In environmental economics, the marginal external cost of emitting a pollutant determines the optimal abatement policy, which might take the form of an emissions tax. But the marginal external cost is often difficult to estimate. This is especially the case when it comes to climate change; estimates of the social cost of carbon (SCC) range from around $10 per metric ton to well over $200 per metric ton, and there has been little or no movement toward a consensus number. Partly as a result, rather than an SCC‐based carbon tax, climate policy has focused on a set of targets that would put limits on temperature increases or atmospheric CO2 concentrations, which in turn imply targets for emission reductions. Economics, however, can tell us little about whether such targets are socially optimal. I discuss the trade‐off between taxes versus targets as the focus of policy, explain why it has been so difficult to estimate a marginal SCC, and suggest an approach to estimating an average SCC through the use of expert elicitation. I argue that such an approach could serve as the basis for a harmonized carbon tax.