Radiative Forcing
Radiative forcing is a key concept in understanding the Earth's energy budget, reflecting the imbalance between incoming solar energy and outgoing infrared radiation due to various factors. It is influenced by both natural phenomena and human activities, such as the emission of greenhouse gases (GHGs) and changes in land use. Positive radiative forcing indicates warming, often caused by substances like carbon dioxide (CO2) and methane (CH4), while negative radiative forcing suggests a cooling effect, typically from aerosols that reflect sunlight.
The measurement of radiative forcing is given in watts per square meter, and it provides insight into the climatic impacts of changes in GHG concentrations, volcanic activity, and other factors. Since the Industrial Revolution, radiative forcing has risen significantly, primarily due to increased GHG levels, with a notable contribution from CO2 emissions. This has led to a net warming effect and stimulated changes in climate systems.
Understanding radiative forcing is crucial for evaluating the extent of climate change, as it establishes a link between human actions and their environmental impacts. However, while it shows trends in energy imbalance, it cannot directly quantify the overall effects of climate change.
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Radiative Forcing
Summary: Radiative forcing is an atmospheric measure of the imbalance in Earth’s energy budget, resulting from human activities or natural phenomena.
Radiative forcing occurs when the balance between the energy reaching Earth through the atmosphere and the amount of energy radiating away from the planet is altered. The word radiative refers to the energy flow that takes place through incoming solar radiation and outgoing infrared rays. The term forcing implies that the imbalance is forced, or caused by external influences, on the atmospheric and climatic systems of our planet. Any changes in the Earth’s energy balance resulting from changes in the internal dynamics of the atmosphere are not considered a part of radiative forcing. The phenomenon is measured between two layers of the atmosphere, the troposphere and stratosphere.
Radiative forcing is measured in the units of net energy incident per unit area of the planet’s surface, generally watts per square meter. A positive value for radiative forcing is associated with an increase, or warming, of surface temperatures. Substances that absorb the solar radiation or trap infrared radiation are responsible for positive forcing. These substances include gases with high global warming potential (GWP). Negative radiative forcing, by contrast, is a cooling effect and is associated with substances that reflect incident radiation. If the rate of incoming energy is equal to the rate at which energy is lost, then the value of radiative forcing will be zero and the planet’s temperature will remain in equilibrium.
Gases and Other Factors
Some of the major factors that have an impact on the value of radiative forcing are changes in the concentration of greenhouse gases (GHGs) in the atmosphere, changes in the area of the planet’s surface covered by forest, the presence of aerosols in the atmosphere, variation in the sun’s radiation output, and volcanic output. Estimating the value of radiative forcing allows us to understand, as well as compare the climatic impacts of, anthropogenic activities and natural phenomena.
The Intergovernmental Panel on Climate Change (IPCC) first used the term radiative forcing in its reports. According to the Fourth Assessment Report (2007), the largest contribution to positive forcing comes from well-mixed GHGs, which include carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), chlorofluorocarbons (CFCs), and hydrochlorofluorocarbons (HCFCs). GHG molecules tend to be extremely persistent, generating cumulative effects. And they circulate well; a “well-mixed” gas is distributed at a given concentration fairly evenly throughout the atmosphere, as by definition its concentration is not temperature-dependent.
The contribution of aerosols, however, is difficult to determine. Certain types have a cooling effect, while aerosols such as black carbon increase the absorption of incoming radiation. The effect of clouds on radiative forcing is also uncertain. Clouds can reflect radiation, but their water vapor absorbs infrared rays. Volcanic eruptions have been observed to cause a cooling effect in the atmosphere; the particles released tend to reflect radiation flowing from the sun.
Human Inputs
Radiative forcing has increased since the beginning of the Industrial Revolution, as a result of the emission of GHGs. For the sake of calculations, scientific studies consider the era between the 18th and the 19th centuries as the base period. This is the period before the beginning of industrialization, for which the value of radiative forcing is considered as zero; the year 1750 correlates to this start point. According to the Fourth Assessment Report, since 1750 a radiative forcing (RF) contribution of plus 2.4 watts per square meter is attributed to the increase in the concentration of well-mixed GHGs such as CO2, CH4, N2O, CFCs, HCFCs, and fluorinated gases.
Of that amount, plus 1.6 watts per square meter—about two thirds of the total— is due to the increase in atmospheric CO2 alone. The direct cooling impact of aerosols is estimated to be minus 0.5 watt per square meter. Human land use patterns taken as a whole have had a minor cooling effect. The values of radiative forcing for each factor have a corresponding uncertainty. For example, the uncertainty in radiative forcing due to linear contrails is minimal, whereas great uncertainty occurs in the RF value due to aerosols.
The Annual Greenhouse Gas Index (AGGI) also tracks changes in RF value due to the concentration of GHGs. The US National Oceanic and Atmospheric Administration maintains this index. According to the AGGI, RF from GHGs increased by 45 percent between 1990 and 2019.
The combined RF due to human activities since 1750, ranging from deforestation to industrial and transportation sector emissions, is pegged at 1.6 watts per square meter. This implies that human existence has resulted in a net warming, and hence stimulated changes in the planet’s climate system.
While RF is intrinsically linked to the phenomenon of climate change, the value of radiative forcing cannot be used on a simple, one-to-one basis to quantify the extent of climate change. However, an increase in concentration of GHGs will cause an increase in radiative forcing, which, in turn, will accelerate climate change and magnify its impacts.
Bibliography
Amer. Chemical Soc. “Radiative Forcing.” ACS.org. ACS, 2015. Web. 28 Dec. 2015.
"Climate Change Indicators: Climate Forcing." US Environmental Protection Agency, 27 June 2024, www.epa.gov/climate-indicators/climate-change-indicators-climate-forcing. Accessed 31 July 2024.
Dawson, B., and M. Spannagle. The Complete Guide to Climate Change. New York: Routledge, 2009. Print.
Intergovernmental Panel on Climate Change. Changes in Atmospheric Constituents and in Radiative Forcing: Contribution of Working Group I to the Fourth Assessment Report of the IPCC. Geneva: IPCC, 2007. Print.
Muller, Rhonda C. et al. "Radiative Forcing Geoengineering under High CO2 Levels Leads to Higher Risk of Arctic Wildfires and Permafrost Thaw than a Targeted Mitigation Scenario." Communications Earth & Environment, vol. 5, no. 180, 5 Apr. 2024, doi.org/10.1038/s43247-024-01329-3. Accessed 31 July 2024.
National Research Council. Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties. Washington, DC: Natl. Academies P, 2005. Print.
Westervelt, D. M., et al. “Radiative Forcing and Climate Response to Projected 21st Century Aerosol Decreases.” Atmospheric Chemistry & Physics 15.22 (2015): 12681–703. Environment Complete. Web. 28 Dec. 2015.