Climate system

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The five components of the climate system all interact. They are the atmosphere, the hydrosphere, the cryosphere, the lithosphere and the biosphere. Climate-system.jpg
The five components of the climate system all interact. They are the atmosphere, the hydrosphere, the cryosphere, the lithosphere and the biosphere.

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.

Contents

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

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]

Flows of energy, water and elements

Earth's atmospheric circulation is driven by the energy imbalance between the equator and the poles. It is further influenced by the rotation of Earth around its own axis. NASA depiction of earth global atmospheric circulation.jpg
Earth's atmospheric circulation is driven by the energy imbalance between the equator and the poles. It is further influenced by the rotation of Earth around its own axis.

Energy and general circulation

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 Imbalance is positive and the climate system is warming. If more energy goes out, the energy imbalance is negative and Earth experiences cooling. [25]

More energy reaches the tropics than the polar regions and the subsequent temperature difference drives the global circulation of the atmosphere and oceans. [26] Air rises when it warms, flows polewards and sinks again when it cools, returning to the equator. [27] Due to the conservation of angular momentum, the Earth's rotation diverts the air to the right in the Northern Hemisphere and to the left in the Southern hemisphere, thus forming distinct atmospheric cells. [28] Monsoons, seasonal changes in wind and precipitation that occur mostly in the tropics, form due to the fact that land masses heat up more easily than the ocean. The temperature difference induces a pressure difference between land and ocean, driving a steady wind. [29]

Ocean water that has more salt has a higher density and differences in density play an important role in ocean circulation. The thermohaline circulation transports heat from the tropics to the polar regions. [30] Ocean circulation is further driven by the interaction with wind. The salt component also influences the freezing point temperature. [31] Vertical movements can bring up colder water to the surface in a process called upwelling, which cools down the air above. [32]

Hydrological cycle

The hydrological cycle or water cycle describes how it is constantly moved between the surface of the Earth and the atmosphere. [33] Plants evapotranspirate and sunlight evaporates water from oceans and other water bodies, leaving behind salt and other minerals. The evaporated freshwater later rains back onto the surface. [34] 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. [35] The evaporation of water requires substantial quantities of energy, whereas a lot of heat is released during condensation. This latent heat is the primary source of energy in the atmosphere. [36]

Biochemical cycles

Carbon is constantly transported between the different elements of the climate system: fixed by living creatures and transported through the ocean and atmosphere. Carbon cycle.jpg
Carbon is constantly transported between the different elements of the climate system: fixed by living creatures and transported through the ocean and atmosphere.

Chemical elements, vital for life, are constantly cycled through the different components of the climate system. The carbon cycle is directly important for climate as it determines the concentrations of two important greenhouse gases in the atmosphere: CO2 and methane. [37] In the fast part of the carbon cycle, plants take up carbon dioxide from the atmosphere using photosynthesis; this is later re-emitted by the breathing of living creatures. [38] As part of the slow carbon cycle, volcanoes release CO2 by degassing, releasing carbon dioxide from the Earth's crust and mantle. [39] As CO2 in the atmosphere makes rain a bit acidic, this rain can slowly dissolve some rocks, a process known as weathering . The minerals that are released in this way, transported to the sea, are used by living creatures whose remains can form sedimentary rocks, bringing the carbon back to the lithosphere. [40]

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. [41] 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. [42]

Changes within the climate system

Climate is constantly varying, on timescales that range from seasons to the lifetime of the Earth. [43] Changes caused by the system's own components and dynamics are called internal climate variability. The system can also experience external forcing from phenomena outside of the system (e.g. a change in Earth's orbit). [44] Longer changes, usually defined as changes that persist for at least 30 years, are referred to as climate changes, [45] although this phrase usually refers to the current global climate change. [46] When the climate changes, the effects may build on each other, cascading through the other parts of the system in a series of climate feedbacks (e.g. albedo changes), producing many different effects (e.g. sea level rise). [47]

Internal variability

Difference between normal December sea surface temperature [degC] and temperatures during the strong El Nino of 1997. El Nino typically brings wetter weather to Mexico and the United States. El-nino.png
Difference between normal December sea surface temperature [°C] and temperatures during the strong El Niño of 1997. El Niño typically brings wetter weather to Mexico and the United States.

Components of the climate system vary continuously, even without external pushes (external forcing). One example in the atmosphere is the North Atlantic Oscillation (NAO), which operates as an atmospheric pressure see-saw. The Portuguese Azores typically have high pressure, whereas there is often lower pressure over Iceland. [49] The difference in pressure oscillates and this affects weather patterns across the North Atlantic region up to central Eurasia. [50] For instance, the weather in Greenland and Canada is cold and dry during a positive NAO. [51] Different phases of the North Atlantic oscillation can be sustained for multiple decades. [52]

The ocean and atmosphere can also work together to spontaneously generate internal climate variability that can persist for years to decades at a time. [53] [54] 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; [55] [56] but also by altering the cloud, water vapour or sea ice distribution, which can affect the total energy budget of the earth. [57] [58]

The oceanic aspects of these oscillations can generate variability on centennial timescales due to the ocean having hundreds of times more mass than the atmosphere, and therefore 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 internal variability helped scientists to attribute recent climate change to greenhouse gases. [59]

External climate forcing

On long timescales, the climate is determined mostly by how much energy is in the system and where it goes. When the Earth's energy budget changes, the climate follows. A change in the energy budget is called a forcing, and when the change is caused by something outside of the five components of the climate system, it is called an external forcing. [60] Volcanoes, for example, result from deep processes within the earth that are not considered part of the climate system. Off-planet changes, such as solar variation and incoming asteroids, are also "external" to the climate system's five components, as are human actions. [61]

The main value to quantify and compare climate forcings is radiative forcing.

Incoming sunlight

The Sun is the predominant source of energy input to the Earth and drives atmospheric circulation. [62] The amount of energy coming from the Sun varies on shorter time scales, including the 11-year solar cycle [63] and longer-term time scales. [64] While the solar cycle is too small to directly warm and cool Earth's surface, it does influence a higher layer of the atmosphere directly, the stratosphere, which may have an effect on the atmosphere near the surface. [65]

Slight variations in the Earth's motion can cause large changes in the seasonal distribution of sunlight reaching the Earth's surface and how it is distributed across the globe, although not to the global and yearly average sunlight. 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. Together these produce Milankovitch cycles, which affect climate and are notable for their correlation to glacial and interglacial periods. [66]

Greenhouse gases

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 CO2, methane and N2O. [67] The dominant contributor to the greenhouse effect is water vapour (~50%), with clouds (~25%) and CO2 (~20%) also playing an important role. When concentrations of long-lived greenhouse gases such as CO2 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. [68] The weathering of carbonates and silicates removes carbon from the atmosphere. [69]

Aerosols

Liquid and solid particles in the atmosphere, collectively named aerosols, have diverse effects on the climate. Some primarily scatter sunlight and thereby cool the planet, while others absorb sunlight and warm the atmosphere. [70] Indirect effects include the fact that aerosols can act as cloud condensation nuclei, stimulating cloud formation. [71] Natural sources of aerosols include sea spray, mineral dust, meteorites and volcanoes, but humans also contribute [70] as human activity such as the combustion of biomass or fossil fuels releases aerosols into the atmosphere. Aerosols counteract a part of the warming effects of emitted greenhouse gases, but only until they fall back to the surface in a few years or less. [72]

In atmospheric temperature from 1979 to 2010, determined by MSU NASA satellites, effects appear from aerosols released by major volcanic eruptions (El Chichon and Pinatubo). El Nino is a separate event, from ocean variability. Msu 1978-2010.jpg
In atmospheric temperature from 1979 to 2010, determined by MSU NASA satellites, effects appear from aerosols released by major volcanic eruptions (El Chichón and Pinatubo). El Niño is a separate event, from ocean variability.

Although volcanoes are technically part of the lithosphere, which itself is part of the climate system, volcanism is defined as an external forcing agent. [73] On average, there are only several volcanic eruptions per century that influence Earth's climate for longer than a year by ejecting tons of SO2 into the stratosphere. [74] [75] The sulfur dioxide is chemically converted into aerosols that cause cooling by blocking a fraction of sunlight to the Earth's surface. Small eruptions affect the atmosphere only subtly. [74]

Land use and cover change

Changes in land cover, such as change of water cover (e.g. rising sea level, drying up of lakes and outburst floods) or deforestation, particularly through human use of the land, can affect the climate. The reflectivity of the area can change, causing the region to capture more or less sunlight. In addition, vegetation interacts with the hydrological cycle, so that precipitation is also affected. [76] Landscape fires release greenhouse gases into the atmosphere and release black carbon, which darkens snow making it easier to melt. [77] [78]

Responses and feedbacks

Some effects of global warming can either enhance (positive feedbacks) or inhibit (negative feedbacks) warming. Observations and modeling studies indicate that there is a net positive feedback to Earth's current global warming. 20220726 Feedbacks affecting global warming and climate change - block diagram.svg
Some effects of global warming can either enhance (positive feedbacks) or inhibit (negative feedbacks) warming. Observations and modeling studies indicate that there is a net positive feedback to Earth's current global warming.

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. [82]

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. [83]

Related Research Articles

<span class="mw-page-title-main">Causes of climate change</span> Effort to scientifically ascertain mechanisms responsible for recent global warming

The scientific community has been investigating the causes of climate change for decades. After thousands of studies, it came to a consensus, where it is "unequivocal that human influence has warmed the atmosphere, ocean and land since pre-industrial times." This consensus is supported by around 200 scientific organizations worldwide, The dominant role in this climate change has been played by the direct emissions of carbon dioxide from the burning of fossil fuels. Indirect CO2 emissions from land use change, and the emissions of methane, nitrous oxide and other greenhouse gases play major supporting roles.

<span class="mw-page-title-main">Climate</span> Statistics of weather conditions in a given region over long periods

Climate is the long-term weather pattern in a region, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorological variables that are commonly measured are temperature, humidity, atmospheric pressure, wind, and precipitation. In a broader sense, climate is the state of the components of the climate system, including the atmosphere, hydrosphere, cryosphere, lithosphere and biosphere and the interactions between them. The climate of a location is affected by its latitude, longitude, terrain, altitude, land use and nearby water bodies and their currents.

<span class="mw-page-title-main">Greenhouse effect</span> Atmospheric phenomenon causing planetary warming

The greenhouse effect occurs when greenhouse gases in a planet's atmosphere insulate the planet from losing heat to space, raising its surface temperature. Surface heating can happen from an internal heat source as in the case of Jupiter, or from its host star as in the case of the Earth. In the case of Earth, the Sun emits shortwave radiation (sunlight) that passes through greenhouse gases to heat the Earth's surface. In response, the Earth's surface emits longwave radiation that is mostly absorbed by greenhouse gases. The absorption of longwave radiation prevents it from reaching space, reducing the rate at which the Earth can cool off.

<span class="mw-page-title-main">Climate variability and change</span> Change in the statistical distribution of climate elements for an extended period

Climate variability includes all the variations in the climate that last longer than individual weather events, whereas the term climate change only refers to those variations that persist for a longer period of time, typically decades or more. Climate change may refer to any time in Earth's history, but the term is now commonly used to describe contemporary climate change, often popularly referred to as global warming. Since the Industrial Revolution, the climate has increasingly been affected by human activities.

<span class="mw-page-title-main">Climate model</span> Quantitative methods used to simulate climate

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<span class="mw-page-title-main">Cloud feedback</span> Type of climate change feedback mechanism

Cloud feedback is a type of climate change feedback, where the overall cloud frequency, height, and the relative fraction of the different types of clouds are altered due to climate change, and these changes then affect the Earth's energy balance. On their own, clouds are already an important part of the climate system, as they consist of water vapor, which acts as a greenhouse gas and so contributes to warming; at the same time, they are bright and reflective of the Sun, which causes cooling. Clouds at low altitudes have a stronger cooling effect, and those at high altitudes have a stronger warming effect. Altogether, clouds make the Earth cooler than it would have been without them.

<span class="mw-page-title-main">Paleoclimatology</span> Study of changes in ancient climate

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<span class="mw-page-title-main">Global dimming</span> Reduction in the amount of sunlight reaching Earths surface

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<span class="mw-page-title-main">Radiative forcing</span> Difference between solar irradiance absorbed by the Earth and energy radiated back to space

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This glossary of climate change is a list of definitions of terms and concepts relevant to climate change, global warming, and related topics.

<span class="mw-page-title-main">Earth's energy budget</span> Accounting of the energy flows which determine Earths surface temperature and drive its climate

Earth's energy budget accounts for the balance between the energy that Earth receives from the Sun and the energy the Earth loses back into outer space. Smaller energy sources, such as Earth's internal heat, are taken into consideration, but make a tiny contribution compared to solar energy. The energy budget also accounts for how energy moves through the climate system. The Sun heats the equatorial tropics more than the polar regions. Therefore, the amount of solar irradiance received by a certain region is unevenly distributed. As the energy seeks equilibrium across the planet, it drives interactions in Earth's climate system, i.e., Earth's water, ice, atmosphere, rocky crust, and all living things. The result is Earth's climate.

Trace gases are gases that are present in small amounts within an environment such as a planet's atmosphere. Trace gases in Earth's atmosphere are gases other than nitrogen (78.1%), oxygen (20.9%), and argon (0.934%) which, in combination, make up 99.934% of its atmosphere.

This is a list of climate change topics.

<span class="mw-page-title-main">Veerabhadran Ramanathan</span>

Veerabhadran "Ram" Ramanathan is Edward A. Frieman Endowed Presidential Chair in Climate Sustainability Scripps Institution of Oceanography, University of California, San Diego. He has contributed to many areas of the atmospheric and climate sciences including developments to general circulation models, atmospheric chemistry, and radiative transfer. He has been a part of major projects such as the Indian Ocean Experiment (INDOEX) and the Earth Radiation Budget Experiment (ERBE), and is known for his contributions to the areas of climate physics, Climate Change and atmospheric aerosols research. He is now the Chair of Bending the Curve: Climate Change Solutions education project of University of California. He has received numerous awards, and is a member of the US National Academy of Sciences. He has spoken about the topic of global warming, and written that "the effect of greenhouse gases on global warming is, in my opinion, the most important environmental issue facing the world today."

<span class="mw-page-title-main">Greenhouse gas</span> Gas in an atmosphere that absorbs and emits radiation at thermal infrared wavelengths

Greenhouse gases (GHGs) are the gases in the atmosphere that raise the surface temperature of planets such as the Earth. What distinguishes them from other gases is that they absorb the wavelengths of radiation that a planet emits, resulting in the greenhouse effect. The Earth is warmed by sunlight, causing its surface to radiate heat, which is then mostly absorbed by greenhouse gases. Without greenhouse gases in the atmosphere, the average temperature of Earth's surface would be about −18 °C (0 °F), rather than the present average of 15 °C (59 °F).

<span class="mw-page-title-main">History of climate change science</span> Aspect of the history of science

The history of the scientific discovery of climate change began in the early 19th century when ice ages and other natural changes in paleoclimate were first suspected and the natural greenhouse effect was first identified. In the late 19th century, scientists first argued that human emissions of greenhouse gases could change Earth's energy balance and climate. The existence of the greenhouse effect, while not named as such, was proposed as early as 1824 by Joseph Fourier. The argument and the evidence were further strengthened by Claude Pouillet in 1827 and 1838. In 1856 Eunice Newton Foote demonstrated that the warming effect of the sun is greater for air with water vapour than for dry air, and the effect is even greater with carbon dioxide.

Tectonic–climatic interaction is the interrelationship between tectonic processes and the climate system. The tectonic processes in question include orogenesis, volcanism, and erosion, while relevant climatic processes include atmospheric circulation, orographic lift, monsoon circulation and the rain shadow effect. As the geological record of past climate changes over millions of years is sparse and poorly resolved, many questions remain unresolved regarding the nature of tectonic-climate interaction, although it is an area of active research by geologists and palaeoclimatologists.

<span class="mw-page-title-main">Atmospheric carbon cycle</span> Transformation of atmospheric carbon between various forms

The atmospheric carbon cycle accounts for the exchange of gaseous carbon compounds, primarily carbon dioxide, between Earth's atmosphere, the oceans, and the terrestrial biosphere. It is one of the faster components of the planet's overall carbon cycle, supporting the exchange of more than 200 billion tons of carbon in and out of the atmosphere throughout the course of each year. Atmospheric concentrations of CO2 remain stable over longer timescales only when there exists a balance between these two flows. Methane, Carbon monoxide (CO), and other human-made compounds are present in smaller concentrations and are also part of the atmospheric carbon cycle.

<span class="mw-page-title-main">Little Ice Age volcanism</span> Massive volcanic activity during the Little Ice Age

Little Ice Age volcanism refers to the massive volcanic activities during the Little Ice Age. Scientists suggested a hypothesis that volcanism was the major driving force of the global cooling among the other natural factors, i.e. the sunspot activities by orbital forcing and greenhouse gas. The Past Global Change (PAGES), a registered paleo-science association for scientific research and networking on past global changes in the University of Bern, Switzerland, suggested that from 1630 to 1850, a total of 16 major eruptions and cooling events had taken place. When a volcano erupts, ashes burst out of the vent together with magma and forms a cloud in the atmosphere. The ashes act as an isolating layer that block out a proportion of solar radiation, causing global cooling. The global cooling effect impacts ocean currents, atmospheric circulation and cause social impacts such as drought and famine. Wars and rebellions were therefore triggered worldwide in the Little Ice Age. It was suggested that the crisis on Ottoman Empire and Ming-Qing Transition in China were typical examples that closely correlated with Little Ice Age.

Ocean dynamical thermostat is a physical mechanism through which changes in the mean radiative forcing influence the gradients of sea surface temperatures in the Pacific Ocean and the strength of the Walker circulation. Increased radiative forcing (warming) is more effective in the western Pacific than in the eastern where the upwelling of cold water masses damps the temperature change. This increases the east-west temperature gradient and strengthens the Walker circulation. Decreased radiative forcing (cooling) has the opposite effect.

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