Earth’s atmosphere hasn’t had this much CO2 in millions of years

Earth’s atmosphere now has more carbon dioxide in it than it has in millions — and possibly tens of millions — of years, according to data released Thursday by the National Oceanic and Atmospheric Administration and scientists at the University of California San Diego.

For the first time, global average concentrations of carbon dioxide, a greenhouse gas emitted as a byproduct of burning fossil fuels, exceeded 430 parts per million (ppm) in May. The new readings were a record high and represented an increase of more than 3 ppm over last year.

The measurements indicate that countries are not doing enough to limit greenhouse gas emissions and reverse the steady buildup of C02, which climate scientists point to as the main culprit for global warming.

“Another year, another record,” Ralph Keeling, a professor of climate sciences, marine chemistry and geochemistry at UC San Diego’s Scripps Institution of Oceanography, said in a statement. “It’s sad.”

Carbon dioxide, like other greenhouse gases, traps heat from the sun and can remain in the atmosphere for centuries. As such, high concentrations of greenhouse gases in the atmosphere contribute to higher global temperatures and other negative consequences of climate change, including rising sea levels, melting polar ice, and more frequent and severe extreme weather events.

Atmospheric carbon dioxide has risen sharply since preindustrial times, owing mostly to human activities that pump greenhouse gases into the air.

Decades ago, crossing the 400 ppm threshold was unthinkable. That meant that for every 1 million molecules of gas in the atmosphere, more than 400 were carbon dioxide. The planet hit that grim milestone in 2013. And now, scientists have warned that levels of CO2 could reach 500 ppm within 30 years.

But human society is already in uncharted territory.

The last time the planet had such high levels of carbon dioxide in the atmosphere was likely more than 30 million years ago, Keeling said, long before humans roamed Earth and during a time when the climate was vastly different.

He said it’s alarming not only how high CO2 levels have climbed, but also how quickly.

“It’s changing so fast,” he told NBC News. “If humans had evolved in such a high-CO2 world, there would probably be places where we wouldn’t be living now. We probably could have adapted to such a world, but we built our society and a civilization around yesterday’s climate.”

Carbon dioxide levels are typically represented on a graph known as the Keeling Curve, named for Keeling’s father, Charles David Keeling, who began taking daily measurements of atmospheric carbon dioxide in 1958 with instruments atop the Mauna Loa Observatory in Hawaii.

The Keeling Curve famously shows a steep climb since the Industrial Revolution, owing to human-caused climate change.

Ralph Keeling and his colleagues at the Scripps Institution of Oceanography found that average concentrations of atmospheric CO2 in May were 430.2 ppm. NOAA’s Global Monitoring Laboratory, which has conducted separate daily readings since 1974, reported an average of 430.5 ppm in May.

Carbon dioxide levels in the atmosphere are closely monitored to gauge how much humans are influencing Earth’s climate. The readings are also an indicator of the planet’s overall health.

“They’re telling you about your whole system health with a single-point measurement,” Keeling said. “We’re getting a holistic measurement of the atmosphere from really a kind of simple set of measurements.”