Seasonal variation in carbon uptake, because most of the worlds landmass is in the northern hemisphere. More details from https://en.wikipedia.org/wiki/Keeling_Curve

> The Keeling Curve also shows a cyclic variation of about 6 ppmv each year corresponding to the seasonal change in uptake of CO2 by the world's land vegetation. Most of this vegetation is in the Northern hemisphere where most of the land is located. From a maximum in May, the level decreases during the northern spring and summer as new plant growth takes CO2 out of the atmosphere through photosynthesis. After reaching a minimum in September, the level rises again in the northern fall and winter as plants and leaves die off and decay, releasing CO2 back into the atmosphere.

Biomas growth in spring acts as a carbon sink, then as it dies in fall it releases the carbon back into the air.

The sensor they plot is in Hawaii, here's 2006 from NASA: https://climate.nasa.gov/climate_resources/142/video-super-h...

(I assume it has to do with prevailing wind and weather in Hawaii more than AC)

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.

Heating homes during the winter is more carbon-intensive than cooling homes during the summer.

Even better it seems to spike in May almost every year. I didn't see if it mentioned where these measurements were taken but typically July/August are the hottest months so if it was AC, we should see the spike in those two months, no?