Methane & Climate
Methane (CH4) is emitted from a variety of both human-related (anthropogenic) and natural sources. Human-related activities include fossil fuel production, animal husbandry (enteric fermentation in livestock and manure management), rice cultivation, biomass burning, and waste management. These activities release significant quantities of methane to the atmosphere. It is estimated that more than 60 percent of global methane emissions are related to human-related activities (IPCC, 2007:a). Natural sources of methane include wetlands, gas hydrates, permafrost, termites, oceans, freshwater bodies, non-wetland soils, and other sources such as wildfires.
When one views the recent geologic past (one million years) and warming, one can see that even in warmer states there is no strong indication of methane release. In our last interglacial, we had warmer temperatures and higher sea level, but there is no indication of large scale methane clathrate release.
One can also look back to periods of extended warm periods and still see nothing like that of the PETM (Paleocene–Eocene Thermal Maximum) event, which is suspected to be a large scale release of methane from hydrate clathrates into the atmosphere.
Methane is released by various human activities and contributes to the excess load of greenhouse gases since the pre-industrial era. The growth rate in the atmosphere stalled for a while, but appears to be rising again. The reason for this is not fully understood.
- One issue is how to reduce human-induced methane emissions in the future to slow the growth or (more hopefully) get its concentration to start to fall.
- Another issue is whether global warming will trigger methane releases from natural reservoirs, thus feeding back to drive further warming.
Although methane has been widely implicated as a possible cause of past climate changes, I think the jury is still out as to whether it was ever a major cause.
Recent studies continue to examine methane and the picture is slowly coming into greater focus.
The main problem is that we don't have any proxies before the ice-core era of past methane changes, and during the ice-core era, the methane changes were too small to be the main cause of the climate changes. The fact that we can't show that methane was a major driver of climate changes in the past doesn't mean we don't know its relative greenhouse effect. This is established securely from the optical properties of the molecule.
There is increasing evidence that the major extinctions of the past several hundreds of millions of years are associated with long lived events following major tectonic disturbances that result in release of greenhouse gases, with associated global warming, ocean anoxia etc.
For example the early Jurassic extinction is associated with events (greenhouse gas induced warming) lasting 200,000 years. Likewise comprehensive analyses shows a coincidence of major tectonic events, and resulting elevation of greenhouse gas levels, are associated with several of the major extinctions of the last 300 million years. Note that CO2 isn’t the only player. Methane is implicated in several of these events (see especially the PETM below) and sulphurous oxides and their effects on ocean acidity and oxygen content are also implicated.
Greenhouse environments are associated with the very delayed (millions of years) recovery of biota following these extinctions.
The lesser extinction associated with the Paleo-Eocene-Thermal Maximum (PETM) 55 MYA is probably the best characterized (not surprisingly since it’s the most recent!) example of massive tectonic processes (the opening up of the N. Atlantic as the plates separated) associated with enhanced atmospheric greenhouse gases, ocean acidification etc.
And even the end-Cretaceous extinction (that did it for the dinosaurs) seems to have had at least a significant component from massive flood basalt events (that resulted in the Deccan Traps in what is now India). In fact there is increasing evidence that the impact that resulted in the Chicxulub crater in the Yucatan post-dates the onset of the extinction by several 100,000’s of years, and the extinction is associated with global warming (including a sudden contribution from the impact into limestone-rich deposits that vaporized massive amounts of carbonate (limestone) back into CO2).
Continued focus on these events and probable causes will likely reveal increasingly accurate understanding.
- 2010/05/28 The Royal Society - Gas hydrates: past and future geohazard?
- 2010/03 RealClimate - Arctic methane on the move
- 2010/03/05 International Arctic Research Centre - Extensive Methane Venting to the Atmosphere from Sediments of the East Siberian Arctic Shelf
- 2010/03/05 Max-Planck-Institut für Biogeochemie - How Stable Is the Methane Cycle?
- 2010/01/15 University of Edinburgh - Large-Scale Controls of Methanogenesis Inferred from Methane and Gravity Spaceborne Data
- 2007/04 Science Magazine - Paleocene-Eocene Thermal Maximum and the Opening of the Northeast Atlantic
- 2007/03 University of Wisconsin-Milwaukee - Elevated atmospheric CO2 and the delayed biotic recovery from the end-Permian mass extinction
- 2007/01 University of Oslo - Hydrothermal venting of greenhouse gases triggering Early Jurassic global warming
- 2005/12 RealClimate - Methane hydrates and global warming
- 2005/12 University of Leeds - The Link between Large Igneous Province Eruptions and Mass Extinctions
- 2005/ Princeton University - Impacts, volcanism and mass extinction: random coincidence or cause and effect?
- 2004/12 University of Plymouth - The palaeoclimatology, palaeoecology and palaeoenvironmental analysis of mass extinction events
- 2002 PNAS - An atmospheric pCO2 reconstruction across the Cretaceous-Tertiary boundary from leaf megafossils