Radiative forcing

Radiative forcing is a concept used to quantify a change to the balance of energy flowing through a planetary atmosphere. Various factors contribute to this change in energy balance, such as concentrations of greenhouse gases and aerosols, and changes in surface albedo and solar irradiance. In more technical terms, it is defined as "the change in the net, downward minus upward, radiative flux due to a change in an external driver of climate change." These external drivers are distinguished from feedbacks and variability that are internal to the climate system, and that further influence the direction and magnitude of imbalance. Radiative forcing on Earth is meaningfully evaluated at the tropopause and at the top of the stratosphere. It is quantified in units of watts per square meter, and often summarized as an average over the total surface area of the globe.
A planet in radiative equilibrium with its parent star and the rest of space can be characterized by net zero radiative forcing and by a planetary equilibrium temperature.
Radiative forcing is not a thing in the sense that a single instrument can independently measure it. Rather it is a scientific concept and entity whose strength can be estimated from more fundamental physics principles. Scientists use measurements of changes in atmospheric parameters to calculate the radiative forcing.
The IPCC summarized the current scientific consensus about radiative forcing changes as follows: "Human-caused radiative forcing of 2.72 W/m² in 2019 relative to 1750 has warmed the climate system. This warming is mainly due to increased GHG concentrations, partly reduced by cooling due to increased aerosol concentrations".
The atmospheric burden of greenhouse gases due to human activity has grown especially rapidly during the last several decades. For carbon dioxide, the 50% increase realized as of year 2020 since 1750 corresponds to a cumulative radiative forcing change of +2.17 W/m². Assuming no change in the emissions growth path, a doubling of concentrations within the next several decades would correspond to a cumulative radiative forcing change of +3.71 W/m².
Radiative forcing can be a useful way to compare the growing warming influence of different anthropogenic greenhouse gases over time. The radiative forcing of long-lived and well-mixed greenhouse gases have been increasing in earth's atmosphere since the industrial revolution. Carbon dioxide has the biggest impact on total forcing, while methane and chlorofluorocarbons play smaller roles as time goes on. The five major greenhouse gases account for about 96% of the direct radiative forcing by long-lived greenhouse gas increases since 1750. The remaining 4% is contributed by the 15 minor halogenated gases.

Definition and fundamentals

Radiative forcing is defined in the IPCC Sixth Assessment Report as follows: "The change in the net, downward minus upward, radiative flux due to a change in an external driver of climate change, such as a change in the concentration of carbon dioxide, the concentration of volcanic aerosols or the output of the Sun."
There are some different types of radiative forcing as defined in the literature:

Stratospherically adjusted radiative forcing: "when all tropospheric properties held fixed at their unperturbed values, and after allowing for stratospheric temperatures, if perturbed, to readjust to radiative-dynamical equilibrium."
Instantaneous radiative forcing: "if no change in stratospheric temperature is accounted for".
Effective radiative forcing: "once both stratospheric and tropospheric adjustments are accounted for".

The radiation balance of the Earth determines the average global temperature. This balance is also called Earth's energy balance. Changes to this balance occur due to factors such 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, which along with its climate feedbacks, ultimately changes the balance. This happens continuously as sunlight hits the surface of Earth, clouds and aerosols form, the concentrations of atmospheric gases vary and seasons alter the groundcover.
Positive radiative forcing means Earth receives more incoming energy from sunlight than it radiates to space. This net gain of energy will cause global warming. Conversely, negative radiative forcing means that Earth loses more energy to space than it receives from the Sun, which produces cooling.

History

Transport of energy and matter in the Earth-atmosphere system is governed by the principles of equilibrium thermodynamics and more generally non-equilibrium thermodynamics. During the first half of the 20th century, physicists developed a comprehensive description of radiative transfer that they began to apply to stellar and planetary atmospheres in radiative equilibrium. Studies of radiative-convective equilibrium followed and matured through the 1960s and 1970s. RCE models began to account for more complex material flows within the energy balance, such as those from a water cycle, and thereby described observations better.
Another application of equilibrium models is that a perturbation in the form of an externally imposed intervention can estimate a change in state. The RCE work distilled this into a forcing-feedback framework for change, and produced climate sensitivity results agreeing with those from GCMs. This conceptual framework asserts that a homogeneous disturbance will be met by slower responses to bring the system to a new equilibrium state. Radiative forcing was a term used to describe these disturbances and gained widespread traction in the literature by the 1980s.

Related metrics

The concept of radiative forcing has been evolving from the initial proposal, named nowadays instantaneous radiative forcing, to other proposals that aim to relate better the radiative imbalance with global warming. For example, researchers explained in 2003 how the adjusted troposphere and stratosphere forcing can be used in general circulation models.
The adjusted radiative forcing, in its different calculation methodologies, estimates the imbalance once the stratosphere temperatures has been modified to achieve a radiative equilibrium in the stratosphere. This new methodology is not estimating any adjustment or feedback that could be produced on the troposphere, for that goal another definition, named effective radiative forcing has been introduced. In general the ERF is the recommendation of the CMIP6 radiative forcing analysis although the stratospherically adjusted methodologies are still being applied in those cases where the adjustments and feedbacks on the troposphere are considered not critical, like in the well mixed greenhouse gases and ozone. A methodology named radiative kernel approach allows to estimate the climate feedbacks within an offline calculation based on a linear approximation

Uses

Climate change attribution

Radiative forcing is used to quantify the strengths of different natural and man-made drivers of Earth's energy imbalance over time. The detailed physical mechanisms by which these drivers cause the planet to warm or cool are varied. Radiative forcing allows the contribution of any one driver to be compared against others.
Another metric 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 that climate change drivers can be placed onto a more 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.

Climate sensitivity

Radiative forcing and climate feedbacks can be used together to estimate a subsequent change in steady-state surface temperature via the equation:
where commonly denotes the climate sensitivity parameter, usually with units K/, and ΔF is the radiative forcing in W/m². An estimate for is obtained from the inverse of the climate feedback parameter having units /K. An estimated value of gives an increase in global temperature of about 1.6 K above the 1750 reference temperature due to the increase in over that time, 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.

Calculations and measurements

Atmospheric observation

Earth's global radiation balance fluctuates as the planet rotates and orbits the Sun, and as global-scale thermal anomalies arise and dissipate within the terrestrial, oceanic and atmospheric systems. Consequently, the planet's 'instantaneous radiative forcing' is also dynamic and naturally fluctuates between states of overall warming and cooling. The combination of periodic and complex processes that give rise to these natural variations will typically revert over periods lasting as long as a few years to produce a net-zero average IRF. Such fluctuations also mask the longer-term forcing trends due to human activities, and thus make direct observation of such trends challenging.
Earth's radiation balance has been continuously monitored by NASA's Clouds and the Earth's Radiant Energy System instruments since year 1998. Each scan of the globe provides an estimate of the total instantaneous radiation balance. This data record captures both the natural fluctuations and human influences on IRF; including changes in greenhouse gases, aerosols, land surface, etc. The record also includes the lagging radiative responses to the radiative imbalances; occurring mainly by way of Earth system feedbacks in temperature, surface albedo, atmospheric water vapor and clouds.
Researchers have used measurements from CERES, AIRS, CloudSat and other satellite-based instruments within NASA's Earth Observing System to parse out contributions by the natural fluctuations and system feedbacks. Removing these contributions within the multi-year data record allows observation of the anthropogenic trend in top-of-atmosphere IRF. The data analysis has also been done in a way that is computationally efficient and independent of most related modelling methods and results. Radiative forcing was thus directly observed to have risen by +0.53 W m⁻² from years 2003 to 2018. About 20% of the increase was associated with a reduction in the atmospheric aerosol burden, and most of the remaining 80% was attributed to the rising burden of greenhouse gases.
A rising trend in the radiative imbalance due to increasing global has been previously observed by ground-based instruments. For example, such measurements have been separately gathered under clear-sky conditions at two Atmospheric Radiation Measurement sites in Oklahoma and Alaska. Each direct observation found that the associated radiative heating experienced by surface dwellers rose by +0.2 W m⁻² during the decade ending 2010. In addition to its focus on longwave radiation and the most influential forcing gas only, this result is proportionally less than the TOA forcing due to its buffering by atmospheric absorption.