275 ppm

For all of human history until about 200 years ago (the start of wide scale industrial processes), our atmosphere contained 275 parts per million of carbon dioxide. Without some CO₂ and other greenhouse gases that trap heat in our atmosphere, our planet would be too cold for humans to inhabit. Evidence suggests that low CO₂ concentrations were the cause of the three Ice Ages experienced on Earth over the last 100,000 years.