En-en adult card 15 radiative forcing: Difference between revisions

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.

* 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.

This card must be removed for the simplified version.

* 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).

* CO2: We can see here that this is quantitatively the main element.

* Aer - Cld Int. [Aerosols - Clouds Interaction] : Aerosols-Cloud interaction. Aerosols are a condensation nucleus of clouds which allows their manufacture. <ref>[https://www.ipcc.ch/site/assets/uploads/2018/03/WG1AR5_SummaryVolume_FINAL_FRENCH.pdf#page=209 Definition of aerosol-cloud interaction, glossary of report 5, working group 1]</ref>This is the indirect effect of aerosols.

* 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 is an acronym for '''R'''epresentative '''C'''oncentration '''P'''athways. 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).

== 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.