Using measurements taken worldwide, scientists estimated that 2015’s global average carbon dioxide concentration was 399.4 parts per million (ppm), a new record high. At Mauna Loa Observatory in Hawai’i, where atmospheric carbon dioxide has been recorded longer than anywhere else in the world, the annual average carbon dioxide concentration was 400.8—also a new record, and a new milestone.

For 800,000 years before the twentieth century, carbon dioxide levels in Earth’s atmosphere never exceeded 300 parts per million. In March 2015, the monthly average went above 400 ppm for the first time.

Restoring a healthy balance between sea otters and their sea urchin prey comes with a modest but meaningful bonus: halting the overgrazing of kelp forests and increasing their ability to remove excess carbon dioxide from the atmosphere.

In response to recent decades' warming, forests in the eastern United States have been "inhaling" more carbon dioxide through photosynthesis than they've “exhaled” through respiration.