The question of how much reasonable carbon tax should be set in each nation is the biggest problem that cannot be calculated in climate change. In previous study, we have proposed environmental thermoeconomics. From thermodynamics, the minimum electricity required to remove carbon 1 ton from the air is calculated as minimum 424 MJ/tonCO2. The medium that connects nature and human is a power plant, and the unit price of industrial electricity in Korea is about $0.066 / kWh. Multiplying the above two values, the cost required to remove carbon 1 ton from the air is calculated as minimum $7.77. In Korea, 80% of carbon emissions are free, and carbon credits are set for 20% of excess. Therefore, the engineering price of carbon 1 ton for 20% excess is calculated as minimum $38.9. Currently, the trading price of Korean carbon market is about $16.5 ~ $33.0. Judging from the uncertainty of climate change, it can be said that the above calculated unit price and trading unit price are similar. Carbon credits are not a means of profit creation through trade, and it is the cost of human obligations through engineering calculation.