Radiative forcing: the basis for climate change

We hear about climate change (almost) every day, but who has ever heard of radiative forcing? It is a notion that is not widely publicized, rarely explained, and this is a pity: it is absolutely central to understanding current climate change.

From the greenhouse effect to radiative forcing

Before getting to the heart of the matter, let me briefly remind you what the greenhouse effect is:

  • The sun emits visible light, some of which is absorbed and warms the Earth.
  • In reaction, it emits infrared radiation (which in physics is called black body radiation).
  • This upward radiation is largely absorbed by gases that have the property of absorbing part of the infrared spectrum (the famous greenhouse gases), which then re-emit them in all directions: a little towards space, and a lot towards the Earth.

The greenhouse effect plays an absolutely major role in the thermal balance of the Earth. Indeed, if it did not exist, the average temperature would be about -20°C, instead of 15°C currently. The Earth would be a ball of ice, frozen for eternity, on which life as we know it would not exist.

Explanation of the greenhouse effect. Credit Youmanity
Source : ADEME
Note: this diagram is for educational purposes and the size of the arrows does not correctly reflect the earth’s radiation balance, see this video for technical explanations

The greenhouse effect is therefore natural: it was not created by man. That of the Earth is well proportioned for the appearance of life (unlike that of Venus or Mars) because it allows liquid water and avoids too great a temperature range. We have all experienced this greenhouse effect: clear nights are cooler than cloudy nights, because the base of the clouds (consisting of liquid and solid particles) acts like the wall of a greenhouse on the infrared radiation rising from the ground, and thus “retains” the earth’s heat.

The scientific term for the greenhouse effect and more generally for all the radiation that leaves and arrives on Earth with its associated energies is the radiation balance. Who says balance, says equilibrium The energy flows compensate each other: the Earth is in a stable state.

Human activities release considerable quantities of greenhouse gases into the atmosphere, which modifies the radiation values: like a blanket, the greenhouse gases prevent part of the infrared radiation from going into space. This creates an imbalance in the balance. This is the radiative forcing.

This imbalance is made in relation to a stable state: it is thus calculated in relative, compared to the year 1750 which is the dawn of the industrial era.its unit is the Watt per square meter (W/m²) (thus a flow of energy by surface). But we’ll leave the values aside for now, to focus on the explanations and we’ll talk about the numbers at the end.

What is this radiative forcing composed of?

First, an overview with the major categories:

Radiative forcing by major category 1750-2011
Source: IPCC 5th report, technical summary

“GHGs” are greenhouse gases (of which the main ones are CO2, CH4, N2O), which create a positive radiative forcing, thus tendency to warming. And for the green bar “Other anthropogenic forcings”, the value is negative which is a tendency to… cooling !

This overview is completed with a natural component, almost without implication in the observed imbalance. The global radiative forcing is thus almost entirely due to the sum of the two anthropogenic (“human”) components:

  • One that heats up (a lot)
  • And the other one that cools (a little)
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Now that the framework is established, here is the complete table with the details of the gases, the values, intervals, degrees of confidence etc.. :

Radiative forcing by component 1750-2011
Source of the graph: IPCC 5th report, Group I chapter 8 “Anthropogenic radiative forcing

Don’t panic! We will detail the 6 blocks one by one, each one being absolutely essential.

Block 1: Greenhouse gases

Greenhouse gases are said to be “homogeneous” because they are sufficiently mixed and persistent in the atmosphere that their concentration can be measured from a small number of sites and be relevant and usable at the global level, despite the fact that their sources (emissions) and sinks (absorption) are necessarily local.

What do we see?

  • CO2 is the main greenhouse gas.
  • Some gases decompose into others (e.g. methane decomposes into : CO2 + stratospheric water vapor + ozone).
  • The degree of confidence is high or very high: their properties and effects are well known.
  • Halogenated hydrocarbons (HFCs, CFCs) destroy O3ozone, which has a negative effect (because tropospheric ozone is a greenhouse gas), but which is largely offset by their own positive greenhouse effect.

Notes :

  • There are in fact 7 greenhouse gases that are accounted for: CO2of course, but also methane (CH4), nitrous oxide (N2O), the two families Hydrofluorocarbons (HFCs) and Perfluorocarbons (PFCs), and two minor but highly warming gases (NF3and SF6), which are not shown in this graph.
  • We must distinguish between good ozone, the one in the stratosphere which protects us from UV rays, and bad ozone, the one in the troposphere which contributes to the greenhouse effect and is also a pollutant.

Block n°2 : short-lived gases

Radiative forcing by component 1750-2011 : block 2

These short-lived gases are considered pollutants. These are, for example, carbon monoxide (CO), Nitrogen Oxides (NOx) and Volatile Organic Compounds (NMVOCs), which come from the combustion of fossil fuels (car engines, boilers, power plants, …) or from certain industrial processes (fertilizer manufacturing, oil refining, …) They are not themselves greenhouse gases, but they react more or less quickly in the atmosphere to form or destroy other compounds that are.

Example with carbon monoxide (CO) emitted by forest fires and the combustion of fossil fuels:

  • 1st effect: CO reacts with oxygen to give CO2
  • And the 2nd effect: it also reacts with other chemical compounds that would otherwise destroy methane and ozone, preventing the concentration of the latter two from decreasing

This double Kiss Cool effect destroys the compounds in question, but the chemical reactions involved produce greenhouse gases, or slow down their purification in the atmosphere, which increases the radiative forcing.

Block n°3 : aerosols

Radiative forcing by component 1750-2011 : block 3

Contrary to what some may have mistakenly thought (e.g. Didier Raoult in his tribunes), aerosols are not gases, but fine particles of all kinds suspended in the atmosphere.

You can see on this beautiful picture 3 types of aerosols: sea spray (typhoons, hurricanes), carbon in the form of soot (mainly from forest fires, here in North America and equatorial Africa), and sand and dust raised by the wind in deserts.

Satellite visualization of different types of aerosols
Credit NASA
Credit : NASA/Joshua Stevens/Adam Voiland

These aerosols have several effects: those on the negative left side of the graph (mineral dust, sulfate, nitrate, organic carbon) will reflect solar radiation and therefore have a tendency to cool (negative radiative forcing). Emitting this kind of particles into the atmosphere is also a way of geo-engineering the climate to reduce the solar radiation reaching the Earth and thus limit global warming.

Aerosols with cooling tendency
Source: IPCC 5th report, technical summary

The right part of the graph, positive, corresponds to soot carbon, resulting from incomplete combustions (unburned), which will absorb solar radiation and thus warm the atmosphere (positive radiative forcing).

Aerosols with a warming trend
Source: IPCC 5th report, technical summary

There is also an additional warming effect when this soot is deposited on snow or ice and darkens it, thus decreasing the reflection of the surface (change of albedo) which reinforces the solar absorption and thus causes an acceleration of the melting.

Carbon in the form of soot on the snow that accelerates melting
Credit : Dark Snow Project

Block n°4 : cloud adjustments due to aerosols

Radiative forcing by component 1750-2011 : block 4

All aerosols suspended in the atmosphere have a multitude of chemical and physical interactions with clouds: formation of ice crystals, triggering of precipitation, condensation, change of lifetime, albedo, …

Aerosol-cloud interactions
Aerosols interact with clouds in many ways
Source: 5th IPCC Report, Group I, Chapter 7

The precise quantification and modeling of these numerous microphysical processes are more complicated than for the other effects, hence the large uncertainty resulting from this part, despite a global radiative forcing that is very likely to be negative (thus a cooling trend).

What about contrails, what about radiative forcing?

While we’re on the subject of clouds: aviation contrails must also be addressed, even if they don’t appear on the graph. Because aviation has multiple impacts on the climate, in addition to CO2 emissions. The most notable effects are the condensation trails resulting from the humidity released by the engines, and also the formation of high altitude clouds called cirrus, resulting from the fine particles emitted during the combustion of kerosene (unburned, NOx, …). Both of these effects increase the greenhouse effect and create a slight positive forcing overall, despite their reflection of sunlight.

Aviation condensation trails

Block #5: Land Use Change

Radiative forcing by component 1750-2011 : block 5

This category is mainly deforestation, which has increased the albedo (reflectivity) of surfaces. You can clearly see on this picture the difference in “clarity” between the two sides. Dark forest absorbs much more radiation than grassland or crops.

Albedo change due to deforestation
Illustration of the difference in albedo (reflectivity) between a wooded area and a grassy area

This change in the reflective surface of the land has therefore induced a slight negative forcing since the pre-industrial era (because brightening of the surface => reflection of part of the solar radiation => cooling). These changes have occurred mainly in populated areas, which is logical (deforestation for the construction of cities, infrastructure, agricultural crops, livestock areas, …).

Soil albedo change 1750-1992
Evolution since 1750 of radiative forcing due to changes in soil albedo.
Source: IPCC 5th report, group I, chapter 8

Block n°6 : Natural causes

Radiative forcing by component 1750-2011 : block 6

There are two main natural causes that influence the climate, on century time scales: first, the change in solar irradiance (= energy that reaches us from the sun).

Solar cycles are 11 years long and have been measured by satellite since the late 1980s. In these cycles the irradiance varies very little: about 0.07% between the top and bottom of a cycle.

Solar irradiance measured by satellite
Solar irradiance measured by satellite since 1979
Source: IPCC 5th report, group I, chapter 8

Reconstructions of solar irradiance since 1750 could be made based on the modeling of magnetic fluxes and relations with sunspots. Depending on the estimates there is either no increase in radiative forcing or very little.

Historical reconstructions of solar irradiance
Reconstruction of historical solar irradiance since 1750, with satellite measurements superimposed since 1979
Source: IPCC 5th report, group I, chapter 8

2nd natural cause: volcanoes. Volcanoes emit CO2 during eruptions, and if these emissions are a major issue on geological time scales (of the order of a million years), they have a minor impact on a century scale. At that time, the main contribution of volcanoes to climate is not CO2 or even ash, but sulfur dioxide (SO2). The latter will form droplets of sulfuric acid in the atmosphere, with a lifetime of a few years if the eruption is strong enough to send it into the stratosphere (between ~10 and ~50 km) and if the volcano is located near the equator. These droplets are in fact aerosols that will reflect solar radiation and thus induce a cooling of the lower atmosphere (below ~10 km).

The eruption of Mount Pinatubo in the Philippines in 1991 was one of the largest eruptions of the 20th century, sending nearly 20 million tons of sulfur dioxide up to 35 km in altitude.

Eruption of Mount Pinatubo
Mount Pinatubo eruption in the Philippines in 1991

These aerosols spread over almost the entire planet in a few months, and caused a drop of about 0.5°C in the average global temperature for 2 years.

Aerosol dispersion from Mount Pinatubo
Propagation on the planet of aerosols linked to the eruption of Pinatubo.
Top left before the eruption, top right the month after the June eruption, bottom left 2 months after, and bottom right 6 months after.

Look at these pictures taken from the space shuttle. On the left a photo a few years before the eruption. And on the right another one two months after the eruption: we can clearly see the layers of aerosols released by the volcano (thick black lines above the clouds).

Visualization in the atmosphere of aerosols from Mount Pinatubo
Credit NASA

A generalized and lasting cooling caused by aerosols released by considerable volcanic eruptions is moreover one of the probable causes of the extinction of the dinosaurs 65 million years ago.

Image

Summary of radiative forcing

Let’s recap the different components of radiative forcing, in order:

1 – Greenhouse gases
2 – Short-lived pollutants
3 – Aerosols
4 – And their interactions with clouds
5 – The change in albedo caused by soil modification
6 – Natural causes: solar variations and volcanic eruptions

When you put it all together, here is the evolution of the components of radiative forcing from the pre-industrial era to 2011:

Evolution of radiative forcing components between 1750 and 2001
Source: IPCC 5th report, group I, chapter 8

On the negative side of the axis (cooling trend):

  • The peaks are the volcanic eruptions: powerful cooling effect but very short and does not influence the trend
  • Albedo change and aerosols are more constant, with a slight stabilization at the end

On the positive side of the axis (warming trend):

  • Greenhouse gases (grey for CO2, light green for the others), which have by far the greatest influence
  • Carbon in the form of soot (BC, for Black Carbon)
  • The condensation trails of the flights
  • Solar variations

The resultant of all these components (black curve) is largely positive, and the value corresponds to that of the red bar of this diagram that we saw at the beginning: 2.3 Watts per m².

Radiative forcing by major category 1750-2011

2.3 Watts per m².

Is that all?

When you put it that way, it doesn’t sound like much and it doesn’t impress anyone. And yet… look at the orders of magnitude that follow. If we apply this radiative forcing on a square of 25m x 25m (i.e. 650 m²), we obtain 1500 W of forcing. 1500 W is the power of this kind of device that is used (absurdly) to heat the terraces in winter.

Example of a 1500 W heater

So this small 2.3 W/m² which does not seem to be much is the equivalent of putting this kind of heating every 25m, on the whole surface of the world, and making it work 7 days a week, 24 hours a day, 365 days a year. 1.2 million billion Watts in all.

If we reason in terms of energy, the figures are even crazier. Over a year, the radiative forcing induced by this additional greenhouse effect sends 37 trillion billion joules back to the surface. It is absolutely colossal, and completely impossible to imagine with our senses.

Watch this video. It is a Soviet nuclear test made in 1955. Well, the energy of the radiative forcing on Earth is this explosion 80 times per second, permanently. That is one in France every 10 seconds: the duration of the video…

To continue on this path, since 1945 there have been about 2500 nuclear tests in the world, for a total energy release of 540 million tons of TNT. Well, all that energy is just 32 minutes of radiative forcing…

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Are you concerned about nuclear weapons? This is legitimate. Climate change should be of equal concern to you.

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