Don't forget that while it's summer in your hemisphere, it's winter in the other one. I believe this has to do with the fact that there's land in the northern hemisphere and hence more biomass. I'm not sure if it has more to do with photosynthesis or cellular respiration. My guess is that the strongly seasonal effect is more photosynthesis in the summer, with the differential effect being stronger for the N hemisphere. I'm guessing respiration is less seasonal, and anthropogenic input is not very seasonal. I took classes about this a decade ago and I remember asking this question at the time, but don't remember the exact answer.
