By 2050 or so, the human population is expected to reach nine billion, essentially adding two Chinas to the number of people alive today. Those billions will be seeking food, water and other resources on a planet where, scientists say, humans are already shaping climate and the web of life. In Dot Earth, which recently moved from the news side of The Times to the Opinion section, Andrew C. Revkin examines efforts to balance human affairs with the planet’s limits. Conceived in part with support from a John Simon Guggenheim Fellowship, Dot Earth tracks relevant developments from suburbia to Siberia. The blog is an interactive exploration of trends and ideas with readers and experts.