Along the efficient emission path calculated by Nordhaus and Boyer (2000) (referred to by Fisher "et al."., 2007), the long-run global average temperature after 500 years increases by 6.2 °C above the 1900 level. Nordhaus and Boyer (2000) stated their concern over the potentially large and uncertain impacts of such a large environmental change. The projected temperature in this IAM, like any other, is subject to scientific uncertainty (e.g., the relationship between concentrations of GHGs and global mean temperature, which is called the climate sensitivity). Projections of future atmospheric concentrations based on emission pathways are also affected by scientific uncertainties, e.g., over how carbon sinks, such as forests, will be affected by future climate change. Klein "et al." (2007) concluded that there were few high quality studies in this area, and placed low confidence in the results of cost–benefit analysis.