| Climate change, deduced by massive greenhouse gas emission, has brought unpredictable risks on natural ecosystem and socio-economic systems by means of global warming, sea-level rise and the change of precipitation. In this sense, it arouses extensive attention; and most governments have been actively negotiating on how to alleviate it.However, it is crucial to identify the uninfluenced trend of carbon emission for governments and policy makers on proposing their abatement targets. While after the targets have been set, the main concern would be choosing the optimal path of economic growth and abatement under the proposed targets which would maximize the social welfare.Therefore, this paper firstly studied the optimal economic growth and the according carbon emission trend for China under no abatement constraint situation. By improving the model of Moon and Sonn (1996), the nessesary condition for the existence of EKC relationship between optimal economic growth rate and energy intensity was deduced, which is that the output elasticity of energy be less than that of capital. Besides, the influence of industrial structure on the descend rate of energy intensity and energy structure on carbon emission have also been taken into account.The projection results showed that the optimal economic growth rate would drop gradually, but the GDP of 2050 will still enlarge 8-fold than that of 2010. Energy consumption and carbon emission would demonstrate an invert-U or bell shaped trend, with the peak years are 2032 and 2031, and peak value are 3255Mtoe and 2636.6MtC respectively. Sensitivity analysis of energy import price, renewable energy policy and energy intensity dropping rate showed that the former two factors could dramatically reduce the emission volumn and the last factor could both reduce the emission and bring the peak year in advance.Succeedingly, aimed at the abament target proposed by China, which is to reduce 40-45 percent of carbon intensity by 2020 compared to the level of 2005, we constructed an optimal economic growth model under the constraint of carbon intensity target. Simulation results showed that the obtainable social welfare under consumption-output ratio (COR) being 55%is larger than that under other COR scenarios. The average optimal economic growth rate under this optimal scenario is 6%, lower than that under the baseline scenario where the COR is 49%. Constrainted by the abatement target, R&D intensity of output showed a former-lower-latter-higher feature, and then a remarkable increase would show up after 2014, carbon intensity would drop accordingly. Besides, energy consumption and carbon emission in optimal scenario were much lower than that in baseline scenario. Sensitivity analyses showed that the increase of R&D investment potential would aggravate the former-lower-latter-higher trend.Furthermore, aimed at the frequently discussed quota allocation scheme, we constructed an optimal economic growth model under the constraint of accumulated emission target. The simulation results showed that the obtainable social welfare was largest with COR is 45%, which suggests that a relative lower COR would be more optimal compared with the former shorter-planning-period model. In this simulation, the averaged economic growth rate in optimal scenario was 8.8%, higher than 7.9% in baseline scenario. The R&D intensity of output showed a drop-rise-drop feature, and accordingly, energy consumption and carbon emission would rise rapidly in the first few years, and then begin to drop in the later period. Carbon intensity undertook a slow rise path in the first few years and would drop dramatically afterwards.Lastly, on the basis of the former three models, this paper developed a simulation system to assist the policy makers to be more professional in doing simulation researches.
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