On a more serious note, the reason for this is most likely the tropical forest absorbing the CO2. The truly worrying part is the fact that we are now way past the 400ppm threshold, maybe permanently, set by scientists as the point of no return.
A workmate of mine, who actually did climate modelling as part of his PhD, made me aware to also keep an eye on Methane, which is a much more potent green house gas with devastating impact on marine life. It is mainly produced by life stock. The upside about this is that if we stop emitting it will relatively quickly leave the atmosphere.