Carbon dioxide (CO2), the No. 1 greenhouse gas leading to man-made global warming, has reached a dubious new milestone.
The level of the gas in the atmosphere, which is measured by instruments on top of Hawaii's Mauna Loa Observatory, topped 410 parts per million (ppm) for the month of April.
This is the highest concentration of the heat-trapping gas ever recorded at the Observatory, where direct measurements have been taking place for more than 60 years, giving us the longest detailed record.
Passing 410 ppm "is important because it punctuates another milestone in the upwards march of CO2," according to Ralph Keeling, head of the Scripps CO2 program at Scripps Institution of Oceanography in California.
"At the recent pace, we'll hit 450 ppm in a mere 16 years, and 500 ppm 20 years after that. That's well within dangerous territory for the climate system," Keeling added.
Keeling's father, Charles David Keeling, began taking the measurements on Mauna Loa in 1958 when the concentration was 315 ppm. The "Keeling Curve," the name given to the the carbon dioxide levels measured in Hawaii, show a drastic and accelerating rise since measurements began.
But the rise of carbon dioxide in the atmosphere began well before the 1950's, which we know thanks to data collected from ice-cores.
Prior to the Industrial Revolution, the levels fluctuated naturally over many thousands of years but had never exceeded 300 ppm at any point in the last 800,000 years.
"The rise is a direct consequence of the large releases of CO2 from fossil-fuel burning," Keeling told CNN.
The oceans and land plants remove a lot of the excess carbon dioxide from the atmosphere, about 50% according to Keeling, but they simply cannot keep up.
And the rest is piling up in the atmosphere.
Carbon dioxide is called a "greenhouse gas" because of its ability to trap solar radiation in the Earth's atmosphere like a blanket and heat up the planet. Without it, the planet would be too cold to support life.
But human activity has increased the levels of the gas to dangerous levels, increasing the thickness of that blanket to the point that the Earth is heating up.
"Our activities keep pumping a steroid into the naturally-varying atmosphere," said Marshall Shepherd, climate scientist and Director of the Atmospheric Sciences program at the University of Georgia.
"Yes, climate has always changed and CO2 has varied but this as an experiment we have not seen because humans are the new kid on Earth's block," Shepherd warned.
The Paris Climate Agreement of 2015, which President Donald Trump pulled the United States out of in 2017, set out a global action plant to reduce greenhouse gas emissions and hopefully keep the global average temperature below two degrees Celsius (3.6 degrees Fahrenheit) over pre-industrial levels.
This will require bringing the Keeling Curve to a peak very soon, something Ralph Keeling is hopeful he will see. "I'm hopeful we will start to see slowing in the next ten years, as renewable energy sources such as wind and solar continue to replace fossil-fuel fuels ... We'll see."
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