"The estimate is that about 50% of total global fossil fuel emissions over the past 100 years have been absorbed by the land and oceans. If the sinks are exhausted or overwhelmed or shallow marine sediment outgassing and permafrost melting occurs, it is possible that, in the worst case scenario, a 50% reduction in the use of fossil fuels (not that there is a realistic strategy to achieve this) would have no effect on the growth rate of atmospheric CO2 (see here). This is how serious the situation is."
http://www.flassbeck-economics.com/climate-change-migration-capitalism-solutions-for-systemic-failure-part-1-systemic-failures/