In a comment on a story on Wattsupwiththat, Julian Flood makes the following interesting suggestion:
No doubt you are familiar with planktonic carbon-fixation paths, but let me remind you anyway. Most plants, phytoplankton included are C3, a process which is discriminatory against the heavier carbon isotopes, so a richly-fed ocean will tend to take up a slightly greater proportion of the light isotope 12C. C3 requires a good level of trace elements, including… zinc and chromium, IIRC, but don’t quote me on that… and without those trace elements the phytos which use C4 will begin to dominate. Indeed, certain flexible phytos will change in those circumstances to C4 from C3. C4 uses much more C13/14 than C3.
A starving, stratified ocean will thus move towards a metabolic pathway which pulls down more heavier isotopes of carbon and the atmosphere will be depleted of those isotopes. I have argued that this is the source of the light isotope atmospheric signal which we are pointing to as the anthropogenic signal.
I am unaware of studies which quantify DMS emissions from the various plankton species, although the C4, starved, types might well be short of the resources to make that notoriously cloud-facilitating chemical. However, reduced planktonic populations will certainly lead to reduced DMS emission. Fewer phytos, less DMS, fewer CCNs, fewer clouds which are not so reflective. Warming.
Incidentally, phyos have diatoms as their most ferocious competitor, a competitor limited by silica availability. Modern agriculture (from about 1750) has been throwing huge amounts of silica into the oceans. Diatoms are C4, which may explain why the ‘anthropogenic’ carbon isotope signal begins in the eighteenth century, long before our burning of fossil fuel can have had an effect.
So, it’s warming, it may even be anthropogenic, but it’s not only about CO2.
Whether Julian has it right or not, the fact that such a possibility exists is illuminating. I have long argued that politicising science is a bad thing, and one reason why is that when a false theory is pushed for unscientific reasons, the true theory is unlikely to be discovered. We know humans are damaging the world, and excess of fertilisers is one such mechanism. If warming is indeed an outcome of over-use of fertilisers, then even those who think warming is bad must surely admit that the global warming scam has allowed decades to pass in which this danger has been allowed to continue, as its serious effects have been ascribed to the wrong cause.