En-en adult card 15 radiative forcing

Card #15: Radiative forcing

Causes		Consequences
Additional Greenhouse Effect Aerosols		Energy Budget

Radiative forcing represents the difference (caused by humans) between the energy that arrives on Earth each second and the energy that is released.
In the 5 th assessment report of IPCC, it is rated at $2.3W/m^{2}$ (Watt per square meter).

Explanation

Together with the coccolithophores, this card is the one that makes the most impression on participants. It is important to explain it well. One way to play it down is to say that Radiative Forcing is just a measure.

Earth receives energy from the Sun and radiates it back to space in the form of infrared. Normally it is in thermal equilibrium and its temperature is constant.

Anything that causes the Earth to move away freom this thermal equilibrium, whether natural (sun, volcanoes) or anthropogenic (aerosols, GHGs), is called radiative forcing.

Caution: the definition has been simplified for educational purposes. A rigorous definition would be as follows: "Radiative forcing is the measure of the imbalance between the energy that arrives every second on Earth and the energy that would leave if the temperature had remained fixed since 1750". As the earth's temperature has risen in the meantime, the delta between instantaneous heat exchanges has decreased. Similarly, in 2050, in the RCP2.6 scenario, the forcing will be 2.6 W/m², but the temperature will have stabilised, meaning that the delta between incoming and outgoing energy will be zero.

On the main graph, we can see the different components of the radiative forcing :

• in the upper part, the warming effects
• in the lower part, the cooling effects

The greenhouse effect (CO2 + Other WMGHG + Trop O3) represents a positive forcing of 3.1 W/m² . It is therefore in the upper part of the graph.

Aerosols (Aer - Rad Int. + Aer - Cld Int.) have a cooling effect and are therefore in the lower part of the graph.

For more details on this graph, see the Radiative Forcing fact sheet.

The secondary graph represents the radiative forcing over two and a half centuries (history and projections). In the 5th IPCC report, the radiative forcing is 2.3 W/m². The values of the forcing in 2100 gave their name to the IPCC scenarios (RCP 2.6, RCP 4.5 etc.). The colours of these scenarios can be found in the graphs of maps n°5, 11, 15, 21, 22 and 24.

For more details on this graph, see the fact sheet on PCR scenarios.

To go further

Key points

• Card 15 suggests that 2.3 W/m2 more energy is coming in than is going out, at any given moment. This is not the case! This is the definition of the "radiation balance" and its value is almost zero: it enters as much energy as it leaves (or 1 W/m2 maximum to take into account the time it takes to reach equilibrium due to warming). In the IPCC report, radiative forcing is the imbalance in energy flux that would exist if the Earth's surface (or the oceans) had been prevented from warming compared to the values that existed in 1750. But the Earth's surface has warmed (by about 1°C) and the "radiative balance", not to be confused with radiative forcing, is almost zero. The legend in Figure 8.18 on page 699 of the IPCC report, on the back of Map 15, should be understood as the flow of energy that would not be returned to space if the Earth's surface had remained stuck at its 1750 temperature. Since the radiation balance is zero, this flow of energy, called "radiative forcing", is the one that has warmed the Earth.
• There is a simple and graphical way to explain the radiative forcing using the image of the greenhouse effect map. Note the arrows on the map from 1 to 4 (1 for reflection, 2 for insolation, 3 for infrared, and 4 for the greenhouse effect arrow). Let's also add a value 5 which is the amount of energy emitted by infrared radiation on the Earth's surface. First of all, it must be said that every warm body emits radiation. The hotter the body is, the more energy it radiates and returns. The amount of energy that reaches the earth is 2-1. The amount of energy leaving the Earth is 3. The amount of energy emitted by the earth in infrared radiation is 5. In 1750, what came in was worth what went out, so 2-1 = 3. We are in equilibrium. As the greenhouse effect increases, arrow 4 grows. This is the contribution of 3.1W/m2. On the other hand, aerosols increase the size of arrow 1. This is the -0.8 W/m2 because it is the amount of energy that does not reach the Earth.
• To understand the previous diagram, we can look at the values directly. Here is a table showing the radiation balance values:

	Source	1750	2020
Recep	ESun	341	341
	ERefl	181	181.8
	ESurf	160	159.2
Emis	EEmisSurf	490	492
	EGreenH	330	333.1
	ESpace	160	158.9
Total	EWarm	490	492.3
	ECool	490	492

The values presented in the table in W/m² are as follows:

• E_Sun the energy emitted by the sun that reaches the Earth
• E_Refl the energy reflected from Earth back into space
• E_Surf the energy that reaches the earth's surface
• E_EmisSurf the energy emitted by the Earth's surface
• E_GreenH the energy that remains on Earth because of the greenhouse effect.
• E_Space the energy that goes back into space
• E_Warm the energy that heats the Earth
• E_Cool that cools the Earth

So we have E_Warm = E_Surf + E_GreenH, and E_Cool = E_EmisSurf . The current radiative balance is E_Warm-2020 - E_Cool2020 which is therefore 0.3 W/m², and the radiative forcing is E_Warm-2020 - E_Cool1750 which is therefore 2.3 W/m².

This card must be removed for the simplified version.

Detail item by item

Warming effects

• Solar: the intensity of solar spots varies over time, with a period of 11 years. Hence the small bumps.
• BC [Black Carbon] on snow: Black carbon is soot (see map 10 Aerosols) that is deposited on snow that is white, and by albedo effect, it warms up.
• Contrails: Contrails are the streaks in aeroplanes due to aerosols and water vapour present and emitted. These trails are like artificial clouds (cirrus clouds, in this case, given their altitude and shape). At this altitude, the warming effect of the clouds (greenhouse effect) outweighs the cooling effect (albedo).
• Strat H2O [stratospheric water vapor]: Aircraft burn kerosene to propel themselves. This combustion, like all combustion, releases CO2 and water vapour. Water vapour is usually not counted in the carbon footprint of hydrocarbons because these water molecules are intended to remain in the atmosphere for one to three weeks before being washed away by rain. As far as aeroplanes are concerned, it's a bit different because they fly at an altitude, close to the stratosphere, where, as the name suggests, the air is stratified. There are no vertical convective movements, almost no clouds, no rain. When water vapour is emitted by aeroplanes, it can stay there for several years and at that point we can start to take into account its greenhouse effect.
• Too much. O3 [Tropospheric Ozone]: Tropospheric ozone. Ozone is like cholesterol : there is good and bad. The "good ozone" is stratospheric ozone, i.e. the ozone layer, which is very high in the atmosphere. It protects us from the sun's ultraviolet rays. The "bad ozone" is ground-level ozone, the ozone that is at ground level in "ozone pollution", especially in cities in hot weather. Ozone is a greenhouse gas, so as our activities produce it, its presence causes a positive radiative forcing. However, ozone is not included in carbon budgets. This is because we do not produce it directly. On the other hand, we do produce ozone precursors such as nitrogen oxides (NOx), volatile organic compounds (VOCs), methane (CH4) and carbon monoxide (CO).
• Other WMGHG [Well Mixed GreenHouse Gases]: Other WMGHG [Well Mixed GreenHouse Gases]: Other well mixed GHGs, or long-lived GHGs (synonymous because if they are long-lived, then they have time to mix well) are mainly methane, nitrous oxide and some other gases such as HFCs.
• CO2: We can see here that this is quantitatively the main element.

Cooling effects

• Aer - Rad Int. Aerosols - Radiation Interaction]: Aerosols-Radiation Interaction. This is the interaction of aerosols with the sun's rays. Clearly, it is the occulting effect. They prevent the sun's rays from reaching the ground. It is also said to be their direct effect.
• Aer - Cld Int. [Aerosols - Clouds Interaction] : Aerosols-Cloud interaction. Aerosols are a condensation nucleus of clouds which allows their manufacture. This is the indirect effect of aerosols.
• Land Use] : Land use. For example, when deforestation, a dark green surface is replaced by a light beige surface. The albedo effect cools the earth.
• Stat. O3 [stratospheric Ozone]: Stratospheric ozone. The ozone in the stratosphere, the "good ozone", which protects us from ultraviolet radiation. As the amount of ozone has decreased because of CFCs (the hole in the ozone layer), the greenhouse effect of this ozone has logically decreased. This is what you can see on this part of the graph.
• Volcanic: large volcanic eruptions send ash into the stratosphere. The ash in the troposphere is washed away by rain in one to three weeks, but the ash that reaches the stratosphere stays much longer. This is because, as the name suggests, the air in the stratosphere is stratified, i.e. vertically stable. There are no vertical convective movements, but there are very powerful horizontal currents, the jetstreams, which mix these ashes over the entire surface of the earth. The result is a cooling of the earth for a few months to a few years. The phenomenon is similar to that of aerosols, it's just that the origin of aerosols is not the same.

RCP

RCP is an acronym for Representative Concentration Pathways. These are the different scenarios proposed by the IPCC. RCPs allow the estimation of greenhouse gas and aerosol concentrations that can be converted approximately into W/m². Approximate equations to do this exist for CO2 (log), CH4 (quadratic) etc. Forcing is not prescribed to climate models, they are imposed either an evolution of concentrations (when they do not represent the carbon cycle) or an evolution of emissions (in this case they also calculate the feedbacks of the carbon cycle). The forcing is the result of the calculation of the atmospheric radiative code specific to each model, which is not a perfect calculation (line by line) but simplified (band by band).

Popularisation content of the subject

• Article: Radiative forcing: at the root of climate change - by Bonpote (Fench)
• Video: LIVE Climate, energy and nuclear with Le Réveilleur, 47th minute

Facilitation tips

• To explain this card, we can use the metaphor of the "Greenhouse Effect" card. On the one hand, the cover around the Earth is thickening, this is the greenhouse effect, so we are accumulating energy. On the other hand, the room cools down, it's the aerosols, we lose energy. What happens? Should it be warmer or cooler under the duvet? Radiative forcing is simply a measure of the impact of both. We can see that the greenhouse effect is more important, so overall, the energy accumulates under the duvet.
• The name of the card may sound scary, but it simply shows radiation that has been altered. Renaming the card makes it simpler. It could be called "Man-made radiation on Earth" or "Forced radiation".

Other possible links

Consequences

• Melting of Sea Ice pack and Deforestation When sea ice melts, a white surface is replaced by a navy blue surface, which has a lower albedo and therefore absorbs more energy. When the forest is cut down and replaced by a meadow, it is the opposite, a dark surface (the foliage) is replaced by a light surface (the meadow). All in all, the artificialization of the soil has a cooling effect on the climate.