The climate-science term radiative forcing (RF) refers to an imbalance in the energy gained and lost by Earth. The capture of more or less solar energy by Earth can becaused by greenhouse gases in the atmosphere, aerosols (small solid particles suspended in the air), clouds, ice and snow, and vegetation. Radiative forcing (often called climate forcing or forcing) is positive if it adds energy to the climate system: Greenhouse gases, dark aerosol particles, and vegetation produce positive radiative forcing. Radiative forcing is negative if it subtracts energy from the climate system; ice, snow, and light-colored aerosol particles produce negative radiative forcing. Scientific understanding of radiative forcing is essential to understanding the history and future of global climate change.
Historical Background and Scientific Foundations
Almost all of the heat energy present in Earth’s soil, air, and water comes originally from the sun in the form of light. A small fraction (about 1/7500) comes from the interior of the planet, which is heated by the decay of radioactive elements; an even smaller fraction comes from the rest of the universe.
Light is a form of radiation, that is, energy traveling freely through space. All light, if absorbed by substance, transfers its energy to that substance, heating it or changing it chemically. However, some materials are transparent to some kinds of light: Glass and air, for example, are transparent to visible light, the kind of light the eyes can see. Also, all objects reflect at least some of the light that falls on them. Snow, for example, reflects most visible light, which is why it is white.
When visible light from the sun arrives at Earth, it passes easily through the cloudless parts of the atmosphere and strikes the ground or ocean. Most of it (65-70%, a global average) is absorbed, warming the surface; the rest (30-35%) is reflected back up into space. As all warm objects radiate or emit a type of invisible light called infrared light, Earth’s surface, warmed by visible light, glows invisibly in the infrared. This infrared glow shines up toward space. Some of it reaches space and its energy leaves Earth forever, but the rest is intercepted by gases in the atmosphere that are opaque to infrared light, that is, which absorb it. The more concentrated these gases are in the atmosphere, the less infrared light escapes into space, the more is absorbed, and the warmer the atmosphere becomes. This entrapment of energy is termed the greenhouse effect, and the gases that cause the greenhouse effect are called greenhouse gases. Greenhouse gases in the atmosphere act like an invisible blanket, keeping Earth warmer than it would be otherwise.
Most of the greenhouse effect is natural. Without the greenhouse effect, Earth’s surface temperature would be well below freezing, even at the equator. However, human activities have been increasing the greenhouse effect for at least 200 years. The burning of fossil fuels and deforestation have been adding greenhouse gases to the atmosphere, warming the globe. At the same time, other human activities, especially the emission of small, light-colored particles of air pollution, have tended to reflect more light from Earth and so cool the globe. Each of these changes in the natural energy balance of Earth is a radiative forcing. Radiative forcings can also be natural, as when a volcano spews millions of tons of light-colored particles into the atmosphere (a temporary negative radiative forcing).
Radiative forcing can be calculated in several different forms. The simplest form, termed instantaneous RF, is the amount of energy gained or lost by Earth per unit of time per square meter of an imaginary surface surrounding the planet at the top of the atmosphere. Energy flow is expressed in watts and area in square meters, so the units of RF are watts per square meter, W/m2.
The largest radiative forcing today is from greenhouse gases present in the atmosphere in amounts above natural levels. These gases include carbon dioxide (CO2), methane (CH4), nitrous oxide (N20), low-altitude ozone, and halocarbons. As of 2005, according to the United Nations’ Intergovernmental Panel on Climate Change (IPCC), anthropogenic greenhouse gases were causing an RF of 2.63 W/m2. Carbon dioxide alone was responsible for 1.66 W/m2, 63% of the total RF from anthropogenic gases and up by 20% from its 1995 value. Low-altitude ozone pollution added another 0.4 W/m2 or so of positive RF. A smaller positive RF, about 0.1 or 0.2 W/m2, was caused by darkening of snow and ice by black carbon particles (soot) released by burning fossil fuel and biofuels such as wood.
Negative or cooling RFs were caused by land-use changes that lightened the land (for example, turning forests into grasslands), adding light-colored aerosol particles to the air, and extra cloud formation due to aerosols. The total positive anthropogenic RF was about 2.9 W/m2, the total negative RF a little more than –1 W/m2, for a total anthropogenic RF of about 1.75 W/m2.
Climate forcings are distinguished from climate feedbacks. A climate feedback is any aspect of the Earth system that is changed by radiative forcing, and then changes the forcing itself, making it stronger or weaker. For example, the melting of ice and snow is a positive climate feedback, because positive forcing tends to increase melting, which exposes dark ground or water where there used to be a reflective surface, which causes Earth to absorb more energy, which increases the radiative forcing that caused the melting initially. Water vapor in the atmosphere, which is the most abundant greenhouse gas, is a feedback, not a forcing, because warmer climate increases evaporation (the shift of water from liquid to vapor), which increases radiative forcing by trapping more solar energy. Also, water vapor does not remain in the atmosphere very long—on the order of weeks, as opposed to decades for other greenhouse gases).
Impacts and Issues
Over Earth’s total area, a positive RF of about 1.75 W/m2 amounts to a large amount of energy—on the order of a thousand trillion watts. This energy is continuously entering Earth’s climate system over and above the level which was entering it before human beings began to unintentionally modify the climate. It accounts for the rise in average global surface air temperatures that has been observed, about 1.33°F (0.74°C) from 1905 to 2005.
There is a high level of scientific confidence in the RF calculated for long-lived greenhouse gases: that is, the chances are small that the real value is very different from the calculated value. These gases are mixed throughout Earth’s whole atmosphere and capture heat in a relatively simple way. However, there are much greater uncertainties for smaller sources of RF, both positive and negative. In particular, the negative RF from aerosols is highly uncertain. In 2005, the National Academy of Sciences went so far as to say that new studies on non-gaseous sources of RF, especially aerosols and their effects on clouds, had “raised doubts as to the continued viability of the radiative forcing concept” and had “raised the question of whether the radiative forcing concept has outlived its usefulness and, if so, what new climate change metrics should be used.”
WORDS TO KNOW
AEROSOL: Liquid droplets or minute particles suspended in air.
ANTHROPOGENIC: Made by humans or resulting from human activities.
EVAPORATION: Change from a liquid (more dense) to a vapor or gas (less dense). When water is heated it becomes a vapor that increases humidity. Evaporation is the opposite of condensation.
INFRARED LIGHT: Portion of the electromagnetic spectrum with wavelengths slightly longer than optical light that takes the form of heat.
However, the National Academy did not conclude that the radiative forcing concept was obsolete, only that it needs continued study. The concept was used by the Intergovernmental Panel on Climate Change in its 2007 report and continues to be a mainstay of computerized climate modeling. Uncertainties in negative RF terms do not call into question the reality of positive anthropogenic climate forcing because the terms scientists are most sure of (RFs from gases) are large and positive, while the terms they are least sure of (aerosol effects) are small and negative. Large positive certainties outweigh small negative uncertainties: Human beings are indeed warming the planet. Without anthropogenic radiative forcing, computer models of climate cannot be made to reflect the amount of warming that has actually been observed.
Solomon. S., et al, eds. Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.
Anderson, Theodore L., et al. “Climate Forcing by Aerosols—A Hazy Picture.” Science 300 (2003): 1103–1104.
National Research Council of the National Academy of Sciences. “Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties.” http://www.nap.edu/openbook.php?isbn=0309095069 (accessed April 11, 2008).