7.0 KiB
| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Climate system | 3/3 | https://en.wikipedia.org/wiki/Climate_system | reference | science, encyclopedia | 2026-05-05T14:48:37.926807+00:00 | kb-cron |
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. The difference in pressure oscillates and this affects weather patterns across the North Atlantic region up to central Eurasia. For instance, the weather in Greenland and Canada is cold and dry during a positive NAO. Different phases of the North Atlantic oscillation can be sustained for multiple decades. The ocean and atmosphere can also work together to spontaneously generate internal climate variability that can persist for 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 vapour or sea ice distribution, which can affect the total energy budget of the earth. 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 larger heat capacity and 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.
=== External climate forcing ===
On long timescales, the climate is determined mainly 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. When the change is caused by something outside of the five components of the climate system, it is called an external forcing. Volcanoes, for example, result from deep processes within the earth that are not considered part of the climate system. Human actions, off-planet changes, such as solar variation and incoming asteroids, are also external to the climate system's five components. The primary 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. The amount of energy coming from the Sun varies on shorter time scales, including the 11-year solar cycle and longer-term time scales. 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. 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.
==== 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. 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, temperature and water vapour increase. Accordingly, water vapour and clouds are not seen as external forcings but as feedback. The weathering of carbonates and silicates removes carbon from the atmosphere.
==== Aerosols ==== Liquid and solid particles in the atmosphere, collectively named aerosols, have diverse effects on the climate. Some primarily scatter sunlight, cooling the planet, while others absorb sunlight and warm the atmosphere. Indirect effects include the fact that aerosols can act as cloud condensation nuclei, stimulating cloud formation. Natural sources of aerosols include sea spray, mineral dust, meteorites and volcanoes. Still, humans also contribute as a human activity, such as the combustion of biomass or fossil fuels, releases aerosols into the atmosphere. Aerosols counteract some of the warming effects of emitted greenhouse gases until they fall back to the surface in a few years or less.
Although volcanoes are technically part of the lithosphere, which is part of the climate system, volcanism is defined as an external forcing agent. 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. 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.
==== 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 precipitation is also affected. Landscape fires release greenhouse gases into the atmosphere and release black carbon, which darkens snow, making it easier to melt.
=== Responses and 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.
== References ==
=== Sources ===
== External links == Media related to Climate system at Wikimedia Commons