Trying to reconcile cost shifting with the discounting of future climate change costs and benefits has taken me on some unexpected detours. I was initially thinking about bills of exchange and their role in the early modern era of concealing church-outlawed "usury" in the guise of a more palatable commercial transaction. Discounting was an arithmetical accounting exercise that arose out of the discounting of bills of exchange.
Both compound interest and discounting partake of the same exponential function -- from different ends of the calculation -- so it is easy (and misleading) to think of the discounting of a bill of exchange as a kind of loan. Discounting a bill of exchange is a sales transaction. The credit involved is commercial credit extended from a supplier to a purchaser. The bank then buys the bill of exchange from the supplier at a discount from its face value.
If one insists on seeing a loan from the banker in the transaction, it would only be an indirect loan to the purchaser of the goods, not to the supplier who sold the bill of exchange to the bank. But that loan would be secured by the goods that were the original object of the transaction that originated the bill of exchange... (Unless, that is, the bill of exchange was only speculative, a circumstance that Marx labeled a swindle.)
The important point is that bills of exchange originated in real transactions of goods, not in purely financial transactions. This has serious implications for the use of "discounting" in cost benefit analysis of public investments.
If the discount rate is meant as a metaphor it is a peculiarly bad one. The goods in question -- costs and benefits of climate change mitigation, for example -- have both negative and positive values but more importantly they have not been contracted for by the interested parties -- there is no "bill of exchange" to be discounted. Furthermore, the beneficiary of the discounted price is not society but the polluting firm who has shifted part of its costs to society and the environment. This perverse distribution of costs and benefits (and incentives) is concealed by the aggregate generality of the climate economy models that construe everything as one big happy economy.
Put it this way: discounting the future costs and benefits of greenhouse gas emissions provides a subsidy to the most prolific emitters of greenhouse gases that they can then reinvest at compound interest. This is hardly a matter of being "neutral" on questions of distribution. Nor is it a question of generational equity. This is simply taking the bankers' perspective on financial accumulation and proclaiming it "socially optimal."