Sam Norton is sceptical about Anthropogenic Global Warming, i.e. the widely accepted conclusion that the world is getting warmer because of human activity. Well, his post suggests that one piece of evidence for this may, or may not, have been debunked. So perhaps we can’t be as certain as we once thought we were that the world today is warmer than it was in the Middle Ages.
But there are things that we can be sure of. One is that the world today is quite a lot warmer than it was in the early twentieth century. That much is clear from temperature records.
Another is that (according to Wikipedia, yes I know this is not the most reliable source) the level of carbon dioxide in our atmosphere is around 20% higher than it was 50 years ago and perhaps 35% higher than the level before the industrial age. The amount of carbon dioxide being added to the atmosphere each year from burning of fossil fuels, around 27 gigatonnes in 2004, is about twice the observed rate of increase of carbon dioxide in the atmosphere, 0.4% of 3 teratonnes which is 12 gigatonnes. Presumably around half of the anthropogenic carbon dioxide is taken up by various “sinks” and half remains in the atmosphere.
Meanwhile the reason I am enjoying eating fresh tomatoes in October is not global warming – but is an effect which is also linked to global warming. My tomatoes are in a greenhouse, which is warmer than the garden outside not because it is heated by burning fossil fuels but because the glass traps the sun’s rays. It is well known and easily demonstrated that carbon dioxide has a similar greenhouse effect. Plug into the equations, or into a simple experimental rig, the increase in the atmospheric concentration of carbon dioxide and the result is clear: an increase in temperature of a few degrees Celsius – just about what has been observed.
So we have observed A, a large and apparently anthropogenic increase in carbon dioxide levels in the atmosphere. And we have observed B, an increase in global temperatures. We can also show by theory and experiment that, everything else being equal, A causes B. I accept that that falls short of proof that the observed A is actually causing the observed B, because there are other factors which makes everything else not equal.
But perhaps it would be fair to say that if the observed B is not being caused by the observed A, but by some other factor outside our control, then we as humanity are extremely fortunate that the predicted greenhouse effect is being cancelled out by some unknown factor and so not causing even faster global warming. To put it in other words, we humans, by pumping all that carbon dioxide into the atmosphere, are doing all that we can to cause global warming, and if we are not causing it that is by luck rather than responsible judgment.