You may have heard that the planet is committed to further warming and sea level rise, irrespective of what choices we now make to reduce carbon emissions. The global warming century trend that was observed from 1906 to 2005 was 0.74°C (with a 90% uncertainty range of 0.56°C to 0.92°C), with more warming occurring in the Northern over Southern Hemispheres, and more over land compared to oceans. Yet, based on our understanding of the climate impact of greenhouse gases (GHG) such as carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and other trace gases, we should have observed even more warming than this. Actually, when you put all the pieces together, the expectation is for much more warming.
But before I tackle the critical issue of just how much more warming is still in the pipeline (in another post), it is important to explain the concept of carbon dioxide equivalents (CO2-e). This term initially confuses a lot of people, but it’s not really that difficult to grasp once it’s been explained.
To start, you need to understand that from a global warming perspective, we are interested in the changes in GHGs – which causes an energy imbalance. The pre-industrial and current concentrations of well-mixed long-lived GHG are 278 parts per million (ppm) for CO2 (now 383), 700 ppb (pp billion) for CH4 (now 1,775), and 270 ppb for N2O (now 320). Most of the other trace greenhouse gases (there are plenty), such as chloroflourocarbons (CFCs) and sulphur hexafluoride (SF6), are almost exclusively a result of industrial activity. Now to quantify their relative contribution to global warming, we need to find a way of putting all of these individual gases (and other climate drivers) on an equal – or equivalent – footing. That’s where CO2-e comes in.
The IPCC has two ways of expressing CO2-e. The first is known as concentration equivalence, which has units of ppm CO2-e. This definition asks: for a given change of a climate forcing agent (such as a greenhouse gas or aerosol), what change in the concentration of CO2 would have been required to have the same effect as the additions of this forcing? ‘Effect’ is here defined in terms of radiative forcing (RF), which is (loosely) the change in the amount of incoming (to Earth) versus outgoing (to space) radiation/energy, measured in watts per square metre (w/m2). A positive RF warms, negative cools.
The other way of representing CO2-e is by emission equivalence, which has units of the mass of CO2-e per unit time. For instance, you could define the climate warming impact over a 100-year period of 1 million tonnes (Mt) of methane as being same as if you’d released 25 Mt of CO2. If you shortened the time period to 20 years, that 1 Mt of methane would be the same as 72 Mt of CO2. The difference between time periods is because methane is more powerful GHG than CO2, but it breaks down more rapidly. This expression is also known as the global warming potential of a GHG.