carbon dioxide in Earth's atmosphere
The quantity of carbon dioxide in Earth's atmosphere has varied widely during Earth's existence. When life first appeared nearly four-billion years ago the atmosphere was mostly carbon dioxide. Five-hundred-million years ago the concentration may have been six-thousand parts-per-million (ppm) (six percent.) By twenty-million years ago the concentration had fallen to less than three-hundred ppm. Over the last eight hundred thousand years the concentration has ranged from 200 ppm during ice ages, increasing to 300 ppm between the ice ages. For the last ten thousand years the concentration has been 260-280 ppm, until the beginning of the industrial era, during which it has risen to the current 400 ppm.
Life thrived during the much higher levels of atmospheric carbon dioxide in the ancient past, partly because the sun was weaker so the greenhouse effect didn't drive temperatures as high, but mostly because life was adapted to that environment. Modern life is adapted to current lower carbon dioxide levels, and the climate is changing so quickly now as the carbon dioxide in the atmosphere increases quickly that many species will not be able to adapt fast enough to the new conditions, and will become extinct.
Several times in the prehistory of Earth the environment has changed too quickly for life to adapt fast enough and there has been a huge reduction of life on Earth. These events are known as extinction events. There have been five major extinction events; the greatest of these occurred 251 million years ago: the Permian–Triassic extinction event in which 90% of species, and a much greater proportion of all life forms, were wiped out.
These major extinctions resulted from environmental changes that occurred over thousands of years. The changes that are happening to our environment now as a result of increasing carbon dioxide levels are happening over decades. The Queensland Museum in Australia says that the current rate of extinction, which is being brought about by human initiated climate change and other human initiated causes, is estimated to be somewhere between 100 and 1000 times greater than during any of the previous extinction events.