Greenhouse effect
About 10,000 years ago, before people started burning large amounts of fossil fuels, there were 260 to 280 parts per million (ppm) of carbon dioxide (CO2) in the atmosphere, but now there is over 400 ppm. Most scientists say that having 350 ppm or less is safe for the environment and that species on the planet can adapt to this level.  Higher levels can make severe problems for animal and marine life that are already being seen today, such as ocean acidification.