In this animation, they show how the arrival of coal and petroleum-based industrialization around the beginning of the 19th century led to a very rapid rise in atmospheric CO₂ levels from a level of about 280 ppm (parts per million), which had been stable for millions of years, to about 412 ppm.

We have to go back about 2.5 million years to find a time when the level was as high as the current value and at that time there were no ice caps and glaciers, and consequently ocean levels were 55 ft higher than they are now. That should give us some idea of how dangerous it would be if we do not do something soon to combat global warming.