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Earth's climate arises from the interaction of five major climate system components: the atmosphere (air), the hydrosphere (water), the cryosphere (ice and permafrost), the lithosphere (earth's upper rocky layer) and the biosphere (living things). Energy, water and different chemical elements are constantly flowing between the different components of the system. Climate is the average of weather, typically over a period of 30 years, and is determined by a combination of processes in the climate system.
Over time, the climate system's pattern 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. The emissions of heat-trapping greenhouse gases from industrial sources and their subsequent accumulation is currently causing global warming. Human activity also releases cooling aerosols, but their net effect is far less than that of greenhouse gases.
Components of the climate system
The atmosphere envelops the earth and stretches out hundreds of kilometres from the surface. It consists mostly of inert nitrogen (78%), oxygen (21%) and argon (1%). The trace gases in the atmosphere, 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 infra-red radiation the Earth's surface emits to balance the Sun's radiation. This causes surface temperatures to rise. The hydrological cycle is the movement of water through the atmosphere. Not only does the hydrological cycle determine patterns of precipitation, it also has an influence on the movement of energy throughout the climate system.
The ocean covers 71% of Earth's surface and has an average depth of nearly 4 kilometres (2.5 miles). It can hold substantially more heat than the atmosphere. The ocean has a salt content of about 3.5% on average, but this varies spatially. Ocean water that has more salt has a higher density and differences in density play an important role in ocean circulation. Ocean circulation is further driven by the interaction with wind. The salt component also influences the freezing point temperature. Vertical movements can bring up colder water to the surface in a process called upwelling, which cools down the air above. The thermohaline circulation transports heat from the tropics to the polar regions.
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. 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.
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. Land closer to open ocean has a more moderate climate than land farther from the ocean. 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.
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. 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. Precipitation and temperatures impact the distribution of different vegetation zones.
Flows of energy, water and elements
The climate system receives energy from the Sun, and to a far lesser extent from the Earth's core, as well as tidal energy from the Moon. The Earth gives off energy to outer space in two forms: it directly reflects a part of the radiation of the Sun and it emits infra-red radiation as black-body radiation. The balance of incoming and outgoing energy, and the passage of the energy through the climate system, determines Earth's energy budget. When the total of incoming energy is greater than the outgoing energy, Earth's energy budget is positive and the climate system is warming. If more energy goes out, the energy budget is negative and Earth experiences cooling. More energy reaches the tropics than the polar regions and the subsequent temperature difference drives the global circulation of the atmosphere and oceans.
Water is stored in all components of the climate system, with the oceans and ice containing the most. Its movement is driven by evaporation from oceans and other water bodies as a consequence of direct and indirect sunlight from the evapotranspiration of water from plants. Precipitation and evaporation are not evenly distributed across the globe, with some regions such as the tropics having more rainfall than evaporation, and others having more evaporation than rainfall.
Chemical elements, vital for life, are also constantly cycled through the different components of the climate system. In the carbon cycle, plants take up carbon dioxide from the atmosphere using photosynthesis; this is re-emitted by the breathing of living creatures. Volcanoes are also part of the extended carbon cycle. Over very long (geological) time periods, they release carbon dioxide from the Earth's crust and mantle, counteracting the uptake by sedimentary rocks and other geological carbon dioxide sinks.
The nitrogen cycle describes the flow of active nitrogen. As atmospheric nitrogen is inert, micro-organisms first have to convert this to an active nitrogen compound in a process called fixing nitrogen, before it can be used as a building block in the biosphere. Human activities play an important role in both carbon and nitrogen cycles: the burning of fossil fuels has displaced carbon from the lithosphere to the atmosphere, and the use of fertilizers has vastly increased the amount of available fixed nitrogen.
The ocean and atmosphere can work together to spontaneously generate internal climate variability that can persist for periods of years to decades at a time. Examples of this type of variability include the El Niño–Southern Oscillation, the Pacific decadal oscillation, and the Atlantic Multidecadal Oscillation. These variations can affect global average surface temperature by redistributing heat between the deep ocean and the atmosphere but also by altering the cloud, water vapor or sea ice distribution which can affect the total energy budget of the earth.
The oceanic aspects of these circulations can generate variability on centennial timescales due to the ocean having hundreds of times more mass than the atmosphere, and thus very high thermal inertia. For example, alterations to ocean processes such as thermohaline circulation play a key role in redistributing heat in the world's oceans. Understanding of internal variability helps scientists to attribute the recent climate change to greenhouse gases.
External climate forcing
Slight variations in the Earth's motion lead to changes in the seasonal distribution of sunlight reaching the Earth's surface and how it is distributed across the globe. There is very little change to the area-averaged, annually-averaged sunshine; but there can be strong changes in the geographical and seasonal distribution. The three types of kinematic change are variations in Earth's eccentricity, changes in the tilt angle of Earth's axis of rotation, and precession of Earth's axis. Combined together, these produce Milankovitch cycles which affect climate and are notable for their correlation to glacial and interglacial periods.
The Sun is the predominant source of energy input to the Earth. Both long- and short-term variations in solar intensity are known to affect global climate. Solar output varies on shorter time scales, including the 11-year solar cycle and longer-term modulations.
Although volcanoes are technically part of the lithosphere, which itself is part of the climate system, volcanism is defined as an external forcing agent.
The eruptions considered to be large enough to affect the Earth's climate on a scale of more than a year are the ones that inject over 100,000 tons of SO2 into the stratosphere. This is due to the optical properties of sulfate aerosols, which strongly absorb or scatter solar radiation, creating a global layer of sulfuric acid haze. On average, such eruptions occur several times per century, and for a period of several years cause cooling by partially blocking the transmission of solar radiation to the Earth's surface. Small eruptions affect the atmosphere only subtly, as temperature changes are comparable with natural variability. However, because smaller eruptions occur at a much higher frequency, in total they too significantly affect Earth's atmosphere.
Over the course of millions of years, the motion of tectonic plates reconfigures global land and ocean areas and generates topography. This can affect both global and local patterns of climate and atmosphere-ocean circulation. 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.
Greenhouse gases trap heat in the lower part of the atmosphere by absorbing longwave radiation. In the Earth's past, many processes contributed to variations in greenhouse gas concentrations. Currently, emissions by humans are the cause of increasing concentrations of some greenhouse gases. such as CO
2, methane and N
2O. The dominant contributor of the greenhouse effect is water vapour (~50%), with clouds (~25%) and CO
2 (~20%) also playing an important role. When concentrations of long-lived greenhouse gases such as CO
2 are increased and temperature rises, the amount of water vapour increases as well, so that water vapour and clouds are not seen as external forcings, but instead as feedbacks.
The different elements of the climate system respond to external forcing in different ways. One important difference between the components is the speed at which they react to a forcing. The atmosphere typically responds within a couple of hours to weeks, while the deep ocean and ice sheets take centuries to millennia to reach a new equilibrium. The initial response of a component to an external forcing can be damped by negative feedbacks and enhanced by positive feedbacks. For example, a significant decrease of solar intensity would quickly lead to a temperature decrease on Earth, which would then allow ice and snow cover to expand. The extra snow and ice has a higher albedo or reflectivity, and therefore reflects more of the Sun's radiation back into space before it can be absorbed by the climate system as a whole; this in turn causes the Earth to cool down further.
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