Cracking a climate conundrumCO2 emissions leveled off between 2014 and 2016. But annual growth of CO2 in the atmosphere rose to more than 50 percent above that of past decades. What explains the contradiction? [image: Email to someone] [image: Share on Facebook]
– ‘Avengers’ casualty: environmental understanding
In 2015, we earthlings – some 7.5 billion of us – discharged 36 billion tons of carbon dioxide into the air from many tailpipes and smokestacks. That is about the same amount of planet-warming gas belched out in 2014, and the figure remained largely unchanged in 2016.
After a century of exponential growth in the mass of carbon dioxide ejected into the air, the leveling-off of the output caught many observers by surprise. It’s explained partly by widespread substitution of natural gas for coal in electricity production and by expanded use of wind and solar energy. [image: Indonesia fire]Forest fires in Indonesia, September 2011. Photo credit: Rini Sulaiman/ Norwegian Embassy for Center for International Forestry Research (CIFOR), via Flickr.
Although the amount of CO2 ejected into the air leveled off in 2015, the quantity accumulating in the atmosphere did not let up. Rather, it spiked. Indeed, the concentration of the gas increased that year by 3 parts per million (ppm), 50 percent more than in the previous year and the average annual increase of the prior four decades. Researchers hadn’t observed an increase so large since they began systematic measurements of CO2 in the atmosphere back in 1958. The amount of CO2 in the air probably hadn’t surged so much in a single year since at least the end of the last ice age – 10,000 years ago. The science explaining the seeming dichotomy
A scientific article published in late 2017 explains this apparent paradox. The extra CO2 came from the world’s tropical forests. Beginning at the end of 2014 and lasting 19 months, the strongest El Niño recorded in more than 50 years warmed and dried the tropics – increasing wildfires, slowing tree growth and speeding-up the rotting of dead vegetation – releasing billions of extra tons of CO2 into the air. The authors of that 2017 paper also reported a surprise: Not all tropical forests reacted to the El Niño the same way, a finding the coauthors say could help improve climate models.