Prediction of the CO2 rise associated with El Niño
October 2018 - 2016 was a milestone year for climate change, marking the first time that the monthly atmospheric CO2 concentration remained above 400ppm for an entire year.
This historic event was predicted months earlier by a collaboration of scientists, who determined the 2015/2016 El Niño to be a significant driver of the increase in CO2 concentration. A paper has now been published providing verification and further explanation of the prediction.
What is El Niño?
The phrase ‘El Niño’ refers to a change in sea surface temperatures across the Tropical Pacific Ocean, linked to a weakening of the usual easterly trade winds. In a non-El Niño year, strong trade winds blowing from east to west maintain a temperature gradient across the Pacific Ocean, with warmer surface water in the West Pacific (near Indonesia) than in the east (near the coast of South America). This warm surface water provides an ample moisture source for cloud formation and precipitation across the Western Tropical Pacific. During an El Niño event, the trade winds slow, and the warm water in the West Pacific extends east, reducing the temperature gradient. This causes a redistribution of energy that changes weather patterns across the entire globe, triggering floods in Ecuador, droughts in Indonesia, and a migration of fish away from the coast of Peru. El Niño even causes an increase in global average temperature. El Niño events are sporadic, taking place every 2-7 years, but have been occurring for at least the last few thousand years.
How does El Niño increase atmospheric CO2 concentrations?
Since the early 1800s, human emission have been causing an increase in atmospheric CO2 concentration. When an El Niño event occurs, a short-term increase in CO2 is added to this background level, causing the extreme weather conditions mentioned above. The flooding in Ecuador and droughts in Indonesia prevent plants from photosynthesising to their full capacity, and so the terrestrial biosphere absorbs less CO2 from the atmosphere than usual. Additionally, the extreme temperatures cause widespread plant death, as well as forest fires which release additional CO2 into the atmosphere.
Global atmospheric CO2 concentration undergoes a yearly fluctuation due to the changing rate of plant photosynthesis throughout the year, with a maximum in May and a minimum in September. Before the industrial revolution, the atmospheric CO2 concentration was steady at around 280ppm (parts per million). However, since the rapid increase in anthropogenic CO2 emissions, this value has been rising, and by late 2015, global CO2 concentrations were fluctuating around the symbolic 400ppm mark. This led to an informal prediction, made in October 2015 by Prof. Ralph Keeling, that the September 2015 CO2 minimum would be the last occurrence of a monthly CO2 concentration below 400ppm. Richard Betts and colleagues took this prediction further, and published a paper in 2016 quantifying these CO2 concentrations.
As is convention, Betts and his colleagues used the atmospheric CO2 concentration of Mauna Loa (Hawaii) as a proxy for global CO2 concentration, as Mauna Loa provides highly accurate data, and has the longest standing CO2 data record.
The 2016 Betts et al. paper used model results to predict an average annual CO2 concentration of 404.53ppm in Mauna Loa for the year 2016, indicating a 3.15ppm increase from the 2015 average. Observations in the year 2016 produced very similar results, with an average annual concentration of 404.28ppm, indicating an annual increment of 3.39ppm from the previous year - significantly greater than the usual 2.1ppm/yr increase in atmospheric CO2 concentration. The difference between the modelled and observed CO2 concentrations was well within the limits of the model, showing that the predictions were made with a high level of accuracy. In addition, two different approaches are included for modelling the CO2 concentration in the year 2016 if there had been no El Niño event. These two approaches were ‘from trend’, in which a simple linear extrapolation of the 2.1ppm/yr increase combined with a monthly adjustment factor was applied to the 2015 values, and the more reliable ‘from regression’, in which more specific increments are also added.
Richard A. Betts, Chris D. Jones, Jeff. R. Knight, Ralph. F. Keeling, John. J. Kennedy, Andrew J. Wiltshire, Robbie M. Andrew and Luiz E. O. C. Aragão (2018): A successful prediction of the record CO2 rise associated with the 2015/2016 El Niño, Phil. Trans. R. Soc. B doi: 10.1098/rstb.2017.0301