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Abstract: This paper assesses the global emission reduction target of halving CO2 emissions by 2050. First, future GDPs with specific uncertainty ranges were developed. The Kaya identity and the developed GDP outlook indicate that halving global CO2 emissions by 2050, which corresponds to almost 450 ppm-CO2eq. stabilization, requires an improvement approximately four times as large as the historical CO2 intensity improvement rate on average up to 2050, if GDP loss should remain within a few percent of potential baseline GDP. In addition, the global energy-related CO2 emission reductions were assessed by using an energy systems model. Marginal abatement cost of CO2 is over 470 $/tCO2 for halving global CO2 emissions by 2050 relative to 2005 even under the lower GDP scenario and technology improvements. Great challenges will have to be met for achieving the 450 ppm-CO2eq. Realistic alternative emission reduction scenarios should be explored in two ways; 1) more innovative technological development than any present imaginable development, and/or drastic social innovations are needed for cheaper carbon costs, e.g., within a few tens of US$ per tonne of CO2, and 2) more modest emission reduction targets, e.g., 550 ppm-CO2eq., and adaptation measures are considered. [Copyright &y& Elsevier]