The atmosphere is made up of five main layers: the troposphere, stratosphere, mesosphere, thermosphere, and exosphere which extends to 10,000 km above the Earth’s surface. However, 50% of the total mass of the atmosphere is located within the lower 5.6 km of the troposphere, along with the majority of the Earth’s water vapour. This means that nearly all of the Earth’s weather happens relatively close to the surface.

The chemical composition of the atmosphere has a significant influence on its behaviour. Reactive greenhouse gases (GHGs) such as water vapour, methane, nitrous oxide, ozone and carbon dioxide (CO₂) are responsible for the majority of global warming as a consequence of then trapping some of the radiation emitted by the Earth’s surface (the so-called greenhouse effect). This process occurs naturally and is partly responsible for maintaining our planet’s temperature at one fit for habitation by living organisms. Incoming short wave solar radiation is either absorbed or reflected by Earth’s surface, depending on its albedo (reflectivity). The land surface then emits infrared radiation (heat energy) with a much longer wavelength that is then absorbed by GHGs in the atmosphere and re-emitted again. As this allows more heat to be absorbed by Earth’s surface, an equilibrium is established at a higher temperature than if the atmospheric GHGs had not been present.

However, with the concentration of GHGs (mainly CO₂) in the atmosphere rising, the strength of the greenhouse effect is increasing and more heat is being kept within the atmosphere. This warming is more than doubled by the water vapour feedback where the saturation vapour pressure (the pressure exerted by water vapour molecules in the atmosphere when the system is at equilibrium i.e. equal numbers of water molecules are condensing and evaporating) is increased by a rise in temperature because the molecules have more energy. This allows more water vapour to be stored in the atmosphere and as a GHG, this amplifies the greenhouse effect. Atmospheric CO₂ is now at over 400 ppm, a huge increase on the pre-industrial level of 280 ppm. With continued output of GHGs from human activity, global warming is inevitable.

Meanwhile, stratospheric ozone is extremely important for absorbing incoming UV radiation (and therefore trapping it in the stratosphere) that could damage the DNA of living organisms at Earth’s surface and increasing the risk of health problems such as skin cancer. Since the 1930s, the use of chlorofluorocarbons (CFCs) in aerosol cans and as a refrigerant led to the gradual chemical destruction of ozone in the stratosphere, resulting in the discovery of the Antarctic ‘ozone hole’ in the late 1970’s. The loss of stratospheric ozone leads to a larger fraction of the short wavelength UVA and UVB solar radiation to reaching the Earth’s surface, particularly over Antarctica during the summer months. Luckily, after the introduction of the Montreal Protocol of 1987, the usage of CFCs has dropped and the ozone layer is repairing itself. In contrast to the importance of stratospheric ozone, when ozone forms at lower levels in the troposphere, it can cause a number of health problems in both animals and plants.

Another vital part of the composition of the atmosphere are aerosols. Aerosols are microscopic particles within the Earth’s atmosphere that can be either natural (e.g. ash from volcanoes, microscopic spores, sea salt etc) or man-made. They can impact the behaviour of incoming and outgoing radiation, as well as the properties of clouds. In this way, they can have an influence on the climate. Aerosol particles such as sulfur from sulfur dioxide (SO₂) emissions or pollutants from transport or fires, are scattered throughout the atmosphere and both reflect and absorb incoming solar radiation. These particles also form cloud condensation nuclei which act as sites for condensation of water vapour to take place, encouraging cloud formation. In doing this, aerosols tend to produce a cooling effect with less solar radiation reaching the Earth’s surface. Over the past 60 years this cooling effect has partly counteracted the warming influence from increasing CO2. Demands for improved air quality have led to the reduction of sulphur emissions from European and North American power stations over the past 15-20 years and hence a reduction in the cooling influence from aerosols.

Read more about the role of Atmospheric Chemistry and Aerosols in UKESM1