Dear Judith,
The ocean-atmosphere carbon cycle behaves as a simple linear feedback system for disturbances, as it did over the past 800,000 years and still does
What the mass balance shows is that nature is a net sink for CO2 over the past 55 years. That is certain. 55 years more sink than source.
That implies that it is near impossible that nature is the main source of the increase in the atmosphere.
There is one and only one possibility that the natural cycle may be the main cause: that is if the natural cycle increased a fourfold in exact lockstep of timing with the fourfold increase of human emissions in the past 55 years. If it was a threefold or fivefold there wouldn’t be a fourfold increase in the atmosphere and net sink rate as is observed. Or, alternatively there was no increase in the natural cycles at all.
(I will work that out in response to Bart’s weird example)
Thus we need another observation to be sure that the natural cycle was constant or increased in lockstep with human emissions.
There are two observations which can show the difference:
– The δ13C level. Substantial increases in natural cycles would either increase the δ13C level in the atmosphere (oceans) or cause an extreme decrease in δ13C level (vegetation). Neither is observed: only a firm decrease in δ13C level in direct ratio to human emissions. Further, vegetation is a net sink for CO2, as is proven by the oxygen balance, thus not the cause of the δ13C decline.
– The residence time. Any substantial increase in any huge natural cycle would reduce the residence time of a CO2 molecule (whatever its origin) in the atmosphere. The newer calculations of residence time show that it is getting slightly longer. Thus a rather stable throughput in an increasing CO2 content of the atmosphere.
The observations show that there is no substantial increase in any of the huge natural cycles and therefore the mass balance is unambiguous that humans are the cause of the increase.
