US reference images
The purpose of the images:

Action on greenhouse gas emissions would be easier if we could see what we were talking about. Not only is carbon dioxide invisible, it is measured in units of mass. Intuitively we make sense of mass in terms of weight and because gases are buoyant this gives us another level of abstraction to deal with.

The simple expedient of illustrating quantities of gas in terms of the space they occupy gives viewers a direct and concrete way to make emissions, savings and targets meaningful to themselves. When viewed this way, audiences who would switch off as soon as soon as numbers get mentioned can engage intelligently quantitative arguments.
When the audience is already engaged with the numbers (e.g. an audience made up of professionals tracking emissions and aiming for targets) then regular charts and tables will be more direct. For other audiences however, physical representations can be a ‘way in’. In short, if the audience is coming to the data to answer their own questions, go for regular charts and tables. Conversely, if you are taking the data to the audience and asking them to engage with it then physical representations like these are more effective.

In summary, the purpose of these images is to:
• Make carbon dioxide ‘real’ (physical) and not just a number
• Provide a physical sense of scale
• Allow people without strong numeracy to engage with quantitative arguments
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