by JoNova from Murry Salby, December 27, 2019

Over the last two years he has been looking at C12 and C13 ratios and CO2 levels around the world, and has come to the conclusion that man-made emissions have only a small effect on global CO2 levels. It’s not just that man-made emissions don’t control the climate, they don’t even control global CO2 levels.

The higher levels of CO2 in recent decades appear to be mostly due to natural sources. He presented this research at the IUGG conference in Melbourne recently, causing great discussion and shocking a few people. Word reached the Sydney Institute, which rushed to arrange for him to speak, given the importance of this work in the current Australian political climate.

The ratio of C13 to C12 (two isotopes of carbon) in our atmosphere has been declining, which is usually viewed as a signature of man-made CO2 emissions. C12 makes up 99% of carbon in the atmosphere (nearly all atmospheric carbon is in the form of CO2). C13 is much rarer — about 1%. Plants don’t like the rarer C13 type as much; photosynthesis works best on the C12 -type -of-CO2 and not the C13-type when absorbing CO2 from the air.

Prof Salby points out that while fossil fuels are richer in C12 than the atmosphere, so too is plant life on Earth, and there isn’t a lot of difference (just 2.6%) in the ratios of C13 to C12 in plants versus fossil fuels. (Fossil fuels are, after all, made in theory from plants, so it’s not surprising that it’s hard to tell their “signatures” apart). So if the C13 to C12 ratio is falling (as more C12 rich carbon is put into the air by burning fossil fuels) then we can’t know if it’s due to man-made CO2 or natural CO2 from plants.

Essentially we can measure man-made emissions reasonably well, but we can’t measure the natural emissions and sequestrations of CO2 at all precisely — the error bars are huge. Humans emits 5Gt or so per annum, but the oceans emit about 90Gt and the land-plants about 60Gt, for a total of maybe 150Gt. Many scientists have assumed that the net flows of carbon to and from natural sinks and sources of CO2 cancel each other out, but there is no real data to confirm this and it’s just a convenient assumption. The problem is that even small fractional changes in natural emissions or sequestrations swamp the human emissions.