The greenhouse effect

The climatic system is changing constantly, and it is clear for scientists that this change is the result of both natural causes (such as volcanic eruptions and changes in solar activity) and human influence. However, scientists also claim, with a very high level of certainty, that it is mainly human influence that has caused the observed unprecedented warming since the mid-20th century.
To understand the link between human influence and the warming trend described above, one should consider the ‘greenhouse’ effect and its interaction with human activity.
The Sun radiates energy towards planet Earth. A certain part of the solar energy that reaches the top of the Earth’s atmosphere is reflected back into space. The rest is absorbed mainly by the Earth’s surface (i.e. land and oceans), and warms the Earth. To balance the incoming energy absorbed, the Earth must, on average, radiate the same amount of thermal energy. The heat from the Earth’s surface is therefore radiated back into space. However, some of this radiated heat is trapped by greenhouse gases (including evaporated water (H₂O), carbon dioxide (CO₂), etc.), contained in the atmosphere and then reradiated in all directions, thus warming the surface of the Earth and the lower atmosphere (Figure 2) This process is known as ‘the greenhouse effect’ and the heat-trapping gases are called ‘greenhouse gases’ (GHGs). In fact, without the atmospheric warming provoked by the greenhouse gases, the average Earth surface temperature would be below water’s freezing point and would thus be hostile to life.