1.2 What is the climate system?

Our climate is one of the largest (if not the largest) complex open systems that we have.

There are five main components for the current climate:

• atmosphere (the envelope of gases surrounding the Earth), e.g. increasing carbon dioxide (CO₂) concentration and radiative forcing

• hydrosphere (oceans, seas, rivers, freshwater, underground water), e.g. changes in the ocean carbon-sink and changes in evaporation

• cryosphere (snow, sea-ice, ice-sheets, glaciers and permafrost), e.g. changes in the Earth's reflectivity (albedo) and melting ice caps

• land surface, e.g. desertification and deforestation

• biosphere (all ecosystems and living organisms), e.g. combustion of fossil fuels and CO₂ fertilisation.

These all interact with each other. For any study of climate change over long timescales we would need to add the lithosphere (the crust and upper mantle involved in plate tectonics), which controls the geological part of the carbon cycle, volcanic activity, the distribution of the continents and oceans and major topographic features such as mountains and high plateaux (and hence major oceanic and atmospheric circulation).

We can identify two main influences on the climate:

• internal changes in the interactions between different components of the climate system (the composition of the atmosphere, oceans, etc.) – human activity can exert a profound influence here

• external changes, such as fluctuations in solar radiation and volcanic activity over which humans have no influence.