So the steel sector emits around 3.7 gigatonnes (Gt) of carbon dioxide (CO₂) a year (see ‘6%, 7%, 8%, 9%, 10%, 11%…’), and perhaps another 18.3 million tonnes (Mt) of methane (see ‘Steel’s methane footprint’).

But the GHG Protocol lists five more greenhouse gases or groups of gases: nitrous oxide (N₂O), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulphur hexafluoride (SF₆) and nitrogen trifluoride (NF₃). What about these?

In the case of steel, thankfully, only CO₂, methane, and nitrous oxide are considered significant¹. Let’s look quickly at steel’s nitrous oxide emissions, and then try to combine these three greenhouse gas footprints into a single, overall footprint.

The steel sector’s main association with nitrous oxide is through the combustion of fossil fuels, collectively responsible for around 1.1 million tonnes (Mt) or 10% of annual human-caused nitrous oxide emissions².  As a back of an envelope approximation, suppose that steelmaking is associated with about 5% of those fossil fuel emissions, mainly through its use of coal and natural gas. That would make it responsible for around 0.055 Mt of nitrous oxide emissions annually.

So we have three separate greenhouse gas footprints for steel: one for CO₂, one for methane and one for nitrous oxide. But these three gases are not directly comparable in terms of their impacts on the climate. Methane is a much more powerful greenhouse gas than CO₂, but it also breaks down faster through natural processes in the atmosphere and soil. Nitrous oxide is even more powerful than methane, and is also relatively long-lasting – but there’s a lot less of it. How can we combine steel’s CO₂, methane and nitrous oxide footprints, into a single ‘greenhouse gas (GHG) footprint’?

The standard way to do this is to compare the effects of different gases in warming the atmosphere, averaged over 100 years, and using CO₂ as a benchmark – the so called ‘global warming potential’ (GWP). Over 100 years one tonne of methane is estimated to warm the atmosphere approximately 28 times as much as one tonne of CO₂, giving methane a GWP of 28³. On the same basis, the GWP of nitrous oxide is 265³. Because it is the benchmark, the GWP of CO₂ itself is, by definition, always 1. Applying these GWP values to the emisions of different greenhouse gases converts them into ‘CO₂ equivalent’ values – shortened to CO₂e or CO₂eq – which can then be combined to calculate an overall GHG footprint.

But there’s a catch. There’s no overwhelming reason to compare the impacts of emissions over 100 years. The time period doesn’t matter too much if two gases remain in the atmosphere for a similar amount of time, as happens to be the case for CO₂ and nitrous oxide. But where they persist for over 100 years, methane only remains in the atmosphere for around twelve years. If you compare the warming impacts of methane and CO₂ over 20 years, rather than 100, you get a ’20-year GWP’, or ‘GWP(20)’, of 84 rather than 28. And that makes a big difference to the value of the methane footprint.

CO2	Methane – CH4			Nitrous oxide – N2O			Steel’s GHG footprint
Gt	Gt	Gt CO2eq (GWP100)	Gt CO2eq (GWP20)	Mt*	Gt CO2eq (GWP100)	Gt CO2eq (GWP20)	Gt CO2eq (GWP100)	Gt CO2eq (GWP20)
3.654	0.0183	0.512	1.537	0.055	0.015	0.015	4.2 (+14.4%)	5.2  (+42.5%)

Table 1 compares steel’s GHG footprints using 100-year and 20-year GWPs. The 100-year and 20-year GWPs for nitrous oxide are the same, and although the warming potential of nitrous oxide is 273 times as high as for CO₂ the total volume of gas emitted is small, and those emissions are equivalent to only 15 Mt of CO₂⁴. Not nothing, but less than 0.5% when compared with the sector’s actual CO₂ emissions.

But including methane makes a huge difference. On a 100-year basis methane emissions add more than 500 Mt of CO₂eq to the steel sector’s GHG footprint – more than the annual emissions of all aviation worldwide. Measured over 20 years, it adds the equivalent of 1,500 Mt of CO₂ – the annual CO₂ emissions of aviation, and then on top of that the CO₂ emissions of France, Germany and the UK combined.

When methane and nitrous oxide are included, the true GHG footprint for the steel sector is around 14.4% higher than when only CO₂ is considered, using a 100-year GWP. On a 20-year basis, it would be a staggering 42.5% higher.

But which is right? And what is special about either 20-year GWPs or 100-years GWPs?

Trying to answer these questions is a major can of worms, and I’ll leave that for another post (or two). The bottom line – if you want to understand the steel sector’s impact on the climate, you have to include methane.

1. Greenhouse Gas Protocol Initiative guidance. Calculating Greenhouse Gas Emissions from Iron and Steel Production ↩︎
2. Global Nitrous Oxide Assessment ↩︎
3. Published GWP values vary (e.g. see this table published by the GHG Protocol), for a range of reasons which I will cover in another post. For this post I have used the values given in the Intergovernmental Panel on Climate Change (IPPC) Fifth Assessment Report (AR5). For methane I have used an average value, rather than a value for fossil methane (which is slightly higher), although the fossil methane value would be justifiable. ↩︎
4. 1.1 Mt of N₂0, with GWP of 273 = 300 Mt CO₂e.  5% of 300 Mt = 15 Mt. 15 Mt out of 3.7 Gt = 0.41% ↩︎