Extracting carbon dioxide from the air is possible. But at what cost?

IN MAY some 250 scientists and policy types from around the world convened in Gothenburg, Sweden, to discuss a dirty secret of the three-year-old Paris climate agreement. Virtually all simulations which chart paths toward meeting that compact's goal--to keep temperature rise "well below" 2°C relative to pre-industrial levels--assume not just a sharp reduction in actual emissions but also the removal of carbon dioxide from the atmosphere on a massive scale. One reason such "negative emissions" h...