Researchers at University of Southampton, UK have studied and cross veined the findings of more than 100 research papers to state the changing pattern of CO2 levels. They stated that the atmospheric level of carbon dioxide never rose above 3000 parts per million. However, some older studies have suggested even higher levels (5000 ppm). But looking at the present scenario of CO2 emission, the researchers believe that the levels may reach as high as 600 ppm by 2050.
Reference Link: https://www.newscientist.com/article/2126776-co2-set-to-hit-levels-not-seen-in-50-million-years-by-2050/
