Carbon dioxide levels in the atmosphere reached a milestone 421 parts per million last weekend, the highest concentration in 3.6 million years according to the US National Oceanic and Atmospheric Administration., ABC News has reported.

"The difference between the warm periods on Earth and the cold periods on Earth is 100 parts per million of carbon dioxide," said Professor Mark Howden, the director of the Institute for Climate, Energy and Disaster Solutions at the Australian National University.

Read the entire article on the ABC News site.