Sir

Your correspondent Geoffrey Hammond is being somewhat pedantic in calling for the 'carbon footprint' — amounts of carbon expressed in tonnes — to be called instead the 'carbon weight', because the essential property of a footprint is its area (“Time to give due weight to the 'carbon footprint' issue” Nature 445, 256; doi:10.1038/445256b 2007).

A true pedant would, however, have gone the full distance! If units of carbon are expressed in kilograms, tonnes and so on, the proper term is 'carbon mass'. Weight is the vertical force acting on a body as a result of gravity. The SI unit of weight is a newton, which has base dimensions of kg m s−2. Balances measure weight, but are calibrated to give the answer in units of mass.

'Footprint' has become an accepted term throughout the ecological and environmental sciences, irrespective of dimensions and units, such as my domestic carbon footprint (tonnes) or my flux-tower carbon footprint (metre radius or hectares). 'Footprint' is evocative and has meaning, with relevant and appropriate units.

A far more serious issue is that journalists and commentators frequently confuse the mass of carbon and the mass of CO2, sometimes within the same article. The mass of a mole of CO2 is almost 4 times (44/12) larger than the mass of a mole of carbon. With errors of this size in articles and reports, it is not surprising that the public at large is confused by apparently conflicting reports on atmospheric CO2 concentrations and terrestrial carbon budgets. Absolute clarity and consistency in definition is essential.