The use of nuclear energy has no significant environmental costs and the possible costs of solar produced electricity are at least different from those the authors consider. We cover this point in the Web page on nuclear energy.
Besides my problems with the definitions, I doubt that environmental impact in any sense is likely to limit human population before other factors like food and a sense of crowdedness come into play. As I argue in the main page on sustainability, this would be at a population in excess of 15 billion, maybe much in excess of 15 billion.