How Do We Know We're Responsible for all the Postindustrial Increase in CO2 Concentrations?
Sometimes I still hear objections to AGW that claim that humans could not be causing the atmosphere to warm because our emissions constitute only a tiny fraction of the CO2 that's in the atmosphere. There are several forms of this claim, but most have to do with the fact that human CO2 emissions are a small fraction of total emissions every year, which of course is true. What they don't tell you is that natural sinks remove what natural sources add each year, and a little more, making the natural carbon cycle a net sink. So while human emissions are small compared to natural, they are responsible for flipping the carbon cycle from being a net sink to a net source of CO2. Human activity is responsible for virtually all the increase in CO2 above preindustrial levels. The graph below shows various components of the carbon cycle with averages for 2011-2020. Here the natural carbon cycle removes about 5.9 GtC annually, while human emissions from fossil fuels and land use change cont...