While humans emit excess carbon into the atmosphere every year, not all of it stays there. Some is absorbed by the ocean, and some by the land and biosphere such that the amount of carbon dioxide in the atmosphere goes up by less than humans emit:
There are plausible reasons to worry about this carbon sink becoming less effective in the future:
- a warming Arctic will melt peat in permafrost which could oxidize and release carbon.
- increasing drought will cause more forest fires which release carbon from forests into the atmosphere.
- humans clear more forests which could cause soils to release carbon.
So far, although these things are happening to varying degrees, they have not resulted in an overall reduction in the sink. To track this, I like to keep tabs on the fraction of carbon emissions reabsorbed each year. This is based on the Mauna Loa carbon data as well as BP's estimates of carbon emissions. My detailed methodology was described here. Since BP came out with 2011 numbers a couple of months back, I decided to update my graph, which came out as follows:
You can see that 2011 was a slightly better than average year, and that overall there is no sign of reduction in the carbon sink at the moment - in fact it's been gradually increasing as a fraction of emissions. If you try to fit a quadratic to the data above, it perfectly reproduces the straight line. The bending down one might worry about is not evident at present.