On May 11, global concentration of carbon dioxide in the atmosphere was measured to have crossed the 415 parts per million (ppm) mark for the first time. On every subsequent day thereafter, the daily average atmospheric concentration of carbon dioxide has remained over that level, touching 415.7 ppm on May 15. On May 18, the daily average carbon dioxide concentration, as measured by sensors at the Mauna Loa observatory in Hawaii, was 415.02 ppm.
The rapidly rising concentration, as measured from Mauna Loa and other observatories, is one of the best indicators of the manner in which the planet has been warming up. The higher the concentration of carbon dioxide, the greater the greenhouse gas effect that causes the Earth’s atmosphere to heat up.
For several thousand years, the carbon dioxide concentration remained constant around 270-280 ppm, before the industrial revolution began to slowly push it up. When direct measurements began at the Mouna Loa observatory in 1958, concentrations were around 315 ppm. It took nearly 50 years for it to reach 380 ppm, a mark first breached in 2004, but thereafter the growth has been rapid.
The first full-day average of more than 400 ppm was achieved on May 9, 2013; two years later, in 2015, even the annual average exceeded 400 ppm. Currently, the carbon dioxide concentration is growing at more than 2 ppm per year, and scientists say the growth rate is likely to reach 3 ppm a year from this year.
Carbon dioxide’s long life
The increase in atmospheric concentrations is caused by the carbon dioxide being constantly emitted in different, mostly man-made, processes. In recent years, the growth in global carbon dioxide emissions has slowed down considerably. It remained almost flat between 2014 and 2016, and increased by 1.6% in 2017 and about 2.7% in 2018. In 2018, the global emission of carbon dioxide was estimated at 37.2 billion tonnes.
The rapid rise in the atmospheric concentrations, however, is due to the fact that carbon dioxide has a very long lifespan in the atmosphere, between 100 and 300 years. So, even if the emissions were to miraculously reduce to zero all of a sudden, it would have no impact on the atmospheric concentrations in the near term.
About half of emitted carbon dioxide is absorbed by plants and oceans, leaving the other half to go into the atmosphere. An addition of about 7.5 billion tonnes carbon dioxide to the atmosphere leads to a 1 ppm rise in its atmospheric concentration. So, in 2018, for example, half the total emissions, or about 18.6 billion tonnes of carbon dioxide, would have been added to the atmosphere, leading to rise of 2.48 ppm in atmospheric concentrations.
The absorption of carbon dioxide by plants follows a predictable seasonal variability. Plants absorb more carbon dioxide during the summer, with the result that a lower amount of carbon dioxide is added to the atmosphere in the summer months of the northern hemisphere, which has considerably more vegetation than southern hemisphere. This variability gets captured in the very rhythmic seasonal fluctuation of atmospheric concentration of carbon dioxide.
The temperature equivalence
The global goal in the fight against climate change has been defined in terms of temperature targets, not carbon dioxide concentrations. The stated effort of the global community is to keep the rise in average surface temperatures below 2ºC higher than during pre-industrial times, and if possible below 1.5°C.
The carbon dioxide concentration level corresponding to a 2ºC rise in global temperatures is generally understood to be 450 ppm. At current rates of growth, that level would be reached in less than 12 years, that is by 2030. Until a few years ago, it used to be understood that this milestone would not be reached till at least 2035. The corresponding carbon dioxide level for a 1.5ºC rise is not very clearly defined.
A special report released by the Intergovernmental Panel on Climate Change last year said the world needed to achieve net zero emissions of all greenhouse gases, not just carbon dioxide, by 2050 to keep alive any realistic chances of restraining the temperature rise to within 1.5ºC. The net zero needs to be achieved by 2075 to attain the 2ºC target.
Net zero is achieved when the total emissions is neutralised by absorption of carbon dioxide through natural sinks like forests, or removal of carbon dioxide from the atmosphere through technological interventions.