Global CO2 levels surge past 400-ppm mark for first time in millions of years

By Gayathri Vaidyanathan | 05/07/2015 08:25 AM EDT

Global CO2 levels rose above 400 parts per million for the first time ever in the modern era in March 2015, marking a significant milestone in the planet’s progression toward a warmer future. The measurements were released by the National Oceanic and Atmospheric Administration.

The last time CO2 levels were this high was 3 million years ago during the Pliocene era. There was little ice at the poles then. Ellesmere Island, which is today part of the Canadian tundra, had forests growing on it.

Scientists have long expected global CO2 levels to breach 400 ppm, since levels regionally have been this high for a while. The Arctic hit the threshold in 2012, and the famed Mauna Loa observatory in Hawaii caught up in 2013 (ClimateWire, March 25, 2014).


"This is significant in the sense that it is another milestone for what humankind is doing to the atmosphere," said Pieter Tans, lead scientist of NOAA’s Global Greenhouse Gas Reference Network.

Humans are raising CO2 levels by 2 ppm every year, a rate that is 100 times greater than witnessed in the past, Tans said.

Wolf hidden in sheep’s clothing?

Scientists say that CO2 levels have to be less than 450 ppm by the end of the century if dangerous climate change is to be avoided (ClimateWire, Oct. 28, 2014). At the present emissions rate, that threshold may be reached in 25 years.

There was a blip of good news in March when the International Energy Agency announced that the CO2 emissions rate had stabilized in 2014, as the world emitted 32.3 billion metric tons of CO2 for the second consecutive year while maintaining economic growth. The IEA spun it as good news ahead of key climate talks to be held in Paris on a global treaty (ClimateWire, March 16).

But Tans said the news was like a wolf hidden in sheep’s clothing. The emissions rate has stabilized between 2013 and 2014 at a very high level, which means people are still emitting lots of CO2 and causing climate change, he said.

Between 2012 and 2014, CO2 concentration in the atmosphere grew by 2.25 ppm, an extremely high growth rate.

To measure global carbon levels, NOAA collects air samples from 40 global sites, which are then analyzed at a laboratory in Colorado.

The sites are extremely remote, ranging from far-flung islands to the decks of cargo ships, and provide a comprehensive global picture, Ed Dlugokencky, the NOAA scientist who manages the global network, said in a statement.

The levels will likely remain high until May, a time when plants actively start using CO2 for photosynthesis. Levels will rise again with the onset of autumn and winter, as the death of organisms triggers decay and the release back to the atmosphere of CO2.