The Paris Agreement within the UN Framework Convention on Climate Change to keep global warming within 1.5–2°С has been effective since November 4, 2016, and will come into force in 2020. According to the calculations of the Intergovernmental Panel on Climate Change, to achieve the above goal, it is necessary to reduce atmospheric greenhouse gas emissions by three times, to the level of 2019, when they amounted to 33.3 Gt, which will allow limiting the temperature rise to 1.5°C by 2050 or to 2°C by 2070. According to the International Renewable Energy Agency, this can be achieved only if by 2040–2050 the so-called Great Energy Transition takes place—a transition from the use of currently dominant fossil hydrocarbon fuels (coal, oil, and natural gas) to renewable energy sources, when their share in the total energy balance reaches 40% or more. Mathematical modeling is shown to help attain the Great Energy Transition and, hence, the goals of the Paris Agreement. However, to create a stable and sustainable energy system of the future, it is necessary not only to expand renewable energy sources rapidly but also to develop nuclear power as a low-carbon source of baseload electricity generation. Only the symbiosis of renewable energy sources and nuclear power plants with highly secure small and medium reactors of the latest generation will make it possible to oust and replace hydrocarbon fuels and solve the problem of climate security without prejudice against economic development.