2.7 What do all of these different measures and units mean?

2.7.1 Measuring greenhouse gases

Carbon of CO2? It is increasingly common to refer to CO2 rather than carbon. Look out for this, because one molecule of carbon is equivalent to 3.67 molecules of CO2; and one molecule of CO2 is equivalent to 0.27 molecules of carbon. Or if you were to place monetary value on CO2, for example, then £100/t of carbon would be equivalent to £27/t of CO2. It might be easier to persuade people to pay £27/t of CO2 rather than £100/t of carbon – even though they are exactly the same!

CO2 or CO2-equivalent (CO2-eq)? CO2-eq is a way to express the sum of the radiative forcing factors arising from the composition of the atmosphere over a given period of time (e.g. 100 or 200 years) as the equivalent amount of CO2. This is complicated by a variety of factors, e.g. different greenhouses gases or aerosols have different residence times in the atmosphere, some have a positive effect and others are negative. Another way to express this would be as the total radiative forcing (which is what the IPCC tend to use), but CO2-eq has become established in policy debates. Look out for this, because some reports cite CO2 (which is strictly only CO2), whereas others use CO2-eq. CO2-eq should express the sum of all forcings (because this is what matters for climate change), but sometimes it is used to refer to the forcings resulting only from the six gases regulated by the Kyoto Protocol.

Definitions and terminology matter here, because the numbers are used to inform policy responses (e.g. emission reduction targets). But too often authors are sloppy and use carbon and CO2 interchangeably, use CO2 sometimes to refer to CO2 only and sometimes as shorthand for all greenhouse gases – leading to all sorts of confusion. It makes a difference – and sometimes the slip conveniently understates the scale of the challenge.

Even then, converting between CO2 and CO2-eq is not straightforward, because CO2-eq is a way of grouping all greenhouse gas forcings and the combined effects are non-linear (they are related by a logarithmic function). The following figures from the UK Climate Change Committee (CCC; 2008, Chapter 1) may be a useful guide:

  • Between 350 and 485 CO2, add 95 ppm for CO2-eq

Thereafter, the relationship is non-linear, so:

Having said all of that, if we use CO2-eq as shorthand for the net enhanced greenhouse effect, which includes both positive and negative forcing factors, then these factors more or less cancel each other out so that current atmospheric CO2 levels are close to CO2-eq.

  • RealClimate explains CO2-eq.

2.7.2 Volume and mass measures

Volume measures. We are familiar with contents listed as percentages (or ‘parts per hundred’) – such as the ingredients list for some foodstuffs, but greenhouse gases occur in tiny amounts in the atmosphere – much less than 1%. So scientists express these tiny amounts as ‘parts per million’ (ppm – 10 000 ppm is 10 000/1 000 000 or 1%). The current concentration of CO2 in the atmosphere is about 380 ppm, compared with the pre-industrial level of 278 ppm.

Mass measures. You'll come across a variety of measures, some familiar (tonnes and grams) – but sometimes prefixed by unfamiliar terms (like ‘mega’, giga and so on). Scientists use a standard system called the SI system. Here's what the prefixes mean:

  • mega – a multiple of 1 000 000 (106, abbreviated to ‘M’)

  • giga – a multiple of 1 000 000 000 (109, abbreviated to ‘G’)

  • tera – a multiple of 1 000 000 000 000 (1012, abbreviated to ‘T’)

  • peta – a multiple of 1 000 000 000 000 000 (1015, abbreviated to ‘P’).

  • 1 tonne (1 t) is 1 000 000 or 106 g

  • 1 million tonnes is sometimes called a megatonne (Mt)

  • 1 billion tonnes is sometimes called a gigatonne (Gt).

Wikipedia has an entry on the SI system of units.

2.6 Does increasing atmospheric CO2 have any other effects?

3 Anthropogenic causes of global warming