A methodology that gives the thermodynamic limits to the gross and the net environmental thermal impacts of the solar synthesis of a fuel is proposed. This methodology is based on the concept of "gross environmental thermal equivalent of a fuel", which is defined as the minimum amount of heat interchanged with the environment needed to reversibly synthesize one mole of the pure fuel, at environmental temperature and pressure, from reactants and with the other products contained in the environment, when this one and the Sun are the only energy sources. The "net environmental thermal equivalent of a fuel" takes into account that the solar energy used to synthesize the fuel does nor heat the environment. Thermodynamic algorithms depends only on Sun radiation equivalent temperature, thermal equations of state of environment and pure fuel, and standard equilibrium constant of formation for each compound taking part in the chemical reaction. Numerical and graphical results are given for the synthesis of graphite, carbon monoxide, methane, ethane, propane, methanol, ethanol, and dihydrogen, all of them synthesized from atmospheric carbon dioxide and water vapour. These algorithms do not depend on any particular technology, and can be considered as an objective reference for selecting reactions, sites, and operational strategies for synthesizing solar fuels. Results and conclusions of this paper can also provide a basis for ecological fiscal taxes of the use of fossil fuels. The results of applying this methodology give an objective criterium to valorize economically thermal and chemical replenishment cost of the environment, related with cost of solar technologies.