Objection: It’s clear from ice cores and other geological history that CO2 fluctuates naturally. It is bogus to assume today’s rise is caused by humans.
Answer: We emit billions of tons of CO2 into the air and, lo and behold, there is more CO2 in the air. Surely it is not so difficult to believe that the CO2 rise is our fault. But if simple common sense is not enough, there is more to the case. (It is worth noting that investigation of this issue by the climate science community is a good indication that they are not taking things for granted or making any assumptions — not even the reasonable ones!)
It is true that CO2 has gone up on its own in the past, most notably during the glacial-interglacial cycles. During this time, CO2 rose and fell by over 100 ppm, ranging between around 180 to 300ppm. But these rises, though they look steep over a 400Kyr timeframe, took 5K to 20Kyrs, depending on the glacial cycle.
By contrast, we have seen an equivalent rise of 100ppm in just 150 years! Check this plot for a dramatic juxtaposition of the slow glacial termination versus the industrial revolution.
There is still more to the case. By analyzing the isotopes of the carbon and oxygen atoms making up atmospheric CO2, in a process similar to carbon dating, scientists can and have detected a human “fingerprint.” What they have found via the isotope signatures can be thought of as “old” carbon, which could only come from fossil fuel deposits, combined with “young” oxygen, as is found in the air all around us. So present day combustion of fossilized hydrocarbon deposits (natural gas, coal, and oil) is definitely the source of the CO2 currently accumulating — just as common sense tells us.
For more of the nitty gritty technicalities straight from the climate scientists, including links to the actual research that established this, visit RealClimate’s article on how we know the CO2 is ours.
Of all the pillars holding up the theory of anthropogenic global warming, this is one of the most unassailable.