Climate: Atmospheric CO2 reaches 400 ppm

Concentration will wane from seasonal high point, but long-term trend is up


Mauna Loa. Photo courtesy USGS.


Carbon dioxide in the atmosphere this week reached a level last recorded 2 to 5 million years ago.

By Summit Voice

FRISCO — Climate scientists have been closely tracking atmospheric carbon dioxide levels for a long time, but this week, the colorless, odorless gas made big headlines.

An atmospheric observatory on Mauna Loa for the first time measured daily concentrations of CO2 at slightly above 400 parts per million, a dubious milestone which, better than any other number, captures the extent to which we are changing the world.

The measurement NOAA reported for Thursday, May 9, 400.03 ppm. The last time there was this much of the heat-trapping pollutant in the air was somewhere between 2 to 5 million years ago, during the Pliocene. The average global temperature was about 3 to 4 degrees Celsius warmer than it is now, and there were forests, horse and giant camels living in what is now the Arctic. Sea levels were 30 to possibly 100 feet higher than they are now.

Global climate may have been in a completely different state, and some scientists are convinced that we are now at the threshold of sudden switch that could through the whole planet into a different climate state. At some point, as the temperatures creep upward, it’s very likely that massive quantities of ice will melt off Greenland and the West Antarctic ice sheet, resulting in a dramatic rise in sea level.

According to NOAA, the milestone is important because 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 pumped into the atmosphere by fossil fuel burning and other human activities is the most significant heat-trapping greenhouse gas. Its concentration has increased every year since scientists started making measurements on the slopes of the Mauna Loa volcano more than five decades ago. The rate of increase has accelerated since the measurements started, from about 0.7 ppm per year in the late 1950s to 2.1 ppm per year during the last 10 years.

“That increase is not a surprise to scientists,” said NOAA senior scientist Pieter Tans, with the Global Monitoring Division of NOAA’s Earth System Research Laboratoryin Boulder, Colo. “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.”

Before the Industrial Revolution in the 19th century, global average CO2 was about 280 ppm. During the last 800,000 years, CO2 fluctuated between about 180 ppm during ice ages and 280 ppm during interglacial warm periods. Today’s rate of increase is more than 100 times faster than the increase that occurred when the last ice age ended.

Researcher Charles David Keeling, of the Scripps Institution of Oceanography, UC San Diego, started measuring carbon dioxide at Mauna Loa in 1958, initiating now what is known as the “Keeling Curve.” His son, Ralph Keeling, also a geochemist at Scripps, has continued the Scripps measurement record since his father’s death in 2005.

“There’s no stopping CO2 from reaching 400 ppm,” said Ralph Keeling. “That’s now a done deal. But what happens from here on still matters to climate, and it’s still under our control. It mainly comes down to how much we continue to rely on fossil fuels for energy.”

NOAA scientists with the Global Monitoring Division have made around-the-clock measurements there since 1974. Having two programs independently measure the greenhouse gas provides confidence that the measurements are correct.

Similar increases of CO2 are seen all over the world by many international scientists. NOAA, for example, which runs a global, cooperative air sampling network, reported last year that all Arctic sites in its network reached 400 ppm for the first time. These high values were a prelude to what is now being observed at Mauna Loa, a site in the subtropics, this year.

Sites in the Southern Hemisphere will follow during the next few years. The increase in the Northern Hemisphere is always a little ahead of the Southern Hemisphere because most of the emissions driving the CO2 increase take place in the north.

Once emitted, CO2 added to the atmosphere and oceans remains for thousands of years. Thus, climate changes forced by CO2 depend primarily on cumulative emissions, making it progressively more and more difficult to avoid further substantial climate change.

On the Web:

NOAA carbon dioxide data:

Scripps Institution of Oceanography carbon dioxide data:

NOAA’s Maua Loa Observatory:

ANIMATION (carbon dioxide levels over 800,000 years):

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  1. […] Climate: Atmospheric CO2 reaches 400 ppm ( […]

  2. […] Climate: Atmospheric CO2 reaches 400 ppm […]

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