evidence: Burning fossil fuels and chopping down trees lead to the release of this greenhouse gas. Both activities exploded after the 19th Century, so it's unsurprising that atmospheric CO2 increased over the same period. [...] Analysis shows that for 800,000 years, atmospheric CO2 did not rise above 300 parts per million (ppm). But since the Industrial Revolution, the CO2 concentration has soared to its current level of nearly 420 ppm. Computer simulations, known as climate models, have been used to show what would have happened to temperatures without the massive amounts of greenhouse gases released by humans.