AP Environmental Science

## Learning Outcomes

- Explain why urbanization can increase atmospheric carbon dioxide
- Describe how land-use change reduces photosynthesis and carbon sequestration
- Identify major urban sources of carbon emissions, including transportation and buildings
- Distinguish between direct carbon increases and related effects such as urban heat islands
- Apply carbon-cycle reasoning to AP Environmental Science free-response questions
- Evaluate how smart growth and urban vegetation can reduce carbon increases from urban development

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand how urbanization can increase atmospheric carbon, with a focus on the following syllabus points:

- Know that urbanization raises atmospheric **CO2** mainly through vegetation removal and greater fossil fuel combustion
- Identify how land-use change from forests, grasslands, or farmland to built environments reduces carbon uptake
- Explain how **photosynthesis** and **carbon sequestration** link urban land conversion to the carbon cycle
- Understand that transportation, buildings, construction, and urban energy use are major sources of urban carbon emissions
- Distinguish atmospheric carbon increase from the **urban heat island** effect, while linking heat islands to extra cooling energy demand
- Compare sprawling, car-dependent growth with compact, transit-oriented development in terms of carbon emissions
- Apply examples such as vehicle miles traveled, electricity use in buildings, and land clearing for roads and housing
- Recognise that mitigation includes smart growth, public transit, urban trees, and green roofs, but these reduce rather than cause emissions

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. What are the two main ways urbanization increases atmospheric carbon dioxide?
2. How does removing vegetation during urban development affect the carbon cycle?
3. Why can urban sprawl increase carbon emissions more than compact development?
4. How can the urban heat island effect indirectly increase atmospheric carbon?
5. Name one strategy that can reduce carbon emissions from urban growth.

## Introduction

Urbanization means the growth of towns and cities and the movement of people into them. In AP Environmental Science, this topic matters because urban growth changes both land use and energy demand. When natural or agricultural land is converted into roads, parking lots, housing, offices, and other infrastructure, the carbon cycle is altered.

The key exam idea is simple but must be explained clearly: urbanization can increase atmospheric carbon dioxide by reducing carbon uptake from vegetation and by increasing carbon emissions from human energy use. Strong answers do not just say “cities make pollution.” They explain the mechanism step by step.

> **Key Term: urbanization**
> The movement of people from rural areas to cities and the land-use changes and resource demands that come with city growth.

### Urbanization and the carbon cycle

To explain why urbanization increases atmospheric carbon, start with the carbon cycle. Carbon moves between the atmosphere, biosphere, hydrosphere, and geosphere. Under natural conditions, plants remove carbon dioxide from the atmosphere through photosynthesis, while respiration and decomposition return some of it.

Urban development interrupts this balance. If forests, grasslands, wetlands, or even farmland are cleared for development, less plant biomass remains to absorb carbon dioxide. At the same time, disturbing soils and removing vegetation can release stored carbon.

> **Key Term: photosynthesis**
> The process by which plants use sunlight to convert carbon dioxide and water into sugars, removing CO2 from the atmosphere.
>
> **Key Term: carbon sequestration**
> The process of capturing and storing atmospheric carbon dioxide, especially in vegetation and soils.

A common exam chain looks like this:

- vegetation removal
- less photosynthesis
- less carbon sequestration
- more CO2 remains in the atmosphere

That chain is often enough for one APES explanation point, as long as each link is clear.

This matters because forests and other vegetation are not just “green space.” They are active carbon sinks. Forests store a large share of above-ground carbon, and soils also store major amounts of carbon. When land is converted to urban uses, the system shifts away from storing carbon and toward releasing it.

Students often miss the role of soil. Urbanization does not only remove trees. It also compacts, disturbs, or seals soils under pavement and buildings. That reduces the ability of soil to hold carbon and support plant growth. So the loss is both above ground and below ground.

A useful AP-style sentence is:

Urbanization increases atmospheric CO2 because clearing vegetated land reduces photosynthesis and carbon sequestration in both biomass and soils.

> **Test Tip:** On FRQs, do not stop at “trees are cut down.” Add the carbon-cycle link: fewer plants means less CO2 uptake by photosynthesis.

Another important distinction: a **carbon sink** is the place or system storing carbon, while sequestration is the process. Forests, wetlands, and soils are sinks. Photosynthesis and long-term storage are part of sequestration.

> **Key Term: carbon sink**
> A reservoir that stores more carbon than it releases, such as forests, wetlands, soils, and oceans.

### Why cities emit more carbon: transportation, buildings, and construction

The second big mechanism is increased fossil fuel combustion. Urbanization usually means more roads, more buildings, more electricity use, more heating and cooling, more goods transport, and more commuting. All of these activities can increase carbon dioxide emissions.

Transportation is one major source. Cars, buses, trucks, and other vehicles burn gasoline or diesel, releasing CO2. This is especially serious in low-density urban sprawl, where homes, jobs, schools, and shops are far apart. When places are separated by design, people usually must drive longer distances.

Urban sprawl has several carbon-related features:

- low-density housing
- single-use zoning
- job sprawl
- heavy car dependence

If a city spreads outward instead of growing more compactly, vehicle miles traveled usually rise. More fuel is burned, so more carbon enters the atmosphere.

A current example helps. Transportation accounted for **29%** of U.S. greenhouse gas emissions in **2022**, according to the EPA. That makes transport one of the clearest urban sources to mention in answers.

Buildings are another major source. Residential and commercial buildings use large amounts of electricity and fuel for lighting, appliances, heating, cooling, and ventilation. In the United States, buildings use about one-third of all energy and about two-thirds of electricity. Building energy use accounts for about one-third of carbon dioxide emissions in the textbook material.

Globally, building-related CO2 emissions also rose strongly over time. A useful figure for context is that building emissions increased by about **50% from 1990 to 2019**.

Construction itself adds more emissions. Making cement and steel is energy-intensive and often depends on fossil fuels. Urbanization therefore adds carbon not only after buildings are occupied, but also during the process of creating the built environment.

A strong APES explanation might therefore split the cause into direct land-use change and energy demand:

1. land clearing removes vegetation and reduces sequestration
2. transportation and buildings increase fossil fuel combustion and emissions

This two-part structure is exactly what examiners want.

> **Key Term: fossil fuel combustion**
> The burning of coal, oil, or natural gas, which releases carbon dioxide and other pollutants.
>
> **Test Tip:** If a question asks for “two ways” urbanization increases atmospheric carbon, give one land-use answer and one energy-use answer. That avoids repeating the same idea twice.

You should also be able to use broad global evidence. Urban emissions were estimated at about **25 GtCO2-eq in 2015** and about **29 GtCO2-eq in 2020**, showing that urban systems are a major share of global emissions.

### Urban heat islands, impervious surfaces, and common exam confusions

Urbanization is also linked to the urban heat island effect, but this is where many students get confused. An urban heat island is when cities are warmer than surrounding rural areas because of asphalt, concrete, buildings, less vegetation, and waste heat from human activity.

The heat island itself is not the same as increasing atmospheric carbon. It is a temperature pattern, not a direct carbon source. However, it can indirectly raise carbon emissions because hotter cities require more air conditioning. More air conditioning means more electricity demand, and if that electricity comes from fossil fuels, then more CO2 is released.

> **Key Term: urban heat island**
> A condition in which urban areas are warmer than nearby rural areas because built surfaces absorb and re-radiate heat and vegetation is reduced.

That gives an important cause-and-effect chain:

- more dark, impervious surfaces
- higher urban temperatures
- greater cooling demand
- more electricity use
- more fossil fuel combustion
- more atmospheric CO2

Impervious surfaces matter for another reason as well. Roads, rooftops, and parking lots replace soil and vegetation. They do not directly emit large amounts of carbon just by existing, but they reduce infiltration, reduce plant cover, and lower the land’s overall ability to store carbon. This is why paved areass usually hold less carbon than undisturbed soils and vegetated land.

Students also often make these mistakes:

- saying cities produce CO2 only through factories
- treating urban heat islands as identical to atmospheric carbon increase
- forgetting that land clearing itself affects the carbon cycle
- claiming urban trees fully offset urban emissions

Urban trees help, but they do not erase the large emissions from transport, buildings, and infrastructure in most cities.

A useful comparison is sprawl versus compact development. Sprawl usually means longer commutes, greater fuel use, and more land conversion. Compact development can reduce these by bringing homes, jobs, schools, and shops closer together and by supporting transit and walking.

A real example is **Curitiba, Brazil**. It grew rapidly but invested in bus rapid transit and transit-oriented planning. That reduced reliance on private cars compared with more sprawling urban models. For APES, you do not need detailed city planning history. The key point is that urban design changes carbon outcomes.

### How smart growth can reduce carbon increases

Urbanization does not have to raise atmospheric carbon by the maximum possible amount. The main mitigation idea is **smart growth**, which means planning cities in ways that reduce sprawl, conserve land, and lower fossil fuel demand.

> **Key Term: smart growth**
> An urban planning approach that reduces sprawl by promoting compact, walkable, mixed-use, transit-oriented development.

Smart growth strategies include:

- mixed-use development
- public transit
- walkability and bike access
- greenbelts around cities
- locating buildings near transit hubs
- replacing abandoned built areas with green space

These strategies reduce carbon in two ways.

First, they reduce transport emissions by lowering car dependence. If people can walk, cycle, or use public transit, fewer miles are driven and less fuel is burned.

Second, they protect carbon sinks by reducing the amount of new vegetated land converted into urban land. If growth is concentrated rather than spread out, less forest, grassland, or farmland is paved over.

Urban vegetation also helps directly and indirectly. Trees and green spaces store carbon and continue removing CO2 through photosynthesis. They also cool nearby areas by shading surfaces and through transpiration. This lowers building cooling demand. EPA materials note that urban trees and vegetation can reduce nearby building energy demand by about **10%**.

Green roofs are another useful example. They reduce heat absorption, support some carbon uptake, and lower cooling energy demand while also reducing runoff.

Still, exam answers must be careful: these are mitigation strategies, not the primary mechanism of carbon increase. If the question asks how urbanization increases atmospheric carbon, start with vegetation loss and fossil fuel use. If it asks how to reduce the impact, then discuss smart growth, trees, and efficient buildings.

A strong final judgement would be:

Urbanization raises atmospheric carbon most when it replaces vegetation with sprawling, energy-intensive, car-dependent development. Its effects can be reduced, though not removed entirely, by compact planning, transit, urban greening, and efficient building design.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Urbanization can increase atmospheric CO2 in two main ways: less carbon uptake and more carbon emissions
- Clearing vegetation reduces photosynthesis and carbon sequestration
- Forests, green space, and soils act as carbon sinks
- Urban land conversion can shift land from a carbon sink toward a greenhouse gas source
- Transportation emissions rise when urban growth is sprawling and car-dependent
- Buildings add carbon through electricity use, heating, cooling, and construction materials
- Transportation accounted for 29% of U.S. greenhouse gas emissions in 2022
- Urban heat islands are not the same as atmospheric carbon increase, but they can raise emissions indirectly by increasing cooling demand
- Impervious surfaces reduce vegetation and soil carbon storage capacity
- Compact, transit-oriented development usually produces fewer emissions than urban sprawl
- Smart growth can reduce carbon increases by lowering driving and protecting undeveloped land
- Urban trees and green roofs help by sequestering carbon and reducing cooling energy use

## Key Terms and Concepts

- urbanization
- photosynthesis
- carbon sequestration
- carbon sink
- fossil fuel combustion
- urban heat island
- smart growth


Impacts of Urbanization - Urbanization increases atmospheric carbon