The uneven heating of the Earth's surface is caused by the sun. This occurs because the Earth is a tilted sphere and the sun does not heat all its surfaces equally and because the sun's heat penetrates land and water at different rates.
The Earth rotates on its tilted axis, meaning that at any given time, half the Earth is having day while the other half is having night. Because it is a sphere, the equator, where it is the widest, heats more evenly than it does at the poles. The Earth is more than 70 percent water, and water and land heat and cool at different rates, causing uneven heating.