This paper presents a simple method for incorporating the time required for new technology to penetrate the market and subsequently substitute for an old one when evaluating the ability of new energy technology to impact global climate change, The methodology is applied to the two largess sources of energy-related carbon dioxide: electricity generation and motor vehicles. Carbon-free road transportation is hypothesized to substitute for petroleum-fueled vehicles and carbon-free electric power generation for fossil-fueled electricity based on empirical analogs for substitution dynamics parameters, beginning in the year 2000. The examples imply that near-term significant reductions to 1990 carbon emissions levels via technology substitution are unlikely. The time scale relevant for realizing reductions in carbon emissions is several times the expected lifetime of the products that new technology is intended to replace.