Fixed anvil temperature hypothesis is a physical hypothesis that describes the response of cloud radiative properties to rising surface temperatures. It presumes that the temperature at which radiation is emitted by anvil clouds is constrained by radiative processes and thus does not change in response to surface warming. Since the amount of radiation emitted by clouds is a function of their temperature, it implies that it does not increase with surface warming and thus a warmer surface does not increase radiation emissions (and thus cooling) by cloud tops. The mechanism has been identified both in climate models and observations of cloud behaviour, it affects how much the world heats up for each extra tonne of greenhouse gas in the atmosphere. However, some evidence suggests that it may be more correctly formulated as decreased anvil warming rather than no anvil warming.
In the tropics, the radiative cooling of the troposphere is balanced by the release of latent heat through condensation of water vapour lofted to high altitudes by convection. The radiative cooling is mostly a consequence of emissions by water vapour and thus becomes ineffective above the 200 hPa pressure level. Congruently, it is at this elevation that thick clouds and anvil clouds – the topmost convective clouds – concentrate. [1]
The "fixed anvil temperature hypothesis" stipulates that owing to energetic and thermodynamic constraints imposed by the Clausius-Clapeyron relationship, the temperature and thus radiative cooling of anvil clouds does not change much with surface temperature. [1] Specifically, cooling decreases below −73 °C (200 K) as the ineffective radiative cooling by CO
2 becomes dominant below that temperature. [2] Instead, the elevation of high clouds rises with surface temperatures. [3]
A related hypothesis is that tropopause temperatures are insensitive to surface warming; however it appears to have distinct mechanisms from the fixed anvil temperature process. [4] They have been related to each other in several studies, [5] which sometimes find a fixed tropopause temperature a more reasonable theory than fixed anvil temperature. [6]
The fixed anvil temperature hypothesis has been widely accepted and even extended to the non-tropical atmosphere. Its strength relies in part on its reliance on simple physical arguments. [7]
The fixed anvil temperature hypothesis was initially formulated by Hartmann and Larson 2002 in the context of the NCAR/PSU MM5 climate model [8] but the stability of top cloud temperatures was already observed in a one-dimensional model by Hansen et al. 1981. [9] It has also been recovered, with limitations, in climate models [10] and in numerous general circulation models. [11] However, some have recovered a dependence on cloud size [12] and on relative humidity [13] or that the fixed anvil temperature is more properly expressed as anvil temperature changing more slowly than surface temperature. [14] Climate models also simulate an increase in cloud top height [15] and some radiative-convective models apply it to the outflow of tropical cyclones. [16]
The fixed anvil temperature hypothesis has also been obtained in simulations of exoplanet climates. [17] At very high CO
2 concentrations approaching a runaway greenhouse however, other physical effects pertaining to cloud opacity may take over and dominate the fixed anvil temperature as surface temperatures reach extreme levels. [18]
The fixed anvil temperature hypothesis has been backed by observational studies [19] for large clouds. Smaller clouds however have no stable temperature and there are temperature fluctuations of about 5 °C (9 °F) [20] which may relate to processes involving the Brewer-Dobson circulation. [13] Xu et al. 2007 found that cloud temperatures are more stable for clouds with sizes exceeding 150 kilometres (93 mi). [21] The ascent of cloud top height with warming is also supported by observations. [15]
Clouds are the second biggest uncertainty in future climate change after human actions, as their effects are complicated and not properly understood. [22] The fixed anvil temperature hypothesis has effects on global climate sensitivity, since anvil clouds are the most important source of outgoing radiation linked to tropical convection [23] and their temperature being stable would render the outgoing radiation non-responsive to surface temperature changes. [24] This creates a positive feedback component of cloud feedback. [25] The fixed anvil temperature hypothesis has also been used to argue that climate modelling should use temperature rather than pressure to model the height of high clouds. [26]
A hypothesis which would have the opposite effect on climate is the iris hypothesis, according to which the coverage of anvil clouds declines with warming, thus allowing more radiation to escape into space and resulting in slower warming. [27] The proportionate anvil warming hypothesis by Zelinka and Hartmann 2010 was formulated on the basis of general circulation models and envisages a small increase of anvil temperature with high warming. [28] The latter hypothesis was intended as a modification to the fixed anvil temperature hypothesis [20] and includes considerations of atmospheric stability and appears to reflect actual climate conditions more closely. [26] Finally, there is a view that cloud top temperatures could actually decrease with surface warming [29] as convection height rises. This may constitute a non-equilibrium response. [30]
As of 2020 [update] further research is needed to properly understand the physics of some cloud feedbacks, [31] as they differ between models, [32] and progress on properly modelling clouds globally is very slow. [22]
Numerical climate models use quantitative methods to simulate the interactions of the important drivers of climate, including atmosphere, oceans, land surface and ice. They are used for a variety of purposes from study of the dynamics of the climate system to projections of future climate. Climate models may also be qualitative models and also narratives, largely descriptive, of possible futures.
Cloud feedback is a type of climate change feedback that has been difficult to quantify in contemporary climate models. It can affect the magnitude of internally generated climate variability or they can affect the magnitude of climate change resulting from external radiative forcings. Cloud representations vary among global climate models, and small changes in cloud cover have a large impact on the climate.
Contrails or vapor trails are line-shaped clouds produced by aircraft engine exhaust or changes in air pressure, typically at aircraft cruising altitudes several miles above the Earth's surface. They are composed primarily of water, in the form of ice crystals. The combination of water vapor in aircraft engine exhaust and the low ambient temperatures at high altitudes causes the trails' formation. Impurities in the engine exhaust from the fuel, including sulfur compounds provide some of the particles that serve as nucleation sites for water droplet growth in the exhaust. If water droplets form, they can freeze to form ice particles that compose a contrail. Their formation can also be triggered by changes in air pressure in wingtip vortices, or in the air over the entire wing surface. Contrails, and other clouds caused directly by human activity, are called homogenitus.
The tropopause is the atmospheric boundary that demarcates the troposphere from the stratosphere, which are the lowest two of the five layers of the atmosphere of Earth. The tropopause is a thermodynamic gradient-stratification layer that marks the end of the troposphere, and is approximately 17 kilometres (11 mi) above the equatorial regions, and approximately 9 kilometres (5.6 mi) above the polar regions.
Global dimming was the name given to a decline in the amount of sunlight reaching the Earth's surface, a measure also known as global direct solar irradiance. It was observed soon after the first systematic measurements of solar irradiance began in the 1950s, and continued until 1980s, with an observed reduction of 4–5% per decade, even though solar activity did not vary more than the usual at the time. Instead, global dimming had been attributed to an increase in atmospheric particulate matter, predominantly sulfate aerosols, as the result of rapidly growing air pollution due to post-war industrialization. After 1980s, global dimming started to reverse, alongside reductions in particulate emissions, in what has been described as global brightening, although this reversal is only considered "partial" for now. This reversal has also been globally uneven, as some of the brightening over the developed countries in the 1980s and 1990s had been counteracted by the increased dimming from the industrialization of the developing countries and the expansion of the global shipping industry, although they have also been making rapid progress in cleaning up air pollution in the recent years.
Radiative forcing is the change in energy flux in the atmosphere caused by natural or anthropogenic factors of climate change as measured in watts per meter squared. It is a scientific concept used to quantify and compare the external drivers of change to Earth's energy balance. These external drivers are distinguished from climate feedbacks and internal variability, which also influence the direction and magnitude of imbalance.
Cloud condensation nuclei (CCNs), also known as cloud seeds, are small particles typically 0.2 µm, or one hundredth the size of a cloud droplet. CCNs are a unique subset of aerosols in the atmosphere on which water vapour condenses. This can affect the radiative properties of clouds and the overall atmosphere. Water requires a non-gaseous surface to make the transition from a vapour to a liquid; this process is called condensation.
Sea surface temperature (SST), or ocean surface temperature, is the ocean temperature close to the surface. The exact meaning of surface varies according to how we measure it. It is between 1 millimetre (0.04 in) and 20 metres (70 ft) below the sea surface. Sea surface temperatures greatly modify air masses in the Earth's atmosphere within a short distance of the shore. Local areas of heavy snow can form in bands downwind of warm water bodies within an otherwise cold air mass. Warm sea surface temperatures can develop and strengthen cyclones over the Ocean. Experts call this process tropical cyclogenesis. Tropical cyclones can also cause a cool wake. This is due to turbulent mixing of the upper 30 metres (100 ft) of the ocean. Sea surface temperature changes during the day. This is like the air above it, but to a lesser degree. There is less variation in sea surface temperature on breezy days than on calm days. Ocean currents, such as the Atlantic Multidecadal Oscillation, can affect sea surface temperatures over several decades. Thermohaline circulation has a major impact on average sea surface temperature throughout most of the world's oceans.
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. Because the Sun heats the equatorial tropics more than the polar regions, received solar irradiance 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.
A circumpolar vortex, or simply polar vortex, is a large region of cold, rotating air; polar vortices encircle both of Earth's polar regions. Polar vortices also exist on other rotating, low-obliquity planetary bodies. The term polar vortex can be used to describe two distinct phenomena; the stratospheric polar vortex, and the tropospheric polar vortex. The stratospheric and tropospheric polar vortices both rotate in the direction of the Earth's spin, but they are distinct phenomena that have different sizes, structures, seasonal cycles, and impacts on weather.
Climate sensitivity is a measure of how much Earth's surface will warm for a doubling in the atmospheric carbon dioxide concentration. In technical terms, climate sensitivity is the average change in global mean surface temperature in response to a radiative forcing, which drives a difference between Earth's incoming and outgoing energy. Climate sensitivity is a key measure in climate science, and a focus area for climate scientists, who want to understand the ultimate consequences of anthropogenic global warming.
The faint young Sun paradox or faint young Sun problem describes the apparent contradiction between observations of liquid water early in Earth's history and the astrophysical expectation that the Sun's output would be only 70 percent as intense during that epoch as it is during the modern epoch. The paradox is this: with the young sun's output at only 70 percent of its current output, early Earth would be expected to be completely frozen – but early Earth seems to have had liquid water and supported life.
Parameterization in a weather or climate model is a method of replacing processes that are too small-scale or complex to be physically represented in the model by a simplified process. This can be contrasted with other processes—e.g., large-scale flow of the atmosphere—that are explicitly resolved within the models. Associated with these parameterizations are various parameters used in the simplified processes. Examples include the descent rate of raindrops, convective clouds, simplifications of the atmospheric radiative transfer on the basis of atmospheric radiative transfer codes, and cloud microphysics. Radiative parameterizations are important to both atmospheric and oceanic modeling alike. Atmospheric emissions from different sources within individual grid boxes also need to be parameterized to determine their impact on air quality.
The iris hypothesis was a hypothesis proposed by Richard Lindzen and colleagues in 2001 that suggested increased sea surface temperature in the tropics would result in reduced cirrus clouds and thus more infrared radiation leakage from Earth's atmosphere. His study of observed changes in cloud coverage and modeled effects on infrared radiation released to space as a result seemed to support the hypothesis. This suggested infrared radiation leakage was hypothesized to be a negative feedback in which an initial warming would result in an overall cooling of the surface.
A runaway greenhouse effect occurs when a planet's atmosphere contains greenhouse gas in an amount sufficient to block thermal radiation from leaving the planet, preventing the planet from cooling and from having liquid water on its surface. A runaway version of the greenhouse effect can be defined by a limit on a planet's outgoing longwave radiation which is asymptotically reached due to higher surface temperatures evaporating water into the atmosphere, increasing its optical depth. This positive feedback means the planet cannot cool down through longwave radiation and continues to heat up until it can radiate outside of the absorption bands of the water vapour.
In climate science, longwave radiation (LWR) is electromagnetic thermal radiation emitted by Earth's surface, atmosphere, and clouds. It may also be referred to as terrestrial radiation. This radiation is in the infrared portion of the spectrum, but is distinct from the shortwave (SW) near-infrared radiation found in sunlight.
The Atlantic Multidecadal Oscillation (AMO), also known as Atlantic Multidecadal Variability (AMV), is the theorized variability of the sea surface temperature (SST) of the North Atlantic Ocean on the timescale of several decades.
Polar amplification is the phenomenon that any change in the net radiation balance tends to produce a larger change in temperature near the poles than in the planetary average. This is commonly referred to as the ratio of polar warming to tropical warming. On a planet with an atmosphere that can restrict emission of longwave radiation to space, surface temperatures will be warmer than a simple planetary equilibrium temperature calculation would predict. Where the atmosphere or an extensive ocean is able to transport heat polewards, the poles will be warmer and equatorial regions cooler than their local net radiation balances would predict. The poles will experience the most cooling when the global-mean temperature is lower relative to a reference climate; alternatively, the poles will experience the greatest warming when the global-mean temperature is higher.
Patterns of solar irradiance and solar variation have been a main driver of climate change over the millions to billions of years of the geologic time scale.
Convective self-aggregation is an atmospheric phenomenon that occurs under idealized conditions such as the tropical ocean where the sea surface temperature is approximately constant. During convective self-aggregation a homogenous atmosphere transition into one region that is very dry and cloud-free and one region with heavy rain and thunderstorm. Convective self-aggregation is, therefore, important in the formation of tropical cyclones, and data indicate that cold pools play a major role in starting it.