I wonder what kind of impact, CO2 or otherwise, do thermal updraft towers have. It's a different kind of solar power that doesn't use photovoltaics - a "solar wind turbine", if you will -, so the production-time impact is limited to the construction cost of the tower and the collector cover, none of which would use particularly special materials. And it can be installed in high-heat arid areas where few plants can grow, limiting operating-time CO2 impact. But what environmental effects they have is a good question.