Some Key Points

• There are other Greenhouse gasses but the concentration of Carbon dioxide (CO₂) in our atmosphere is the key metric, measurements are taken at the Mauna Loa Observatory in Hawaii. It is a high altitude measurement in the well mixed troposphere in a pollution free environment.
• In September 2016 the level reached 400ppm, this was seen as key as it marks a point of no return in our lifetimes.
• The CO₂ metric is an important metric as it is directly related to the global temperature deviation, as one increases the other generally follows.
• In the past 10 years CO₂ levels in the atmosphere have increased by 35ppm, and given the status quo of 13% increases CO₂ emission each decade we are likely to reach levels of 450ppm by 2030.
• The high/low projections to 2040 simply follows this trend in CO₂ emissions, it is the nothing changes scenario.

References

Notes

Figure 5.1 shows the atmospheric CO₂ record based on ice core data before 1958, (Ethridge et. al., 1996; MacFarling Meure et al., 2006) and yearly averages of direct observations from Mauna Loa and the South Pole after and including 1958 the units are ppm (parts per million). It is difficult to miss the huge increase since the beginning of the industrial revolution. A clever Swede called Arrhenius predicated in 1896 that the globe may warm with CO₂ emissions however he thought it would take thousands of years to be a problem, since CO₂ emission were much lower back then.

Half Our CO₂ is reabsorbed - Figure 5.2

Analysing the persistence of atmospheric CO₂ is complex, there is a time lag between emissions and temperature increase, and there is a natural half life of CO₂ residence in the atmosphere. In Figure 5.2 we have plotted actual atmospheric CO₂ levels, against a calculation* for CO₂ levels where no emissions are reabsorbed by the planet, and against a case where they are all reabsorbed. With reference to the data showing temperature increase from 1980 where we see greater than 18Gt CO₂ emissions, the data implies that the planet can take up to 18Gt or so of CO₂ a year from human activity, but that anything above this will raise the temperature. However even this CO₂ uptake is greater than the planet can handle without consequences for the land and the oceans for example ocean acidifications and the effect that has on sea life. It is not as simple as this but it serves as an approximation.