What is the difference between natural greenhouse gases and anthropogenic greenhouse gases?

Answer 1

The important greenhouse gases, water vapour, carbon dioxide, methane and ozone are naturally occurring, but can also be added to the atmosphere by human activities.

Natural sources of carbon dioxide do not add to the the atmospheric concentration, and the atmospheric concentration has remained in the range of 260 to 280 parts per million (ppm) for hundreds of thousands of years, with animals breathing out carbon dioxide and plants absorbing it from the atmosphere again, producing food which animals eat. Anthropogenic sources include fossil fuels, such as coal, oil and natural gas, which have sequestered carbon underground for millions of years. By burning fossil fuels for industry, production of electricity and for transport, we are actually adding to the level of carbon dioxide in the atmosphere, thereby creating an enhanced greenhouse effect. Cement manufacture releases carbon dioxide which was also sequestered in mineral form. The other major source of additional greenhouse gases is deforestation. Forests hold large stores of carbon for thousands of years, so by removing trees without allowing new ones to grow and absorb as much carbon as the previous trees, we add to the level of atmospheric carbon dioxide.

Methane is produced naturally from the digestive systems of ruminants and from anaerobic decomposition, and anthropogenically by industry and as a byproduct of coal mining. Because of increasing human populations, we now maintain large herds of ruminants such as sheep and cattle, thereby adding considerably to methane emissions. Scientists have found ways to alter the diets of cattle so as to markedly reduce their production of methane, but these techniques are not in widespread use.

The one greenhouse gas that humans can not add directly to the atmosphere is water vapour, since any excess would precipitate within days, maintaining a global equilibrium. However, water vapour is indirectly implicated in global warming. As average global temperatures rise in response to increasing levels of carbon dioxide and methane, the atmosphere takes up more water vapour. This in turn adds a little more to the greenhouse effect and therefore to global warming, providing an example of positive feedback.

So, the essential difference is that anthropogenic sources increase the atmospheric concentration of greenhouse gases and create an enhanced greenhouse effect.