NOAA’sMauna Loa observatory in Hawaii is the oldest continuous carbon dioxide (CO2.) measurement station in the world and a primary global benchmark site for tracking the increase of this potent heat-trapping gas. (CREDIT: Mary Miller, Exploratorium)
NOAA’s Mauna Loa observatory in Hawaii is the oldest continuous carbon dioxide (CO2.) measurement station in the world and a primary global benchmark site for tracking the increase of this potent heat-trapping gas. (CREDIT: Mary Miller, Exploratorium)

Heat-trapping gas concentrations top 400 ppm, two months earlier than last year

March 21, 2014

By Monica Allen

Over the last five days beginning on March 16, 2014, carbon dioxide levels have surpassed 400 parts per million at NOAA's Mauna Loa Observatory in Hawaii. This is nearly two months earlier than last year when the concentration of this greenhouse gas was first recorded above 400 parts per million on May 9, at the historic NOAA observatory.

We caught up with James Butler, Ph.D., Director of ESRL's Global Monitoring Division, to ask about what it means that we reached this milestone earlier than last year. To track carbon dioxide concentrations daily click here.

Atmospheric CO2 graph at Mauna Loa Observatory, HI What does it mean that carbon dioxide levels topped 400 ppm on March 16 this year at Mauna Loa Observatory, nearly two months earlier than last year?

JB: 400 ppm is essentially a milestone along the way, reminding us that carbon dioxide continues to increase in the atmosphere, and at faster rates virtually every decade. This is consistent with rising fossil fuel emissions.

Why is it earlier this year?

JB: Seasonal swings in atmospheric carbon dioxide with highs in the Spring and lows in the Fall make “400 ppm” an annual event that must come earlier every year with rising atmospheric carbon dioxide.

Is there a tipping point, or carbon dioxide concentration that sets off severe consequences for human and planetary health?

JB: 400 ppm is not a tipping point. It is a milestone, marking the fact that humans have caused carbon dioxide concentrations to rise 120 ppm since pre-industrial times, with over 90 percent of that in the past century alone. We don't know where the tipping points are.

How long do you expect these higher levels to last?

JB: Two to three months; the peak should occur again in May and this year may be over 402 ppm. Next year we expect it will be over 404 ppm, etc.

Are we seeing the increase accelerate with time?

JB: Carbon dioxide concentrations have increased every year since Dave Keeling of the Scripps Institution of Oceanography started making measurements on the slopes of Mauna Loa volcano in 1958. The rate of increase has accelerated from about 0.7 ppm per year in the late 1950s to 2.1 ppm per year during the last decade.

Please explain the role of the natural cycle for carbon dioxide emissions?

JB: Plant growth drives the uptake of carbon dioxide from the atmosphere and is strongest in the early to mid-summer. Planetary respiration from decaying plant matter puts carbon dioxide back into the atmosphere all year long, but the fall and winter drop in photosynthesis allows respiration to dominate during those months, which brings carbon dioxide back up.

Do you expect carbon dioxide levels to top 400 ppm even earlier next year?

JB: Yes. Every year going forward for a long time.

What are we seeing globally at other measuring sites?

JB: Arctic sites all reached 400 ppm about a year before Mauna Loa last year. Southern hemispheric sites will follow with South Pole reaching 400 ppm in a few years.

What would it take to reverse the upward trend of carbon dioxide concentration?

JB: Elimination of about 80 percent of fossil fuel emissions would essentially stop the rise in carbon dioxide in the atmosphere, but it would not start decreasing until even further reductions are made.and then it would only do so slowly.