Aerosols

Aerosols consist of particles in the atmosphere that are formed by emissions from factories, cars, forest fires, volcanic eruptions and other sources. There are many different types of aerosols, and they influence the the climate in various ways. Scientists do not fully understand the effects of aerosos, but there is agreement that aerosols generally have a cooling effect. This is demonstrated by climate models that only take into account greenhouse gases and ignore aerosols. Such models show stronger global warming than is observed [L88].

Aerosols affect radiation to and from the earth through various mechanisms. In part, the radiation is directly affected by the fact that aerosols either absorb the radiation or reflect it. This is called in scientific literature aerosol-radiation interaction [L90]. Another mechanism is called aerosol-cloud interaction [L91]. This is about how aerosols affect clouds and cloud formation, and how this change of clouds in turn affects radiation to and out from the planet. A third mechanism is the effect of soot (black carbon) deposited on snow and ice, which changes the way light is reflected from the ground (albedo).

A table (8.4 page 683) from IPCC's 5th Assessment Report (AR5) [L89] contains an overview of the direct effect of some aerosols (only the aerosol-radiation interaction mechanism is taken into account). The figures shown in the table are radiative forcing caused by different types of aerosols. Radiative forcing is expressed in W/m² and although aerosols do not directly cause radiation, this concept is used to compare the effect of aerosols with the effect of direct solar radiation towards the top of the atmosphere. For comparison, radiative forcing caused by CO₂ is between 1.33 and 2.03 W/m² and by methane between 0.74 and 1.20 W/m² [L92]. The radiative forcing figures given here represent the difference between the present (approx. 2010) and pre-industrial times (1750). Positive radiative forcing has a warming effect, while negative radiative forcing has a cooling effect. A simplified version of the table (showing only the latest estimates from the AR5 report) is shown below:

Sulfate	Between -0.60 and -0.20
Soot from fossil fuel combustion and biofuels (Black Carbon)	Between +0.05 and +0.80
Other Organic Aerosols (Primary Organic Aerosols)	Between -0.16 and -0.03
Combustion of biomass	Between -0.20 and +0.20
Combustion products from organic substances (Secondary Organic Carbon)	Between -0.27 and +0.20
Nitrate	Between -0.30 and -0.03
Dust	Between -0.30 and +0.10

As mentioned above, the table only shows the direct effects of aerosols (whether they absorb or reflect radiation). Another table from the IPCC [L92], with radiative forcing figures from greenhouse gasses and other sources, shows the total radiative forcing caused by the influence of aerosols on clouds (aerosol-cloud interaction mechanism) to be between -1.33 and -0.06 W/m².

These tables clearly demonstrate the great uncertainty when effects of aerosols are to be assessed. The ranges which the radiative forcing is assumed to be within are quite large, and some ranges cover both the negative (cooling) and the positive (warming) parts of the scale.

Aerosols generally have a short lifespan in the atmosphere. For example, soot will only remain in the atmosphere for a few days or weeks [L93], while emissions from volcanoes remain in the atmosphere for some years. Therefore, the effects of aerosols depend on new supplies (emissions) all the time. As soon as such emissions are reduced, the effect on the climate will also be reduced relatively fast (although there is some inertia in the system). Greenhouse gases, on the other hand, remain in the atmosphere for about ten years (CH₄) or many hundreds of years (CO₂). Reducing the emission of aerosols with a heating effect (mainly soot) will therefore reduce the global warming only in the short term.

Emissions of soot also have a negative effect on people's health, and a reduction of these emissions will therefore have a double positive effect. The Climate & Air Coalition website [L94], has an overview of emission sources which shows that households, when burning biomass for heating and cooking, account for 58% of the total emissions of soot (Black Carbon). Then follows transport (24%), industrial production (6%) and agriculture (5%). These are figures from 2015.

Emission of SO₂ also has a negative impact on people's health, as well as other adverse consequences, for example acid precipitation [L109]. Reduction of these emissions is therefore desirable, but this will unfortunately reduce the cooling effect of sulphate in the atmosphere.

Latest update: 2021-07-14