Why is earth's surface heated unevenly?
Differences in albedo.
The majority of the heating of the Earth's surface is from the sun. Albedo, or reflectiveness, is different for different surfaces. As a result a black asphalt parking lot is going to absorb a lot more sunlight than an ice covered pond. Since the different surfaces of the Earth absorb different amounts of energy they will heat unevenly.
In addition to albedo there is also the angle that the sun rays strike the Earth. Since the Earth is spherical, sunlight will strike some points much more directly than others. The more direct the angle the sunlight hits the Earth, the more energy the Earth gets for a given area.