Revised version of paper uploaded, making clearer the range of options for metric values and that GTPs are not in any way better than GWPs. Also addressed and accepted all other comments. Only comment left un-addressed is question about whether milk solids are fat corrected. Stewart says no, but this would make a negligible difference. So we proposed to leave as is. Will send to MPI once I have your approval to submit. Will need to double check the manuscript length now still meets journal requirements!