The natural greenhouse effect
The Earth’s weather and climate are driven by incoming solar radiation. Around half the radiation is absorbed by the Earth’s surface, which then heats up. A further 20 percent is absorbed by the atmosphere, which also warms. Thirty percent is reflected back into space and is lost. It is essential for life on Earth that the heat radiated by the Earth’s surface does not immediately leave the atmosphere. Greenhouse gases such as water vapour, carbon dioxide, methane and nitrous oxide absorb the heat radiation and, in turn, radiate them back again. Thanks to this greenhouse effect (german) , the global mean temperature in the lower atmosphere increases by around 32°C, from -18 to +14°C. This is what makes the Earth inhabitable. We have known about these relationships for over 150 years and verified them through experiments and observations.
Humans are intensifying the greenhouse effect
Since the beginning of industrialisation in the 19th century, there have been substantial increases in atmospheric greenhouse gases as a result of human activity. The main culprits are the combustion of coal, oil and natural gas, intensive agriculture, the loss of large areas of forest and moorland, and changes in land use. For instance, in the last 150 years or so, the carbon dioxide content of the atmosphere has increased by almost 50 percent, from around 280 ppm (CO2 particles per million air molecules) to 415 ppm (as of December 2021). This is the highest CO2 content our atmosphere has seen in the last two million years.
Because human activity is intensifying the greenhouse effect, there is additional heat in the overall climate system. The majority of this heat is accumulating in the oceans and warming them. Only around one to two percent remains in and warms the atmosphere. Almost all of the total observed warming since industrialisation began is attributable to human activity. Solar and volcanic activity and internal fluctuations in the climate system play only a very minor role. These findings are partly based on simulations using climate models that have been adept at capturing the interactions between basic physical laws for several decades.