Carbon dioxide levels reach global milestone

Worldwide levels of carbon dioxide — the gas scientists say is most responsible for global warming — reached a significant milestone for the month of March, the National Oceanic and Atmospheric Administration said Wednesday.

The global monthly average for carbon dioxide hit 400.83 parts per million in March, the first time the average surpassed 400 ppm for an entire month since such measurements began in the late 1950s, NOAA said.

"It's both disturbing and daunting," said NOAA chief greenhouse gas scientist Pieter Tans. "Daunting from the standpoint on how hard it is to slow this down."

The burning of the oil, gas and coal for energy releases "greenhouse" gases such as carbon dioxide and methane. These gases have caused the Earth's temperature to rise over the past century to levels that cannot be explained by natural variability.

The last time carbon dioxide reached 400 ppm was millions of years ago, according to the journal Nature Geoscience. A 2009 report in the journal found evidence of CO2 levels of 365 ppm to 415 ppm roughly 4.5 million years ago.

"Reaching 400 parts per million as a global average is a significant milestone," Tans said.

CO2 levels were around 280 ppm prior to the Industrial Revolution in the late 1800s, when large amounts of greenhouse gases began to be released by the burning of fossil fuels.

Carbon dioxide is invisible, odorless, and colorless, yet it's responsible for 63% of the warming attributable to all greenhouse gases, according to NOAA's Earth System Research Laboratory in Boulder, Colo.

Levels of carbon dioxide go up and down each year, reaching their highest levels in May and then going back down as plants absorb the gas. But the long-term trend is up, according to NOAA.