There are many different greenhouse gases (for a primer on what a greenhouse gas is, see my earlier post), some of which last longer in the atmosphere than others, and each of which has a different heat-trapping capacity. So how do we know where to focus our time and attention? For example, suppose we run an industrial process that creates 1000 tonnes of carbon dioxide each year, but only half a tonne of sulphur hexafluoride (SF6); what should we prioritise in order to have the biggest impact on our carbon footprint?
This is where the concept of Global Warming Potentials (GWPs) comes in. A GWP value indicates how much heat the gas traps compared to a standard unit of carbon dioxide, over a defined period of time (usually 20, 50 or 100 years – a GWP20, GWP50 or GWP100). Think of a GWP like a currency exchange rate; just as every currency has an ‘exchange rate’ against the US dollar, so does every gas have an exchange rate against CO2. A gas’s GWP is used to convert it into the common currency of CO2e – carbon dioxide equivalents – which allows us to directly compare the climate impact of different emission releases.
So in our simple example, it turns out that SF6 has a GWP100 of 22,800 (IPCC 4th Assessment Report). In other words, it’s an incredibly potent greenhouse gas, with an impact on our climate almost twenty three thousand times higher than an equivalent mass of carbon dioxide!
In our example, therefore, the half tonne of SF6 equates to (0.5 tonnes * 22,800) = 11,400 tonnes CO2e, whereas the 1000 tonnes of CO2 remains 1000 tonnes. So, despite initial appearances, tackling the small mass of SF6 released will make a far more significant contribution to reducing our carbon footprint than would tackling the larger release of CO2.
Ok, so now we have a basic understanding of what a GWP is, and how we can use them to direct our resources and effort as we seek to mitigate climate change. But wait, didn’t I just say there are different GWPs – GWP20, GWP50, GWP100, etc – for each gas? I did. And I’ll talk about why that matters in my next blog post!