A common practice in remote sensing studies of the aerosol indirect effect is to consider the correlation between the droplet effective radius and the aerosol index. However, this correlation is not unique as the relationship depends also on the liquid water path. Moreover, it differs for oceanic and continental clouds because clouds over land generally have higher cloud bases, are thinner and have a higher average cloud droplet number concentration for a given aerosol index than marine clouds. Only when using aerosol number concentration at cloud base instead of aerosol index and using an effective radius representative for the whole cloud rather than just at cloud top, do the defined susceptibilities of oceanic and continental clouds obtained from ECHAM4 climate model simulations agree within the standard errors. This has important consequences for properly interpreting the magnitude of the anthropogenic indirect aerosol effect on climate.