Peter Tans approved the above verbatim quote to be directly attributed to him. I contacted him by email today on the basis of this material:
“The rate of CO2 growth over the last decade is 100 to 200 times faster than what the Earth experienced during the transition from the last Ice Age,” Pieter Tans, lead scientist of NOAA’s Global Greenhouse Gas Reference Network, said in a statement. “This is a real shock to the atmosphere.”
The annual rate of increase in atmospheric carbon dioxide over the past 60 years is about 100 times faster than previous natural increases, such as those that occurred at the end of the last ice age 11,000-17,000 years ago.
https://e360.yale.edu/digest/co2-levels-continue-to-increase-at-record-rate https://www.climate.gov/news-features/understanding-climate/climate-change-atmospheric-carbon-dioxide
The ignition and continuous burning of millions of miles of exposed coal beds. You would need to burn approximately ~3 teratons of coal. coal beds can be ignited by natural causes but we would notice a coal fire larger than a continent.
A drop in sea level by hundreds of feet would do it. large sea level drop cause outgassing of methane and co2, from wetlands and sea muds. but again New York would notice it if long island stopped being an island.
Some of these have actually happen in earth's geologic history just none recently, and not as consistently as we have managed (at least not since life evolved). Really scientists have spent a lot of time and money looking for other causes, and no one has found any. Scientists concluded it was human activity by eliminating every other conceivable cause.