There are two isotopes of carbon in our atmosphere: C12 and C13. C12 makes up 99% and C13 is a paltry 1% of the atmospheric carbon (which is almost all in the form of CO2). Fossil fuels are a bit richer in the C12 isotope and that is a crutch that the AGW proponents lean on but there is a wrinkle. Plants are also rich in the C12 isotope - because, really, fossil fuels are made essentially from plants - with a small variance. So, it's hard to identify the source of the CO2 from the carbon isotope signature.
Man-made emissions are fairly well estimated at 5 Gigatonnes (Gt) per year. But, as we've mentioned before, through sequestration, there are other natural sources of CO2: the oceans (emitting about 90Gt per year) and land-based plant life (emitting about 60Gt per year). It is however, difficult to measure and the error thresholds on these numbers are wide. E.M. Smith presents it as follows:
“It is often asserted that we can measure the human contribution of CO2 to the air by looking at the ratio of C12 to C13. The theory is that plants absorb more C12 than C13 (by about 2%, not a big signature), so we can look at the air and know which came from plants and which came from volcanos and which came from fossil fuels, via us. Plants are ‘deficient’ in C13, and so, then, ought to be our fossil fuel derived CO2.
The implication is that since coal and oil were from plants, that “plant signature” means “human via fossil fuels”. But it just isn’t that simple. Take a look at the above chart. We are 5.5 and plants are putting 121.6 into the air each year (not counting ocean plants). There is a lot of carbon slopping back and forth between sinks and sources. Exactly how closely do we know the rate of soil evolution of CO2, for example?”So if the natural sources vary by 3%, which is entirely possible in a year, that would constitute the entire industrialized contribution of CO2 to the atmosphere. Murray Salby, the Chair of Climate Science at Macquarie University and a visiting professor at a number of international universities and a former member of the Bureau of Meterology in Australia and the US National Center for Atmospheric Research, gave a speech ahead of a paper that is due out this year. Salby found that the largest increases year-to-year occurred when the world warmed fastest due to El Nino conditions. The smallest increases correlated with volcanoes which pump dust up into the atmosphere and keep the world cooler for a while. In other words, temperature controls CO2 levels on a yearly time-scale, and according to Salby, man-made emissions have little effect.
Read that again: temperature controls CO2 levels on a yearly time-scale.
The climate models assume that most of the rise in CO2 (from 280 ppmv in1780 to 392 ppmv today) was due to industrialization and fossil fuel use. But the globe has been warming during that period (in fact since the depths of the Little Ice Age around 1680), so warmer conditions could be the reason that CO2 has been rising.
Salby does not dispute that some of the rise in CO2 levels is due to man-made emissions, but found that temperature alone explains about 80% of the variation in CO2 levels.
The up and coming paper with all the graphs will be released in about six weeks. It has passed peer review, and sounds like it has been a long time coming. Salby says he sat on the results for six months wondering if there was any other interpretation he could arrive at, and then, when he invited scientists he trusted and admired to comment on the paper, they also sat on it for half a year. His speech created waves at the IUGG conference, and word is spreading.
According to Salby, science is about discourse and questioning. He emphasized the importance of debate: “Excluding discourse is not science”. He felt that it was not his position to comment on policy, saying the scientists that do are more activist than scientist.
He finishes his presentation saying that “anyone who thinks the science is settled on this topic, is in fantasia”.
Salby was once an IPCC reviewer, and comments, damningly, that if these results had been available in 2007, “the IPCC could not have drawn the conclusion that it did.”
So it is a matter of degrees: temperature controlling CO2 levels and the amount of natural CO2 emissions compared to industrialized sources.