The principles of these calculations are simple. The sun emits short-wave radiation, heating the Earth's surface, which then sends long-wave heat radiation back into space. However, part of this long-wave radiation is absorbed by the atmosphere and returned to the earth. This contributes to a heating of the surface, and allows the average temperature at the surface to be about 15°C. Without the greenhouse effect the temperature of the earth would have been 18°C. There are several components in the atmosphere that causes this absorption, including water vapor (H2O), methane (CH4), nitrous oxide (N2O), ozone (O3) and carbon dioxide (CO2). The term greenhouse effect is due to the similar function as the glass roof of a greenhouse.
Svante Arrhenius simple calculations showed a temperature increase of 5°C for a doubling of atmospheric CO2 content. Today sophisticated climate models have been developed that run on supercomputers around the world. The various models show somewhat different results for different processes, but throughout the results we see an average increase of 2-4°C in the global temperature caused by a doubling of the levels of CO2 in the atmosphere, i.e. a result that is surprisingly similar to Arrhenius' calculations. It is therefore very likely that the earth will warm significantly as a result of increased CO2 content in the atmosphere. After the consumption of fossil fuels in earnest started after the industrial revolution, the proportion of CO2 in the atmosphere has increased from 280 to 390 ppm (parts per million), an increase of about 39%. Global temperatures during the same period saw an increased by 0.8°C. With the current consumption of fossil fuels, we will reach a doubling of the CO2 content in the second half of this century. The IPCC (Intergovernmental Panel on Climate Change), consisting of several thousand of the world's leading climate scientists, completely agree that we are experiencing anthropogenic climate change with rising global temperatures, and increasing incidence of extreme weather events across the globe. The uncertainty is however larger when it comes to the regional consequences of global warming.