Where has all this Carbon Dioxide come from?

For the most part, the human impact on the atmospheric burden of natural greenhouse gases can be traced back to activities that effectively add a new source of the gas and/or increase natural emissions in various ways. Take CO2, for example. Despite being the feature that characterises the industrial age, burning fossil fuels is not the only anthropogenic source of CO2. For centuries, people have been clearing forests, burning the wood and turning vast tracts of land over to agricultural use in order to feed an ever-expanding population. The process of ‘deforestation and land-use change’ also adds to the CO2 content of the atmosphere.

Sources of other greenhouse gases – the human connection

Methane is generated during the breakdown of organic matter by bacteria that thrive in anaerobic (i.e. oxygen-free) environments – principally in waterlogged soils (bogs, swamps and other wetlands, whence methane's common name of ‘marsh gas’) and in the guts of termites and grazing animals. But today, only some 30% of global CH4 emissions come from natural sources, with natural wetlands accounting for about two-thirds of the total. Rice paddies, effectively artificial marshes, contribute a further 11%, and an astonishing 16% is due to the flatulence of grazing livestock (cattle, sheep, etc.)! While such sources are undoubtedly biogenic in origin, they also clearly have an anthropogenic element – closely linked to human food production, in this case.

Waste management (e.g. organic matter rotting in landfill sites) adds a further anthropogenic source of CH4 (around 17% of global emissions). And since natural gas is mainly methane, so too does leakage from natural gas pipelines and the common practice of venting the gas to the atmosphere at oil production sites and from coal mines (a further 19%). Finally, burning vegetation can also generate CH4, depending on the way it burns (i.e. smouldering as opposed to flaming).

Nitrous oxide
is part of the natural nitrogen cycle; it is produced by the activities of micro-organisms in soils and sediments. Again, the increase in its atmospheric concentration is thought to result mainly from agricultural activities, such as the application of nitrogenous fertilisers to boost crop yields; some of the nitrogen ends up in the air as N2O. In addition, the high-temperature combustion of fossil fuels (or indeed, any kind of vegetation) in air produces some N2O (through reaction between N2 and O2 in the air), along with other nitrogen oxides (notably nitric oxide, NO).

Ozone is also a natural component of the lower atmosphere (due in part to transport down from the stratosphere), but the normal background level is low. However, enhanced concentrations of tropospheric ozone are now found in many polluted environments, especially over densely populated industrialised regions. Here, ozone is generated close to the surface by the action of sunlight on the mix of gaseous pollutants that is typically found in vehicle exhaust fumes: un-burnt hydrocarbons, carbon monoxide (CO) and nitric oxide (NO). Ozone is one of the more noxious components of ‘photochemical smog’, since exposure to enhanced levels of the gas is harmful to both human health and plant growth.
Unfortunately, gains made in reducing vehicle emissions of the key ozone ‘precursors’ (by fitting catalytic converters) are being outweighed by the worldwide growth in car usage. And there are many other anthropogenic sources of these pollutants as well – including power stations, industrial processes, and the burning of vegetation.

Relative contributions of various gases to the total greenhouse
effect on climate over the period 1750 to 2000.

How sure are scientists that the Earth really is warming up?

  • The Earth's climate system has demonstrably changed on both global and regional scales since the pre-industrial era, with some of these changes attributable to human activities.
  • Globally, it is very likely that the 1990s was the warmest decade and 1998 the warmest year in the instrumental record [1861–2000].
  • New analyses of proxy data for the Northern Hemisphere indicate that the increase in temperature in the 20th century is likely to have been the largest of any century during the past 1000 years. It is also likely that the 1990s was the warmest decade and 1998 the warmest year [of the millennium].
  • In the light of new evidence and taking into account the remaining uncertainties, most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations.

Source: ‘Summary for Policymakers’ (SPM) included in the report from the scientific working group in the IPCC TAR (IPCC, 2001a).

‘Global Warming’. An OpenLearn chunk used/reworked by permission of The Open University copyright © (2007).’ http://creativecommons.org/licenses/by-nc-sa/2.0/uk/