Carbon dioxide is a primary greenhouse gas, efficient at trapping heat from the sun. The colorless gas is released from power plants and vehicles as they burn coal, oil and gas.
“[The] increase is not a surprise to scientists,” said Pieter Tans, a senior scientist at the National Oceanic and Atmospheric Administration. “The evidence is conclusive that the strong growth of global [carbon dioxide] emissions from the burning of coal, oil and natural gas is driving the acceleration.”
Climate scientist Joanna Haigh of Imperial College London said the particular figure reached Friday — 400 parts per million — holds no particular significance except as a milestone. “It gives us the chance to mark the ongoing increase in [carbon dioxide] concentration and talk about why it’s a problem for the climate.”
Scientists have firmly linked rising atmospheric carbon dioxide to higher global temperatures, which have increased nearly a degree Fahrenheit, on average, since 1950.
Larger temperature increases have occurred in the Arctic. In 2009, an international agreement sought to limit temperature increases to 3.6 degrees Fahrenheit (2 degrees Celsius) by 2100 to avoid catastrophic effects on the climate.
Estimates from the 1700s show atmospheric carbon dioxide at about 270 parts per million — about 40 percent lower than today. Air bubbles trapped in Antarctic ice cores show that, in the past 800,000 years, airborne concentrations remained lower than 400 parts per million. And scientists at the Scripps Institution of Oceanography and elsewhere have estimated that about 3 million to 5 million years have passed since so much carbon dioxide wafted in the Earth’s atmosphere.
The temperature during that period, known as the Pliocene Epoch, was 5 to 7 degrees warmer than today, with seas tens of feet higher.
Airborne concentrations of carbon dioxide vary by season and location on Earth. But the measurements from the Mauna Loa monitor, which is run by Scripps, are considered the gold standard.
Concentrations there are plotted on the iconic Keeling Curve, named after scientist Charles David Keeling, who initiated the measurements in 1958. At that time, the carbon dioxide level was 316 parts per million.
Concentrations of the gas have been rising steadily since — a reflection of the world’s fossil fuel economy. In 2012, global carbon dioxide emissions soared to a record high of 35.6 billion tons, up 2.6 percent from 2011.