According to dr. jameel ahmad, chairman of the civil engineering department at new york's cooper union, things will begin to fall apart during the first month of march after humans vacate manhattan. each march, temperatures normally flutter back and forth around 32°f as many as 40 times (presumably, climate change could push this back to february). whenever it is, the repeated freezing and thawing make asphalt and cement split. when snow thaws, water seeps into these fresh cracks. when it freezes, the water expands, and cracks widen. .

as pavement separates, weeds like mustard, shamrock, and goosegrass blow in from central park and work their way down the new cracks, which widen further. in the current world, before they get too far, city maintenance usually shows up, kills the weeds, and fills the fissures. but in the post-people world, there's no one left to continually patch new york. the weeds are followed by the city's most prolific exotic species, the chinese ailanthus tree. even with 8 million people around, ailanthus — otherwise innocently known as the tree-of-heaven — are implacable invaders capable of rooting in tiny chinks in subway tunnels, unnoticed until their spreading leaf canopies start poking from sidewalk grates. with no one to yank their seedlings, within five years powerful ailanthus roots are heaving up sidewalks and wreaking havoc in sewers — which are already stressed by all the plastic bags and old newspaper mush that no one is clearing away. as soil long trapped beneath pavement gets exposed to sun and rain, other species jump in, and soon leaf litter adds to the rising piles of debris clogging the sewer grates.

what claim does weisman make in "the city without us"? how does the text structure of this excerpt support weisman's claim? be sure to refer to examples from the text to support your analysis.