Greenhouse gas

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A greenhouse gas (GHG) is a gas or vapor such as carbon dioxide, methane, or water vapor which contributes to global warming by absorbing infrared radiation coming from earth then re-radiating part of it back to earth. Water vapor and carbon dioxide account for most of this green house effect. The gas of greatest concern is carbon dioxide because humans have raised its concentration in the atmosphere greatly by the burning of fuels and the cutting down of forests. (Forests take carbon dioxide out of the atmosphere by photosynthesis when they are alive and release it into the atmosphere when they are killed and decompose or are burnt.) The concentration of carbon dioxide in the atmosphere prior to the industrial era was about 280 parts per million (ppm) by volume. Over the past 200 years humans have introduced about 400 petagrams (petagram = 1015g) of carbon into the atmosphere. Part of this has been taken out again by oceans and terrestrial vegetation, and part remains. The concentration of carbon dioxide in the atmosphere is now about 387 ppm (2010 figure). Most experts place the safe limit of carbon dioxide in the atmosphere somewhere between 350 and 550 ppm (although some outlying figures, maybe no longer current, are as high as 750 ppm). As of 2011, we were pumping nine petagrams of carbon into the atmoshere per year.[1]

Life of pollutants in the atmosphere

Methane Wp→ and black carbon Wp→, soot, are relatively short lived in the atmosphere. Both offer opportunities for control which exceed those for carbon dioxide which is long-lived in the atmosphere. (According to the IPCC, `It is very likely that more than 20% of emitted CO2 will remain in the atmosphere longer than 1000 years after anthropogenic emissions have stopped.'[2]) Black carbon, soot from fires, power plants and engines, lasts only a few weeks. It plays a major role in the melting of Arctic sea ice Wp→ and ice sheet Wp→s which may result in rise in sea level Wp→. Methane often enters the atmosphere through pipeline leaks, which, if the effort is taken, can be sealed, saving both money and the atmosphere.[3] Methane may also be flared, burned off.[4]


  1. Lee Kump, "The Last Great Global Warming," Scientific American, July 2011, p 161. This may only be the contribution from fuel use. Carbon dioxide concentrations and safe limits are from Jonathan Foley, "Boundaries for a Healthy Planet, Scientific American, July 2011. The 400 Pg figure is from Sabine et al., "Current Status and Past Trends of the Global Carbon Cycle" in Christopher Field and Michael Raupach, The Global Carbon Cycle (2004), p 18.
  2. The IPCC continues: `This extremely long time required by sinks to remove anthropogenic CO2makes climate change irreversible on human time scale.' -- IPCC, Climate Change 2013, chap. 6, p 8.
  3. "Cutting Short-lived Pollutants Can Slow Sea Level Rise" article by Andrew Freedman on Climate Central April 14th, 2013
  4. "Coming to sites across the UK soon – fracking flares: IGas chief warns that any production of shale gas would involve 'flaring off' leakages" article by Tom Bawden in The Independent May 9, 2013


Intergovernmental Panel on Climate Change (IPCC), Climate Change 2013, Chapter 6, `Carbon and Other Biogeochemical Cycles' (a.k.a. `Working Group I contribution to the IPCC 5th Assessment Report "Climate Change 2013: The Physical Science Basis" ').