The earth's climate depends on a balance of many factors. The main ones are not changeable by any human actions. They include such things as the luminosity of the sun, the distance between earth and sun, the precise shape of earth’s orbit, any intervening space dust, the angle between the earth’s axis of rotation and the plane of its orbit, the speed of its own rotation (affecting the length of each day), and the size and composition of the atmosphere.
Over the planet’s long history (about 4.5 billion years) most of these factors have varied. We know that the sun's output has changed, and that earth’s orbit has long-term cycles that cause subtle changes to the shape of its orbit.
But the composition of the atmosphere can also have a dramatic effect on the climatic conditions of a planet. For confirmation we need look no further than our own moon. At the same average distance from the sun as the earth is, the moon has a vastly different climate. Its daytime mean surface temperature is greater than 100°C, but shortly after sunset temperatures will plummet to minus 150°C. With no insulating atmosphere, the moon gets too hot by day, but the same lack of insulation means it will quickly cool at night.
Not all atmospheres offer the same insulation. Some gases are particularly good at holding in heat at the surface. On earth, the principal heat-retaining gases are water vapour and carbon dioxide.
The presence of these gases creates what is called a greenhouse effect. It is a common misconception to consider the greenhouse effect as caused by humans. The greenhouse effect is a natural process by which an atmosphere can retain heat from the sun. Without our natural greenhouse effect, the average temperature on earth would be about 33°C colder.
An atmosphere, therefore, is like a blanket. Too thick a blanket, however, can cause overheating. If the concentration of the heat-retaining gases (so-called greenhouse gases) increases, then more heat will remain in the lower atmosphere and at the surface. A good example of an extreme greenhouse effect is the planet Venus, whose atmosphere is about 90 per cent carbon dioxide. It surface temperature is much hotter than would be expected given its distance from the sun. In fact, Venus is hotter than Mercury, even though the latter planet is far closer to the sun.
Human activities are increasing the concentration of carbon dioxide in the earth’s atmosphere. This is the main reason for concern. At the same time, average temperatures on earth are rising. This is called the enhanced greenhouse effect, or anthropogenic (human-caused) global warming.
Carbon dioxide (CO2) is not a problem of itself; in fact all plant matter relies on it as the starting point for photosynthesis, by which complex biological molecules are made and distributed throughout the food chain. Without some CO2 in air, all life would cease. But too much CO2 brings changes to the heat balance at the surface of our planet.
In the past, CO2 levels and average global temperatures have fluctuated for natural reasons. The current changes appear to be more rapid than any global-scale changes known before, and the main cause is almost certainly the release of large volumes of CO2 from the burning of carbon-containing fuels (coal, oil and methane gas). There could also be natural factors continuing to act, but since the mid-20th century human-driven factors have dominated.
Further details on the science of all these issues are provided in the web site pages under this section.