That is because it had a sudden start at the beginning of the twentieth century but there was no concurrent increase of carbon dioxide in the air. (tamino.wordpress.com)
The problem is the rate of increase of carbon dioxide levels, because the ocean does not have time to transfer it from the surface to the deep ocean. (realclimate.org)
The real concern lies with increase of both carbon dioxide emissions and temperature that can not be predicted using the models of past climate fluctuations and patterns. (hubpages.com)