The climate system is the highly complex global system consisting of 5 major components: the atmosphere, the oceans, the cryosphere (snow and ice), the land surface, the biosphere, and the interactions between them. The interactions of these components determine not only day-to-day weather, but also long-term averages that we refer to as the climate.
The atmosphere is the most unstable and rapidly changing of the 5 components. Its composition has changed many times throughout the history of the Earth, and it currently is made up primarily of nitrogen (78.1%) and oxygen (20.9%). These gases have limited interactions with incoming sunlight, and do not interact with the infrared radiation emitted by the Earth. There are a number of trace gases present in the atmosphere that do interact with this light however, notably carbon dioxide (CO2) and methane (CH4) which are referred to as greenhouse gases. The section below discusses the effects these gases have on the global climate system.
The cryosphere impacts the climate system greatly through its high albedo (reflectivity), its low thermal conductivity, and most importantly its critical role in driving deep ocean water circulation. Also, because the ice sheets store a large amount of water, variations in their volume contribute to sea level rise.
There are numerous interactions between the components of the climate system, as they are all open systems with the freedom to exchange mass, heat and momentum with one another. An example of an interaction is the ocean-atmosphere system, which is a strongly-coupled system exchanging water vapour and heat through evaporation, among other things. This is part of the hydrologic cycle which leads to condensation, cloud formation, and precipitation, while also supplying energy to weather systems. The ocean-atmosphere system also exchanges gases like carbon dioxide, where the ocean acts as a large carbon sink.
The climate system is driven by energy that is received from the sun. Some of this energy is reflected back into space, while the rest is absorbed by the land and ocean and re-emitted as radiant heat; this is known as the heat balance of the Earth. Not all of the energy that is reflected or re-emitted by the land and ocean makes it back into space however, as the Earth's greenhouse effect acts to trap in some of the light. This heat balance is what determines the temperature of the Earth.
This balance is not so simple, however. One important complication stems from the uneven heating of the Earth, as the Equator and the Poles do not receive the same amounts of energy from the sun. This non-uniform heating leads to temperature differences throughout the globe, which the atmosphere and ocean act to reduce by transporting heat from the warm tropics to the cool poles. This heat transportation is what gives rise to ocean currents, wind, evaporation, precipitation, and global weather patterns.
The carbon cycle of the Earth is constantly added to by humans through the burning of fossil fuels which emit greenhouse gases. The emission of greenhouse gases alters the balance of heat on the Earth, as it enhances the greenhouse effect which traps infrared radiation. Land-use change by deforestation and agriculture practices also influences the climate system.
When the balance of incoming and outgoing energy is altered (see radiative forcing), the amount of heat at different points in the climate system is changed and therefore so are the heat transport processes. These changes have a potential impact on regional and global climate. They can also result in extreme weather events, but even minor changes in global weather patterns have far-reaching consequences.