Carbon dioxide levels race past troubling milestone

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 the 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," says 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 three 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 United States, 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," says Bruce Bauer, a scientist with NOAA's National Centers for Environmental Information.

The race towards greater warming

What's more troubling, says 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 all changed starting in the 1850s with massive deforestation around the world. Then in the 1950s, a dramatic increase in the burning of fossil fuels -- coal to make electricity and steel, oil for vehicles and manufacturing -- vastly accelerated the rate of CO₂ being pumped into the atmosphere.

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

Fact: 10,000: The number of Empire State Buildings that could be filled with just the carbon from all the CO₂ humans emit every year, according to NOAA scientist Jim Butler.