According to the measurement of Mona Loa laboratory, in 2016, CO2 level reached 400 parts per million (ppm) and starting from September 2016, the usual annual low hovered above 400 ppm(1). This reminds us of the stark reality of how hard it will be for the world to keep the global temperature increase well below 2 degree Celsius or even 1.5 degree Celsius.
The fluctuation of this curve (Keeling curve) in a year is due to the fluctuation in the photosynthesis activity in spring and winter. In Northern hemisphere, with the advent of spring, temperature becomes warmer, snow starts melting, soil moisture gets replenished and plants increase their photosynthetic activity. This leads to reduction of CO2 in the atmosphere. Again, at the time of winter, photosynthesis decreases and respiration increases which lead to increase in CO2.
The annual rate of increase of CO2 was about 0.7 ppm per year in the late 1950s which accelerated to 2.1 ppm per year in 2005-14. Global temperatures rose in parallel with CO2 increase. 2016 was the hottest year in record after 1880 and 2016 was 1.1 degree Celsius warmer than pre-industrial level. The last time, the world witnessed consistent level of above 400 ppm, was the warmer part of the Miocene, some 16 million years ago.
Below we can see a forecast of CO2 made by ERI using Holt-Winters Exponential Smoothing method. We have provided both the forecast (red line) and the upper and lower limit for this forecast (shaded area). We can see that CO2 concentration will be above 410 ppm in 2018 and starting from 2019, it will be consistently above 410 ppm. This can increase to 427.55 ppm in May, 2023. The real level of CO2 concentration can be lower than the prediction if major emitters like China and USA decrease their CO2 emissions in future.