News•May 10, 2013

Carbon Dioxide Passes 400 PPM Milestone, NOAA Finds

On May 9, the amount of carbon dioxide in the air exceeded 400 parts per million (ppm) for the first time in the observational record since 1958, and very likely for the first time in at least 800,000 years, the National Oceanic and Atmospheric Administration (NOAA) announced on Friday. This marks the first time the daily average CO2 concentration has risen past 400 ppm in the iconic record kept by the Mauna Loa Observatory in Hawaii, where daily observations go back to 1958,

Climate scientists recognize this 400 ppm mark as a symbolic milestone, illustrating the rapid increase of human-caused CO2 emissions over the past century. Numerous other climate data, gleaned from ice cores, ocean sediment, and other sources show that this is the highest CO2 concentration in the air in all of modern human history, possibly as far back as 15 million years ago.

Record of atmospheric carbon dioxide concentrations dating back 800,000 years (left) and during the course of the Mauna Loa record (right inset).
Click on the image to enlarge.

Carbon dioxide is the most important long-lived global warming gas. Once CO2 is emitted by burning fossil fuels such as coal and oil, a single CO2 molecule can remain in the atmosphere for hundreds of years. Global CO2 emissions reached a record high of 35.6 billion tons in 2012, up 2.6 percent from 2011. Carbon dioxide and other greenhouse gases warm the planet by absorbing the sun’s energy and preventing heat from escaping back into space.

Mauna Loa, as the oldest continuous carbon dioxide (CO2) measurement station in the world, is the primary global benchmark site for monitoring the increase of this potent heat-trapping gas. Carbon dioxide concentrations have steadily increased since scientists started making measurements on the slopes of the Hawaiian volcano more than five decades ago.

Reflecting the increase in manmade emissions of the gas, the rate of increase in CO2 levels has accelerated since the measurements began, from about 0.7 ppm per year in the late 1950s to 2.1 ppm per year during the last 10 years, NOAA said in a press release. Today’s rate of increase is more than 100 times faster than the increase that occurred when the last ice age ended, and this is driving a rapid increase in global average surface temperatures.

Before the start of the Industrial Revolution, global average CO2 was about 280 ppm, and it fluctuated between about 180 ppm and 280 ppm during the past 800,000 years, NOAA reported.

“The evidence is conclusive that the strong growth of global CO2 emissions from the burning of coal, oil, and natural gas is driving the acceleration,” said Pieter Tans, with the Global Monitoring Division of NOAA’s Earth System Research Laboratory in Boulder, Colo., in a press release.

As Climate Central reported on May 3, there is no single, agreed-upon answer to when CO2 concentrations were last at this level, as studies show a wide date range from between 800,000 to 15 million years ago. The most direct evidence comes from tiny bubbles of ancient air that act as time capsules, sealing ancient air in the vast ice sheets of Antarctica and Greenland. By drilling for ice cores and analyzing the air bubbles, scientists have found that, at no point during at least the past 800,000 years have atmospheric CO2 levels been as high as they are now.

A 2011 study in the journal Paleoceanography found that atmospheric CO2 levels may have been comparable to today’s as recently as sometime between 2 and 4.6 million years ago, during the Pliocene epoch, which saw the arrival of Homo habilis, a possible ancestor of modern homo sapiens, and when herds of giant, elephant-like Mastadons roamed North America. Modern human civilization didn’t arrive on the scene until the Holocene Epoch, which began 12,000 years ago.

For a 2009 study, published in the journal Science, scientists analyzed shells in deep sea sediments to estimate past CO2 levels, and found that CO2 levels have not been as high as they are now for at least the past 10 to 15 million years, during the Miocene epoch. At this time, megatoothed sharks prowled the seas, which were up to 100 feet higher than they are now.

Although the daily average CO2 benchmark has been established, it is unlikely that the monthly average or annual average CO2 level will surpass 400 ppm this year. Typically, CO2 levels in the Northern Hemisphere peak each year in May, after which they drop slightly as trees and plants around much of the world suck up more CO2 as they grow more each summer. But with global carbon emissions showing no signs of slowing, it may only take another one to three years before the annual average surpasses 400 ppm as well. By the middle of the century, CO2 levels are expected to climb to 450 ppm or higher, depending on emissions.

Editor's Note: After this story was published, NOAA revised their numbers for May 9 to 399.89 ppm, and the Scripps Institution of Oceanography reported 399.73 ppm. The revision is due to additional analysis and differences in the methods used by these two institutions. As of May 14, NOAA data showed a reading of 400.07 ppm on May 13, indicating the 400 ppm threshold was still exceeded.