Question

Over the last 800,000 years before humans existed, CO2 levels in the atmosphere have stayed below ________ PPM. Today, CO2 levels are over ________ PPM.

fill in the blanks

Answer 1

I guess the first blank is either 170 or 280 and second is 450

For the 800,000 years we have records of, average global CO2 levels fluctuated between about 170 ppm and 280 ppm. Once humans started to burn fossil fuels in the industrial era, things changed rapidly.

Regardless of which estimate is correct, it is clear that CO2 levels are now higher than they have ever been in mankind’s history. With global CO2 emissions continuing on an upward trajectory that is likely to put CO2 concentrations above 450 ppm or higher, it is extremely unlikely that the steadily rising shape of the Keeling Curve is going to change anytime soon.