The balance of energy entering and leaving our planet, measured in W•m-2, is crucial for sustaining life. The term radiative forcing refers to changes in the net radiative flux relative to 1750 due to the effects of external drivers of climate change. Radiative forcing, along with climate feedback loops, determines Earth’s energy imbalance. Substances that are positive radiative forcers increase Earth’s energy imbalance and contribute to warming. Negative radiative forcers decrease Earth’s radiation imbalance and contribute to cooling. Radiative forcing is a control variable for the climate change Earth system process in the planetary boundaries framework. Increases in Earth's energy imbalance have caused this system process to be in a region of definitely high risk.
Increases in radiative forcing drive changes to climate and affect extreme weather, food security, ocean currents, and land availability. Impacts of these changes form the basis of the global need to keep Earth's average surface temperature increase below 1.5°C, as per the Paris Agreement.
Explore the contributions of each forcer by adjusting the bar graph sliders. Click on the information button beside each forcer to read more about that substance, including its sources, impacts, and solutions. Investigate how the contributions of each forcer and the overall net value have progressed over time using the timeline below the graph.
Learning Outcomes
Explain what is meant by "Earth's energy balance" and connect the concept to the effects of changes to albedo and greenhouse gases on climate.
Define positive and negative radiative forcers and give examples of each.
Identify the reference point in time used for changes to Earth's radiation balance.
Quantify the relative contributions of greenhouse gases as positive radiative forcers.
Differentiate between global warming potential (GWP) and effective radiative forcing (ERF) as measures of the significance of a greenhouse gas.
Articulate the role sulfate aerosols play in cooling the planet and their beneficial and harmful implications on climate and human health, including the time scale for their effects.
Visualize individual and combined effective radiative forcing values over time.
Correlate effective radiative forcing values to change in global surface temperature and to the Paris Accord 1.5°C and 2.0°C targets.
Connect change in Earth's radiative forcing to the second control variable for climate change in the planetary boundaries framework.
CO2
Carbon dioxide (CO2) is a greenhouse gas that is emitted into the atmosphere by a number of different anthropogenic and natural activities.
There are both natural and anthropogenic sources of CO2. Natural sources include outgasing from the ocean, decomposing vegetation and other biomass, volcanic eruptions, and wildfires. However, most of the CO2 emitted to the atmosphere since the Industrial Revolution comes from fossil fuel combustion. As of 2024, CO2 is present in our atmosphere at a concentration of roughly 422 parts per million (ppm). This number has greatly fluctuated over the history of the Earth, but has been drastically increasing in recent years, largely due to anthropogenic emissions.
CO2 is the largest contributor to positive radiative forcing since 1750. As a greenhouse gas, CO2 absorbsIR radiation emitted by Earth, preventing it from escaping out of the atmosphere. By trapping IR radiation, CO2 acts as a positive radiative forcer.
CO2 is also involved in a positive feedback loop that increases its potential impact. There are around 1,500 gigatons of carbon stored in permafrost from the remnants of plants and other organic matter that froze over before they could fully decompose. As permafrost begins to thaw due to increasing temperatures, bacteria break down the organic matter, releasing CO2 into the atmosphere, which leads to further warming.
Reducing CO2 emissions is the most effective solution to lowering its positive radiative forcing. This can be accomplished by using renewable sources of energy, such as wind and solar, building more energy efficient technology, and converting to electric transportation powered by renewable energy. Carbon capture is a solution that will contribute to reducing CO2 emissions into the atmosphere. Carbon capture focuses on capturing CO2 from unavoidable industrial processes and storing it deep underground (Carbon capture and storage, CCS) or utilizing it as a feedstock for other processes (Carbon capture and utilization, CCU) to offset their atmospheric warming effect.
CH4
Methane (CH4) is a greenhouse gas that is emitted into the atmosphere by a number of different anthropogenic and natural activities.
The main anthropogenic sources of methane are agriculture, fossil fuel combustion, and decomposition of landfill waste. Wetlands are the largest natural contributor to methane emissions. Methane has a short atmospheric lifetime of 12 years, which is relatively small compared to other greenhouse gases, due to its tendency to be readily broken down by the hydroxyl radical. However, the global warming potential of methane is 28 times larger than that of carbon dioxide over a 100 year timescale. Though it has a much larger global warming potential, its radiative forcing impact is much lower than CO2 because it is present in much smaller concentrations, roughly 1900 parts per billion (ppb).
Methane is the second largest contributor to changes in positive radiative forcing since 1750. As greenhouse gas, methane IR radiation, thereby acting as a positive radiative forcer.
Methane leads tropospheric ozone production and is a potent greenhouse gas, causing methane to have an increased impact on radiative forcing. Methane is removed from the troposphere by reacting with the hydroxyl radical. One of the consequences of this reaction is the production of tropospheric ozone, a greenhouse gas that further contributes to positive radiative forcing.
Current solutions to reducing methane emissions include moving to renewable energy sources; practicing sustainable manure management, such as anaerobic digestion with capture of methane; and reducing the use of animal products in human diets.
N2O
Nitrous oxide (N2O) is both a greenhouse gas and ozone depleting substance.
The largest anthropogenic source of N2O is from overapplication of fertilizer. Globally, N2O production is predominantly due to microbial processes, which can occur naturally in soil and oceans, but has been greatly enhanced due to human activity. Livestock waste, fossil fuel combustion, and industrial processes are much smaller sources of the release of N2O into the atmosphere. Once N2O is in the stratosphere, it is photochemically converted to NO. NO can enter a cycle in which it reacts with ozone and destroys thousands of ozone molecules. Currently, N2O is one of the most significant substances that causes the depletion of stratospheric ozone.
In the mid-20th century, atmospheric levels of N2O began to increase rapidly after the creation of synthetic N-based fertilizer via the Haber-Bosch process. This is due to fungi and bacteria that carry out nitrification and denitrification reactions, transforming the ammonia that is used as fertilizer into N2O. Today 80% of the world's N2O emissions are from agriculture.
Nitrous oxide (N2O) is a potent greenhouse gas with a global warming potential that is 298 times greater than carbon dioxide over a 100-year period. In addition to being a greenhouse gas in the troposphere, N2O is also a powerful stratospheric ozone depleting substance. Atmospheric levels of N2O are currently higher than they have been for the past 45,000 years. In 1750, the atmospheric concentration of N2O was 270 parts per billion (ppb), but, as of 2024, the concentration has now increased to 337 ppb. Because it is a potent greenhouse gas, nitrous oxide acts as a positive radiative forcer.
In order to mitigate N2O emissions, changes to agricultural processes are required. A large amount of N2O emissions can be mitigated by simple strategies such as only applying fertilizer at appropriate times of the season and alternative fertilizer application strategies. Another strategy involves using fertilizers that contain less ammonia. Implementing solutions such as these can aid in reducing N2O emissions while maintaining the productivity required to feed a planet with 8 billion people.
F-Gases
Fluorinated gases (F-gases) are a group of chemical compounds that contain fluorine atoms and sometimes chlorine and/or bromine atoms. These have been used as refrigerants and fire extinguishers and some other applications. Chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and hydrofluorocarbons (HFCs) are all F-gases.
F-gases are important in our daily lives. They are used in domestic and commercial refrigeration; household, commercial, and vehicle air conditioning; aerosol propellants; insulation; and fire extinguishers. F-gases are released into the atmosphere when these systems leak and when appliances containing refrigerants are not properly disposed of at the end of their lifetime. All emissions of F-gases are anthropogenic. Due to the changes implemented by the Montreal Protocol and Kigali Amendment, most F-gases are no longer allowed to be produced or are being phased out, which reduces their radiative forcing potential. However, due to their long atmospheric lifetime and high global warming potential, their environmental impact remains a global concern.
F-gases contribute to positive radiative forcing as they have a high global warming potential, a long atmospheric lifetime, and are potent greenhouse gases. However, many F-gases contain a chlorine or bromine atom, which can undergo photochemical decomposition in the stratosphere, releasing chlorine or bromine free radicals. These free radicals cause a chain reaction that catalyzes stratospheric ozone depletion, which results in indirect tropospheric cooling. While F-gases contribute to both global warming and cooling, its positive radiative forcing will always be greater than its negative radiative forcing.
F-gases are released into the atmosphere when domestic and commercial air refrigerant systems leak and when refrigerants are not correctly captured and recycled. Creating systems to minimize leakage and handling F-gases with more care would reduce emissions. Another solution is to replace F-gases with substances that have lower global warming potentials. Scientists are in an ongoing search for better substitutes for F-gases. The replacement compound must undergo vapour-compression cycles to transfer heat in refrigeration and air conditioning units while not posing a threat to the environment.
Aerosols
Aerosols are small particles suspended in the atmosphere. This section considers sulfate aerosols, nitrate aerosols, organic carbon, and black carbon aerosols. Aerosols affect cloud formation and can absorb or reflect incoming solar radiation, impacting the earth's radiation balance.
Aerosol particles are present in the atmosphere with ranges in size from a few nanometers to tens of micrometres. They can be a result of direct emission (primary aerosols like organic carbon, sea salt, and mineral dust) or as a product of chemical reactions (secondary aerosols like sulfate, nitrate, and ammonium compounds). SO2 and the sulfate aerosols they form are large contributors to the change in negative radiative forcing of our planet. SO2 is emitted from natural sources, for example volcanoes emit directly into both the troposphere and stratosphere. SO2 also has anthropogenic sources that include the burning of high sulfer bunker fuel in ships and other forms of transportation. Sulfate aerosols are mostly found in compounds that have counter-ions, such as ammonium ions, in the atmosphere.
Nitrate aerosols originate from the release of nitrogen oxides (NOx) by combustion engines such as vehicles and powerplants. Organic carbon is also included in this category of aerosols as they are a complex mixture of chemical compounds containing carbon-carbon bonds that reflect radiation. They may be produced from fossil fuel and biofuel burning as well as natural biogenic emissions. Black carbon, more commonly known as soot, becomes suspended in the atmosphere and absorbs solar radiation, acting as a positive radiative forcing aerosol.
There are two important interactions that aerosols take part in: aerosol-cloud and aerosol-radiation. Aerosol-cloud interactions represent how aerosols can increase the number of cloud droplets, which results in brighter and more reflective clouds. Aerosol-radiation interactions are ones in which light-coloured aerosols scatter radiation and reflect it back out into the atmosphere, therefore contributing to cooling. Black carbon makes the aerosol radiative forcing value less negative, but when all aerosols are considered, the effect of black carbon as an aerosol will never outweigh the effects of other aerosols as negative radiative forcers.
Many aerosols also contribute to respiratory, cardiovascular, and lung diseases. These health effects are most evident in coastal port cities due to the amount of sulfur historically present in ship fuel, along with other urban sources of sulfur combustion products.
Lowering the percentage of sulfur in bunker fuel and other types of fuel as well as adding “sulfur scrubbers” to industrial processes will limit the production of sulfate aerosols. Decreasing the amount of aerosols in the atmosphere will have positive effects on human health, but may lead to significant temperature increase because of the reduction of negative radiative forcing.
Port cities have high rates of respiratory illnesses and premature deaths caused by pollution from ships burning high sulfur bunker fuel. Key chemical substances include harmful sulfur oxides that produce sulfate aerosols. But sulfate aerosols in “ship tracks” over the ocean also contribute to whitening and brightening of clouds, which has a beneficial effect on radiative forcing that drives climate change. What are sulfate aerosols and why are they important? Find out in the Aerosols & Shipping case study.
Ozone
Ozone (O3) in the stratosphere filters out incoming ultraviolet radiation that is harmful to biological systems. In the troposphere, O3 is a greenhouse gas that is formed by ozone precursors, such as volatile organic compounds (VOCs), carbon monoxide (CO), and nitrogen oxides (NOx).
Often when ozone is mentioned, we think of the stratospheric ozone layer that protects us from harmful ultraviolet radiation. Yet, ozone is also present in the troposphere, where it is a potent greenhouse gas.
Ozone is formed in the troposphere from chemical reactions involving nitrogen oxides (NOx), volatile organic compounds (VOCs), and carbon monoxide (CO) in the presence of sunlight. NOx, VOCs, and CO are emitted from sources like cars and industrial power plants. These compounds control the oxidizing capacity of the atmosphere and form ozone in the troposphere, giving them their name as ozone precursors.
Stratospheric ozone is formed when an oxygen molecule (O2) is broken apart by ultraviolet radiation and then each oxygen atom collides with another O2 molecule to form ozone.
Tropospheric ozone acts as a greenhouse gas and absorbs IR radiation which leads to warming. The majority of tropospheric ozone is created by short lived compounds, such as VOCs, CO, and NOx, that act as ozone precursors. When ozone in the stratosphere is depleted, it has a small cooling effect as ozone depletion limits the amount of ozone that is responsible for heating by conversion of incoming solar radiation into heat radiation. This cooling effect is very small, however, so it does not compete with the fact that, as a greenhouse gas in the troposphere, ozone is a positive radiative forcer.
One technology that will limit the concentration of tropospheric ozone is catalytic converters. As a requirement, catalytic converters are included on vehicles to convert VOCs, NOx, and CO leaving the exhaust into water, carbon dioxide, and nitrogen gas. If these ozone precursors are reduced, it limits the amount of ozone being produced.
To decrease the warming effect of ozone, it is essential that the stratospheric ozone concentration maintains steady. To do this, we need to limit the amount of ozone-depleting substances emitted into our atmosphere from certain applications, such as refrigeration, where inert halogen-containing gases make their way up to the stratosphere and take part in catalytic ozone depleting cycles.
Black Carbon on Snow
Black carbon, also called soot, is fine particulate matter formed from the incomplete combustion of fossil fuels, biofuels, and biomass. The radiative forcing values used here are calculated exclusively for when black carbon is on snow or ice.
Black carbon is formed through incomplete combustion of carbon-based sources. It is emitted into the atmosphere and stays there until it is removed by wet or dry deposition. Since it is from combustion of fuels, black carbon is a fully anthropogenic emission.
Black carbon is very efficient at absorbing solar radiation, which increases the temperature. Snow and ice typically have a very high albedo, but when black carbon deposits on top of the snow, it can greatly reduce its reflectivity. Since decreasing albedo causes less light to be reflected, black carbon on snow acts as a positive radiative forcer.
Black carbon absorbs light as an aerosol before it is removed via deposition, however, this forcing is not considered in this variable, but rather is factored into the forcing of aerosols.
Black carbon has a short lifetime, so reducing emissions would generate quick results regionally. Ceasing open burning in agriculture, practicing waste management, altering certain tools used in industrial production, and implementing cleaner diesel vehicles would reduce black carbon emissions with relatively quick results.
Scenarios
Let's imagine that it is now 2050. The solutions for mitigating carbon dioxide (CO2) emissions haven't made enough difference and anthropogenic emissions are out of control. Drag the CO2 bar to its maximum value. How does this affect the net radiative forcing value compared to the 2023 value? If CO2 emissions are allowed to increase to extreme levels, then what does that mean for surface temperatures in different places of the world? What might happen if all the permafrost melts? How would this affect some people's quality of life? (HINT: Navigate your way to the CO2 impacts tab for more information)
Now, let's look at another extreme. It is 2050 and we have not only reached "net zero" for CO2 emissions, but we have also captured enough carbon dioxide and stored it to return to pre-industrial levels. However, humans have not focused on mitigating other anthropogenic emissions besides CO2, so the other bars remain unchanged. Drag the CO2 bar to zero, which is its pre-industrial level. How does this affect the net radiative forcing value compared to the 2023 value? Would net zero carbon “fix” climate change? Why is the net radiative forcing value still positive? While CO2 is a large piece of the puzzle, it is important to note that there are other contributors to radiative forcing that must be mitigated. Check out the other scenarios to explore this further.
As the second most important positive radiative forcer, methane (CH4) is currently an increasingly large target for mitigating climate change. Let's say there is a scientist named James that views methane as a relatively unimportant target for mitigating climate change. Why might scientist James decide that methane is less worthy of time and resources? What properties of methane would James argue makes it a less important target? Scientist Claire fights this decision and states that all available funds and resources should be focused on methane. What properties of methane might scientist Claire refer to to support her disagreement with scientist James? What scientist do you agree with?
Scientist Claire decides to prove that methane is the most important radiative forcer to focus on. By 2050, scientist Claire has been able to get anthropogenic methane radiative forcing down to net zero. Slide the methane radiative forcing bar to zero. Scientist James argues that the Earth is still warming up at the same rate as it is in 2023, but scientist Claire disagrees. Who is correct? How do you know?
Continue with scenarios involving other radiative forcers to get the full picture.
Imagine it is 2050 and the population has reached around 10 billion people. Agriculture capacity has increased substantially to feed a larger world population. One approach has been to use much bigger fertilizer spreaders that distribute nitrogen based fertilizers over a large area in a short period of time. However, with such a high demand for food, these tools are used without care for where fertilizer has previously fallen or the total amount of nitrogen fertilizer reaching the soil. How would this impact the radiative forcing of N2O? Move the N2O bar to +1.0 W/m2. Does the net radiative forcing value increase or decrease? Why?
Along with the increase in land use for agriculture, the amount of livestock has grown to feed the population as well. Move the N2O bar in the direction you think it would shift. This increase in livestock agriculture also results in an increase of CH4 emissions. Does the net radiative forcing value increase or decrease with the increase of these two compounds? Assuming the population remains at 10 billion, how would you decrease the emission of N2O? What would you implement now to avoid this in the future?
Continue with scenarios involving other radiative forcers to get the full picture.
Fluorinated gases (F-gases) have a long atmospheric lifetime and a high global warming potential. This means that we could still be affected by some of the first F-gases that were ever released into the atmosphere! Using the timeline bar, make an estimate of when F-gases were first invented and emitted. Now, starting at that date, drag the timeline bar while watching the F-gases bar. When are the largest “jumps” in the F-gas radiative forcing value? Why do you think this is?
Continue to drag the timeline bar toward 2023. When does the rate of increase in F-gas radiative forcing value begin to slow down? Why do you think that the rate of increase of the effect of F-gases began to decrease? Can you predict whether the value of the F-gases bar will be bigger or smaller than its 2023 value in 50 years? 100 years? (HINT: Navigate to the sources tab in F-gases for more information)
Continue with scenarios involving other radiative forcers to get the full picture.
Let's imagine that recently a company was started that wanted to mimic the aerosol effects of a volcano, so as to cancel out the effects of the large amounts of CO2 it produces. What would be a good candidate for the primary aerosol released by this company? Would the company be releasing into the troposphere, stratosphere, or both? (HINT: read the impacts tab in the aerosols section) How would the aerosols bar change as this company gets started? Would this have any effects on human health? Now, the company gains traction and continues to release aerosols. Starting with all other radiative forcers at 2023 levels, adjust the aerosols bar to -2.0 W/m2 and note the change in net radiative forcing. Is the net radiative forcing value positive or negative? How would this impact Earth's temperature? Using your answers to the previous questions, would you support the expansion of this company? Can you anticipate any unintended consequences of this "climate engineering"?