What specific environmental problem does designing cities around public transit aim to reduce by lowering car use?

Designing cities around public transit primarily aims to reduce atmospheric pollution, which encompasses both the degradation of local and regional air quality and global climate change. This specific environmental problem stems from the combustion of fossil fuels—such as gasoline and diesel—in the internal combustion engines of private vehicles. When these fuels burn, they release a variety of substances into the atmosphere. One major group consists of greenhouse gases (GHGs), predominantly carbon dioxide (CO2), but also methane (CH4) and nitrous oxide (N2O). These gases are crucial because they trap heat within the Earth's atmosphere, a process called the greenhouse effect. An escalated concentration of these gases in the atmosphere leads to an increase in global temperatures, resulting in climate change, which manifests as disruptions to global weather patterns, rising sea levels, and a greater frequency of extreme weather events. The second group comprises criteria air pollutants, which directly affect local and regional air quality and human health. These include nitrogen oxides (NOx), volatile organic compounds (VOCs), various sizes of particulate matter (PM2.5 and PM10), and carbon monoxide (CO). Nitrogen oxides and volatile organic compounds react in sunlight to form ground-level ozone, a primary ingredient of smog, which impairs visibility and respiratory health. Particulate matter consists of microscopic solid or liquid particles that, when inhaled, can penetrate deep into the lungs, contributing to respiratory and cardiovascular diseases. Carbon monoxide is a toxic gas that hinders the blood's ability to transport oxygen. By reducing the overall number of private car trips, public transit directly decreases the collective emission of these harmful substances, thereby alleviating both the drivers of climate change and the local burdens of air pollution.