Climate system
Earth's climate system is a complex system with five interacting components: the atmosphere (air), the hydrosphere (water), the cryosphere (ice and permafrost), the lithosphere (earth's upper rocky layer) and the biosphere (living things).[1] Climate is the statistical characterization of the climate system.[2] It represents the average weather, typically over a period of 30 years, and is determined by a combination of processes, such as ocean currents and wind patterns.[3][4] Circulation in the atmosphere and oceans transports heat from the tropical regions to regions that receive less energy from the Sun. Solar radiation is the main driving force for this circulation. The water cycle also moves energy throughout the climate system. In addition, certain chemical elements are constantly moving between the components of the climate system. Two examples for these biochemical cycles are the carbon and nitrogen cycles.
The climate system can change due to internal variability and external forcings. These external forcings can be natural, such as variations in solar intensity and volcanic eruptions, or caused by humans. Accumulation of greenhouse gases in the atmosphere, mainly being emitted by people burning fossil fuels, is causing climate change. Human activity also releases cooling aerosols, but their net effect is far less than that of greenhouse gases.[1] Changes can be amplified by feedback processes in the different climate system components.
Components[edit]
The atmosphere envelops the earth and extends hundreds of kilometres from the surface. It consists mostly of inert nitrogen (78%), oxygen (21%) and argon (0.9%).[5] Some trace gases in the atmosphere, such as water vapour and carbon dioxide, are the gases most important for the workings of the climate system, as they are greenhouse gases which allow visible light from the Sun to penetrate to the surface, but block some of the infrared radiation the Earth's surface emits to balance the Sun's radiation. This causes surface temperatures to rise.[6]
The hydrological cycle is the movement of water through the climate system. Not only does the hydrological cycle determine patterns of precipitation, it also has an influence on the movement of energy throughout the climate system.[7]
The hydrosphere proper contains all the liquid water on Earth, with most of it contained in the world's oceans.[8] The ocean covers 71% of Earth's surface to an average depth of nearly 4 kilometres (2.5 miles),[9] and ocean heat content is much larger than the heat held by the atmosphere.[10][11] It contains seawater with a salt content of about 3.5% on average, but this varies spatially.[9] Brackish water is found in estuaries and some lakes, and most freshwater, 2.5% of all water, is held in ice and snow.[12]
The cryosphere contains all parts of the climate system where water is solid. This includes sea ice, ice sheets, permafrost and snow cover. Because there is more land in the Northern Hemisphere compared to the Southern Hemisphere, a larger part of that hemisphere is covered in snow.[13] Both hemispheres have about the same amount of sea ice. Most frozen water is contained in the ice sheets on Greenland and Antarctica, which average about 2 kilometres (1.2 miles) in height. These ice sheets slowly flow towards their margins.[14]
The Earth's crust, specifically mountains and valleys, shapes global wind patterns: vast mountain ranges form a barrier to winds and impact where and how much it rains.[15][16] Land closer to open ocean has a more moderate climate than land farther from the ocean.[17] For the purpose of modelling the climate, the land is often considered static as it changes very slowly compared to the other elements that make up the climate system.[18] The position of the continents determines the geometry of the oceans and therefore influences patterns of ocean circulation. The locations of the seas are important in controlling the transfer of heat and moisture across the globe, and therefore, in determining global climate.[19]
Lastly, the biosphere also interacts with the rest of the climate system. Vegetation is often darker or lighter than the soil beneath, so that more or less of the Sun's heat gets trapped in areas with vegetation.[20] Vegetation is good at trapping water, which is then taken up by its roots. Without vegetation, this water would have run off to the closest rivers or other water bodies. Water taken up by plants instead evaporates, contributing to the hydrological cycle.[21] Precipitation and temperature influences the distribution of different vegetation zones.[22] Carbon assimilation from seawater by the growth of small phytoplankton is almost as much as land plants from the atmosphere.[23] While humans are technically part of the biosphere, they are often treated as a separate components of Earth's climate system, the anthroposphere, because of human's large impact on the planet.[20]
