Why does global warming happen naturally?

Answer 1

It's not! There is a natural greenhouse effect, which keeps the earth warm enough for life, but this is not what we mean by global warming.

As long as greenhouse gas concentrations remain at their natural, long term levels, uninfluenced by human activity, the warming of the earth is natural and has in fact helped maintain the world's temperature at a comfortable level suitable for human existence. The increase in global temperatures above this level is accepted by most climate research scientists as principally the result of human activity.

Greenhouse gases are gases that build up in the Earth's atmosphere and absorb infrared radiation. This causes a warming effect in the lower atmosphere and on the surface of the planet. The main naturally occurring greenhouse gases are: water vapour (H2O), carbon dioxide (CO2), methane (CH4) and ozone (O3). Water vapour does not refer to clouds that are lower in the atmosphere, as clouds are liquid water and reflect solar radiation, which would have a cooling effect.

Answer 2

It is part of earth's natural process that greenhouse gases regulate the temperature at the surface.

Normal cycles of warming and cooling are closely tied with orbital periods known as Milankovitch Cycles. According to the orbital periods earth should actually be cooling at the present time, ever so slightly, not warming.

Besides orbital cycles there are other factors that can affect global climate. While volcanic eruptions spew material into the air which has the immediate effect of cooling the earth for a few years, after that ash settles out the remaining gases generally have an overall warming effect.

Mountain building activity along coastal areas create vast rain shadows that cause forest vegetation to die. Forests can sequester large amounts of carbon, which when released would contribute to warming. Forests also have a lower albedo than desert sand. Higher albedo areas reflect more heat.

One more natural warming effect concerns melting ice. Ice has a very high reflectivity. The ocean water beneath it tends to be much darker, so as the ice melts, the larger ocean surface area absorbs more heat, thus boosting temperature.

At the present time, however, artificial effects exceed natural warming effects by about two orders of magnitude.