There are three main factors deciding the climate of a planet:
- Solar input. The total solar influx, depending on distance from the sun, angle of the planet's axis and solar activity
- Albedo - or reflections of solar rays from the Earth and back into space
- Chemical composition of the atmosphere
There are several other factors and mechanisms having impacts as well, such as changes in ocean currents and maybe cosmic radiation, but as a rule they are not the primary forcing factors, but rather parts of feedback systems that enforce the trend in either cooling or warming the planet. As a rule the chemical composition of the atmosphere acts as a feedback system to the two other factors, but there are exceptions to this.
Any change in the climate (not the weather!) can only be explained by a change in the radiative forcing from these three factors.
If there has been no change in the solar forcing, the change must find its explanation in one or both of the other two factors.