re: Hemispheres
Maybe Antarctica and the Arctic Ocean are factors? There is getting to be more and more open water in the Arctic in the summer, allowing more absorption/less reflection than ice. Antarctica, even if it were to lose a lot of ice, would still be a huge ice-covered body, reflecting a lot.
Around here, you can tell when there is still a good snowpack in the mountains in late spring because, even though there might be lots of sunshine and warm surface temperatures, the air stays cool. When most of the snowpack is gone, then we start to get hot weather.