By Michael Le Page
We will pass yet another unwelcome milestone this year. The average concentration of carbon dioxide in the atmosphere is likely to rise by 2.8 parts per million to 411 ppm in 2019 – passing 410 ppm just a few years after first passing the 400 ppm mark.
That’s the forecast of Richard Betts of the Met Office Hadley Centre in the UK, who began making annual CO2 forecasts a few years ago to test our understanding of the factors involved. The Met Office began publicly releasing the forecasts last year.
The rise in atmospheric CO2 is the main cause of global heating. Before the industrial age began the CO2 concentration was around 280 ppm, and had not risen much higher for hundreds of thousands of years.
Now it is rising ever faster because human emissions of the gas continue to increase. The average annual increase has risen from less than 1 ppm a year in the 1950s, when measurements began, to well over 2 ppm today.
While the long term trend is remorselessly clear, the annual rise varies greatly from year to year depending on how the weather affects the balance between plants taking up CO2 as they grow and releasing it as they decay or burn. During El Nino years, for instance, there can be extensive droughts and wildfires, leading to big jumps in CO2 levels.
The biggest ever annual rise, of 3.4 ppm, occurred in 2016 during a strong El Nino. The forecast rise of 2.8 would also be one of the largest rises on record.
CO2 concentrations at any one place also vary over the year, peaking at the end of the winter and falling as plants grow in summer. Betts’ forecast is for the monthly average at Mauna Loa in Hawaii to peak at 415 ppm in May and drop back to 408 ppm in September before rising even higher in 2020.
Last year Betts forecast an annual rise of 2.3 ppm plus or minus 0.6 ppm, and the observed rise was 2 ppm. “So the observed rise was within the forecast range but slightly lower than the central estimate,” he says.
New Scientist pointed out that CO2 looked set to pass the 410 mark in its 2019 previews.
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