AP Environmental Science

## Learning Outcomes

- Explain how changes in glacial ice affect sea level over geologic time and today
- Distinguish between **land ice** melt and **sea ice** melt
- Understand how **thermal expansion** and **albedo** help drive climate-related sea level change
- Describe how rising sea level disrupts estuaries, wetlands, barrier islands, and coastal biodiversity
- Explain how glacier retreat can create new habitat through **primary succession**
- Apply these ideas to AP Environmental Science multiple-choice and free-response questions

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand sea level and glacial ice, with a focus on the following syllabus points:

- Know that sea level has changed over geologic time because of changes in glacial ice, ocean temperature, and land movement
- Identify the main modern causes of sea level rise, including melting **land ice** and **thermal expansion**
- Explain the difference between melting **land ice** and melting **sea ice**
- Understand how loss of ice lowers **albedo** and increases warming through positive feedback
- Describe ecological impacts of sea level rise, especially on estuaries, coastal wetlands, mangroves, and barrier islands
- Explain **saltwater intrusion**, wetland migration, and why some wetlands are lost through coastal squeeze
- Understand that retreating glaciers expose bare rock and sediment, leading to **primary succession**
- Distinguish between **global mean sea level rise** and **relative sea level rise**

## 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. Why does melting land ice raise sea level, but melting sea ice usually does not?
2. How does warmer ocean water raise sea level even if no extra water is added?
3. What is albedo, and why does ice loss speed up warming?
4. How can sea level rise damage wetlands and estuaries?
5. Why does glacier retreat lead to primary succession rather than secondary succession?

## Introduction

Sea level and glacial ice are a strong example of how physical Earth systems and living systems connect. Ice stored on land affects coastlines, freshwater supplies, habitat area, ocean circulation, and the stability of many natural communitiess. When AP Environmental Science asks about natural disruptions, this topic often appears as a chain: climate change alters ice, altered ice changes sea level, and sea level change disrupts natural communitiess.

This topic also tests several easy-to-mix-up ideas. Students often confuse land ice with sea ice, assume all coasts experience the same sea level rise, or forget that glacier retreat can destroy habitat in one place while creating new habitat in another. Getting those distinctions right is a good way to gain marks.

> **Key Term: sea level rise**
> An increase in the height of the ocean relative to land or relative to the global average ocean surface.

### Why sea level changes

Sea level has never been perfectly fixed. Over geologic time, it has changed because Earth itself changes. The main long-term drivers include shifts in ocean and atmospheric circulation, plate tectonics moving continents to different latitudes, volcanic activity, and changes in atmospheric carbon dioxide. Sea level is also affected by the shape of ocean basins, tectonic uplift, land subsidence, and how much water is stored as ice, groundwater, lakes, and rivers.

For APES, the key modern drivers are more specific. Today, sea level rises mainly because warming increases the volume of seawater and because land-based ice melts and adds water to the oceans.

> **Key Term: thermal expansion**
> The increase in water volume that occurs when ocean water warms and becomes less dense.
>
> **Key Term: land ice**
> Ice stored on land, such as glaciers and ice sheets, that raises sea level when it melts and flows to the ocean.

A useful exam chain is:

- increased greenhouse gases
- warmer atmosphere and oceans
- more melting of glaciers and ice sheets on land
- more thermal expansion of seawater
- higher sea level
- more flooding, erosion, habitat loss, and saltwater intrusion

The textbook material gives a helpful breakdown for present sea-level change:

- about **30%** from melting glaciers and ice sheets on land
- about **30%** from thermal expansion
- about **40%** from coastal land subsidence

That last point matters. Sometimes the sea is not rising especially fast at a location; instead, the land is sinking. To organisms on that coast, the effect is still greater flooding.

> **Key Term: relative sea level rise**
> Sea-level change measured at a specific coast, including both ocean rise and local land uplift or subsidence.

Students often write only about global sea level rise. But many environmental system impacts depend on relative sea level rise. A wetland on a sinking coast may experience much worse flooding than one on a stable coast, even if global mean sea level rise is the same.

> **Test Tip:** If an FRQ asks for a cause of sea level rise, give a direct mechanism such as **melting land ice** or **thermal expansion**, then explain how it changes ocean volume.

### Land ice, sea ice, and albedo: the distinction students must know

One of the highest-yield distinctions in this topic is between land ice and sea ice.

**Land ice** includes glaciers and ice sheets on Greenland and Antarctica. When this ice melts, the meltwater enters the ocean and increases ocean volume. That raises sea level directly.

**Sea ice** is already floating in the ocean. Because it already displaces water, melting it causes little direct sea-level rise. That does not mean sea ice is unimportant. It matters greatly because of **albedo**.

> **Key Term: sea ice**
> Floating ice on the ocean surface that contributes little direct sea-level rise when it melts.
>
> **Key Term: albedo**
> The fraction of incoming solar radiation reflected by a surface.

Ice and snow have high albedo. Open ocean has much lower albedo. When sea ice melts, a bright reflective surface is replaced by darker water that absorbs more solar energy. That extra absorbed energy warms the region further, which melts more ice. This is a classic positive feedback loop.

The same basic pattern applies to land ice too. As glaciers retreat, exposed rock or soil usually reflects less sunlight than snow and ice did. More absorption means more warming.

This is why the polar regions respond quickly to warming. Less ice means lower albedo, which means more absorbed energy, which means even less ice later.

The textbook also notes major ice stores:

- Antarctica holds about **90%** of the world’s ice and about **70%** of its freshwater
- if all Antarctic ice melted, sea level would rise by about **60 m**
- Greenland ice would add about **7 m**

You do not need all those numbers for every response, but they show the scale of the cryosphere’s connection to sea level.

A related benchmark is that sea level at the Last Glacial Maximum, around **20,000 years ago**, was much lower than today. The textbook states that present sea level is about **130 m** higher than the historical minimum around that time. This shows how strongly glacial ice volume and sea level are linked.

> **Test Tip:** If a question asks you to distinguish two ice types, write it as a contrast: **land ice melt adds water to oceans; sea ice melt mainly affects climate through albedo feedback**.

### Coastal habitat impacts: why sea level rise matters for biodiversity

Sea level rise is not just a physical measurement. It changes habitat structure, salinity, oxygen conditions, and species interactions. Coastal systems are especially sensitive because even small vertical changes can flood broad flat areas.

Estuaries and coastal wetlands are among the first habitatss affected. Estuaries depend on a balance between freshwater and seawater. Rising sea level pushes saltwater farther inland, shifts salinity zones, and changes which species can survive in each part of the estuary.

> **Key Term: saltwater intrusion**
> The movement of seawater into freshwater aquifers, soils, rivers, or wetlands.

Saltwater intrusion can damage both natural systemss and human systems. In wetlands, it can stress or kill freshwater plants. In aquifers, it can contaminate drinking water. In agricultural soils, it can reduce crop productivity.

Sea level rise also increases flooding frequency and the reach of storm surge. A higher baseline sea level means a storm starts from a higher platform, so overwash and erosion become more severe.

Barrier islands are important in this process.

> **Key Term: barrier island**
> A long, narrow offshore sand body that helps protect mainland coasts from waves and storm surge.

Barrier islands are not fixed. They naturally erode, overwash, breach, and migrate landward. Rising sea level and stronger storm impacts can speed up these changes. When barrier islands are damaged, the estuaries and wetlands behind them lose part of their physical protection.

Wetlands respond to sea level rise in two main ways:

- they can build upward through sediment and organic matter accumulation
- they can migrate inland if there is space
  > **Key Term: accretion**
  > The buildup of sediment and organic matter that raises wetland elevation.

If accretion keeps pace with relative sea level rise, a marsh may persist. If not, it may drown and convert to open water. If landward migration is blocked by roads, seawalls, housing, or other development, the wetland gets trapped.

> **Key Term: coastal squeeze**
> The loss of coastal habitat when rising seas push habitatss inland but development blocks their migration.

This is why some APES questions focus on “wetland migration” and others on “wetland loss.” Both can happen, and the difference depends on sediment supply, plant productivity, rate of sea level rise, and whether inland space is available.

The source material also highlights further coastal impacts:

- altered species composition
- reduced dissolved oxygen
- more algal blooms
- worsening dead zones
- fewer nesting, spawning, and egg-laying areas
- more invasive species
- reduced water quality from polluted runoff

That is a full ecological cascade, not just “flooding.”

One subtle point students miss: sea level rise can create some new shallow marine habitats on flooded continental shelves. But in exam answers, you should usually stress the stronger and more common disruption to existing estuaries, marshes, mangroves, beaches, and human infrastructure.

### Glacier retreat as ecological disruption and habitat creation

Glaciers affect natural systemss in two different ways. At the global scale, their melting contributes to sea level rise. At the local scale, their retreat changes land surfaces and freshwater flows.

The textbook notes that the total surface area of glaciers worldwide has decreased by about **50%** since the end of the nineteenth century. Glacier melting can cause:

- flash floods
- glacial lake overflow
- changed river flow patterns
- landslides
- displacement of people

These effects matter for biodiversity. Species in rivers downstream may depend on cold, steady meltwater flow. If glaciers shrink, rivers may first receive extra water, but later may suffer lower summer flow as the long-term ice reserve disappears. Fish such as salmon can be affected if migration routes or spawning conditions change.

But glacier retreat also exposes new land. Freshly uncovered rock, till, or sediment has little or no soil. That means succession begins from the start.

> **Key Term: glacier retreat**
> The backward movement of a glacier’s terminus when melting exceeds accumulation.
>
> **Key Term: primary succession**
> Ecological succession that begins on a surface with little or no soil, such as land newly exposed by retreating glaciers.

This is primary succession, not secondary succession. Secondary succession begins where soil remains after a disturbance such as fire.

The usual post-glacial sequence is:

1. bare rock or sediment is exposed
2. pioneer species such as lichens and mosses colonize
3. weathering and organic matter formation begin
4. thin soil develops
5. grasses, herbs, and shrubs establish
6. later, trees may appear if climate allows

This process can increase local biodiversity over time. That is why glacier retreat is a good APES example of a disturbance that can create new habitat even while climate change is causing habitat loss elsewhere.

A strong response might say: glacier retreat contributes to global sea-level-related habitat loss, but locally it can expose new substrate and begin primary succession.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Sea level changes over geologic time because of glacial ice, ocean temperature, tectonics, and basin shape
- Modern sea level rise is driven mainly by melting **land ice** and **thermal expansion**
- Melting **land ice** raises sea level directly because it adds water to the ocean
- Melting **sea ice** causes little direct sea-level rise but increases warming through reduced **albedo**
- **Relative sea level rise** includes both ocean rise and local land subsidence or uplift
- Rising sea level affects estuaries, wetlands, mangroves, beaches, and barrier islands
- **Saltwater intrusion** can move seawater into aquifers, soils, rivers, and wetlands
- Wetlands can survive if **accretion** and inland migration keep pace with sea-level rise
- **Coastal squeeze** happens when development blocks wetland migration inland
- Glacier retreat can alter river flow, increase hazards, and disrupt freshwater habitatss
- Retreating glaciers expose bare substrate and begin **primary succession**
- A key APES distinction is **land ice raises sea level; sea ice mainly matters through albedo feedback**

## Key Terms and Concepts

- sea level rise
- thermal expansion
- land ice
- relative sea level rise
- sea ice
- albedo
- saltwater intrusion
- barrier island
- accretion
- coastal squeeze
- glacier retreat
- primary succession


Natural Disruptions to Ecosystems - Sea level and glacial ice