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Worst-case scenario for global CO2 levels realized over the coming decades?

In May 2013 the world reacted to a disturbing milestone: Daily averages of atmospheric carbon dioxide hit 400 parts per million (ppm) for the first time in modern history, compared with around 250 parts per million around the time of the Industrial Revolution. Except for a one-day reprieve in late August, daily averages have been above 400 ppm since December 2015. The milestone was noted, analyzed, and mourned by climate scientists who speculated that their children and grandchildren might never again see carbon dioxide concentrations drip below 400 ppm.

Carbon dioxide concentration has been climbing ever since measurements began at Hawaii's Mauna Loa Observatory in 1958. And the climb is accelerating: in the 1960s and 70s, carbon dioxide concentration rose by around 1 ppm per year. Last year the concentration growth rate was 3 ppm per year.

The milestone of 400 ppm is mostly symbolic, but signifies that earth is rapidly approaching the 450 ppm threshold seen by some as a climate stabilization target.

The point at which the global carbon dioxide concentration is projected to reach 500 ppm depends on which so-called "representative concentration pathway" humanity follows. The scenarios range from RCP 2.6, in which CO2 emissions peak and then decline by the mid-21st centry, to RCP 8.5, in which CO2 emissions continue unabated. In the best-case projections, earth never hits 500 ppm, peaking below 450 ppm around 2050. In the worst-case scenario, 500 ppm occurs between 2040 and 2050.

Will earth's carbon dioxide emissions follow the worst-case scenario, reaching a global carbon dioxide concentration of 500 ppm before 2050?

For this question to resolve as positive, the Mauna Loa Observatory must report a monthly carbon dioxide concentration above 500 ppm on or before December 31, 2050.

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