to be sure, effective green infrastructure is more complex than simply planting more trees as trees have ecosystem disservices as well: roots can interfere with sewer pipes, tree pollen can exacerbate allergies, and branches can fall over in storms. but even if no one ends up volunteering to water trees or weed tree pits, subway announcements can still bring awareness of and appreciation for city trees to show how the urban environment is not just buildings and people but a multidimensional ecosystem.

today the subway serves 4.5 million people daily and has a 24-hour operating schedule, making it the only rapid transit system in the world that always runs in its entirety — fitting for the city that never sleeps. even as the largest underground transit system in the u.s., there are still proposals and works in progress to expand the reach of the subway to make it more convenient for commuters.

unfortunately, the outlook is not bright for underground trains in coastal cities — especially in the big apple, which is far enough below sea level that it is already threatened whenever a tropical storm happens to travel north. as global warming causes an increase in sea level, the city will face a higher threat of flooding each year. even as recently as 2012, the subway system was devastated by hurricane sandy resulting in $5 billion worth of damage.

if the sea level should rise by a meter or more over the next century, new york city may find itself underwater with a waterlogged and salt corroded transit system after every storm. of course, this is a problem for every coastal city as the u.s. falls behind in innovations to combat rising sea levels. hopefully, progress will catch up with the problem and the many subway-dependent commuters will still have their trains for many years to come.

(image at top: the city hall subway station in the early 1900s. source: library of congress)