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Carbon dioxide levels race past troubling milestone
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Carbon dioxide levels race past troubling milestone

Relentless rise of this greenhouse gas locks in a warmer future, scientists say

Carbon dioxide  levels in the Earth’s atmosphere passed a troubling milestone for good this summer, locking in levels of the heat-trapping gas not seen for millions of years.

Every year, the amount of carbon dioxide (CO2) rises during winter and then falls slightly during the Northern Hemisphere’s growing season, as plants take up this greenhouse gas during photosynthesis.

But this year, for the first time since before the Ice Age, CO2 will not fall below 400 ppm.

“It’s unlikely we’ll ever see CO2 below 400 ppm during our lifetime and probably much longer,” said Pieter Tans, lead scientist of NOAA's Global Greenhouse Gas Reference Network.

Measurements taken at NOAA’s atmospheric observatories on Mauna Loa and at the South Pole both indicate that CO2 has passed 400 ppm for good.

What’s so important about 400 ppm?

Four hundred parts per million is an arbitrary milestone, but it also may be a window on our future.

The last time CO2 levels were this high was the mid-Pliocene warm period — about 3 million years ago. Paleoclimate research suggests that there was a lot less ice to cool the planet then. The extent of the Greenland Ice Sheet and the West Antarctic Ice Sheet were severely reduced.

Ditto for the Arctic.

Three million years ago, sea levels were up to 65 feet higher than today. Forests replaced tundra as trees marched toward the North Pole. Tropical rainforests were squished into a narrow band around the equator. In the U.S., the Central and Southern plains might not have had sub-freezing temperatures in winter, let alone white Christmases.

“There were some differences in continent locations, and in Earth's orbit around the sun, but the Pliocene is considered a bellwether for what future climate might be like,”
said Bruce Bauer, a scientist with NOAA’s National Centers for Environmental Information.

The race towards greater warming

What’s more troubling, said Tans, is that the rate of CO2 increase is more than 100 times faster than anything observed in the ice core record that goes  back 800,000 years. This will continue as long as fossil fuel consumption remains at its current high level worldwide.

For most of human evolution, CO2 levels hovered around 278 ppm, helping to maintain the global climate in a relatively stable state conducive to agriculture and the growth of human populations. That changed in the 1850s, first  through massive deforestation. However, since the 1950s, the burning of coal to make electricity and steel, of oil for vehicles and manufacturing, and now of natural gas, has vastly accelerated the CO2 increase.

“About 85 percent of all fossil fuel consumption since the start of the industrial revolution took place during my lifetime,” said Tans.

Learn more about NOAA’s greenhouse gas research.

For more information, please contact Theo Stein, NOAA public affairs at NOAA's Earth System Research Laboratory, at 303-497-6288 or by email at

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Oceanic and Atmospheric Research (OAR) - or "NOAA Research" - provides the research foundation for understanding the complex systems that support our planet. Working in partnership with other organizational units of the NOAA, a bureau of the Department of Commerce, NOAA Research enables better forecasts, earlier warnings for natural disasters, and a greater understanding of the Earth. Our role is to provide unbiased science to better manage the environment, nationally, and globally.


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