There's more carbon dioxide in the atmosphere today than at any time in the past 800,000 years – 402 parts per million, in measurements earlier this week – according to a report from the mountaintop observatory in Hawaii that measures concentrations of the greenhouse gas.
Carbon dioxide (CO2) the most common and most important of the human-produced greenhouse gases, because it is released from so many sources around the world (like factories, cars, airplanes and power plants) and because it can remain in the atmosphere for centuries, adding to global warming for many human lifetimes.
The CO2 recordings were made at the National Oceanic and Atmospheric Administration's Mauna Loa Observatory, which lies about two miles above sea level near the volcano that shares its name on the island of Hawaii.
Carbon dioxide levels passed 400 ppm for the first time in recorded history last May and reached that milestone two months earlier this year, on March 16. They rose just above 402 ppm on April 7, and are projected to remain above 400 ppm for the next couple of months, according to Mashable.
Though the number is largely a symbolic milestone, it's a reminder "that carbon dioxide continues to increase in the atmosphere, and at faster rates virtually every decade," said Dr. James Butler, director of NOAA's Global Monitoring Division. "This is consistent with rising fossil fuel emissions," he added.
This month's measurements are believed to be the highest in at least 800,000 years – by far the highest since humans have walked on Earth – based on CO2 readings taken from air bubbles in ice cores drilled deep within many glaciers around the world, and from corals taken from deep in the ocean, Mashable notes.
In an interview on NOAA's website announcing this year's reading, Butler noted that "400 ppm is not a tipping point. It is a milestone, marking the fact that humans have caused carbon dioxide concentrations to rise 120 ppm since pre-industrial times, with over 90 percent of that in the past century alone."
NOAA has been measuring atmospheric CO2 since 1958, when Dave Keeling of California's Scripps Institution of Oceanography began taking readings near the Mauna Loa volcano. "The rate of increase has accelerated from about 0.7 ppm per year in the late 1950s to 2.1 ppm per year during the last decade," Dr. Butler said.
Here's what the full record of CO2 measurements since 1958 looks like:
MORE: Scenes From Hawaii's Mauna Loa Observatory
Mauna Loa Observatory
Sunset on the horizon at the mountaintop observatory on Mauna Loa, Hawaii. Monitoring of atmospheric carbon dioxide began here in 1958, when the world's CO2 concentrations were at 313 ppm. (Photo by LCDR Eric Johnson, NOAA Corps)