Click the image, or right click to open in new tab ..
CO2 readings at Mauna Loa began in 1958. For the first ten years or so scientists observed what appeared to be a linear increase in the CO2 concentration. At the time the CO2 concentration was increasing by less than one part per million (ppm) per year, as you can see from the graph by examining the first decade. From 1958 to the 68 it increases from about 316 to 324, or eight ppm in 10 years.
If that were true, that ppm of CO2 was rising linearly by less than one ppm per year, the current ppm of CO2 (58 years after 1958) would be less than (316 + 58 =) 374 ppm. But it's not, it's just over 400 ppm now.
So the rate of increase is not linear. It's exponential - the rate of increase is increasing year after year, like compounding interest. You can see that the graph has a slight upward bend, and that upward bend is gradually increasing over time.
Actually, by the late 60s or early 70s scientists had already abandoned their original idea that the rate of increase was linear, and began ringing the alarm bells about global warming.
Recent research has revealed this exponential rate of increase began in the early 1800s, as the industrial revolution picked up steam.