Robert Kunzig
National Geographic News
Published May 9, 2013
An instrument near the summit of Mauna Loa in Hawaii has recorded a long-awaited climate milestone: the amount of carbon dioxide in the atmosphere there has exceeded 400 parts per million (ppm) for the first time in 55 years of measurement—and probably more than 3 million years of Earth history.
The last time the concentration of Earth's main greenhouse gas reached this mark, horses and camels lived in the high Arctic. Seas were at least 30 feet higher—at a level that today would inundate major cities around the world.
The planet was about 2 to 3 degrees Celsius (3.6 to 5.4 degrees Fahrenheit) warmer. But the Earth then was in the final stage of a prolonged greenhouse epoch, and CO2 concentrations were on their way down. This time, 400 ppm is a milepost on a far more rapid uphill climb toward an uncertain climate future.
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The measurement NOAA reported for Thursday, May 9, 400.03 ppm, was for a single day. Each data point on the Keeling curve, however, is actually an average of all the measurements made at Mauna Loa over an entire month. The CO2 concentration at Mauna Loa is unlikely to surpass 400 ppm for the whole month of May.
It certainly won't exceed 400 for all of 2013. CO2 peaks in May every year. By June the level will begin falling, as spring kicks into high gear in the Northern Hemisphere, where most of the planet's land is concentrated, and plants draw CO2 out of the atmosphere to fuel their new growth. By November, the CO2 level will be 5 or 6 ppm lower than it is now.
Then the curve will turn upward again: In the winter, plants stop making new carbohydrates but continue to burn the old, respiring CO2 back into the atmosphere.
This seasonal sawtooth—think of it as the breath of northern forests—is the natural part of the Keeling curve. The man-made part is its steady upward climb from one year to the next. Both were discovered at Mauna Loa.
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