Radiative forcing, also called climate forcing, is a way to measure changes in the balance of energy moving through a planet's atmosphere. Factors like the amounts of greenhouse gases and aerosols in the air, changes in how much sunlight is reflected by Earth's surface (called albedo), and changes in the sun's energy reaching Earth all affect this energy balance. In technical terms, radiative forcing is the change in the net amount of energy (measured in watts per square meter) moving into and out of Earth's atmosphere due to changes in climate drivers. These drivers are different from feedbacks and natural changes that happen within the climate system and can affect how much energy is gained or lost. Scientists evaluate radiative forcing at specific layers of Earth's atmosphere, such as the tropopause and stratopause. It is measured in watts per square meter and is often described as an average over Earth's entire surface.
A planet that is in balance with its star and space has no net radiative forcing and a stable temperature.
Radiative forcing is not something that can be directly measured by a single tool. Instead, it is a scientific idea that scientists estimate using basic physics and data from the atmosphere. They use changes in atmospheric conditions to calculate radiative forcing.
The IPCC summarized scientific agreement about radiative forcing as follows: "Human activities have caused a radiative forcing of 2.72 watts per square meter in 2019 compared to 1750. This has warmed Earth's climate system, mainly because of higher greenhouse gas levels, partly offset by cooling from more aerosols in the air."
Human activities have greatly increased greenhouse gas levels since about 1950. For carbon dioxide, a 50% increase in concentration since 1750 (from 1.5 times the 1750 level) has led to a radiative forcing change of +2.17 watts per square meter. If current trends continue, doubling carbon dioxide levels (to 2 times the 1750 level) in the next few decades would result in a radiative forcing change of +3.71 watts per square meter.
Radiative forcing helps compare the warming effects of different human-caused greenhouse gases over time. Since the industrial revolution, the levels of long-lived and well-mixed greenhouse gases in Earth's atmosphere have increased. Carbon dioxide has the largest impact on total radiative forcing, while methane and chlorofluorocarbons (CFCs) contribute less over time. The five main greenhouse gases—water vapor, carbon dioxide, methane, nitrous oxide, and ozone—account for about 96% of the direct radiative forcing from long-lived greenhouse gases since 1750. The remaining 4% comes from 15 other, less common gases.
Definition and fundamentals
Radiative forcing is explained in the IPCC Sixth Assessment Report as the change in the net amount of energy (measured in watts per square meter) that reaches Earth compared to what leaves Earth. This change happens because of factors that affect Earth's climate, such as changes in carbon dioxide levels, volcanic particles in the air, or changes in the Sun's energy output.
There are different types of radiative forcing:
– Stratospherically adjusted radiative forcing: This is calculated by keeping the air near Earth’s surface unchanged and allowing the air in the upper atmosphere (stratosphere) to adjust to new energy conditions.
– Instantaneous radiative forcing: This does not consider changes in the upper atmosphere’s temperature.
– Effective radiative forcing: This includes changes in both the upper and lower atmosphere.
The balance between energy Earth absorbs from the Sun and energy it releases into space determines Earth’s average temperature. This balance is also called Earth’s energy balance. Changes in this balance happen because of things like how much sunlight reaches Earth, how much sunlight is reflected by clouds or gases, how much heat is trapped by greenhouse gases, or how much heat is released by Earth’s surface. These changes are called radiative forcing. These changes, along with other reactions in the climate system, affect Earth’s temperature continuously as sunlight hits Earth, clouds and particles form, gas levels in the air change, and seasons alter Earth’s surface.
Positive radiative forcing means Earth receives more energy from the Sun than it sends back into space. This extra energy causes Earth to warm. Negative radiative forcing means Earth sends more energy into space than it receives, which causes Earth to cool.
The movement of energy and matter in Earth’s atmosphere is explained by rules about how energy behaves in balanced and unbalanced systems. In the early 1900s, scientists developed a way to describe how energy moves through radiation, which they later used to study the atmospheres of stars and planets. Later, scientists studied how energy moves through radiation and convection (heat movement through air or water), and this research improved by the 1960s and 1970s. These models included more details, such as how water moves through the atmosphere, to better match real-world observations.
One use of these models is to predict how Earth’s climate might change if something changes, like the amount of a gas in the air. These models helped create a system to understand how changes in energy balance lead to changes in Earth’s temperature. This system, called the forcing-feedback framework, explains that when energy balance is disturbed, Earth’s climate will slowly adjust to reach a new balance. The term "radiative forcing" was used to describe these disturbances and became widely used in scientific writing by the 1980s.
The idea of radiative forcing has changed over time. It started with a method called "instantaneous radiative forcing" and later evolved to better connect energy imbalances with changes in Earth’s temperature. For example, in 2003, scientists showed how adjusting for changes in the upper and lower atmosphere can improve climate models.
A method called "adjusted radiative forcing" calculates energy imbalances after the upper atmosphere’s temperature has changed to reach a new balance. This method does not consider changes in the lower atmosphere, so another term, "effective radiative forcing," was created to include both. Effective radiative forcing is the main method used in climate studies today, though other methods are still used for certain cases, like changes in greenhouse gases or ozone. A method called the "radiative kernel approach" helps estimate climate reactions using simple calculations based on energy changes.
Uses
Radiative forcing is a tool used to measure how different natural and human-caused factors affect Earth's energy balance over time. These factors can cause the planet to warm or cool in various ways. Radiative forcing helps compare the effects of each factor to understand their overall impact.
A related measure called effective radiative forcing (ERF) removes the influence of quick changes in the atmosphere that do not affect long-term temperature trends. This makes it easier to compare the effects of different climate change factors and better understand how Earth's surface temperature responds to human activities.
Radiative forcing and climate feedbacks can be used together to estimate how much Earth's temperature might change in the future. This is done using an equation that relates the climate sensitivity parameter (λ̃), usually measured in K/(W/m²), to the amount of radiative forcing (ΔF) in W/m². The climate sensitivity parameter is calculated as the inverse of the climate feedback parameter (λ), which has units (W/m²)/K. For example, if λ̃ is estimated to be about 0.8, this suggests a global temperature increase of approximately 1.6 K above the 1750 reference temperature due to rising CO₂ levels (from 278 to 405 ppm, causing a radiative forcing of 2.0 W/m²). It also predicts an additional warming of 1.4 K above current temperatures if CO₂ levels were to double from pre-industrial values. These calculations assume no other factors are involved.
Historically, radiative forcing has been most useful for predicting changes caused by greenhouse gases. It is less effective for other human influences, such as soot.
Calculations and measurements
Earth's global radiation balance changes as the planet spins on its axis and moves around the Sun, and as large-scale temperature changes occur in the land, oceans, and atmosphere (such as El Niño-Southern Oscillation, or ENSO). Because of these changes, Earth's "instantaneous radiative forcing" (IRF) also changes naturally over time, sometimes showing warming and sometimes cooling. These changes are caused by regular and complex processes that usually return to balance over a few years, resulting in an average IRF of zero. These natural changes can hide long-term trends caused by human activities, making it harder to see those trends clearly.
Since 1998, NASA's Clouds and the Earth's Radiant Energy System (CERES) instruments have measured Earth's radiation balance continuously. Each full scan of Earth provides an estimate of the total (all-sky) instantaneous radiation balance. This data includes both natural changes and human influences on IRF, such as changes in greenhouse gases, tiny particles in the air (aerosols), and land surfaces. The data also includes delayed responses to energy imbalances, which happen mainly through Earth system processes like changes in temperature, surface reflectivity (albedo), water vapor in the air, and clouds.
Scientists have used measurements from CERES, AIRS, CloudSat, and other satellite instruments in NASA's Earth Observing System to separate the effects of natural changes and system processes. By removing these effects from the data over many years, scientists can observe the human-caused changes in the top of the atmosphere (TOA) IRF. The analysis was done in a way that is efficient for computers and does not rely on most modeling methods. The data shows that radiative forcing increased by +0.53 W m² (±0.11 W m²) from 2003 to 2018. About 20% of this increase was linked to fewer tiny particles in the air, and the remaining 80% was due to more greenhouse gases in the atmosphere.
A growing imbalance in radiative forcing caused by rising global carbon dioxide levels has been observed before using ground-based instruments. For example, measurements taken under clear skies at two Atmospheric Radiation Measurement (ARM) sites in Oklahoma and Alaska showed that the amount of heat from infrared radiation at Earth's surface increased by +0.2 W m² (±0.07 W m²) during the decade ending in 2010. This result focuses only on longwave radiation and the most important forcing gas, carbon dioxide, and is smaller than the TOA forcing because the atmosphere absorbs some of the heat.
Radiative forcing can be studied based on factors outside the climate system. Basic estimates in the following sections are based on the physics of matter and energy. Forcings (ΔF) are measured over Earth's entire surface and a specific time period. These estimates may be important for understanding global climate changes over decades or longer. Gas forcing estimates in the IPCC's AR6 report include adjustments for "fast" feedbacks (positive or negative), which happen through atmospheric responses (called effective radiative forcing).
Forcing due to changes in atmospheric gases
To calculate the effect of a well-mixed greenhouse gas on Earth's climate, scientists use tools that analyze how light interacts with the atmosphere. These tools can determine the radiative forcing (ΔF), which measures how much a gas changes the planet's energy balance, based on changes in its concentration. These calculations can be simplified into a math formula specific to each gas.
For carbon dioxide (CO₂), a simplified formula is:
ΔF = α × ln(C / C₀)
Here, C₀ is the starting concentration of CO₂ in parts per million (ppm) by volume, and ΔC is the change in concentration (in ppm). In some studies, like those about climate sensitivity, C₀ is set to 278 ppm, which was the CO₂ level in the year 1750 before major human-caused changes.
Human activities have increased greenhouse gas levels rapidly since about 1950. For CO₂, the 50% increase in concentration (C/C₀ = 1.5) from 1750 to 2020 results in a radiative forcing change (ΔF) of +2.17 W/m². If CO₂ concentrations double (C/C₀ = 2) in the coming decades, the radiative forcing change would be +3.71 W/m².
The warming effect of CO₂ increases slowly as its concentration rises, following a logarithmic pattern up to about eight times the current level. This means that each additional unit of CO₂ has a smaller warming effect than the previous one. However, this simplified formula becomes less accurate at very high concentrations. CO₂ does not fully absorb all infrared radiation, even at high levels. Scientists believe the shape of CO₂'s absorption spectrum, especially a broadening of its 15-μm band caused by a molecular feature called Fermi resonance, explains this logarithmic pattern.
Other greenhouse gases, such as methane and N₂O, have different formulas for calculating radiative forcing. Methane follows a square-root relationship, while chlorofluorocarbons (CFCs) follow a linear relationship. These formulas are detailed in reports from the Intergovernmental Panel on Climate Change (IPCC). A 2016 study suggested changes to the methane formula used in IPCC reports. Information about the radiative forcing from major greenhouse gases is included in sections about recent trends and the IPCC list of greenhouse gases.
Water vapor is currently Earth's most important greenhouse gas, contributing to about half of all atmospheric warming. Its concentration depends mostly on Earth's average temperature. For every degree Celsius of warming, water vapor levels could increase by up to 7%. Over long time periods, water vapor acts as a feedback system that strengthens the warming caused by other gases like CO₂.
Forcing due to changes in solar irradiance
The total amount of sunlight, including all colors of light, that reaches Earth from the Sun is called Total Solar Irradiance (TSI). On average, this value is known as the solar constant and is about 1361 watts per square meter (W/m²) at Earth's average distance from the Sun, which is one astronomical unit. This value is measured at the top of Earth's atmosphere. Earth's TSI changes over time due to two factors: the Sun's activity, such as sunspots, and the shape of Earth's orbit around the Sun. Since 1978, several satellite instruments, including ERB, ACRIM 1-3, VIRGO, and TIM, have measured TSI with increasing accuracy.
If Earth is considered a perfect sphere, the area of Earth that faces the Sun (πr²) is one-fourth the size of Earth's total surface area (4πr²). Because of this, the average amount of sunlight that reaches each square meter of Earth's atmosphere over a full year is one-fourth of TSI. This value, called I₀, is nearly constant and equals about 340 W/m².
Earth's orbit around the Sun is not a perfect circle but an ellipse. This means the distance between Earth and the Sun changes throughout the year. At its farthest point (aphelion, in early July), TSI is about 1321 W/m². At its closest point (perihelion, in early January), TSI is about 1412 W/m². This yearly change in TSI is about ±3.4%. However, this small variation has little effect on Earth's seasons and climate zones, which are mostly shaped by Earth's tilt. These repeating patterns do not add or remove energy over long periods, so they have no net effect on climate changes over decades.
Over the course of an 11-year cycle of sunspots, the average TSI changes slightly, between about 1360 W/m² and 1362 W/m² (±0.05%). Observations of sunspots began around 1600 and show longer patterns, such as the Gleissberg and DeVries/Seuss cycles, which influence the 11-year cycle. Despite these complex patterns, the 11-year cycle remains the most noticeable variation in long-term records.
Changes in TSI caused by sunspots contribute a small but non-zero effect on climate changes over decades. Some studies suggest these changes might have played a role in climate shifts during the Little Ice Age, along with other factors like volcanic eruptions and deforestation. Since the late 20th century, average TSI has slightly decreased, matching a decline in sunspot activity.
Long-term changes in Earth's orbit, called Milankovitch cycles, have influenced solar irradiance over tens of thousands to hundreds of thousands of years. These cycles include changes in Earth's orbital shape (eccentricity), tilt (obliquity), and the direction of Earth's tilt (precession). The 100,000-year cycle in orbital shape causes TSI to change by about ±0.2%. Today, Earth's orbit is becoming more circular, which causes TSI to slowly decrease. Scientists predict Earth's orbit will remain stable, including these changes, for at least the next 10 million years.
Since forming about 4.5 billion years ago, the Sun has used about half of its hydrogen fuel. As the Sun ages, TSI will slowly increase by about 1% every 100 million years. This change is too slow to be measured and has no noticeable effect on human timescales.
The table in the text summarizes the maximum changes in Earth's solar irradiance over the last decade. Each change discussed contributes a forcing value calculated using the formula:
where R = 0.30 is Earth's reflectivity. Changes in solar energy are expected to have only small effects on Earth's climate, even if new solar physics is discovered.
Forcing due to changes in albedo and aerosols
Some of the sunlight that reaches Earth is reflected by clouds, aerosols, oceans, land, snow, ice, plants, and other natural or human-made surfaces. The amount of sunlight reflected is called Earth's bond albedo (R). This is measured at the top of Earth's atmosphere and averages about 30% (0.30) globally each year. The remaining 70% (0.70) of sunlight is absorbed by Earth.
Atmospheric gases and particles contribute about three-fourths of Earth's albedo, with clouds alone accounting for about half. Clouds and water vapor play major roles because most of Earth's surface is covered by liquid water. Patterns of cloud formation and movement are complex, influenced by ocean heat flows and wind patterns like jet streams. Scientists have observed that the albedo of Earth's northern and southern hemispheres is nearly equal (within 0.2%), even though more than two-thirds of Earth's land and 85% of its human population are in the northern hemisphere.
Satellites such as MODIS, VIIRS, and CERES have measured Earth's albedo since 1998. Landsat imagery, available since 1972, has also been used in some studies. Recent improvements in measurement accuracy have made it easier to analyze how changes in Earth's albedo affect climate over the past decade. However, the available data is not yet long enough to predict long-term trends or fully understand other related questions.
Earth's albedo changes seasonally due to Earth's tilt and the movement of sunlight. These changes are linked to shifts in vegetation, snow, and sea ice. Over the course of a year, Earth's albedo varies by about ±7% (±0.02) around its average value, with the highest values occurring twice yearly near the time of the equinoxes. This cycle does not contribute to long-term climate changes because it balances out over time.
Regional albedo changes occur yearly due to natural processes, human activities, and system feedbacks. For example, cutting down forests can increase Earth's reflectivity, while adding water storage or irrigation to dry areas can decrease it. In the Arctic, losing ice reduces albedo, while expanding deserts in lower latitudes increases it.
Between 2000 and 2012, no clear overall trend in Earth's albedo was found within the 0.1% accuracy of measurements taken by CERES. Some scientists believe the small yearly changes suggest that complex feedbacks in Earth's systems may be keeping albedo stable. However, historical records show that rare events, such as major volcanic eruptions, can significantly change Earth's albedo for several years or longer.
Satellite data show that Earth's albedo has remained stable despite recent natural and human-caused changes. Over longer timescales, it is unclear whether these changes will have a larger impact on Earth's climate.
Recent growth trends
The IPCC summarized scientific findings about changes in radiative forcing as follows: "Human activities have caused a radiative forcing of 2.72 [1.96 to 3.48] W/m² in 2019 compared to 1750, which has warmed the climate system. This warming is mainly due to higher levels of greenhouse gases (GHGs), partially offset by cooling from increased aerosol concentrations."
Radiative forcing is a helpful tool for comparing how different human-caused greenhouse gases contribute to warming over time.
Since the Industrial Revolution, the radiative forcing from long-lived and well-mixed greenhouse gases in Earth’s atmosphere has increased. A table includes the direct contributions from carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), chlorofluorocarbons (CFCs) 12 and 11, and fifteen other halogenated gases. This data does not include the significant effects from shorter-lived gases, aerosols, or indirect influences like methane decay and some halogen reactions. It also does not consider changes in land use or solar activity.
The data show that CO₂ contributes the most to total radiative forcing, while methane and CFCs contribute less over time. The five major greenhouse gases account for about 96% of the direct radiative forcing from long-lived greenhouse gases since 1750. The remaining 4% comes from the 15 minor halogenated gases.
In 2016, the total radiative forcing was 3.027 W/m². Using the commonly accepted climate sensitivity parameter (λ) of 0.8 K/(W/m²), this would suggest a global temperature increase of 2.4 K, which is much higher than the observed increase of about 1.2 K. Part of this difference is due to a delay in the Earth’s temperature reaching equilibrium with the forcing. The rest may be due to cooling effects from aerosols, a lower climate sensitivity than expected, or a combination of these factors.
The table also includes an "Annual Greenhouse Gas Index" (AGGI), which compares the total direct radiative forcing from long-lived greenhouse gases in any given year to the level in 1990. This baseline year was chosen because it is the reference for the Kyoto Protocol. The AGGI measures yearly changes in factors like CO₂ emissions and absorption, methane and nitrous oxide sources and sinks, the decline in ozone-depleting chemicals linked to the Montreal Protocol, and the rise in their substitutes (hydrogenated CFCs (HCFCs) and hydrofluorocarbons (HFCs)). Most of the increase in radiative forcing since 1990 is due to CO₂. In 2013, the AGGI was 1.34, meaning total direct radiative forcing had increased by 34% since 1990. CO₂ alone contributed a 46% increase in radiative forcing during this time. The decrease in CFCs reduced the overall rise in radiative forcing.
Another table, prepared for climate model comparisons under the IPCC, includes all types of forcings, not just those from greenhouse gases.