With more than half the world’s population now living in cities, it seems safe to say that pavement is in, and in a big way. Ecologically speaking, that may be good news—at least in theory. Dense areas like Manhattan and Hong Kong can help contend with the perils of global warming through lower per-capita energy use as residents walk to work, take public transit, and live in apartments with shared walls. It’s even been pointed out that the entire world population could squeeze into Texas if developers filled the state with townhouses, creating a Lone Star megacity while leaving the rest of the earth to revert to wild land.

But before we pack our bags for Texas and climb aboard the hyperdensity train, there is something to consider. Cities may well be green machines when it comes to global warming and world ecology. But what about the local environment?

The virtues of compact living have been hailed as far back as Jane Jacobs, author of the classic The Death and Life of Great American Cities (1961). Dense cities, she argued, \”are natural generators of diversity and prolific incubators of new enterprises and ideas of all kinds.\”

Jacobs wrote before the emergence of the environmental movement, so there is no real concern in her book with the ecological impact of all these people crammed into one spot on the earth—whatever merits density might have for generating vibrant life among the masses. But a new generation of writers, taking their cue from Jacobs, has retooled her argument. As they see it, apart from its social and economic benefits, hyperdensity also contributes to the health of the environment.

The case for density’s environmental merits has been made most pointedly by David Owen, a staff writer for The New Yorker, in Green Metropolis: Why Living Smaller, Living Closer, and Driving Less Are the Keys to Sustainability (2009). Residing in rural Vermont might seem greener than life in gritty old New York City. But according to Owen, New York, and Manhattan in particular (where population density is over 69,000 people per square mile), has enormous energy and natural-resource efficiencies. At least as measured on a per-capita basis. \”Placing one and a half million people on a twenty-three-square-mile island,\” Owen writes, \”sharply reduces their opportunities to be wasteful, enables most of them to get by without owning cars, encourages them to keep their families small, and forces the majority to live in some of the most inherently energy-efficient residential structures in the world: apartment buildings.\” Move over Portland, Ore. New York City is the real green machine.

In his paean to urban life, Triumph of the City: How Our Greatest Invention Makes Us Richer, Smarter, Greener, Healthier, and Happier (2011), the economist Edward L. Glaeser extends the case for hyperdensity. Glaeser reports that he and the environmental economist Matthew E. Kahn compared households in census tracts with more than 10,000 people per square mile with those a tenth or less as dense. They found that households in the denser tracts used just 687 gallons of gasoline per year, while those in the thinly settled areas used almost twice as much. \”Residing in a forest might seem to be a good way of showing one’s love of nature,\” Glaeser writes, \”but living in a concrete jungle is actually far more ecologically friendly.\” The threat to world ecology posed by climate change and the clear benefits that city life affords with respect to per-capita carbon emissions cause Glaeser to conclude that \”there is nothing greener than blacktop.\”

The architect and urban planner Vishaan Chakrabarti, an associate professor at Columbia University, has recently added his support to this line of argument in A Country of Cities: A Manifesto for an Urban America (2013). \”If you love nature,\” he writes, \”don’t live in it.\” With less regulation and more market-driven urban growth, he maintains, the world can be a greener place. Think how hyperdensity in Texas could free up the rest of the planet for nature to take root again.

But wait a minute. Imagine seven billion people all flushing toilets in Texas and what that would do to the surrounding waters. The pitfalls of hyperdensity come into view. Cities are an ecological good—except when it comes to their own ecology.

Consider land-cover change in Greater New York. The region was once a watery world chiefly defined by the wetlands that sprawled across it. These magnificent tidal marshes served as nurseries for fish, filtering impurities from the water. But from the early 1800s to 1980, marshland equivalent in area to four times the size of Manhattan was obliterated around New York Harbor to make way for real-estate development, roads, parks, and landfills in which to deposit the solid waste produced by the booming population. In some parts of the harbor, the changes were phenomenal. Flushing Meadows was once really a meadow (more precisely, a tidal marsh), not a park and tennis center. Between 1900 and 1966, the marshland there declined from 2,409 acres to zero.

Dense New York may well be an exemplar to some planners of what environmentally minded cities can do. But its path to greenness was a bloody one. Fresh Kills, on Staten Island, is a case in point. It was once a low-lying tidal marsh of grass, birds, and fish. But after World War II, with the economy booming, this home to snapping turtles and great egrets experienced a makeover. In 1948, city garbage scows first headed to Fresh Kills to deposit their loads; over the course of the next four decades, tons of solid waste came streaming into the marsh, burying it under what became several towering mountains of garbage.

The radical decline in marshland had a profound effect on biodiversity. A recent review of 26 studies on species richness focused on terrestrial, freshwater, and marine habitats within 100 miles of midtown Manhattan. Seventeen of the studies uncovered declines in biota very likely related to anthropogenic factors like development and land-cover changes.

Nor is dense New York such a green dream when viewed from a biochemical standpoint. Nutrient pollution of the nearby waters has proved a troublesome and longstanding problem. If density is indeed the environmental benefit some have made it out to be, then Manhattan experienced ecological nirvana in 1910, when the borough’s population peaked at 2.3 million people. Was the concentration of so many people in one place an ecological virtue? Not from a water-quality perspective. All the human waste sent sloshing into New York Harbor produced a toxic stew of nutrients inhospitable to many life forms. Between 1900 and 1916, the dissolved-oxygen saturation of the harbor, a key indicator of its ecological health, declined from almost 100 percent to 47 percent. Even such figures, however, obscure the fact that some of the waters had become hypoxic, containing an oxygen saturation below 30 percent, a condition in which most fish cannot survive.

This early, \”brown\” history matters because while water quality in New York has improved, and Fresh Kills over the next 30 years will be turned into a city park, biological diversity has suffered. Indeed, levels of key nutrient pollutants in New York Harbor, even with the reforms of the Clean Water Act, are far above what they were in the early 20th century. As a result, the species that flourish tend to be only those able to survive in one of the most engineered and highly disturbed environments imaginable: plants, for example, like Phragmites australis, an invasive reed found all over Greater New York.

The urge to reflexively endorse more growth and development under the simple assumption that dense urban life is an unqualified environmental good should be resisted. In truth, sustainable cities require more than mere density. They depend on a vital expanse of wetlands and a system of agriculture that can recycle the nutrients in waste back to the land, goals that most developed cities, at least, have fallen short of achieving. Moreover, more urban development wedded to \”market-based choices,\” to quote Chakrabarti, is not likely to move cities in that ecological direction—for the simple reason that the \”choices\” are mainly being made by powerful developers, who see land chiefly as a tool for capital accumulation.

By the middle of the 21st century, it is projected, nearly seven out of every 10 people on the planet will live in a city. Density seems more inevitable today than at any other point in human history. But the hard reality is that, from an ecological perspective, simply having developers build more Times Squares across the globe won’t get the planet to where it ought to be.

Author Bio: Ted Steinberg is a professor of history and law at Case Western Reserve University. He is the author, most recently, of Gotham Unbound: The Ecological History of Greater New York (Simon & Schuster, 2014).