Aerosols are small particles in the atmosphere originating from natural processes and emissions of air pollutants from human activities (e.g., burning fossil fuels). Aerosols can affect the climate by reflecting or absorbing heat and light from the sun and influencing cloud formation. In this article, learn more about the influence of aerosols on climate and recent trends in atmospheric aerosol concentrations.
Aerosols are small particles in the atmosphere of both natural and human-made origin. Aerosols can affect the climate by directly interacting with solar radiation (energy in the form of sunlight) (Box 1) and by playing a role in cloud formation and precipitation. How a given aerosol will influence the climate depends on the material it is composed of, its colour, size, and where it is in the atmosphere (i.e., vertical location and whether it is over land or ocean)1.
Natural aerosols include dust, wildfire smoke, volcanic emissions (ash and sulphates) and sea spray2. Human-made aerosols result from industrial activity and the burning of fossil fuels and wood. Two types of aerosols have a notable influence on climate: black carbon and sulphate aerosols. Black carbon is produced when coal, gas and wood are burned. Aerosols containing black carbon tend to be dark. Sulphate aerosols are emitted during volcanic eruptions (Box 2) and the burning of fossil fuels and industrial processes such as smelting. Aerosols comprised of sulphates tend to be light in colour. Darker-coloured aerosols absorb radiation, resulting in atmospheric warming; lighter-coloured aerosols reflect radiation, resulting in atmospheric cooling. Darker-coloured aerosols can also cause additional warming when they settle on snow and ice because they make the surface darker, causing more radiation to be absorbed on these otherwise reflective surfaces.
The length of time that aerosols stay in the atmosphere is known as the residence time. Unlike greenhouse gases, which typically reside in the atmosphere for decades3, the residence time of aerosols is generally between 2 to 30 days, with shorter residence times in rainy conditions and longer residence times in dry conditions. Volcanic aerosols are the exception; they are typically emitted high in the atmosphere and, consequently, have much longer residence times (Box 2). Since most aerosols have short residence times, concentrations of these particles are highest around the sources of emissions. As such, the climate impacts of aerosols are typically confined to local or regional scales.
Aerosols influence the climate by reflecting and absorbing radiation and by playing a role in cloud formation and the occurrence of precipitation. Sunlight (solar radiation) is the main source of energy on Earth (Figure 1 and Box 1), so anything that changes the amount of sunlight reflected or absorbed by the Earth will affect the climate.
Figure 1: Components of the Earth’s radiation budget (the balance of incoming and outgoing radiation) [Source NASA]. See Box 1 for more details.
Sunlight is a form of energy known as radiation. When the sun’s energy (in the form of shortwave ultraviolet radiation, yellow in Figure 1) enters the atmosphere, some is reflected back to space, some is absorbed by the atmosphere, and some is absorbed by the Earth’s surface (land and oceans). The Earth and atmosphere re-emit some of the energy that is absorbed (orange in Figure 1) in the form of longwave infrared radiation, with some of this radiation going back to space and some being reflected back towards the Earth’s surface. In addition, some of the longwave radiation is absorbed by the atmosphere resulting in atmospheric heating (Figure 1). When radiation is reflected back to space, the amount of energy that reaches the Earth’s surface is reduced, resulting in a cooling effect (leftmost cloud of Figure 1).
Clouds play an important role in the Earth’s energy budget. Clouds at lower altitudes are usually thick and therefore reflect incoming radiation back to space, resulting in a cooling effect. Clouds at higher altitudes tend to be thin, allowing radiation to reach the Earth’s surface. These clouds also tend to absorb heat from the atmosphere and the surface, contributing to their warming effect. On average, clouds reflect more incoming radiation than the amount of outgoing energy they trap, resulting in an overall cooling effect4.
The reflection and absorption of radiation by aerosols is affected by the colour of the surface of the aerosol, the aerosol’s chemical composition, and the presence and type of clouds5. The presence of aerosols, and the corresponding warming and cooling of the atmosphere, can change where clouds form or whether they form at all. Aerosols also alter cloud properties and affect precipitation.
Aerosols act as “seeds” for cloud formation. It is difficult for water to condense into droplets on its own, but when a seed particle is present, moisture can condense onto the particle allowing clouds to form. When the concentration of aerosols increases, there are more small cloud droplets for the same amount of water. Smaller cloud droplets make clouds brighter and more reflective, causing enhanced atmospheric cooling5, 6.
Aerosols affect precipitation by influencing the absorption and reflection of radiation, and through cloud formation. The influence of aerosols on precipitation is an active area of research, with the general scientific consensus being that aerosols tend to decrease global precipitation7. Overall, aerosols in the atmosphere decrease the amount of radiation reaching the Earth’s surface, which lowers the energy available for evaporation of surface moisture and water, thus, reducing precipitation7. As noted, when there are more aerosols, cloud droplets tend to be smaller5,7,9. Smaller droplets require a longer time to reach a size where they fall as precipitation. In addition, smaller droplets evaporate more easily, resulting in smaller clouds with shorter lifetimes than larger clouds. For these reasons, the amount of precipitation from clouds is reduced when there are more aerosols5.
While more aerosols generally lead to a reduction in precipitation, it is important to note that aerosols vary in their radiative properties and in their ability to serve as cloud seeds7. As such, there are situations where aerosols can increase precipitation, especially at the regional scale7,10.
Aerosols emitted from volcanoes have different climate impacts compared to aerosols emitted from other sources since they can have longer residence times in the atmosphere. When volcanoes erupt, they inject ash and sulphate aerosols into the atmosphere. Ash can remain in the atmosphere for up to a few months, shading sunlight and causing cooling before being washed out by rain. If an eruption is particularly powerful, ash and aerosols may reach the stratosphere, the layer of the atmosphere between 10 and 50 km above the Earth’s surface. Sulphate aerosols in this layer have much longer residence times and travel vast distances, often around the globe, reflecting sunlight and causing cooling. The long residence time of sulphate aerosols in the upper atmosphere is the reason that large volcanic eruptions can cause global cooling in the years following an event. Several major volcanic eruptions are labelled in blue in Figure 2. More information on volcanic aerosols and their climate impacts can be found on the US Geological Survey.
Figure 2: Average annual Earth land temperature from 1750-2024 with major volcanic eruptions labelled. [Data Source:Berkeley Earth, figure source: Ruth Moore, Environment Climate Change Canada.]
Overall, aerosols have a cooling effect on the climate since they predominantly reflect sunlight and increase cloud reflectivity. Since the industrial era, high concentrations of aerosols from human-caused emissions of air pollutants have masked some of the warming from greenhouse gases. Efforts to reduce emissions of air pollutants to protect human and ecosystem health have reduced the concentrations of aerosols in the atmosphere11. As a result, some amount of recent global warming can be attributed to the reduction of aerosols11,12. Since human emissions of aerosols cause smog and acid rain, which have detrimental effects on environmental and human health, it is important to continue to reduce aerosol emissions alongside efforts to reduce greenhouse gas emissions.
[Figure 1] NASA. Earth’s energy budget. Accessed on 2024-10-30. Available at: https://mynasadata.larc.nasa.gov/basic-page/earths-energy-budget