CO2 may be past turning point for our lifespans

May not register below 400 ppm again

Scientists who measure and forecast the concentrations of greenhouse gases in the atmosphere said Monday that we may have passed a key turning point.

Humans walking the Earth today will probably never live to see carbon dioxide concentrations in the atmosphere once again fall below a level of 400 parts per million (ppm), at least when measured at the iconic Mauna Loa Observatory in Hawaii, where the longest global record of CO2 has been compiled.

“Our forecast supports the suggestion that the Mauna Loa record will never again show CO2 concentrations below the symbolic 400 ppm within our lifetimes,” write the researchers, led by Richard Betts of the UK Met Office’s Hadley Center, in Nature Climate Change. The study was conducted with colleagues from the Hadley Centre and Ralph Keeling of the Scripps Institution of Oceanography in La Jolla, California.

Carbon dioxide concentrations in the pre-industrial atmosphere were around 280 parts per million. But concentrations began to rise with the early growth of industry and continually climbed throughout the 20th century, as documented by the famous Keeling curve, based on observations taken at Mauna Loa dating back to the late 1950s.

This record is referred to often as a “saw-toothed curve,” because every year, concentrations go up and down somewhat (due to the life cycles of plants across the globe, which drawn in carbon dioxide through the process of photosynthesis). But nonetheless, the longterm trend is steadily upward because humans are putting more carbon dioxide into the atmosphere than plants and other natural “sinks” can pull back out again.

Concentrations have crossed 400 parts per million on a temporary basis. It began as brief excursions, and last year, 2015, the annual average concentration at Mauna Loa was more than 400 parts per million for the first time (it was 400.9). Nonetheless, during the course of the yearly cycle in carbon dioxide concentrations, there were still some parts of the year last year when they remained below that level.

What the new study suggests is that those days are over — carbon dioxide will never fall below 400 ppm this year, nor next, nor the next. The reason is that the strong 2015-16 El Niño event has pushed concentrations upward more than usual for a given year — El Niños tend to do that, because they dry out tropical regions, lessening tree growth and sparking vast wildfires. That means that even in September of this year, when annual concentrations are typically at their lowest (as northern hemisphere trees lose their leaves and vegetation growth declines heading into winter), they’ll likely still be slightly over 400 parts per million, scientists forecast.