Radiative Forcing of Climate: The Historical Evolution of the Radiative Forcing Concept, the Forcing Agents and their Quantification, and Applications

Abstract

We describe the historical evolution of the conceptualization, formulation, quantification, application, and utilization of “radiative forcing” (RF) of Earth’s climate. Basic theories of shortwave and longwave radiation were developed through the nineteenth and twentieth centuries and established the analytical framework for defining and quantifying the perturbations to Earth’s radiative energy balance by natural and anthropogenic influences. The insight that Earth’s climate could be radiatively forced by changes in carbon dioxide, first introduced in the nineteenth century, gained empirical support with sustained observations of the atmospheric concentrations of the gas beginning in 1957. Advances in laboratory and field measurements, theory, instrumentation, computational technology, data, and analysis of well-mixed greenhouse gases and the global climate system through the twentieth century enabled the development and formalism of RF; this allowed RF to be related to changes in global-mean surface temperature with the aid of increasingly sophisticated models. This in turn led to RF becoming firmly established as a principal concept in climate science by 1990. The linkage with surface temperature has proven to be the most important application of the RF concept, enabling a simple metric to evaluate the relative climate impacts of different agents. The late 1970s and 1980s saw accelerated developments in quantification, including the first assessment of the effect of the forcing due to the doubling of carbon dioxide on climate (the “Charney” report). The concept was subsequently extended to a wide variety of agents beyond well-mixed greenhouse gases (WMGHGs; carbon dioxide, methane, nitrous oxide, and halocarbons) to short-lived species such as ozone. The WMO and IPCC international assessments began the important sequence of periodic evaluations and quantifications of the forcings by natural (solar irradiance changes and stratospheric aerosols resulting from volcanic eruptions) and a growing set of anthropogenic agents (WMGHGs, ozone, aerosols, land surface changes, contrails). From the 1990s to the present, knowledge and scientific confidence in the radiative agents acting on the climate system have proliferated. The conceptual basis of RF has also evolved as both our understanding of the way radiative forcing drives climate change and the diversity of the forcing mechanisms have grown. This has led to the current situation where “effective radiative forcing” (ERF) is regarded as the preferred practical definition of radiative forcing in order to better capture the link between forcing and global-mean surface temperature change. The use of ERF, however, comes with its own attendant issues, including challenges in its diagnosis from climate models, its applications to small forcings, and blurring of the distinction between rapid climate adjustments (fast responses) and climate feedbacks; this will necessitate further elaboration of its utility in the future. Global climate model simulations of radiative perturbations by various agents have established how the forcings affect other climate variables besides temperature (e.g., precipitation). The forcing–response linkage as simulated by models, including the diversity in the spatial distribution of forcings by the different agents, has provided a practical demonstration of the effectiveness of agents in perturbing the radiative energy balance and causing climate changes. The significant advances over the past half century have established, with very high confidence, that the global-mean ERF due to human activity since preindustrial times is positive (the 2013 IPCC assessment gives a best estimate of 2.3 W m−2, with a range from 1.1 to 3.3 W m−2; 90% confidence interval). Further, except in the immediate aftermath of climatically significant volcanic eruptions, the net anthropogenic forcing dominates over natural radiative forcing mechanisms. Nevertheless, the substantial remaining uncertainty in the net anthropogenic ERF leads to large uncertainties in estimates of climate sensitivity from observations and in predicting future climate impacts. The uncertainty in the ERF arises principally from the incorporation of the rapid climate adjustments in the formulation, the well-recognized difficulties in characterizing the preindustrial state of the atmosphere, and the incomplete knowledge of the interactions of aerosols with clouds. This uncertainty impairs the quantitative evaluation of climate adaptation and mitigation pathways in the future. A grand challenge in Earth system science lies in continuing to sustain the relatively simple essence of the radiative forcing concept in a form similar to that originally devised, and at the same time improving the quantification of the forcing. This, in turn, demands an accurate, yet increasingly complex and comprehensive, accounting of the relevant processes in the climate system.

Assessment of pre-industrial to present-day anthropogenic climate forcing in UKESM1

“…interactions when quantifying PD climate forcing. In particular, we consider that rapid adjustments included in the definition of ERF should include chemical as well as physical adjustments, consistent with Ramaswamy et al (2019). They concluded https://doi.org/10.5194/acp-2019-1152 Preprint.…”

“…of the historical evolution of the RF concept, its quantification for different forcing agents, and applications of RF can be found in Ramaswamy et al (2019).…”

Radiative forcing of climate change from the Copernicus reanalysis of atmospheric composition

“…The initial top-ofatmosphere imbalance is the instantaneous radiative forcing. Several decades ago, it was realized that for comparison of climate change mechanisms the radiative flux change at the tropopause, or equivalently at the top of the atmosphere after stratospheric temperatures are adjusted to equilibrium, was a better predictor for the surface temperature change and defined as radiative forcing (RF) (Ramanathan, 1975;Shine et al, 1990;Ramaswamy et al, 2019). The adjustment time in the 50 stratosphere is of the order of 2 to 3 months and is several orders of magnitude shorter than the time required for the surfacetropospheric system to equilibrate after a (time independent) perturbation.…”

Glyoxal's impact on dry ammonium salts: fast and reversible surface aerosol browning

“…We then multiply this energy fraction times the magnitude of global direct radiative forcing due to sulfate scattering, estimated by the IPCC to be -0.4 ±0.2 W/m 2 (Ramaswamy et al, 2018), to quantify a global radiative forcing of +7.6 ´10 -5 W/m 2 by dry browning of ammonium sulfate aerosol. This climate forcing is negligible compared to the global net aerosol direct effect (-0.45 ±0.5 W/m 2 ) or absorption by black carbon (+0.4 +0.4 -0.35 W/m 2 ) (Ramaswamy et al, 2018), and is less than 1% of estimates of radiative forcing by secondary brown carbon (+0.015 to +0.081 W/m 2 ) (Mukai and Ambe, 1986;Hecobian et al, 230 2010;Shamjad et al, 2015;Tuccella et al, 2020).…”

“…We then multiply this energy fraction times the magnitude of global direct radiative forcing due to sulfate scattering, estimated by the IPCC to be -0.4 ±0.2 W/m 2 (Ramaswamy et al, 2018), to quantify a global radiative forcing of +7.6 ´10 -5 W/m 2 by dry browning of ammonium sulfate aerosol. This climate forcing is negligible compared to the global net aerosol direct effect (-0.45 ±0.5 W/m 2 ) or absorption by black carbon (+0.4 +0.4 -0.35 W/m 2 ) (Ramaswamy et al, 2018), and is less than 1% of estimates of radiative forcing by secondary brown carbon (+0.015 to +0.081 W/m 2 ) (Mukai and Ambe, 1986;Hecobian et al, 230 2010;Shamjad et al, 2015;Tuccella et al, 2020). While dry browning of ammonium sulfate aerosol in the presence of ambient glyoxal thus does not appear to be globally significant in terms of radiative forcing, it may be regionally significant in polluted areas where glyoxal concentrations can greatly exceed 70 ppt (Volkamer et al, 2005a), where larger loadings of AS aerosol are present, or where aerosol browning by glyoxal occurs in the upper troposphere (Zhang et al, 2017).…”