Digesting 400 ppm for global mean CO2 concentration

On 2 May 2013, it was reported in Nature that hourly values of CO2 concentrations at Mauna Loa, Hawaii crossed the symbolic milestone of 400 ppm (parts per million) in April 2013 several times, and could record a daily-mean concentration of more than 400 ppm within another month. Indeed, daily mean value of 400 ppm was recorded on 9 May 2013. Interestingly, this value has not been reached for a few million years at this site. Humanity’s fossil fuel (e.g. coal, petroleum) emissions in the industrial era and large-scale modification of the landscape (e.g. conversion of forests to croplands) in the past several millennia have been implicated for this rise in CO2. In the most recent decade, fossil fuel emissions and land cover change have respectively, contributed approximately 90% and 10% to total CO2 emissions. CO2 absorbs infrared radiation emitted by the planet and hence has a tendency to increase the heat content and temperature of the planet. What does this symbolic milestone of 400 ppm represent? Should we be concerned? First, we should recognize that the value reported in Nature refers to the global mean atmospheric CO2 concentration. Values much larger than this have been recorded on local and regional scales depending on the strength of local sources and sinks of CO2 and local atmospheric stability conditions. For instance, cities with large industrial and vehicular emissions could record local values twice this symbolic number when mixing of atmospheric air is highly restricted. The Mauna Loa Observatory is located at an altitude of 3400 m in the northern subtropics in a remote location. CO2 measured here truly represents the globally averaged, horizontally and vertically, CO2 concentration since there is very little local influence at this site. Next, to appreciate the magnitude of humanity’s impact on atmospheric composition, one needs to know how big a ppm is on a global scale. Using simple arithmetic involving the mass of the global atmosphere and molecular weights of dry air and CO2, one can show that 1 ppm is equivalent to approximately 2 billion tonnes of carbon or 7.5 billion tonnes of CO2. In the recent years, humanity’s CO2 emissions have averaged at about 10 billion tonnes of carbon per year. Out of this, about 50% (5 billion tonnes of carbon) has stayed in the atmosphere and the other half has been removed by terrestrial plants and oceans. Hence, the atmospheric loading of carbon is increasing at about 5 billion tonnes of carbon or 2.5 ppm of CO2 per year (Figure 1). The effect of global-scale photosynthesis by terrestrial plants can be appreciated by looking at the regular annual rhythm (like heart beats) in the time series (red curve in Figure 1) – the global mean CO2 has lower values in summer when plants actively take up CO2 and higher values in winter when plants are dormant. The average difference between winter and summer is about 6 ppm. Had the distribution of land and hence terrestrial plants been equal between northern and southern hemispheres, this seasonal see-saw would have been absent. Because of these seasonal swings, it will be a while, perhaps a few years, before the global CO2 concentration averaged over an entire year, passes 400 ppm. The annual mean CO2 values for 2011 and 2012 were 390.48 and 392.5 respectively. It is likely that the annual mean value of 400 ppm will be recorded in 2015. In what way is this current trend in CO2 unique? In the past several million years, in the absence of human influence, CO2 has naturally fluctuated between 180 and 280 ppm during glacial and interglacial periods. Thus, the current level of CO2 is unusual and signifies the footprint or mark of human civilization on this planet. One might even wonder if we have taken the destiny of the planet in our hands. People could be endlessly debating the role of humans on the recent global warming because of the inherent variability in surface temperature, but there can be very little doubt that anthropogenic emissions are responsible for the CO2 rise and humans are changing the chemical composition of the atmosphere. The increased atmospheric CO2 has already invaded the global oceans altering ocean chemistry and resulting in ocean acidification and disruptions to marine life. If we continue to burn fossil fuels, CO2 concentration in the atmosphere will reach levels last seen in the Cretaceous period (150–66 million years ago) when