Seeking solutions as pandemic race for space exacerbates a climate-unfriendly trend. You might think that cramped and crowded cities, dense with roads and highrise towers, are bad for the environment and bad for addressing climate change. But it is a long-established principle of environmental economics that while the land beneath urban cores has been largely stolen from nature, cities provide ecological benefits. Packing people all into one place, called “densification,” makes carbon-friendly public transit work. It also allows us to concentrate services such as sewage treatment and energy systems. Perhaps most important, environmentalists have hoped that concentrating people into dynamic cities such as London, New York, Montreal and Vancouver would take the pressure off surrounding green spaces that are so essential for keeping the environment healthy. But there are increasing signs that those hopes have faded and that COVID-19 has just made things worse. Climate scientists say it is time to take that into account.