One way to state this idea is that when comparing between options A and B we need to produce an optimal estimate of utility(A) - utility(B) rather than an optimal estimate of each separately.
However, the examples you use are somewhat unrealistic in the following sense. When comparing e.g. two options to reduce carbon emissions, we usually also need to take into account the difference in cost, not only the difference in impact on carbon emissions, because the residual money can be invested in something else. This means there is no avoiding estimating the utility of reducing carbon emissions, even if there’s large error involved. Still it makes sense to be consistent.
One way to state this idea is that when comparing between options A and B we need to produce an optimal estimate of utility(A) - utility(B) rather than an optimal estimate of each separately. However, the examples you use are somewhat unrealistic in the following sense. When comparing e.g. two options to reduce carbon emissions, we usually also need to take into account the difference in cost, not only the difference in impact on carbon emissions, because the residual money can be invested in something else. This means there is no avoiding estimating the utility of reducing carbon emissions, even if there’s large error involved. Still it makes sense to be consistent.