Objection: According to the IPCC, 150 billion tonnes of carbon go into the atmosphere from natural processes every year. This is almost 30 times the amount of carbon humans emit. What difference can we make?
Answer: It’s true that natural fluxes in the carbon cycle are much larger than anthropogenic emissions. But for roughly the last 10,000 years, until the industrial revolution, every gigatonne of carbon going into the atmosphere was balanced by one coming out.
What humans have done is alter one side of this cycle. We put approximately 6 gigatonnes of carbon into the air but, unlike nature, we are not taking any out.
Thankfully, nature is compensating in part for our emissions, because only about half the CO2 we emit stays in the air. Nevertheless, since we began burning fossil fuels in earnest over 150 years ago, the atmospheric concentration that was relatively stable for the previous several thousand years has now risen by over 35%.
So whatever the total amounts going in and out “naturally,” humans have clearly upset the balance and significantly altered an important part of the climate system.