"Owing mainly to anthropogenic activities including land use change and fossil fuel burning, the 13C/12C [carbon isotopes] ratio of CO2 in the atmosphere has changed over the last 200 years by 1.5 parts per thousand."

p. 1 . ²

Annual CO₂  increases compiled from Monthly Average Mauna Loa CO₂ levels.

"Plant photosynthesis discriminates against carbon 13 isotopes. In other words, plant carbon tends to to have less ¹³C than the CO² from which it is formed."

p. 2.

Trends in Atmospheric Carbon Dioxide. NOAA.

isotope.

"A precise determination of the isotopic composition can help separate CO2 fluxes into terrestrial and marine components."

p. 13.

"...current fossil fuel emissions of about 6.1 Gt [gigatons] C [carbon] per year result in a long term change in the CO2 mixing ratio in the atmosphere of about 1.5 ppm per year. The carbon 13 isotope of CO2 changes about 0.02% per year."

p. 15.

"While it has been possible for the past 50 years to measure the CO2 mixing ratio and the isotopic composition of air samples directly, this information must be recovered by more indirect means for the more distant past. Much of our quantitative knowledge of climate fluctuation in the past has been derived from records preserved in the ice sheets, and deep sea sediments."

p. 16.

"It represents one of the earliest results from ice cores exhibiting a general increase in CO2 concentrations with time since 1750 together with a decrease in carbon 13 isotope values. Using these data it was possible to directly compare measurements of CO2 concentrations in air obtained from Mauna Loa since 1958 by Keeling et al. with longer term ice core archive record from Antarctica. The average carbon 13 isotope value of samples before 1800 A.D. is –6.41% in the ice core."

p. 16.

Prosenjit Ghosh, Willi A. Brand, "Stable isotope ratio mass spectrometry in global climate research,"

Why global warming is not a natural trend.

"Another, quite independent way that we know that fossil fuel burning and land clearing specifically are responsible for the increase in CO2 in the last 150 years is through the measurement of carbon isotopes.

 Isotopes are simply different atoms with the same chemical behavior (isotope means “same type”) but with different masses.

Atomic Nuclei and electron shells depicted.

Carbon is composed of three different isotopes, ¹⁴C, ¹³C and ¹²C. ¹²C is the most common. ¹³C is about 1% of the total. ¹⁴C accounts for only about 1 in 1 trillion carbon atoms.

CO2 produced from burning fossil fuels or burning forests has quite a different isotopic composition from CO2 in the atmosphere. This is because plants have a preference for the lighter isotopes (¹²C vs. ¹³C); thus they have lower ¹³C/¹²C ratios.
Since fossil fuels are ultimately derived from ancient plants, plants and fossil fuels all have roughly the same ¹³C/¹²C ratio – about 2% lower than that of the atmosphere.
As CO2 from these materials is released into, and mixes with, the atmosphere, the average ¹³C/¹²C ratio of the atmosphere decreases.

Furthermore, the  ¹³ C / ¹² C ratios begin to decline dramatically just as the CO₂ starts to increase — around 1850 AD.

• This is exactly what we expect if the increased CO₂ is in fact due to fossil fuel burning.

Furthermore, we can trace the absorption of CO₂ into the ocean by measuring the  ¹³ C / ¹² C ratio of surface ocean waters.

√ While the data are not as complete as the tree ring data (we have only been making these measurements for a few decades) we observe what is expected: the surface ocean ¹³C/¹²C is decreasing.

√ Measurements of ¹³C/¹²C on corals and sponges — whose carbonate shells reflect the ocean chemistry just as tree rings record the atmospheric chemistry — show that this decline began about the same time as in the atmosphere; that is, when human CO₂ production began to accelerate in earnest."

Real Climate

Threshold conceptually illustrated as a bathtub.

Isotopes can't lie; these heavy istopes are evidence of human interference