A table of global sources of the three main greenhouse gases – carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O, also abbreviated as NOx) – is reportedly no longer readily available from the UN Intergovernmental Panel on Climate Change. Here it is as reproduced by the US Energy Information Administration in the December 2004 report, “Emissions of Greenhouse Gases in the United States 2003”.
The table shows that human (anthropogenic) CO₂ emissions in the 1990s were less than 3% of the total, ie, 97% of CO₂ emissions were natural, although more than half of the human emissions exceeded the amount that could be naturally absorbed. For the other greenhouse gases, human CH₄ emissions were 50% greater than natural CH₄, representing 60% of the total, and only 6% of the human emissions exceeded what could be naturally absorbed. Human N₂O emissions represented about 55% of that total, and 55% exceeded what could be absorbed.
One thing that the table does not indicate is the different greenhouse effect levels of the three gases. CH₄ has 20 times the greenhouse effect of CO₂, N₂O 300 times. Therefore, the annual increase in greenhouse gases by effect is about 88% due to CO₂, 3% to CH₄, and 9% to N₂O.
Combining that information with what the table indicates, to halt the annual increases in these greenhouse gases, humans would have to reduce CO₂ emissions to 51% of the level specified here for the 1990s, N₂O to 25%, and CH₄ to 94%.
If the annual increase in greenhouse effect were to be halted by reducing CO₂ alone, humans would have to reduce emissions to less than 43% of their 1990s level. If, however, human CH₄ emissions were halved (relatively easy to achieve by, eg, reducing animal agriculture and capturing leakage at natural gas wells), human CO₂ emissions would have to be reduced to 58% of their 1990s level.
Another important consideration is the very different half-lives of these greenhouse gases. Most strikingly, CO₂ persists for centuries, even millennia, in the atmosphere, whereas CH₄ persists for only about 10 years. In other words, changes to CO₂ emissions would not have an effect for hundreds of years, but the effect of changes to CH₄ emissions would be relatively immediate. (N₂O lasts about 100 years.) (It may well be that the climate change effects we are experiencing today are due to coal burning in the 19th century, which at the time was mitigated by the cooling effect of soot.)
In summary, halting the increase of greenhouse gas emissions remains a formidable challenge, let alone that of reducing their levels in the atmosphere. But N₂O and CH₄ are easy targets for reduction that must not be ignored, particularly because their reduction would have a much more immediate effect than reduction of CO₂.