March was the first month in modern records that levels of the heat-trapping gas exceeded 400 parts per million at sites across the entire globe.

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WASHINGTON — Global levels of carbon dioxide, the most prevalent heat-trapping gas, have passed a daunting milestone, federal scientists say.

The National Oceanic and Atmospheric Administration (NOAA) says that in March the global monthly average for carbon dioxide hit 400.83 parts per million.

That is the first month in modern records that the entire globe broke 400 ppm, reaching levels that haven’t been seen in about 2 million years.

“It’s both disturbing and daunting,” said NOAA chief greenhouse-gas scientist Pieter Tans. “Daunting from the standpoint on how hard it is to slow this down.”

He said it is disturbing because it is happening at a pace so fast that it seems like an explosion compared to Earth’s slow-moving natural changes.

Carbon dioxide isn’t just higher, it is increasing at a record pace, 100 times faster than natural rises in the past, Tans said.

Pushed by the burning of coal, oil and gas, global carbon dioxide is 18 percent higher than in 1980, when NOAA first calculated a worldwide average.

In 35 years, carbon-dioxide levels rose 61 parts per million.

In pre-human times, it took about 6,000 years for carbon dioxide to rise about 80 parts per million, Tans said.

Monthly levels fluctuate with the season, peaking in May and then decreasing as plants absorb carbon dioxide. But they are increasing on a year-to-year basis.

Levels are also higher in the Northern Hemisphere because that’s where carbon dioxide is being spewed by power plants and vehicles, Tans said.

The first time levels passed the 400 ppm milestone was for just a few weeks in the Arctic in 2012. Last year the monthly Northern Hemisphere average measured in Hawaii exceeded 400, and now the global average has as well, said James Butler, head of NOAA’s global monitoring division.

The United Nations has said that greenhouse gases should peak at no more than 450 ppm this century to maximize the chance of limiting the global temperature rise to 2 degrees Celsius (3.6 degrees Fahrenheit).

The recording was based on air samples taken from 40 sites around the world, NOAA said.

Increasing CO2 emissions are blamed for global climate change that causes stronger storms, melting Arctic ice and rising sea levels, according to scientists.

This is the first time the emissions have reached that level on a global basis — sites in the Arctic and Hawaii recorded CO2 concentrations over 400 ppm in 2012 and 2013, respectively.

“This marks the fact that humans burning fossil fuels have caused global carbon-dioxide concentrations to rise more than 120 parts per million since preindustrial times,” said Tans. Half of that rise has occurred since 1980, he said. Concentrations of CO2 are rising at about 2 to 3 ppm a year.