The cause of the world's problems isn't rich families of Chelsea after all, it is the rear end of a cow

Thanks for the opportunity to bring up one of my favourite examples of climate-change policy, JG: the New Zealand fart tax that nearly was. This is not an urban myth. The government of New Zealand really did get close to bringing in a tax on livestock flatulence, before their plans were brought to a halt by irate farmers walking their animals up the steps of the parliament building. I guess Treasuries will never be able to resist sticking a tax on things they know people love too much to give up. In our case, that's cars, and in New Zealand's case, it's sheep. And as both cases showed, this is a dangerous political game that can backfire.

If you are interested in the contribution of animal rear-end emissions to atmospheric greenhouse-gas concentrations, you might also want to have a look at termites, ants and other insects. And as well as the plant emissions noted by John, don't forget worms and their emissions of nitrous oxide.

This research is hardly ground-breaking. Efforts to find ways to get cows to fart less have been under way for some time. But I'm sure a Welsh solution will have innumerable advantages.

But perhaps we shouldn't be trying to reduce this source of methane. It is, after all, a valuable energy-source (unlike carbon dioxide) if it can be harnessed. There are businesses out there hoping to do exactly that.

This sort of thing is a common theme amongst sceptics. Thinking about other sources of greenhouse gases helps to keep things in perspective, but it is often treated more than a little simplistically. For example, people usually just trot out the figure of 23 (or 21, depending which IPCC report you were relying on) for the Global Warming Potential of methane when estimating its impact. But this does not mean that methane is 23 times stronger a greenhouse gas than carbon dioxide. Different greenhouse gases have different residence times in the atmosphere. The GWP tries to take account of this. Methane's average lifespan in the atmosphere, at approximately 12 years at the moment, is much shorter than that of carbon dioxide. Its radiative forcing, on the other hand, is more than a hundred times stronger than carbon dioxide. That figure of 23 is based on a time horizon of 100 years. Over a 20-year time horizon, the GWP is 62, and over a 500-year horizon, it is only 7.

This is of more than academic interest. If we were genuinely worried about approaching a tipping point, it would be rational to focus far more on reducing methane emissions than we currently do, as the short-term impact of emissions is greater and the impact of reductions is felt more quickly. On the other hand, if we see anthropogenic global warming as only a long-term cumulative problem, we ought to value methane emissions less highly than implied by that figure of 23, as current emissions will have mostly been reabsorbed before their impact is felt. The fact that Kyoto and its various subsidiary mechanisms can't even start to take account of temporal issues like this is one of the many reasons why it is a bad mechanism.

And on a broader level, there is a temptation for sceptics to argue that, as there are many natural sources of greenhouse gases, human emissions are relatively inconsequential. But that is the classic error of looking at aggregate rather than marginal changes. If a system is in equilibrium, it may not take a large input from outside the system to unbalance that equilibrium. A good amount of carbon dioxide and other greenhouse gases are both emitted and absorbed naturally by the Earth's ecosystem each year. Sources exogenous to the natural cycle, such as anthropogenic emissions, can have a relatively significant impact on the balance of emission and absorption, even though they are only a small part of the total.

This doesn't mean that the rest of global-warming theory follows, just because this part of the theory holds some water. The great uncertainty is in cause and effect. Sensible sceptics do not deny either that atmospheric concentrations of some greenhouse gases have been increasing, or that global average temperatures increased for good chunks of the twentieth century (though with a significant retrenchment for a period after the war). The questions still to be answered are to what extent one is connected to the other, in what direction that cause-effect mechanism occurs (i.e. does the temperature lead the atmospheric concentrations or vice versa), and to what extent do other factors (such as solar activity) play a part? There is still a great deal of uncertainty around these issues, whatever politicians, the media and certain parts of the scientific establishment might say.

And it is worth pointing out that scepticism cuts both ways. When we say we don't know, that is neither proof positive nor negative. Sceptics have no more ruled out the possibility of anthropogenic global warming than alarmists have proved it. In my view, we should treat this as a risk, just as we treat the possibility (however unlikely) that our house might burn down as a risk. None of the current mechanisms to deal with global warming do so - they all treat it as though impacts and solutions were calculable and shares of responsibility were rationally assignable. We need to start from scratch to come up with a sensible, adaptable replacement for Kyoto for the period from 2012.

I overlooked the nitrogen comment, assuming it was obvious that what JG meant was nitrous oxide. You're probably right to pick him up on it, though, for fear that some people reading this might conclude that we ought to be scrubbing nitrogen from our atmosphere. To be fair to JG, this mistake is inherited from the Times article on which his post was based (see final paragraph), though the following sentence in The Times clarifies that N₂O is what they really had in mind.

All the same, I agree it is worrying that people who think they are qualified to comment on these sorts of things (e.g. Lewis Smith, "Environment Reporter" at The Times) don't know the difference between nitrogen and nitrous oxide. It's like another bugbear of mine - when someone doesn't know the difference between MW and MWh, or uses MW/h when they mean MWh, I take it as a sure sign that they haven't got a fundamental understanding of the concepts of energy and power.

And I also agree with your inference that this is a sign of a broader problem of the dumbing-down (democratization?) of education and the lack of regard for rationalism in our society. Have you tried to have a reasoned argument on a blog or with people employed by the government recently? It certainly gives me cause for pessimism. Nice to know there's someone else out there who can see the way we're heading. Not sure what we do about it, though.