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General concepts

The global climate system can conceptually be divided into several spheres. Often the following spheres are recognised: the atmosphere, the oceans, the lithosphere (the soil layer and the uppermost part of the solid Earth), the biosphere (plants and animals on land, in the soil, and in the oceans), the hydrosphere, and the cryosphere (ice sheets, glaciers, permafrost).

Essential to this conceptual view is how energy and matter flows from one sphere to another to create cycles. For example, the hydrosphere is also the hydrological cycle because water flows from one sphere to the other, and is indeed an integral part of all the other spheres. Also, the carbon cycle is a key cycle in the global climate system because of the central role of carbon dioxide (CO2) as a greenhouse gas.

As all spheres meet and interact at the surface of the Earth the surface energy balance is a key determinant of the climate, from the very local scale to the global scale.

Conceptual sketch of the climate system, it's spheres and some selected interactions. Image source: IPCC AR4, WG1, FAQ 1.2 2007

Because of these cycles and web of interactions that hook into each other the global climate system is a complex one and 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 – weeks for the global atmosphere to thousands of years for the deep oceans and the largest ice sheets. The combination of widely differerent response times and the non-linearity of the system create internal variations.

This natural or internal variability is present at many times-scales, from variations in the daily weather to (multi-)decadal – and slower – variations related to the oceans. To study the processes and analyse the response resulting from to various forcings numerical models of the system, global climate models (GCMs) are needed.

External forcings, that is factors outside the climate system, that may influence the global climate system are variations in solar output, astronomical influences such as Earth orbit around the Sun (i.e. the Milankowitch cycles), volcanic eruptions (because they involve the deep Earth rather than the lithosphere), and anthropogenic influences.

To understand the factors underlying a climate change (as opposed to internal/natural variability) the concept of radiative forcing is useful because it explains how the different constituents of the atmosphere influences the surface energy balance. Thus, the radiative forcing resulting from different emission scenarios are used as input to GCM simulations of climate change.

Read more in the IPCC reports:

 

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