Global warming refers to the warming of the general climate on earth. Winter is a season like summer, fall or spring. Due to global warming, winter seasons tend to be warmer than previous winters. The same is true for the other seasons of the year. But the seasonal differences are less affected. 
This is because seasons are caused by the earth rotating around an axis which is not parallel to the axis of the solar system. Thus the same area on earth receives a different amount of sunlight depending on which part of it's orbit around the sun the earth currently is. Global warming in turn is caused by greenhouse gases like CO2 or methane. These gases reduce the amount of heat escaping to space by absorbing and reflecting it back to the surface of the earth. This causes the atmosphere to heat up like a greenhouse, hence the naming for the gases.

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