The northern hemisphere has an outsized effect on global temperatures, because there's more land area in the northern hemisphere. As mentioned in other comments

The northern hemisphere has an outsized effect on global temperatures, because there's more land area in the northern hemisphere. As mentioned in other comments, albedo (reflectivity) from sea ice (as well as clouds) reduces solar input into the Earth climate system. But also, more importantly, land ice and snow extent also can have an effect on Earth's average albedo. If less warm water from the tropics is being pulled up towards the Arctic via the AMOC, not only will sea ice form more easily, but temperatures will tend to stay colder in northern latitudes in North America and Europe in particular, which is conducive for longer winters, great chance of standing snow cover, etc. "Crippled Atlantic currents triggered ice age climate change" https://www.sciencemag.org/news/2016/06/crippled-atlantic-currents-triggered-ice-age-climate-change https://www.sciencemag.org/news/2016/06/crippled-atlantic-cu... "What Caused the Younger Dryas Cold Event?" https://pubs.geoscienceworld.org/gsa/geology/article/38/4/383/130267/What-Caused-the-Younger-Dryas-Cold-Event https://pubs.geoscienceworld.org/gsa/geology/article/38/4/38...