More Information- Climate Change

Human Activities that Contribute to Greenhouse Gas Emissions

Many factors contribute both to positive and negative changes in the concentration of greenhouse gases in the atmosphere (e.g. burning fossil fuels adds CO2 to the atmosphere - a positive flux; planting trees removes CO2 – a negative flux). Human activity has impacts on the concentrations of four main types of greenhouse gas:

carbon dioxide, methane, nitrous oxide and certain fluorinated gases (some chemical compounds containing the element fluorine):

  • Carbon dioxide (CO2) concentration in the atmosphere increases mainly as a result of:
  1. the combustion of fossil fuels – widely used in power generation, heating and cooling buildings, transportation and the manufacture of cement etc.
  2. deforestation and land clearing which emits CO2 as well as reducing its uptake by plants life
  3. release from decaying plant matter.
  4. any weakening of the substantial uptake of CO2 by the oceans.
  • Methane (CH4) is emitted in human activities related to agriculture (e.g. from grazing livestock), natural gas distribution and from garbage dump sites. Human activities impacting on some natural ecosystems such as wetlands also have an impact on methane emissions. According to the IPCC 2007 report, CH4 concentrations have not been increasing in the atmosphere over the last two decades. This situation could be reversed if rising global temperatures were to cause unforeseen release of large methane deposits stored deep in ocean sediments and frozen soils.
  • Nitrous oxide (N2O) is emitted by human activities such as fertiliser use and the burning of fossil fuels. Natural processes in soils and oceans that can be affected by human activities also release N2O.
  • Fluorinated gases are emitted mainly as a result of human activities. Natural processes are only a small source. Principal fluorinated gases include the chlorofluorocarbons which were used as refrigeration agents and in other industrial processes before it was recognised that their presence in the atmosphere causes ozone depletion in the stratosphere. The abundance of chlorofluorocarbon gases is decreasing as a result of international regulations.

Significant increases in atmospheric concentrations of all of these gases have occurred since the beginning of the industrial era in the late eighteenth century but as noted above concentrations of methane and chlorofluorocarbons have stabilised or fallen in recent decades.

Graph 1 from  the IPCC 2007 report shows the contributions to the total global anthropogenic greenhouse gas (GHG) emissions from each of these different gases (expressed in terms of carbon dioxide equivalents)  and from different human activities over time.

In summary,

Global increases in CO2 concentrations are due primarily to fossil fuel use, with land-use change providing another significant but smaller contribution. It is very likely that the observed increase in CH4 concentration is predominantly due to agriculture and fossil fuel use. CH4 growth rates have declined since the early 1990s, consistent with total emissions (sum of anthropogenic and natural sources) being nearly constant during this period. The increase in N2O concentration is primarily due to agriculture.’


Graph 1: Global anthropogenic greenhouse gas (GHG) emissions - (a) Global annual emissions of anthropogenic GHGs from 1970 to 2004.5 (b) Share of different anthropogenic GHGs in total emissions in 2004 in terms of carbon dioxide equivalents (CO2-eq). (c) Share of different sectors in total anthropogenic GHG emissions in 2004 in terms of CO2-eq.

Note 1. The anthropogenic GHGs include only carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and sulphurhexafluoride (SF6), whose emissions are covered by the United Nations Framework Convention on Climate Change. These GHGs are weighted by their 100-year Global Warming Potentials. Note 2. Forestry includes deforestation. (Source: IPCC 2007 b