against global warming is also very delicate. However, this is an important issue as terrestrial systems – that is to say, forests, as well as soil, meadows, wetlands etc – store three times the amount of carbon in the atmosphere.
But this natural mechanism is almost at breaking point. On the one hand, the temperature increase is slowing photosynthesis down. The 2003 European heat wave thus blocked it momentarily. On the other hand, in oceans, the temperature increase and acidification might disrupt biodiversity and absorption capacity. The sinks could become “sources”. This means that they could emit some of the carbon they have stocked and therefore worsen the phenomenon they previously regulated.