A technique similar to carbon dating has shown that the increase in atmospheric carbon dioxide levels is due to the burning of fossil fuels since the beginning of the industrial revolution. How does we know this?

The correct explanation relies on the isotopic signature of carbon in fossil fuels. Plants preferentially absorb the lighter carbon isotope, Carbon-12 ($^{12}\text{C}$), during photosynthesis, leading to organic matter (and thus fossil fuels) being depleted in Carbon-13 ($^{13}\text{C}$) relative to the atmosphere or inorganic carbon sources. When these fossil fuels are burned, they release carbon dioxide that has a lower $^{13}\text{C}$/$^{12}\text{C}$ ratio. This influx of $^{13}\text{C}$-depleted carbon dioxide into the atmosphere causes the overall atmospheric $^{13}\text{C}$/$^{12}\text{C}$ ratio to decrease, which is precisely what has been observed since the industrial revolution.

Therefore, the correct option is:

• Fossil fuels contain a lower ratio of Carbon 13 to Carbon 12 than rocks and other inorganic substances. When fossil fuels are burned, releasing carbon dioxide, there would be a lower amount of carbon 13 in the atmosphere compared to pre-industrial levels.