The climate system
Conceptual sketch of the climate systemclimate system
The climate system is the highly complex system consisting of five major components: the atmosphere, the hydrosphere, the cryosphere, the lithosphere and the biosphere, and the interactions between them. The climate system evolves in time under the influence of its own internal dynamics and because of external forcings such as volcanic eruptions, solar variations and anthropogenic forcings such as the changing composition of the atmosphere and land use change., it's spheres and some selected interactions. Image source: IPCC AR4, WG1, FAQ 1.2 2007
When working with climate and the processes involved, the global climate system has to be taken into consideration. While the climate represents the mean state of the atmosphere over a long (decadal to multi-decadal) period of time, the climate system actually involves the atmosphere, the oceans, the soil layer, plants and animals, water and ice. All these components interact while energy and matter flow between them.
The components of the climate system can be described as spheres: atmosphere, lithosphere, biosphere, hydrosphere and cryosphere. Flows between and within the spheres can be seen as cycles. One of these is the carbon cycle, an important cycle due to the central role played by carbon dioxide (CO2) which is the most important greenhouse gas (GHG)greenhouse gas (GHG)
Greenhouse gases are those gaseous constituents of the atmosphere, both natural and anthropogenic, that absorb and emit radiation at specific wavelengths within the spectrum of thermal infrared radiation emitted by the Earth's surface, the atmosphere itself, and by clouds. This property causes the greenhouse effect. Water vapour (H2O), carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4) and ozone (O3) are the primary greenhouse gases in the Earth's atmosphere. Moreover, there are a number of entirely human-made greenhouse gases in the atmosphere, such as the halocarbons and other chlorine- and bromine-containing substances, dealt with under the Montreal Protocol. Beside CO2, N2O and CH4, the Kyoto Protocol deals with the greenhouse gases sulphur hexafluoride (SF6), hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs).. Another essential cycle is the hydrological cycle, following the water in its different states (ice, liquid, gas) from sphere to sphere.
The atmosphere is the most unstable and rapidly changing component of the climate system. It is composed of a wide variety of gases, some of them highly reactive. But it also contains solid and liquid particles, water in its various phases and clouds. The hydrosphere is the component comprising all liquid surface and subterranean water, both fresh and saline. The oceans store and transport a large amount of energy and dissolve and store large quantities of CO2. The cryosphere derives its importance to the climate system from the high reflectivity for solar radiation and the large thermal inertia of its ice. The marine and terrestrial biospheres have a major impact on the atmosphere’s composition. Through the photosynthetic process both marine and terrestrial plants (especially forests) store significant amounts of carbon. Vegetation and soils on the land surface control how energy received from the Sun is returned to the atmosphere.
Because of these cycles and interactions, the complex global climate system responds in a highly non-linear way to changes and forcing factors. The response time or characteristic time-scale of the components of the system varies widely, from days to weeks for the global atmosphere to thousands of years for the deep oceans and the largest ice sheets.
The climate system is an interactive system forced or influenced by various external forcing mechanisms, the most important of which is the Sun; other important forcing agents are greenhouse gases (GHG). To study the processes and analyse the response resulting from various forcings, including the increased content of GHG in the atmosphere, numerical models describing the climate system are needed. These are called climate models.