News•October 28, 2015

El Niño Could Push CO2 Permanently Above Milestone

El Niño has its fingers in a lot of pies this year: Not only is it helping to boost 2015 toward the warmest year on record, but it is also a major factor in blockbuster hurricane activity in the Pacific and is contributing to a major worldwide coral die-off.

By this time next year we’ll probably be able to add another effect to that list: This El Niño is likely to tip us over into a world with carbon dioxide concentrations permanently above 400 parts per million.

“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,” Ralph Keeling, the director of the CO2 Program at the Scripps Institution of Oceanography, said in an Oct. 21 blog post.

Rise and Fall

Because of unabated manmade emissions, the levels of carbon dioxide in Earth’s atmosphere are rising every year, which is, in turn, driving up the planet’s temperature.

Since measurements began atop Hawaii’s Mauna Loa volcano in 1958 (an effort begun by Keeling’s father, Charles Keeling), atmospheric CO2 concentrations have risen from about 315 ppm to nearly 400 ppm. The graph showing that continuous uptick was dubbed the Keeling Curve. (Before the Industrial Revolution began, CO2 was around 280 ppm.)

Embedded in the overall rise of CO2 on the graph are the annual wiggles that show the change of CO2 with the seasons. The rise and fall trails the spring bloom and fall die-off of plants in the Northern Hemisphere, as they suck up and then release CO2. Levels usually peak in May and bottom out in September.

But man made emissions are continuously adding more CO2 to the atmosphere, and the plants can only suck up a finite amount. That leads to excess CO2 that then accumulates in the atmosphere, to the tune of about 2 ppm per year currently. (That rate has accelerated since the Mauna Loa record began, as emissions have ramped up over time. The yearly rise at the beginning of the record was only about 0.75 ppm.)

Since the beginning of the 20th century, all that extra CO2 has lead to a global temperature rise of 1.6°F. Depending on if and when emissions are curtailed, the temperature rise by the end of this century could see another 3°F to 9°F of warming.

The first time the Mauna Loa record saw a measurement above 400 ppm was in May 2013. Because of the yearly buildup, that mark was passed earlier in 2014, and CO2 levels stayed above it for longer — that April was the first month with an average above 400 ppm. This year, levels were above that value for six months before declining and bottoming out in September.

Scientists like Keeling have been waiting for the year when they no longer drop below 400 ppm. He thinks that because of the influence of El Niño, this coming year will be the one.

El Niño’s Push

CO2 levels rise much faster during and just following an El Niño event, he said. Much of this is because the shift in the location of warm tropical Pacific Ocean waters that is a hallmark of El Niño leads to a shift in the location of tropical rains. Many tropical forests end up in drought and there can often be an uptick in forest fires (as has been the case with the terrific forest fires in Indonesia this year), meaning even more CO2 builds up in the atmosphere.

The last El Niño of comparable strength to the current event came in 1997 and 1998. While the difference in CO2 levels from September 1996 to September 1997 was about 0.74 ppm, the rise from September 1997 to September 1998 was 3.7 ppm.

Nearly 20 years later, the growth in emissions means that “if this El Niño is comparable, the rise from September 2015 to September 2016 could easily be 4.4 ppm,” Keeling wrote in his blog post.

That would take the September average from about 397 ppm this year to 402 ppm next year, he said.

“Without the El Niño, it seems most likely we would have dipped below 400 for the last time in September of next year,” Keeling said in an email.

The 400 ppm mark is of course somewhat arbitrary, and more important is the buildup of CO2 in the atmosphere and the fact that levels this high haven’t been present on Earth in likely about 3.5 million years, said Jason Smerdon, a climate researcher at Columbia University’s Lamont-Doherty Earth Observatory.

“That was a long time ago when the world was a very different place. We also show no signs of stopping at 400 ppm and in the absence of any action to curb fossil fuel emissions we will be having this same discussion about 500 ppm in each of our lifetimes,” he said in an email, adding that Keeling’s calculations looked reasonable.

But while the 400 ppm mark is a fairly symbolic one, it comes with a psychological shift, like rolling your car odometer over the 100,000-mile mark, even for someone who has been watching the miles tick upward his whole career.

“It takes some getting used to, but just like getting older, you have to accept it as the new reality,” Keeling said. And that reality is a much warmer one.