We have discussed Carbon Dioxide (CO2) and Methane (CH4) as major Green House Gases (GHGs). In addition to these two, Water Vapour (H2O) is a very major contributor. More minor inputs also from Nitrous Oxide (N20) and ozone. Many other gases such as CFS, Sulphur dioxide, Nitrous oxides and Carbon monoxide play their part. Exactly what each part is, how they interact and their presence in different atmospheric levels are the areas of major research and, to some degree, are poorly understood.
Other substances including water droplets (in clouds) and smoke and dust (including volcanic ash) are also generally put into the GHG category.
Water as a gas or as water droplets are the prime GHG. Both humidity levels (gaseous water) and clouds affect the amount of heat retained in the atmosphere. Desert nights are cold as the heat escapes while day and night are similar temperatures in the humid, cloudy tropics. Up to 85% of the greenhouse effect on earth can be attributed to water vapour.
In a warming atmosphere we can expect more water vapour and an increase in warming. This is generally described as a positive feedback effect. If this was the only feedback loop then we would already have had a “runaway greenhouse effect” where we just keep heating until we lose our atmosphere completely.
A recent paper in “the Journal of Geophysical Research Atmospheres” by Marc Morano suggests that while water vapour levels increase in the lower atmosphere (troposphere) they are actually observed to decrease in the upper atmosphere (stratosphere) creating a negative feedback situation. His paper states that this effect may completely offset any projected warming from projected CH4 and CO2 increases.
This, and other studies suggesting similar results, are very important to the GHG debate as current warming models generally have a positive feedback effect added in for water vapour. In fact many models leave out water vapour entirely.
Global Climate Models (GCMs)
While we know a lot about the physics that describes how a water or CO2 molecule can warm the atmosphere, we know very little about how the whole, complex, earth atmosphere works. We can predict accurately how much warming extra CO2 in the atmosphere creates but we can only model what we expect to actually happen.
Models are mathematical processes where we enter all the known physics (or at least simple physics) into a computer and press a button to find out the result. As we only partially know the inputs, we are left with a probabilistic range of outcomes. If we are somewhere near right on the inputs then our output may be fairly accurate.
The IPCC models to date have been pretty good. All in all they have done a lot to advance our knowledge of Climate Science. A lack of predicted atmospheric warming in recent years has been explained as a period when the oceans took up excess heat and this is now expected to reverse, accelerating atmospheric warming.
The problem here is that the heating of the oceans was not predicted as it was not included in the GCMs. This leaves us wondering what other major input has not been included in climate models. Water vapour is a big one. The IPCC generally play down the effect of water vapour, with statements like “Water vapour is the most abundant and important greenhouse gas in the atmosphere. However, human activities have only a small direct influence on the amount of atmospheric water vapour”.
Planetary cycles (Milankovitch) and solar activity are seen to have had significant influence on the creation of ice ages (and mini ice ages) in the past. These are viewed as having little influence in current GCMs. This assumption is most likely correct but until we fully understand what happened previously our models are likely to be way off the mark.
In 1750 Carbon dioxide levels in the atmosphere (troposphere to be precise) are estimated at 280 ppm (parts per million) and this is now measured to be about 400 ppm (ie 0.028 % to 0.04%). As previously discussed low CO2 levels have helped create the ice ages of the past. Increasing CO2 levels will definitely have an influence in warming the future.
After water vapour it is the most effective GHG. This is due to both its longevity in the atmosphere and its abundance. A methane molecule has about 25 times the effect of a CO2 molecule but there is significantly less methane in the atmosphere and its longevity is about a quarter.
The increase in CO2 can be largely put down to human activity (anthropogenic) including burning fossil fuels, reducing biomass, farming and industrial practices. Cement manufacture is estimated to create 3% of CO2 emissions.
For the fossil fuels, burning liquid fuels account for about 40% of increased CO2, coal about 35% and gas about 20%. Europe and the US account for about 60% of the increased levels of atmospheric CO2. Today, China is responsible for about 23% of CO2 emitted.
If we look at CO2 levels for each joule of energy produced then if we assign methane a level of 10, oil (petrol / gasoline) is 13, wood burning is 17 and coal burning is 18 times as much. So if gas replaced coal as a fuel source we would have nearly half the CO2 emissions (not counting transport, engineering issues). On the same scale solar cells are about 1.
Basically plants take up CO2 and animals expel it. Creating “carbon sinks” diminishes the amount of CO2 in the atmosphere. This means planting more trees, vegetation, gardens etc, together with more wetlands (ie starter coal swamps) and burying biochar (charcoal for soil improvement)
As discussed previously increased CO2 levels also increase ocean acidity.
Methane levels in the atmosphere are estimated to have risen from 0.7 ppm to 1.8 ppm from 1750 with spikes at around 3 ppm. The significant increases and continued current increases are somewhat of a concern, particularly as they are poorly understood.
Methane level measurements have an annual spike corresponding to northern hemisphere summer. As the level of this spike against northern hemisphere winters has remained similar over time it is likely that it is a seasonally related increase. Culprits could include rice paddies, forest fires and melting arctic ice allowing methane escape.
Other sources of excess methane, not corresponding to a summer spike are from fossil fuel production, landfill and animals (mostly ruminants). Satellite images show small, intense spikes around major activity including the Three Corners coal and coal seam gas activity in the US and the recent methane gas leak in California. These appear to be obvious leaks that should be controlled both physically and legislatively. Broad anomalies also occur over eastern US, Europe and China and are likely to be from intense human activity including coal and gas production and burning, car exhausts, landfill etc.).
Satellite images do not seem to show broad hotspots over oil production areas like the middle east nor over Australia (some small spots over the eastern states population and coal centres can be seen). This may be somewhat subjective as the images do change quite a lot. This would suggest that oil and gas production and large land animals are not the major contributors. Vegetation rotting or burning is likely to be a greater contributor than the ruminants ( the grass grows and dies off whether the animal is there or not). Intense farming and crops grown to feed these animals would increase CH4 levels. Estimates of methane from fossil fuel operations vary from about 5 to 10 % of world emissions.
Rice is being grown in increasing quantities and rice paddies behave like swamps giving off significant methane gas. It is estimated that up to 20% of CH4 emissions come from rice paddies. This is consistent with the cyclic nature of methane emissions and the intense broad spike over southern China and SE Asia.
The most worrying is the increase in methane emissions from the arctic region. Methane is trapped below permafrost in the arctic land mases and as this ice melts the methane escapes. As arctic temperatures increase this happens to a larger extent. We know that we measure less sea ice each year currently and I suspect the significant increase in methane levels is largely coming from this source.
As (or if?) the planet continues to warm other sources of buried methane will also be disturbed. This happens to some extent without warming when tectonic movement allows large gas leaks to occur. Huge amounts of gas are contained in gas hydrate deposits (frozen methane called clathrates) under the deep ocean. When (or if) these are freed up there will be some immense methane spikes! I don’t believe that the deep ocean will change much in temperature with the current warming oceans. This heating occurs over the top 100 metres or so.