TheInfoList

Almost all of the energy that affects Earth's climate is received as radiant energy from the Sun. The planet and its atmosphere absorb and reflect some of the energy, while Outgoing longwave radiation, long-wave energy is radiated back into space. The balance between absorbed and radiated energy determines the average global temperature. Because the atmosphere absorbs some of the re-radiated long-wave energy, the planet is warmer than it would be in the absence of the atmosphere: see greenhouse effect. The radiation balance is altered by such factors as the intensity of solar energy, reflectivity of clouds or gases, absorption by various greenhouse gases or surfaces and heat emission by various materials. Any such alteration is a radiative forcing, and changes the balance. This happens continuously as sunlight hits the surface, clouds and aerosols form, the concentrations of atmospheric gases vary and seasons alter the groundcover.

# IPCC usage

The Intergovernmental Panel on Climate Change (IPCC) IPCC AR4, AR4 report defines radiative forcings as:
"Radiative forcing is a measure of the influence a factor has in altering the balance of incoming and outgoing energy in the Earth-atmosphere system and is an index of the importance of the factor as a potential climate change mechanism. In this report radiative forcing values are for changes relative to preindustrial conditions defined at 1750 and are expressed in Watts [sic] per square meter (W/m2)."
In simple terms, radiative forcing is "...the rate of energy change per unit area of the globe as measured at the top of the atmosphere." In the context of climate change, the term "forcing" is restricted to changes in the radiation balance of the surface-troposphere system imposed by external factors, with no changes in stratospheric dynamics, no surface and tropospheric feedbacks in operation (''i.e.'', no secondary effects induced because of changes in tropospheric motions or its thermodynamic state), and no dynamically induced changes in the amount and distribution of atmospheric water (vapour, liquid, and solid forms).

# Basic estimates

Radiative forcing can be evaluated for its dependence on different factors which are external to the climate system. Except where necessary and noted, the basic estimates which follow do not include indirect feedbacks (positive or negative) which also occur via Earth system responses. Forcing changes (ΔF) are expressed as yearly averages over the total surface of the planet. They may be significant in the context of global climate forcing for times spanning decades or longer.

## Forcing due to changes in solar irradiance

The intensity of solar irradiance including all wavelengths is the Total Solar Irradiance (TSI) and on average is the solar constant. It is equal to about 1361 W m−2 at the distance of Earth's annual-mean orbital radius of one astronomical unit and as measured at the top of the atmosphere. Earth TSI varies with both solar activity and planetary orbital dynamics. Multiple satellite-based instruments including Nimbus 7, ERB, ACRIMSAT, ACRIM 1-3, Solar and Heliospheric Observatory, VIRGO, and Solar Radiation and Climate Experiment, TIM have continuously measured TSI with improving accuracy and precision since 1978. Approximating Earth as a sphere, the cross-sectional area exposed to the Sun ($\pi r^2$) is equal to one quarter the area of the planet's surface ($4\pi r^2$). The globally and annually averaged amount of solar irradiance per square meter of Earth's atmospheric surface ($I_0$) is therefore equal to one quarter of TSI, and has a nearly constant value of $I_0=340~~\mathrm~\mathrm^$.

### Annual cycles

Earth follows an elliptical orbit around the Sun such that TSI received at any instance fluctuates between about 1321 W m−2 (at aphelion in early July) and 1412 W m−2 (at perihelion in early January), or thus by about +/-3.4% during each year. The change in instantaneous radiative forcing has minor influences on Earth's seasonal weather patterns and its climate zones, which primarily result from the annual cycling in Earth's relative tilt direction. Such repeating cycles contribute a net-zero forcing (by definition) in the context of decades-long climate changes.

### Sunspot activity

Average annual TSI varies between about 1360 W m−2 and 1362 W m−2 (+/-0.05%) over the course of a typical 11-year sunspot cycle, sunspot activity cycle. Sunspot observations have been recorded since about year 1600 and show evidence of lengthier oscillations (Gleissberg cycle, Devries/Seuss cycle, etc.) which modulate the 11-year cycle (Schwabe cycle). Despite such complex behavior, the amplitude of the 11-year cycle has been the most prominent variation throughout this long-term observation record. TSI variations associated with sunspots contribute a small but non-zero net forcing in the context of decadal climate changes. Some research suggests they may have partly influenced climate shifts during the Little Ice Age, along with concurrent changes in volcanic activity and deforestation. Since the late 20th century, average TSI has trended slightly lower along with a downward trend in sunspot activity.

### Milankovitch shifts

Climate forcing caused by variations in solar irradiance have occurred during Milankovitch cycles, which span periods of about 40,000 to 100,000 years. Milankovitch cycles consist of similarly long-duration cycles in Earth's orbital eccentricity (or ellipticity), orbital obliquity, and tilt direction. Among these, the 100,000 year cycle in eccentricity causes TSI to fluctuate by about +/-0.2%. Currently, Earth’s eccentricity is nearing its least elliptic (most circular) causing average annual TSI to very slowly decrease. Simulations also indicate that Earth's orbital dynamics will stability of the solar system, remain stable including these variations for least the next 10 million years.

### Sun aging

Our Sun has consumed about half its hydrogen fuel since forming approximately 4.5 billion years ago. TSI will continue to slowly increase during the aging process at a rate of about 1% each 100 million years. Such rate of change is far too small to be detectable within measurements and is insignificant on human timescales.

### TSI forcing summary

The maximum fractional variations (Δτ) in Earth's solar irradiance during the last decade are summarized in the accompanying table. Each variation previously discussed contributes a forcing of: : $\Delta F = ~I_0 \times \left(1-R\right) \times \Delta \tau ~~ = ~ 238 \times \Delta \tau ~~\left(\mathrm~\mathrm^\right) \,$, where R=0.30 is Earth's reflectivity. The radiative and climate forcings arising from changes in the Sun's insolation are expected to continue to be minor, notwithstanding some as-of-yet undiscovered solar physics.

## Forcing due to changes in albedo

A fraction of incident solar radiation is reflected by clouds & aerosols, oceans and landforms, snow & ice, vegetation, and other natural & man-made surface features. The reflected fraction is known as Earth's bond albedo (R), is evaluated at the top of the atmosphere, and has an average annual global value of about 0.30 (30%). The overall fraction of solar power absorbed by Earth is then (1-R) or 0.70 (70%). Atmospheric components contribute about three-quarters of Earth albedo, and clouds alone are responsible for half. The pronounced roles of clouds and water vapor are linked with the majority presence of liquid water covering Earth's crust, the planet's crust. Global patterns in cloud formation and circulation are highly complex phenomena with couplings to ocean heat flows, and with jet streams assisting their rapid transport. Moreover, the albedos of Earth's northern and southern hemispheres have been observed to be essentially equal (within 0.2%). This is noteworthy since more than two-thirds of land and 85% of the human population are distributed north. Multiple satellite-based instruments including Moderate Resolution Imaging Spectroradiometer, MODIS, Visible Infrared Imaging Radiometer Suite, VIIRs, and Clouds and the Earth's Radiant Energy System, CERES have continuously monitored Earth's albedo since 1998. Landsat program, Landsat imagery available since 1972 has also been used in some studies. Measurement accuracy has improved and results have converged in recent years, enabling more confident assessment of the recent decadal forcing influence of planetary albedo. Nevertheless, the existing data record is still too short to support longer-term predictions or to address other related questions.

### Annual cycles

Seasonal variations in planetary albedo can be understood as a set of system feedbacks that occur largely in response to the cycle of solar forcing. Along with the atmospheric responses, most apparent to surface dwellers are the changes in vegetation, snow, and sea-ice coverage. Intra-annual variations of about +/-0.02 (+/- 7%) around Earth's mean albedo have been observed throughout the course of a year, with maxima occurring twice per year near the time of each solar equinox. This repeating cycle contributes net-zero forcing in the context of decades-long climate changes.

### Interannual variability

Regional albedos change from year to year due to shifts arising from natural processes, human actions, and system feedbacks. For example, human acts of deforestion typically raise Earth's reflectivity while introducing water storage and irrigation to arid lands may lower it. Likewise considering feedbacks, Ice–albedo feedback, ice loss in arctic regions decreases albedo while expanding desertification at low to middle latitudes increases it. During years 2000-2012, no overall trend in Earth's albedo was discernible within the 0.1% standard deviation of values measured by CERES. Along with the hemispherical equivalence, some researchers interpret the remarkably small interannual differences as evidence that planetary albedo may currently be constrained by the action of complex system feedbacks. Nevertheless, historical evidence also suggests that infrequent events such as major volcanic winter, volcanic eruptions can significantly perturb the planetary albedo for several years or longer.

### Albedo forcing summary

The measured fractional variations (Δα) in Earth's albedo during the first decade of the 21st century are summarized in the accompanying table. Similar to TSI, the radiative forcing due to a fractional change in planetary albedo (Δα) is: : $\Delta F = ~-I_0 \times R \times \Delta \alpha ~~ = ~-102 \times \Delta \alpha ~~\left(\mathrm~\mathrm^\right) \,$. Satellite observations show that various Earth system feedbacks have stabilized planetary albedo despite recent natural and human-caused shifts. On longer timescales, it is more uncertain whether the net forcing which results from such external changes will remain minor.

## Forcing due to changes in atmospheric gas

For a well-mixed greenhouse gas, atmospheric radiative transfer codes, radiative transfer codes that examine each spectral line for atmospheric conditions can be used to calculate the forcing change ΔF as a function of a change in its concentration. These calculations may be simplified into an algebraic formulation that is specific to that gas.

### Carbon dioxide

A simplified first-order approximation expression for carbon dioxide is: : $\Delta F = 5.35 \times \ln ~~\left(\mathrm~\mathrm^\right) \,$, where ''C'' is the concentration in parts per million (ppm) by volume and ''C''0 is the reference concentration (278 ppm in year 1750}) prior to substantial anthropogenic changes. The atmospheric burden of greenhouse gases due to human activity has grown especially rapidly during the last several decades (since about year 1950). The 50% increase (C/C0=1.5) for realized as of year 2020 corresponds to $\Delta F=+2.1~~\mathrm~\mathrm^$. By comparison, a sustained 1% increase in TSI or 2% decrease in albedo might be required to induce a similar magnitude of forcing, as per these basic estimates. Assuming no change in the emissions growth path, a doubling (C/C0=2) within the next several decades would correspond to ΔF=+3.7 W m−2. The relationship between and radiative forcing is logarithmic scale, logarithmic at concentrations up to around eight times the current value. Increased concentrations thus have a progressively smaller warming effect. However, the first-order approximation is inaccurate at higher concentrations and there is no saturation in the absorption of infrared radiation by .

### Other trace gases

Somewhat different formulae apply for other trace greenhouse gases such as methane and (square-root dependence) or CFCs (linear), with coefficients that may be found for example in the Intergovernmental Panel on Climate Change, IPCC reports. A year 2016 study suggests a significant revision to the methane IPCC formula. Forcings by the most influential trace gases in Earth's atmosphere are included in the section describing #Recent growth trends, recent growth trends, and in the IPCC list of greenhouse gases.

### Water vapor

Water vapor is Earth's primary greenhouse gas currently responsible for about half of all atmospheric gas forcing. Its overall atmospheric concentration depends almost entirely on the average planetary temperature, and has the potential to increase by as much as 7% with every degree (°C) of temperature rise (see also: Clausius–Clapeyron relation). Thus over long time scales, water vapor behaves as a system feedback that amplifies the radiative forcing driven by the growth of carbon dioxide and other trace gases.

# Climate sensitivity

Radiative forcing can be used to estimate a subsequent change in steady-state (often denoted "equilibrium") surface temperature (Δ''T''s) arising from that forcing via the equation: : $\Delta T_s =~ \lambda~\Delta F$ where λ is commonly denoted the climate sensitivity parameter, usually with units K/(W/m2), and Δ''F'' is the radiative forcing in W/m2. A typical value of λ, 0.8 K/(W/m2), gives an increase in global temperature of about 1.6 K above the 1750 reference temperature due to the increase in over that time (278 to 405 ppm, for a forcing of 2.0 W/m2), and predicts a further warming of 1.4 K above present temperatures if the mixing ratio in the atmosphere were to become double its pre-industrial value; both of these calculations assume no other forcings. Historically, radiative forcing displays the best predictive capacity for specific types of forcing such as greenhouse gases. It is less effective for other anthropogenic influences like soot. A new framework called ‘effective radiative forcing’ or ERF removes the effect of rapid adjustments within the atmosphere that are unrelated to longer term surface temperature responses. ERF means different factors driving climate change can be placed onto a level playing field to enable comparison of their effects and a more consistent view of how global surface temperature responds to various types of human forcing.

# Related metrics

* Climate sensitivity * Anthropogenic heat * Emission standard * Global warming potential

# References

IPCC glossary

CO2: The Thermostat that Controls Earth's Temperature
by NASA's Goddard Institute for Space Studies, October, 2010, Forcing vs. Feedbacks * Intergovernmental Panel on Climate Change’s IPCC Fourth Assessment Report, Fourth Assessment Report (2007), Chapter 2
"Changes in Atmospheric Constituents and Radiative Forcing,"
pp. 133–134 (PDF, 8.6 MB, 106 pp.). * Environmental Protection Agency, U.S. EPA (2009)
Climate Change – Science
Explanation of climate change topics including radiative forcing. * United States National Research Council (2005),