Global warming by definition is an increase in the average temperature of both land and sea.
To understand how global warming works you must first have an understanding of how the sun, earth, and atmosphere interact:
How the sun, earth, and atmosphere layers interact:
The basic science of global explains that the suns rays enter the atmosphere and warm the Earth. These rays are reflected back out into outer space, but the atmosphere keeps prevents some of these rays from escaping to keep the Earth warm.
Global warming is caused by carbon emissions thickening our atmosphere. As our atmosphere thickens, more and more sun rays are trapped, which increases the temperature of the Earth's land masses and oceans.