I have remarked before that ‘climate change’ is full of unknowns, about which people often make extraordinarily confident assertions. A correspondent reported one such the other day — he had been listening to a lecturer in the UK who said that it would take ‘a million years for silicate weathering to remove the extra CO2 being put into the atmosphere’. He (the listener) thought that an extraordinary claim, and asked for comment. I think it is extraordinary too, and even Wikipedia, which has a definite tendency towards AGW orthodoxy, hums and haws a bit, before saying that ‘[t]he atmospheric lifetime of CO2 is estimated of the order of 30–95 years’. That is plainly a lot less.
Why is it important? Well, the AGW hypothesis says that adding carbon dioxide to the atmosphere must increase global temperature, which must be bad for everything and everybody. Hidden in that assertion is the notion that once a carbon dioxide molecule is added, it just stays up there. But moment’s thought indicates that since CO2 is the fuel for all plants, more CO2 must be good news to the plant world, which might use the added gas as a sort of dessert. How much do the plants take, anyway? There are lots of estimates, and no one seems to be really sure. Then there’s the ocean, which releases carbon dioxide as it warms.
But back to the thousand-years claim. I know where that comes from (more in a moment), but it stimulated Tom Segalstad in Norway to write a long email on the subject, which I am summarising here, because it provided me with both the history and the sources.
Before the establishment of the IPCC the conventional estimate of CO2 residence time was accepted to be five years. When that short time-span didn’t seem to fit what the early models projected, Tom says, ‘[t]he IPCC next constructed an “artificial” residence time for atmospheric CO2 to fit their model, of 50-200 years (IPCC 1990, Table 1.1)’. By and large the IPCC has stuck to this rather generously wide estimate ever since.
One of the leading figures in the IPCC, Susan Solomon, has argued for a thousand-year residence time, but her paper starts with a model, and ignores observations. It is a good example of the ‘scary’ AGW paper that assumes that it is completely right, despite the lack of supporting evidence. Moreover, if carbon dioxide had a life of a thousand years in the atmosphere, would we still be able to make carbonated drinks? Would plants to able to use CO2 so efficiently in photosynthesis? Tom points out, in any case, that ‘the measured rise in the atmospheric CO2 level is just half of that expected from the amount of anthropogenic CO2 supplied to the atmosphere’. In short, something is chewing up the carbon dioxide, and that makes any estimate of a thousand-year residence somewhat mysterious, if not head-scratching.
It seems as though the IPCC has a static model of the atmosphere, in which carbon dioxide is in some sort of natural balance, with the gas moving in and out of the ocean, the atmosphere and the ecosystem — an equilibrium model. When humans add more CO2 the system goes out of balance, which is bad. Humans must stop doing that, so that the system can return to equilibrium. You’ve heard this before, you say. Yes, so have I.
Tom has his own papers on the subject, and one of them argues that carbon isotope analysis supports a five-year residence time (yes, perhaps that early estimate was a good one), and that all the human-produced fossil-fuel carbon dioxide is only 4 per cent of the whole, the remaining 96 per cent having come from what was there last time we looked, outgassing from volcanoes and the oceans. Do you wonder about the volcanoes? Well, about 1500 are known to have been active on land in the last 10,000 years, and there are thousands under the seas. CO2 comes out from major fractures and dormant volcanoes, too.
Another paper of his lists nearly 40 articles, all of them between the 1950s and the 1990s, that calculated the residence time of carbon dioxide in the atmosphere; the range was from about one year to seven years. But none of these, as we know, fits with the modelling carried out for the purposes of the IPCC reports, and they have been discarded.
Does it matter? The current lack of significant warming, more than 17 years in the case of the RSS dataset and of varying smaller lengths in all of the other global temperature datasets, tells us that CO2 cannot be the principal driver of temperature. What is more, the apparent greening of the globe that has occurred in the last decades suggests that more carbon dioxide in the atmosphere is good for plants, and by extension, for us — at least so far.
But I learned a lot from reading these papers, and considering the argument again. For the moment, it seems to me that we know much less than the orthodox think we do about what happens when CO2 is added to the atmosphere, and that the outcome may actually be a beneficial one.