AP Environmental Science

## Learning Outcomes

- Explain how climate change alters soil through temperature, rainfall, and disturbance changes
- Distinguish between **runoff**, **infiltration**, and **leaching** in soil-related climate questions
- Describe how climate change increases both water erosion and wind erosion
- Understand how desertification, salinization, and permafrost thaw damage soil systems
- Explain how loss of soil organic matter reduces fertility and weakens soils as carbon sinks
- Apply these ideas to agriculture, environmental system stability, and climate feedback loops
- Use clear cause-and-effect chains in AP Environmental Science free-response answers

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand how climate change affects soil, with a focus on the following syllabus points:

- Know that changes in temperature and precipitation can reduce soil viability and increase erosion
- Identify how heavier precipitation increases runoff and topsoil loss when rainfall exceeds infiltration
- Explain how hotter, drier conditions can lower soil moisture and increase wind erosion
- Understand how climate change can intensify **desertification** in arid and semiarid regions
- Distinguish between general soil degradation and specific processes such as **salinization** and permafrost thaw
- Explain how warming can reduce soil organic matter and release stored carbon
- Understand the positive feedback loop in which thawing permafrost releases **CO2** and **CH4**
- Connect soil changes to reduced agricultural productivity, biodiversity impacts, and wider climate effects

## 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. How can climate change increase both runoff erosion and wind erosion?
2. What is the difference between **infiltration**, **runoff**, and **leaching**?
3. Why can heavier rainfall still leave soils with less usable moisture for plants?
4. How does permafrost thaw create a positive feedback loop?
5. Name two ways climate change can reduce crop productivity by damaging soil.

## Introduction

Climate change affects soil mainly by changing temperature, precipitation patterns, and disturbance frequency. Soil is not just loose material on the ground. It stores water, holds nutrients, supports plant growth, filters water, and stores carbon. When climate change damages soil, the effects spread into agriculture, freshwater quality, biodiversity, and the climate system itself.

For AP Environmental Science, you need to move beyond simply saying that “climate change harms soil.” Strong answers explain the process: warming changes evaporation and rainfall patterns, this changes plant cover and water movement, and those shifts lead to erosion, drying, salinization, or thawing. The exam often rewards cause-and-effect chains more than isolated facts.

> **Key Term: soil erosion**
> The removal and movement of soil, especially nutrient-rich topsoil, by water or wind.

### Why climate change changes soil moisture and soil stability

Climate change does not affect every region in the same way, but it often changes the balance between water entering soil and water leaving it. Higher temperatures increase evaporation from soil and evapotranspiration from plants. In many places, that means soils dry out faster.

Drying matters because soil moisture helps hold natural systemss together. Moist soils support vegetation, and vegetation protects soil from erosion. When soils dry, plants become stressed, root systems weaken, and the ground is left more exposed. Bare soil is much easier for wind to move and for rain to wash away.

> **Key Term: soil moisture**
> The water stored in soil that is available for plants and important for soil health and drought resistance.

A common mistake is to assume that more rainfall always means wetter soil. That is not always true. A warmer climate may increase total rainfall in some places, but if the rain comes in intense bursts, much of it may run off instead of entering the ground. Also, higher temperatures can increase water loss so much that soils still end up drier overall.

This is why climate change can produce two opposite-looking conditions in different places, or even in the same place at different times:

- longer dry periods that lower soil moisture
- heavier storms that saturate the surface briefly, then produce runoff

The AP exam likes this contrast because it tests whether you understand process, not just memorized trends.

> **Key Term: infiltration**
> The movement of water from the ground surface into the soil.
>
> **Key Term: runoff**
> Water flowing over the land surface when precipitation exceeds the rate at which soil can absorb it.

When precipitation rate is greater than infiltration rate, runoff forms. That simple rule explains many climate-soil questions.

For example:

- A storm drops a large amount of rain in a short time.
- The soil cannot absorb water quickly enough.
- Water moves across the surface as runoff.
- Runoff carries sediment and topsoil away.
- The remaining soil has lower fertility and lower water-holding capacity.

That last point is often missed. Erosion is not just the loss of “dirt.” It removes the most productive layer of soil, which contains much of the organic matter and plant-available nutrients.

> **Test Tip:** In FRQs, write the chain clearly: **intense rainfall → runoff increases → erosion increases → topsoil is lost → fertility and crop productivity decrease**. This is much stronger than simply stating that climate change “damages soil.”

### How climate change increases erosion, topsoil loss, and land degradation

Climate change can increase both major forms of erosion: water erosion and wind erosion. Students often remember only one.

Water erosion increases when heavy precipitation becomes more frequent or more intense. Fast-moving water strips away loose particles, especially from exposed slopes, recently tilled fields, overgrazed land, or land with little plant cover. This is especially damaging because topsoil forms very slowly but can be lost quickly.

Wind erosion increases under hotter, drier conditions. If vegetation dies back and the soil surface dries out, wind can lift and transport fine particles. The result is not only soil loss but also reduced water-holding capacity afterward, making future drought stress worse.

This can become a feedback cycle:

- drought reduces plant cover
- exposed soil erodes more easily
- erosion lowers the soil’s ability to hold water
- plants struggle even more in the next dry period

That is a positive feedback within the land system because the original problem strengthens itself.

> **Key Term: topsoil**
> The upper, most fertile soil layer, rich in organic matter and nutrients needed for plant growth.

The Dust Bowl of the 1930s is a useful comparison point, even though it was not caused only by climate. Severe drought combined with poor farming practices left soil exposed, and wind removed huge amounts of topsoil. APES questions may use this kind of historical example to test whether you can connect drought, bare soil, and erosion.

Climate change can produce similar risks when drought combines with:

- overgrazing
- monoculture
- deforestation
- intensive tillage
- post-fire bare ground

Wildfire is a useful link here. Hotter, drier conditions increase wildfire risk in many places. After vegetation burns, roots no longer stabilize the soil. Heavy rain soon after a fire can create extreme erosion and mudslides because the ground has lost plant cover.

> **Key Term: soil degradation**
> The decline in soil quality and productive ability due to erosion, nutrient loss, salinization, compaction, or other damaging processes.

Soil degradation is broader than erosion. Erosion is one mechanism, but degraded soils may also have lower organic matter, poorer structure, worse infiltration, and reduced fertility.

Another point AP students should know: degraded soil also harms water quality. Eroded sediment can enter streams and lakes, increasing turbidity. Sediments may carry nutrients such as nitrogen and phosphorus, adding to eutrophication risk.

> **Test Tip:** If an FRQ asks for an aquatic consequence of climate-driven soil change, mention **sediment runoff** or **nutrient runoff** into streams, lakes, wetlands, or estuaries.

### Desertification, salinization, and why dry regions are especially vulnerable

Drylands are especially sensitive to climate change because they already sit near the edge of water limitation. A moderate shift in heat or rainfall can push them into much more severe degradation.

> **Key Term: desertification**
> The degradation of productive land into more desert-like conditions, especially in arid and semiarid regions.

Desertification is not simply “the spread of deserts.” It means productive land loses vegetation, soil quality, and agricultural usefulness. Climate change can intensify desertification through a sequence like this:

- temperatures rise
- evaporation increases
- drought becomes more severe or more frequent
- vegetation cover decreases
- soil becomes exposed
- wind and water erosion remove topsoil
- less water is retained in the soil
- regrowth becomes more difficult

This is why desertification is both an ecological and an agricultural problem. Once vegetation is lost, the soil may no longer support seed germination well, and the land becomes far less productive.

Salinization is another major issue in hot, dry regions.

> **Key Term: salinization**
> The buildup of dissolved salts in soil, often made worse by evaporation and irrigation in dry climates.

Here is the process:

- irrigation or shallow groundwater brings water containing dissolved salts into the soil
- strong heat causes water to evaporate from the surface
- the water leaves, but the salts remain
- salt concentration in topsoil increases
- plants have more difficulty taking up water
- yields fall, and severe cases can kill vegetation

Climate change can worsen salinization because higher temperatures raise evaporation rates. In coastal regions, sea-level rise can also increase saltwater intrusion into soils and aquifers, making the problem worse.

Students often confuse salinization with desertification. They are related, but not the same.

- **Desertification** is broad land degradation toward desert-like conditions.
- **Salinization** is specifically salt buildup in soil.
- Both reduce productivity, but through different immediate mechanisms.

In APES answers, that distinction matters.

### Soil organic matter, carbon storage, and permafrost thaw

Soil is also part of the carbon cycle. That makes climate change and soil change tightly linked.

> **Key Term: soil organic matter**
> Decomposed plant and animal material in soil that improves fertility, structure, and water-holding capacity.

Soil organic matter is valuable for several reasons:

- it improves soil structure
- it helps soil retain moisture
- it provides nutrients
- it supports soil organisms
- it stores carbon

When temperatures rise, decomposition often speeds up. Microbes break down organic matter more quickly, and this can release carbon dioxide through soil respiration. If soils lose organic matter, they usually become less fertile, hold less water, and erode more easily.

This means climate change can damage soils while also weakening soils as carbon sinks.

> **Key Term: soil carbon sequestration**
> The storage of carbon in soil rather than in the atmosphere as carbon dioxide.

Healthy soils can store carbon, but this storage is not unlimited. Disturbance, tillage, erosion, and warming can all reduce stored soil carbon. That is why land management can help climate mitigation, but it cannot replace cutting fossil fuel use.

Permafrost thaw is one of the clearest examples of climate-soil feedback.

> **Key Term: permafrost**
> Ground that remains frozen for at least two consecutive years.

Large areas of Arctic and sub-Arctic soils contain permafrost. These frozen soils store huge amounts of organic carbon. When warming causes permafrost to thaw:

- the ground structure becomes unstable
- some areas become waterlogged or collapse
- previously frozen organic matter becomes available to microbes
- microbes decompose that material
- **CO2** and **CH4** are released
  > **Key Term: positive feedback loop**
  > A process in which an initial change causes further changes that strengthen the original change.

Permafrost thaw is a positive feedback loop because:

- warming causes thaw
- thaw releases greenhouse gases
- greenhouse gases cause more warming
- more warming causes more thaw

That is both a local soil problem and a global climate problem.

Permafrost thaw can also damage roads, pipelines, and buildings because the soil no longer has the same frozen support structure. So the impact is ecological, climatic, and economic.

### Why soil impacts matter for agriculture and how to respond

The clearest exam link is to agriculture. Climate-damaged soils reduce agricultural productivity through several connected pathways:

- topsoil loss lowers nutrient availability
- reduced organic matter lowers fertility
- lower water-holding capacity increases drought stress
- salinization reduces plant water uptake
- erosion damages root zones and seedbeds
- unstable thawed soils disrupt high-latitude environmentss and land use

In cold regions, some crop yields may increase at first because of warming. But that is not the overall global picture. Gains in some places may be offset by drought, erosion, and land loss elsewhere.

Students should avoid making absolute claims such as “climate change reduces soil moisture everywhere.” Regional variation matters. Some places get drier, some wetter, some both seasonally.

The main management responses are soil-health practices that improve infiltration, protect topsoil, and maintain organic matter. These include:

- cover crops
- reduced or no-till farming
- contour plowing
- terracing
- strip cropping
- windbreaks
- riparian buffers
- adding compost or manure
- careful irrigation and drainage management

These practices matter because they do more than “protect the soil.” They change the same processes climate change is stressing:

- they increase infiltration
- they reduce runoff velocity
- they hold soil in place
- they improve moisture retention
- they reduce erosion risk

If an FRQ asks for a solution, tie the method to the mechanism. For example, do not stop at “use cover crops.” Say that cover crops keep soil covered, reduce runoff and wind erosion, and improve soil organic matter.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Climate change affects soil by altering temperature, precipitation, and disturbance patterns
- Higher temperatures often increase evaporation and can lower soil moisture
- Heavy rainfall can still leave soils with less usable water if runoff increases faster than infiltration
- Water erosion removes topsoil during intense rain and runoff events
- Wind erosion increases when soils become dry, bare, and poorly protected by vegetation
- Soil degradation is broader than erosion and includes fertility loss, structural decline, and reduced water-holding capacity
- Desertification is the degradation of productive land into more desert-like conditions
- Salinization is the buildup of salts in soil, often worsened by evaporation and irrigation in dry climates
- Soil organic matter improves fertility, structure, and water retention while storing carbon
- Warming can speed decomposition, reducing soil organic matter and releasing more carbon dioxide
- Permafrost thaw destabilizes soil and releases stored carbon as **CO2** and **CH4**
- Permafrost thaw is a positive feedback loop because warming causes greenhouse gas release that drives further warming

## Key Terms and Concepts

- soil erosion
- soil moisture
- infiltration
- runoff
- topsoil
- soil degradation
- desertification
- salinization
- soil organic matter
- soil carbon sequestration
- permafrost
- positive feedback loop


Global Climate Change - Climate change affects soil