The climate change during Earth’s history: Some changes were worldwide, while others simply affected an area or a hemisphere. In addition, a number of natural factors contribute to modify the Earth's climate during various periods
The quantity of energy emitted by the Sun is not constant. There are evidences revealing that the Earth's temperature corresponds to a solar cycle. Long term changes can occur.
The orbit of the Earth around the Sun changes slowly. This influences the quantity of energy which is reflected and absorbed. It is thought that these variations of Earth's orbit are one of the factors that triggered the ice ages.
Approximately 1/3 of the energy emitted by the Sun returns to space after penetrating Earth's atmosphere. A fraction of what remains is then absorbed by the atmosphere, but the major part is absorbed by the Earth's surface. The surface returns infra-red energy and while part of this energy is lost in space, another part is absorbed again and re- emitted by the clouds and gases. This contributes to heat Earth's surface and the troposphere
These are very fine particles that remain in suspension in the atmosphere during a very long time. They reflect the solar radiation and also absorb it. By modifying the quantity of the aerosols in the atmosphere, one modifies the quantity of the reflected and absorbed solar energy.
Scientific studies reveal that various human activities, like combustion of fossil fuels for producing electrical energy, heating and transport, produce greenhouse gases. Increase in the concentrations of these greenhouse gases leads to global warming.
By replacing forests with arable lands or the natural vegetation by asphalt and concrete, humanity modifies the way in which terrestrial surface reflects sunlight and releases heat. All these changes can also modify the regional configurations of evaporation, streaming and rains.
Due to the agricultural and industrial activities, humanity adds great quantities of fine particles called aerosols to the atmosphere. Most of the aerosols are quickly falling due to gravity and precipitations, but they do not less influence the atmosphere radiation absorption. It is the quantity and the nature of these particles as well as the nature of underneath surface (land or water) that determine if this have a heating effect or not.
Some direct effects include deaths and illnesses related to excessive heat or cold exposure. Indirect effects of climate on health may involve respiratory disorders due to air pollution, including spores and pollens. Incidences of waterborne diseases, such as cholera, as well as food productivity and its relation to nutrition are other indirect effects of climate on health.