The world is likely days away from a "sobering milestone" in our planetary history.
Concentrations of the greenhouse gas carbon dioxide will likely reach 400 parts per million (ppm) for first time in human history, say scientists at the Scripps Institution of Oceanography, and unless drastic action is taken, we're on track to hit 450 ppm in the near future.
"I wish it weren't true, but it looks like the world is going to blow through the 400-ppm level without losing a beat," said Scripps geochemist Ralph Keeling, whose father Charles David (Dave) Keeling began the "Keeling Curve" to track daily CO2 levels recorded at Mauna Loa Observatory.
The last time the greenhouse gases were at 400 ppm was likely the Pliocene epoch, between 3.2 million and 5 million years ago.
The current reading is at 399.72 ppm -- far past the 350 ppm level many, including noted climate scientist James Hansen, have warned is the upper safe limit before the planet hits a tipping point.
"At this pace we'll hit 450 ppm within a few decades," warned Keeling. Indeed, the rate of rise of CO2 over the past century is "unprecedented."
The figure should serve as a call to act on the deadly emissions caused by our fossil fuel addiction, the scientists say.
"The 400-ppm threshold is a sobering milestone, and should serve as a wake up call for all of us to support clean energy technology and reduce emissions of greenhouse gases, before it's too late for our children and grandchildren," said Tim Lueker, an oceanographer and carbon cycle researcher and part of the Scripps CO2 Group.