Hundreds of scientists urge Trump to..., page-42

As mjp2 said, the annual decline during northern hemisphere Summer is matched every year by an incline in the winter. It's quite well understood. During the Summer there's lots of plant growth taking up CO2. In the winter the plants die back/drop their leaves and the vast majority of the carbon goes back to CO2 as they decompose. Only a tiny fraction remains solid over the long term.