Why is the southern hemisphere warmer than the northern?
The southern hemisphere is warmer than the northern hemisphere because more of its surface area is water.
Water has a high specific heat capacity so it loses heat slowly. Most of the area of the southern hemisphere is ocean. The oceans warm up in the summer and retain heat during the winter.
The northern hemisphere has much more land mass which loses its heat quickly. This is demonstrated by the continental effect where the central regions of the northern continents have very cold winters.
The Earth is at perihelion around 3 January and at aphelion around 3 July. The heat retained in the southern hemisphere oceans makes the average temperature of the Earth a few degrees higher in July when Earth is furthest from the Sun than it is in January when it is its closest.