Hof "et al." (2008) (referred to by World Bank, 2010:8) examined the sensitivity of the optimal climate target to assumptions about the time horizon, climate sensitivity, mitigation costs, likely damages, and discount rates. The optimal target was defined as the concentration that would result in the lowest reduction in the present value (i.e., discounted) of global consumption. A set of assumptions that included a relatively high climate sensitivity (i.e., a relatively large global temperature increase for a given increase in GHGs), high damages, a long time horizon, low discount rates (i.e., future consumption is valued relatively highly), and low mitigation costs, produced an optimum peak in the concentration of CO2e at 540 parts per million (ppm). Another set of assumptions that assumed a lower climate sensitivity (lower global temperature increase), lower damages, a shorter time horizon, and a higher discount rate (present consumption is valued relatively more highly), produced an optimum peaking at 750 ppm.