See Eric Monnin et al, Atmospheric CO2 Concentrations over the Last Glacial Termination Science 291, 112 (2001) and https://courses.seas.harvard.edu/climate/eli/Courses/global-change-debates/Sources/Temperature-leads-CO2-in-ice-cores/more/Monnin-2001.pdf
These cycles are also evident in the historical record, going back to the pre-Roman Egyptian cooling, the Roman warning, the Dark Ages, the Medieval warming, and the Little Ice Age, from which we are just now emerging. People didn't write “gee, the climate is cooling (or warming)” but Egyptians wrote about needing to build dams and canals, Romans noted that grapes were growing in Britain, Slavs moving up the Eastern Alps thought the Germans had abandoned the land because they got fat and lazy — and then had to abandon the same land when the glaciers returned, ….
The cycles appeared in the 1995 IPCC report, but were removed in the 2001 report, on the insistence of the editor, Michael Mann, to preserve the illusion of his infamous “Hockey Stick” graph.
Infamous “Hockey Stick” Graph that Eliminates the Dark Ages, the Medieval Warming, and the Little Ice Age
Figure 2.20 in IPCC's Climate Change 2001
Fossil fuels have been underground, not exposed to cosmic rays, for millions of years. They therefore do not contain any carbon 14. Burning fossil fuels dilutes the atmospheric concentration of carbon 14 in CO2.
If emissions of CO2 caused by burning fossil fuels were the only source for the observed decrease in the concentration of carbon 14 in CO2 in the atmoshphere since 1750, and therefore the only source of increase of the total concentration of CO2 in the atmosphere, the trend of its dilution, as measured in the geological record, lake bed and ocean bottom sediments, bogs, …, would be much more than observed.
Kenneth Skrable, George Chabot, and Clayton French analyzed this in World Atmospheric CO2, Its 14C Specific Activity, Non-fossil Component, Anthropogenic Fossil Component, and Emissions (1750-2018), which appeared in Health Physics 122,2, pp 291-305 (1 Feb 2022), doi: https://doi.org/10.1097/HP.0000000000001485. From the abstract:
After 1750 and the onset of the industrial revolution, the anthropogenic fossil component and the non-fossil component in the total atmospheric CO2 concentration, C(t), began to increase. Despite the lack of knowledge of these two components, claims that all or most of the increase in C(t) since 1800 has been due to the anthropogenic fossil component have continued since they began in 1960 with "Keeling Curve: Increase in CO2 from burning fossil fuel." Data and plots of annual anthropogenic fossil CO2 emissions and concentrations, C(t), published by the Energy Information Administration, are expanded in this paper. Additions include annual mean values in 1750 through 2018 of the 14C specific activity, concentrations of the two components, and their changes from values in 1750. The specific activity of 14C in the atmosphere gets reduced by a dilution effect when fossil CO2, which is devoid of 14C, enters the atmosphere. We have used the results of this effect to quantify the two components. All results covering the period from 1750 through 2018 are listed in a table and plotted in figures. These results negate claims that the increase in C(t) since 1800 has been dominated by the increase of the anthropogenic fossil component. We determined that in 2018, atmospheric anthropogenic fossil CO2 represented 23% of the total emissions since 1750 with the remaining 77% in the exchange reservoirs. Our results show that the percentage of the total CO2 due to the use of fossil fuels from 1750 to 2018 increased from 0% in 1750 to 12% in 2018, much too low to be the cause of global warming.