The Sun bathes the Earth in radiation. Much of this radiation reaches the surface, warming the planet and driving the climate. The surface then emits heat (as infrared radiation), most of which does not escape to space, but is instead absorbed by gases in the Earth’s atmosphere. These gases then warm, emitting further heat radiation both to space and back towards the Earth’s surface. This is what we all know as the greenhouse effect.
The Earth’s energy balance
In order for the radiative budget of the Earth to be in equilibrium – that is heat radiation leaving the planet to equal the absorbed solar energy – atmospheric temperatures must adjust. The different portions of this equilibrium can be quantified in an energy balance, demonstrating that the solar radiation adsorbed by the Earth is equal to heat radiation emitted to space.
Averaged across the globe, the Earth receives 341 W of incoming radiation per m2 (Wm-2) of its surface. Of this 341 Wm-2, 30% is reflected back to space by bright surfaces with high albedo (such as ice, snow, desert sand, and clouds), leaving 239 Wm-2 available to the climate system. A further 78 Wm-2 is absorbed by the atmosphere, leaving 161 Wm-2 to be absorbed at the surface. As well as this, the surface is also warmed by 333 Wm-2 of back radiation, emitted from the atmosphere due to the presence of Greenhouse gases, meaning the surface receives 161 + 333 = 494 Wm-2. As energy cannot be created or destroyed, the Earth needs to emit 494 Wm-2 back from the surface in order to keep the energy balance in equilibrium. A very small amount is emitted through thermal transfer and evapotranspiration (17 and 80 Wm-2 respectively), and the remaining 396 Wm-2 is emitted by the surface as heat radiation. However, only 239 Wm-2 leaves the top of the atmosphere due to trapping by Greenhouse gases – equal to the initial amount of solar radiation absorbed.
Each greenhouse gas in the atmosphere has a different ability for trapping radiation. The graph below shows calculations of the intensity of heat radiation leaving the atmosphere at one location, in this case the tropics. The black curve represents the present-day composition of the atmosphere, with total emitted radiation of 287.9 Wm-2. This theoretical curve matches well with measurements of the radiation made from satellites. The red curve indicates a hypothetical situation in which all atmospheric gases have their present-day concentrations, except water vapour which is removed. The result demonstrates the importance of water vapour in trapping radiation – without it emitted radiation increases by 58 Wm-2, an increase of 20%. The green curve demonstrates absorption with an atmosphere where CO2 has been removed. This causes a large increase in emitted radiation, 30.7 Wm-2 higher than with CO2 included. Although this increase is not as large as the change observed when we remove water vapour, this still represents an increase in emitted radiation by over 10% -a considerable impact for a gas which has an atmospheric concentration of about 0.04%.
The response of the climate
These calculations clearly demonstrate that changes in atmospheric greenhouse gas concentrations have important impacts on radiation emission, and therefore temperature. Physical calculations tell us that a 3.6 Wm-2 increase in radiation trapping, which would arise through a doubling of the atmospheric concentration of CO2, would lead to a 1°C temperature rise. However, this simple relationship is complicated by other factors, enhancing or reducing the response. One example is water vapour, which is increasingly evaporated with a warming climate. Water vapour further enhances the greenhouse effect, approximately doubling the impact of CO2. Another is cloud cover. Increased cloud cover (through increased evaporation) can either: reduce warming through an increase in the reflectance of solar radiation to space; or increase warming through the trapping of surface emitted heat radiation. However, cloud response to climate change remains a great uncertainty, and contributes to the wide range of temperatures (2.0°C to 4.5°C) predicted for a CO2 doubling.
The concentration of CO2 in the atmosphere has varied over time but it remained below about 0.03% from several million years ago (before the first primates appeared on Earth) until the beginning of the nineteenth century. It has risen steadily since then as a result of the burning of fossil fuels, reaching 0.04% in 2013. Predictions for the date when it will reach double its pre-industrial value vary according to assumptions on future emissions ranging from 2050, with no abatement, to 2100 if international agreements could produce really significant reductions.