Three factors
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.

  1. The output from the sun varies.
    There is a regular fluctuation in the number of sunspots; darker areas with lower temperature.
    Solar energy output increases with the number of sunspots, since these spots are balanced by development of bright areas; facuale . The heating effect of these outweigh the cooling effect of sunspots.

    The energy output from the sun as a rule rises and falls in line with an 11-year sunspot cycle.
    (There are other cycles as well. There is e.g. a 22-year cycle that may be linked to development of magnetic fields associated with sunspots and that may affect cosmic rays penetration effect and cloud formation.)
    (Source: WMO: Climate into the 21st Century, p.90 ).

    Last time there was a high sunspot activity was in 1998.
    Since 2003 sunspot activity has been unusually low.
    If the sun follows the usual 11-year cycle, sunspot activity should increase again around 2011 to 2014.
  1. The Earth's atmosphere mainly consists of Nitrogen, Oxygen and inert gases.
    In addition there is water vapor and other greenhouse gases like CO2, methane, nitrous oxides, ozone, fluorides etc.
    When the amount of greenhouse gases (GHG) increases, the greenhouse effect will also increase correspondingly.
  1. Albedo
    Albedo is the ability to reflect sunlight out into outer space.
    Ice and snow, deserts, clouds, aerosols (particles in the air) all reflect sunlight.
    The higher albedo, the more solar energy reflected; the colder.
    When it is getting colder, ice cover expands, thus also increasing the albedo.

    A cooling trend thus enforces itself, as a warming trend likewise enforces itself by causing less albedo and releasing more greenhouse gases, above all, increasing the amount of water vapour in the air.