"Of the many moral conundrums presented by climate change, the issue's intergenerational nature is perhaps trickiest to sort out. It boils down to this: We benefit mightily from burning cheap coal and will shoulder most of the expense associated with switching the global economy to low-carbon fuel sources. But our grandchildren and great-grandchildren will pay the price for our profligate energy ways and will reap the majority of the benefit of our shift to cleaner-burning fuels."

Source: "Ethical Dilemma Profoundly Sways Economics of Climate Change", Climate Central, Jan 27, 2011.

http://www.climatecentral.org/news/ethical-dilemma-profoundly-sways-economics-of-climate-change/

This article explains how and why economists use a disocunt rate to determine the cost of carbon. It also highlights the different schools of thought on what discount rate should be used.