The Greenhouse Gases
The greenhouse gases are components of the atmosphere that can 'absorb' and release infrared rays (heat energy). These gases have three or more atoms in their molecules. This in contrast to the gases making up more than 99% of the atmosphere: nitrogen, oxygen and argon having only two atoms in their molecules.
  1. Water
    The most important greenhouse gas is water vapor, which at times can approach 2% in parts of the atmosphere.
    However, when water vapor condenses into clouds, the white upside reflects sunlight back into space, which has a negative forcing effect.
    Water vapor cannot initiate radiative forcing, it can only respond to radiative forcing through feedback mechanisms.

    Water vapor in the air is fairly constant, and human activities have little impact on this.
    (Except maybe for jet aeroplanes leaving condense stripes -jet contrails- at high altitudes.)

    Since humans have little or no direct impact on the amount of water in the air, and since it is not a forcing factor, water vapor is not included in negotiations on the reduction of emissions of greenhouse gases.

    With increasing temperatures there will be increasing evaporation and increased amounts of water vapor in the air.
    In addition to enhancing the greenhouse effect it means that rainstorms can become more violent, as more water has to come down.

    The fact that clouds have a warming as well as cooling effect, is confusing to some, and frequently used by people who want to bring doubt on climate research. "These researchers can't even manage to make up their minds whether clouds are cooling or warming! How can we trust in people like that?".

    It is of course somewhat problematic that science as a rule does not give 'black or white' - answers.
    Media, propaganda- and PR companies have an easier time there.
  1. Carbondioxide (CO2)
    Carbon dioxide is the gas that almost all living creatures breathe out.
    Green vegetation absorbs the CO2 during daylight hours, assimilating it in the process called photosynthesis.
    At the same time the plants release the gas oxygen (O2 ), which animals and humans breathe in.
    Without oxygen we would suffocate.
    The photosynthesis is the basis for all higher forms of life on Earth, as it in addition to oxygen produces glucose, a simple sugar that is the basis for more complex compounds building up the plants.
    Without plants, animals would no longer have food.
    Increasing amounts of CO2 in the air may have a fertilizing effect on many plants if other essential requirements for photosynthesis are present. (Water, soil, minerals, right temperature etc)

  1. Carbon everywhere - but in different cycles
    The global carbon cycle shows the carbon reservoirs in GtC (gigatonne= one thousand million tonnes) and fluxes in GtC/year.
    The indicated figures are annual averages over the period 1980 to 1989.
    Current fossil fuel emissions are higher than indicated here.
    The component cycles are simplified and the figures present average values.
    The riverine flux, particularly the anthropogenic portion, is currently very poorly quantified and is not shown here. Evidence is accumulating that many of the fluxes can fluctuate significantly from year to year.
    In contrast to the static view conveyed in figures like this one, the carbon system is dynamic and coupled to the climate system on seasonal, interannual and decadal timescales.
    As seen from the figure, there is a lot of carbon stored as coal in the soil, and as CO2, carbonates and other compounds in the deep oceans.
    As can be seen, there are mainly three carbon sinks:
    1) The oceans
    2) The soil, and
    3) Forests and vegetation.

    Graphic by UNEP/GRID-Arendal:
    Climate graphics library (UNEP/GRID-Arendal

    Read more:
    Carbon, carbon everywhere (UNEP/GRID-Arendal
    Center for International Climate and Environmental Research - Oslo (CICERO)
    Fact sheets about climate change (CICERO)