Greenhouse gas emissions from transportation in the United States have remained persistently high. One cause is common low-density land use patterns that make most Americans dependent on automobiles. Reducing these emissions requires increasing density, which U.S. local government law makes difficult to achieve through the political process. Mount Laurel, a 1975 New Jersey Supreme Court case that addressed an affordable housing crisis by restraining local parochialism, provides a potential solution. Environmental advocates may be able to mount similar state-law challenges against low-density zoning based on the high carbon emissions it produces. Such a challenge is legally and normatively defensible in New Jersey and other states.