The One Planet radio programme on the BBC's World Service came to Carbon Visuals with simple question: "what does our carbon footprint look like?" Hear the show (first broadcast on Thursday 8 April 2010 at 09:30 GMT / 10:30 BST) by visiting the One Planet website.

One Planet carefully documents the impact the making of the programme has on the atmosphere. It turns out that their carbon footprint is about 34.9 tonnes per year. But what does that mean? Measuring carbon dioxide in terms of mass (tonnes) makes sense from an accounting point of view but doesn't give us a sense of scale we can relate to. We at Carbon Visuals came up with a variety of alternative ways of representing One Planet's footprint. Some use our on-line tools: the Carbon Widget, which allows anyone to place a beautiful visualisation of carbon dioxide in their own web page and the Carbon Quilt, which offers a range of beautiful and informative visualisations.

Most people think of mass in terms of weight - the force with which heavy things push downwards - but that doesn't really work with gases. Gases are buoyant - they float supported by the air itself - so it doesn't 'feel' like they push down at all. For an intuitive sense of scale we at Carbon Visuals generally prefer to represent quantities of gas in terms of volume - the space gas occupies. But there are different ways of representing volumes. The one to choose depends on the story you want to tell and who you want to tell it to. The aim is to help your viewers to make the quantity meaningful for themselves, which means giving them something they can relate to personally. 

A simple way to represent a volume is with a sphere. This is what 34.9 tonnes of carbon dioxide gas looks like as a sphere:

BBC One Planet's annual emissions as a sphere (actual volume of gas)

Spheres are quite good for representing single quantities but less good for comparisons because comparing volumes by eye is something few of us are any good at - even with a handy Routemaster bus to provide a sense of scale. On the other hand, we are much better at 'counting' by eye, so it is often good to break a volume up into standard sized units.

BBC One Planet's annual emissions as a pile of one-tonne cubes (actual volume of gas)

In the picture above, One Planet's carbon footprint is represented as a pile of one-tonne cubes. It turns out that one tonne of carbon dioxide would fill a cube 8.12 metres high. But comparison is even easier if, as well as using standard sized units we vary the size of the volume in just one dimension. We could, for instance, easily compare the carbon footprint of two shows by representing them as different sized 'towers' with equal base areas. In the picture above, One Planet's footprint is represented as a tower of 10 tonne cubes. 10 tonnes of carbon dioxide would fill a cube 17.5 metres high.

BBC One Planet's annual emissions as a tower of 10-tonne cubes (actual volume of gas)

Giving the viewer a way to relate to the volume is very important. Despite being an internationally established standard of comparison, a Routemaster bus isn't quite up to the task. We need something of comprable size that viewers have seen before. Preferably it will be something in viewers' own environment that they can relate to on a physical level because they know what it's like to share space with such an object. For Londoners, Nelson's Column works well in this instance:

BBC One Planet's annual emissions as a tower of 10-tonne cubes (actual volume of gas)

Nelson's Column is well known around the world, so it can also help people who haven't been to London to relate to the volume. But there are still many options. Choosing the right one depends on the context of the visualisation. For some audiences, it's most useful to keep things as simple as possible, as above.  But we could also choose to show more of the surrounding area:

BBC One Planet's annual emissions as a tower of 10-tonne cubes (actual volume of gas)

Or we could embed the volume in an actual photograph, which gives many more ways for viewers to put themselves in the space and relate to the volume:

BBC One Planet's annual emissions (actual volume of gas). The photograph we used was by Christian Bortes. 

In many instances, an annual footprint is just to big to provide insight. Breaking it up into smaller units can help. One Planet for instance could visualise their footprint show by show with the Carbon Widget. The widget makes visualisation easy - all you have to do put a number in, give the picture a title and description and the widget generates a few lines of code that you can copy and paste into your webpage.

We decided to break One Planet's footprint into units of time. If the programme is responsible for 34.9 tonnes of carbon dioxide a year, this is equivalent to an average of 96 kg every day:

BBC One Planet's daily emissions (actual volume of gas)

We can break this down even more and ask how much does BBC One Planet emit every second?

Emissions from BBC One Planet every second! (Actual volume of gas)

It turns out that One Planet accounts for just over one-billionth of the World's total annual emissions of carbon dioxide. As a patch of the 'Carbon Quilt' the One Planet footprint looks like this:

Screengrab from the Carbon Quilt site 

So the simple question, 'what does our carbon footprint look like?' has many possible answers. Investigate more carbon footprints and even see what your own looks like on the Carbon Quilt site: www.carbonquilt.org.