Last year, the world flirted with a depressing milestone: for the first time in human history, the atmospheric concentration of carbon dioxide peaked at 400 parts per million. This is the most basic sign of humanity's effect on the global greenhouse, and last year, it made only a quick dash up to 400 ppm, with the peak concentration during the seasonal high sneaking just over the mark. This year, two months earlier, we've already hit 400 parts per million, and unlike last time, the concentration of carbon dioxide is expected to stay over that level for a while.
In the northern hemisphere, says Brian Kahn for Climate Central, “[a]tmospheric carbon dioxide usually peaks in May”—a reflection of the seasonal growth of plants. Carbon dioxide builds up over the winter, peaking just before northern hemisphere plants really kick it into gear in late spring.
At the Mauna Loa Observatory in Hawaii, the reading is currently sitting at 401.19 parts per million of atmospheric carbon dioxide.
“If levels continue to rise in the next few months — and there’s no reason to believe they won’t — April or May will likely be the first time the monthly atmospheric carbon dioxide average will be above 400 ppm,” says Kahn. “Estimates for when the atmosphere last contained this much carbon dioxide range from 800,000 years ago all the way to 15 million years.”
The global average atmospheric concentration of carbon dioxide needs to stay below 420 parts per milioon by the year 2100, says Nature, “to keep global warming below 2°C.”
In the time before the Industrial Revolution, atmospheric carbon dioxide was at 280 parts per million. In the late 1950s, when observations began at Mauna Loa, it was over 300. In the year 2000, it was at 370.