Humanity's Greenhouse Footprint
Writing about web page http://cdiac.esd.ornl.gov/pns/current_ghg.html
Let's tackle the 'humanity not responsible for most of the greenhouse gas' thing, now.
First port of call – the actual figures.
Carbon dioxide % contribution: 25.3%
Methane % contribution: 60.2%
N2O % contribution: 14.8%
Ozone (not in ozone layer) % contribution: 26.5%
These aren't small numbers. And there are currently no competing theories to explain them except human factors – many of these figures are unprecedented for 400,000 years, a time period which encompasses grapes in England, ice skating on the Thames and the last mini-ice age.
Now, we need to tackle the claim that the above doesn't matter. The gist of this particular argument is that water vapour allegedly dominates greenhouse gases, so these don't have an effect.
There is some partial truth in this – depending on who you ask, water vapour accounts for between 88% and 98% of the overall effect. But this does not affect the case for climate change. (This should be unsurprising, since these figures are from IPCC, who have built it into their models.) Why not?
As should be obvious by now, it's because our models are more complicated than what the public imagines. Firstly, there is a non-linear aspect to this – small perturbations can have a greater impact than you think. After all, our modern, human temperature scale is entirely subjective – a disastrous change of 5 C is only a tiny wobble if we look at things from the scale of a planet which would be really really cold without any greenhouse effect. But there is a broader argument to be made. What is significant is not just the power and volume of greenhouse gases of each type, but also their longevity. This determines how long the gas stays in the atmosphere, and also how quickly it returns to an equilibrium.
Carbon dioxide has a variable lifetime. To fast acting sinks are dominant, then 5 years. If slow acting sinks are effective (and this is increasingly the case due to rainforest damage and so on), then up to 100 years. Methane has a lifetime of 12 years. N2O has a lifetime of 114 years.
Water vapour has a lifetime of 1 week.
What does this mean? It means that any excess or deficiency is quickly balanced out. We can see this with things like rain, and snow. It means also that water vapour cannot have a forcing role on the climate. Unlike the other greenhouse gases, which can pool and accumulate, water vapour doesn't hang around long enough to have a real effect. For that, you would need a steady, long term influx of water vapour, and no such source exists, either natural or man made. (Hmm, human factories come close, but we can mostly ignore this for now…)
This limits water vapour to a reactive greenhouse gas. I.e. it is implausible for it to directly cause any climate change, but it is perfectly plausible for something to change the atmosphere, and so change the water vapour equilibrium, which can then have an effect on the climate. The question, then, we should be asking is what this reaction is.
The result is well known to meteologists. Warm air can hold more humidity than cold air. And sustained increase in temperature, such as that caused by carbon dioxide greenhouse effects, then, shifts equilibrium to more water vapour in the air, which has a further greenhouse effect. The role of water vapour then, is that of a positive feedback. It makes climate change worse. That's why our climate models are spot-on.
So the human effect on the greenhouse gases do matter.