Worldwide atmospheric levels of carbon dioxide, the gas scientists say is most responsible for global warming, surpassed 400 parts per million for the month of March, the National Oceanic and Atmospheric Administration said Wednesday.
Though there have been readings this high before, this is the first time that global concentrations of the CO2 gas have averaged 400 ppm for an entire month. Measurements of carbon dioxide in our atmosphere began in the late 1950s.
"It was only a matter of time that we would average 400 parts per million globally," Pieter Tans, lead scientist of NOAA's Global Greenhouse Gas Reference Network, said in a statement. "Reaching 400 parts per million as a global average is a significant milestone."
The burning of the oil, gas and coal that provides the energy for our world releases "greenhouse" gases such as CO2 and methane. These extra 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 that carbon dioxide reached 400 ppm was millions of years ago. How do we know this?
Scientists can analyze the gases trapped in ice to reconstruct what climate was like in prehistory, but that record only goes back 800,000 years, according to the Scripps Institute of Oceanography.
It's harder to estimate carbon dioxide levels before then, but in 2009, one research team reported in the journal Nature Geoscience that it had found evidence of CO2 levels that ranged from 365 to 415 ppm roughly 4.5 million years ago.
CO2 levels were around 280 ppm prior to the Industrial Revolution, when we first began releasing large amounts into the atmosphere through 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.
