Leader of Keeling Curve measurement says temporary bump from El Niño could push atmospheric CO₂ levels above symbolic threshold for good

The Mauna Loa CO₂ record is a saw-tooth pattern, with CO₂ concentrations typically falling from May through September, and rising over the rest of the year. This cycle is caused by the natural exchanges of CO₂ with vegetation and soils. Each year, the values are higher than the year before, as CO₂ continues to pile up in the atmosphere from fossil fuel burning. This year, as expected, we hit the annual low point back in September and CO₂ concentrations are starting up again. The lowest point this year was well below 400 parts per million (ppm). The lowest daily minimum this year was 395.83 ppm and the average for the month of September, was around 397.1 ppm. By sometime in the next month or two, CO₂ will again rise above 400 ppm. Will daily values at Mauna Loa ever fall below 400 ppm again in our lifetimes? I’m prepared to project that they won’t, making the current values the last time the Mauna Loa record will produce numbers in the 300s.

The background for my forecast:

In recent years, CO₂ has been increasing by around 2.2 ppm, per year. Barring anything unusual, we would therefore expect next year’s September value to be around 399.3 ppm, just barely below 400 ppm, and we’d expect the lowest daily minima to be around 398 ppm or so. But we seem now to be on the verge of the largest El Niño event since 1997. This is significant because CO₂ tends to rise much faster during and just following El Niño events. From September 1997 to September 1998, for example, CO₂ rose by a whopping 3.7 ppm. If this El Niño is comparable, the rise from September 2015 to September 2016 could easily be 4.4 ppm, allowing for an El Niño boost and allowing that fossil-fuel emissions rates globally are larger now than in 1998. Taking these factors into account, a reasonable forecast for next year’s September minimum is around 402 ppm, with the lowest daily minima also over 400 ppm.

The El Niño growth spurt in atmospheric CO₂ is mostly caused by drought in the tropics. Rainfall that normally falls over tropical landmasses shifts to the oceans during El Niño events. This slows the normal growth of tropical forests and increases forest fires. Indonesia suffered severe fires during the 1997 event and, from recent news, is already being hit hard this year.

The loss of carbon from tropical forests in El Niño years is temporary as the forests tend to regrow in normal years, building back their biomass and sucking CO₂ out of the air in the process. But the eventual recovery from this El Niño won’t bring us back below 400 ppm, because its impact will be dwarfed by the global consumption of fossil fuels, pushing CO₂ levels ever higher.

­ – Ralph Keeling