climate science
Science of Global Warming: Why the Climate Changes: Emissions of Heat-Trapping Gases and Aerosols
Why is the climate changing? What causes global warming? What
creates the so-called greenhouse effect?
The answers to these questions all have to do with the chemical composition of the Earth's atmosphere. The atmosphere around us is made up of gases. Some of these gases function like the panes of a greenhouse: they let some radiation from the sun in but also retain heat in the atmosphere, that is: they don't let all radiation back out. As a result of this natural effect, it is warmer on Earth than it would be without these heat-trapping gases. Human contributions of certain gases to the atmosphere have increased this greenhouse effect.
Carbon dioxide (CO2) is one of those heat-trapping (greenhouse)
gases that have increased significantly in atmospheric concentration since pre-industrial times and thus has raised the greatest concern. But several others contribute significantly to the warming of the lower part of the atmosphere, such as - methane (CH4)
- nitrous oxide (N2O)
- ozone (O3)
- halocarbons
The combined warming effect (positive radiative forcing) from these other trace gases is approximately equal to that of CO2. In addition, there are small particles (aerosols) that can directly or indirectly help cool the atmosphere (negative radiative forcing). Examples include - sulphates (e.g., produced by burning coal)
- volcanic dust.
some of these gases and aerosols are very effective in trapping heat or blocking incoming radiation, but are relatively quickly removed from the atmosphere. Others can trap less heat, but stay in the atmosphere longer. The combination of these factors determines what scientists call the direct Global Warming Potential of an emitted compound -- a measure the IPCC has devised to show "the possible warming effect on the surface-troposphere system arising from the emission of each gas."
To be able to compare the Global Warming Potential (GWP) of
different gases, a base unit of CO2 is said to have the GWP of 1.
The chart below shows, for example, that each ton of methane will have 12 times the global warming impact over a hundred-year period as a ton of carbon dioxide. Even though total emissions of chlorofluorocarbons are quite small compared with emissions of carbon dioxide, their impact is significant since their global warming potential is so large. And even bigger than that, scientists recently identified a rare and previously unreported gas -- trifluoromethyl sulfur pentafluoride (SF5CF3) -- that is long-lived (an atmospheric residence of several hundred to several thousand years) and may have a Global Warming Potential that could be as much as 18,000 times that of CO2. Nevertheless, carbon dioxide emissions still account for about half of the total Global Warming Potential of emissions from human sources.
| | Anthropogenic sources | Total global emissions (for 1990, in metric tons) | Atmospheric lifetime (in years) | Direct global warming potential over 100 years | | Carbon dioxide | Burning of fossil fuels, cement manufacture, deforestation and other land-use changes | 7.1 Gt
(in C equiv.)*
(1 Gt = 1 gigaton or 1 billion tons) | 120 | 1 | | Methane | Livestock, wet rice agriculture, solid waste, coal mining, oil & gas production, biomass burning | 310 Mt CH4
(1Mt = 1 million tons) | 12 | 21 | Nitrous
oxide | Nylon production, nitric acid production, biomass burning, cultivated soils, automobiles with three-way catalysts | 6.7 Mt
(in N equiv.)** | 120 | 270 | Chlorofluoro-carbons
(CFCs | Chemical products & processes, including refrigeration, industrial solvents, blown-foam insulation | 1,672 Mt
(for all CFC/HFC/HCFC total in C equiv.) | 50->100 | 3,800 -- 8,100 | | CFC substitutes (HCFCs and HFCs) | Same usage as CFCs | See above | Varies by orders of magnitude | Varies by orders of magnitude | | Sulphur hexafluoride (SF6) | Chemical products and processes | 37.7 Mt
(in C equiv.) | 3,200 | 23,900 | Trifluoro-
methyl sulphur pentafluoride (SF5CF3) | Unknown | Minimal, growing fast | ˜1,000 | ˜18,000 |
CO2 measured in "C equivalent" means that only the mass of the
carbon is measured, not the mass of carbon dioxide. This is a scientific convention accounting for the fact that carbon cycles through the air, oceans, biosphere, fuels etc. and in the process occurs together with different elements (e.g., CO2, CO, CH4, CaCO3, CFC-12). Emissions expressed in units of elemental C can be easily converted to emissions
in CO2 units by adjusting for the mass of the attached oxygen atoms, that is by multiplying by the ratios of their molecular weights (44/12) or 3.67.
- NOx measured in "N equivalents" follows the same logic.
Sources: Carbon Dioxide Information Analysis Center. 2000. Current Greenhouse Gas Concentrations. (available online at http://cdiac.esd.ornl.gov/pns/current_ghg.html; Sturges, W.T. et al.
2000. A potent greenhouse gas identified in the atmosphere: SF5CF3. Science 289: 611-613 IPCC. 1995. Climate Change 1995: The Science of Climate Change, Radiative Forcing of Climate Change. New York: Cambridge University
Press, pp. 65-132; IPCC. 2000. IPCC Special Report: Emission Scenarios, Summary for Policymakers. (available online at http://sres.ciesin.org/. Contents: |
