Kartik Chandran, an environmental engineer at Columbia, will discuss some of his urban wastewater treatment projects at a panel discussion Friday following the screening of a new film about Rio de Janeiro’s Guanabara Bay.

This is the 100th blog I’ve written for the State of the Planet. It seemed like a good occasion to take a look at my five most popular blogs to see what has changed in the years since they were written. Is the news better or worse for seawater greenhouses, plastic pollution, turning wastewater into drinking water, coral reefs and rare earth metals?