Measurements of carbon dioxide, the chief human-caused greenhouse gas, averaged 417.1 parts per million at Mauna Loa, Hawaii, for the month of May, when carbon levels in the air peak, the National Oceanic and Atmospheric Administration said.

And carbon dioxide levels are now nearly 100 parts per million higher than then.

“The rise in atmospheric carbon dioxide levels is relentless, and this means the costs of climate change to humans and the planet continue to rise relentlessly as well," said University of Michigan environment dean Jonathan Overpeck.

Carbon dioxide levels peak in May because starting in late May, because growing plants suck up more of heat-trapping gas, causing carbon amounts in the air to drop, Tans said.