Radiative forcing

Radiative forcing (or climate 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 (expressed in W/m2) 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 stratopause. 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 (since about year 1950). For carbon dioxide, the 50% increase (C/C0 = 1.5) realized as of year 2020 since 1750 corresponds to a cumulative radiative forcing change (ΔF) of +2.17 W/m2. Assuming no change in the emissions growth path, a doubling of concentrations (C/C0 = 2) within the next several decades would correspond to a cumulative radiative forcing change (ΔF) 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 (CFCs) play smaller roles as time goes on. The five major greenhouse gases (water vapor, carbon dioxide, methane, nitrous oxide and ozone) 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 (expressed in W/m2) due to a change in an external driver of climate change, such as a change in the concentration of carbon dioxide (CO2), 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 (i.e. the balance between absorbed and radiated energy) 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 (global dimming).

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 (RCE) 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 (effectively imposed onto the top-of-atmosphere energy balance) will be met by slower responses (correlated more or less with changes in a planet's surface temperature) 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.

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