Radiative forcing
Different factors make average temperature on earth rise or fall. There are positive and negative radiative forcing factors. The main forcing factors are solar input, albedo, greenhouse gases, grading of earth's axis, particles (aerosols), Milankovich cycluses.

These forcing factors influence other factors impacting climate, mainly in feedback systems. Such factors can be sea currents and variations like El Nino Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO), again impacting on albedo and greenhouse gases.

  1. Of the three main factors: sun, albedo and atmospheric chemistry, the only factor that has changed significantly the last 100 years is the atmospheric chemistry.
    Carbon dioxide (CO2) has increased by more than 30%, from 280 ppm to 390 ppm.
    Methane has more than doubled, rising from 715 ppb (parts per billion) to around 1800 ppb.
    Nitrogen oxide (N2O) has increased from 270 ppb to over 320 ppb.

    Graphic: UNEP/GRID-Arendal: Vital Climate Graphics, 2001

  1. Radiative forcing is the change in the balance between radiation coming into the atmosphere and radiation going out.
    A positive radiative forcing tends on average to warm the surface of the Earth, and negative forcing tends on average to cool the surface.

    The current net solar radiation has been calculated to 168 Watts per square meter on the Earth's surface. If this figure is reduced, we experience a negative radiative forcing. If the figure increases, the radiative forcing is positive.

    Currently the balance of radiative forcing is positive, giving a net average extra energy corresponding to around 2 watt per every square meter of the Earth. (Just to relate this to something familiar: one small christmas tree light yields 1 watt of energy.)

    For the time being this extra energy entails an average annual global temperature rise of 0.02 - 0.03 °C.

    Graphic: UNEP/GRID-Arendal: Vital Climate Graphics, 2001