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We're creeping closer to a milestone: the highest concentration of atmospheric CO₂ in human history. The U.S.’s greenhouse gas emissions are down to —due largely to the recession, the natural gas boom displacing much dirtier coal, and increased fuel efficiency in vehicles—all good news for the Obama administration, which promised to cut emissions by 17 percent from 2005 levels by 2020. But the amount of carbon in the global atmosphere is creeping higher and higher. 

The amount of carbon in the atmosphere is well beyond the 350 parts per million that scientists, , say will create an unstable climate. (And if you’re still a climate change doubter, .) In fact, it’s right around 400 ppm. The last time it was that high was during the Pliocene epoch 3.5 to 5 million years ago.

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As for what life was like then, scientists rely on fossil records to recreate where plants and animals lived and in what quantity. Pliocene fossil records show that the climate was generally warmer and wetter than today. Maps of Pliocene vegetation record forests growing on Ellesmere Island in the Canadian Arctic, and savannas and woodlands spreading over what is now North African desert.

Scripps Institution of Oceanography climate scientist Charles Keeling began taking measurements of carbon dioxide from the Mauna Loa volcano on the Big Island of Hawaii in 1958. That record, called “The Keeling Curve,” is now our longest running record of carbon dioxide in the atmosphere (which you can see at and  on Twitter).

“The rate of rise of CO₂ over the past century is unprecedented; there is no known period in geologic history when such high rates have been found. The continuous rise is a direct consequence of society’s heavy reliance on fossil fuels for energy,” Scripps .

Although it may be a while before the CO₂ concentration stays at or above 400 ppm, the number is significant, says Ralph Keeling, a geochemist at Scripps who oversees the monitoring. (He’s also Charles’s son.)

“It’s a time to take stock of where we are and where we’re going,” he Nature.com.

Climate change is a global problem, but the U.S. is one of the largest greenhouse gas emitters, a reason for us to be a policy leader when it comes to the issue. The U.S. government has dragged its feet on creating a national climate-change policy, but states have been busy. California has strict fuel economy standards, states in the Northeast have an emissions trading market, and 30 states have mandatory renewable energy standards. And maybe they can make even deeper cuts as time goes on. But as CO₂ emissions climb this benchmark is a sobering reminder of why more should be done to reduce emissions—and soon.

One action that would make a big difference: regulate greenhouse gas emissions from existing power plants. Last year the EPA proposed carbon standards for future power plants, but in April the agency missed its deadline for finalizing rules for existing power plants, which are responsible for of our nation’s emissions.