Earth is warmer in northern hemisphere because of a couple of factors, such as the the tilt of the earth, and where it is at in comparison to the sun since Earth doesn't orbit a perfect circle.
The earth's tilt gives us season,and when the northern hemisphere is tilted toward the sun, it makes it summer there, and the placement of earth is close to the sun.
Now, the closer you go to the equator, the hotter you get, because the sun us always hitting the equator.