AP Environmental Science

## Learning Outcomes

- Explain how increased greenhouse gases lead to major climate change effects through linked cause-and-effect chains
- Identify major effects on coasts, oceans, weather, natural systemss, agriculture, and human health
- Distinguish between ocean acidification and ocean warming, and between coral bleaching and coral death
- Understand why sea-level rise is driven mainly by thermal expansion and melting land ice
- Describe positive feedback loops such as ice-albedo feedback and permafrost methane release
- Apply this knowledge to AP Environmental Science multiple-choice and free-response questions

## AP Environmental Science Syllabus

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

- Know that sea-level rise is driven mainly by **thermal expansion** of warming seawater and melting **land-based ice**
- Identify major coastal impacts, including flooding, erosion, saltwater intrusion, wetland loss, and stronger storm-surge damage
- Explain how climate change affects extreme heat, drought, wildfire risk, heavy precipitation, and some storms
- Understand how **ocean acidification** occurs when atmospheric CO₂ dissolves in seawater and lowers pH
- Distinguish coral bleaching from coral death and explain why warming and acidification together threaten reefs
- Describe **species range shifts** and changing seasonal timing, and explain why these can disrupt food webs
- Identify one human health effect, one environmental effect, and one agricultural effect of climate change
- Understand positive feedback loops, especially ice-albedo feedback and greenhouse gas release from thawing permafrost

## 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 causes of sea-level rise?
2. Why does climate change increase flood risk even if annual rainfall does not rise very much?
3. How is ocean acidification different from ocean warming?
4. What is coral bleaching, and why is it not exactly the same as coral death?
5. Name two ways climate change can affect human health.

## Introduction

Major climate change effects are a high-yield AP Environmental Science topic because they connect atmospheric change to real biological, physical, and social outcomes. On the exam, you are often asked to explain chains such as increased greenhouse gases to warming to ecological change, or CO₂ increase to ocean acidification to shell damage.

What matters most is not memorising a loose list of impacts. You need to understand how the effects fit together. Warming changes sea level, precipitation, ocean chemistry, species distributions, agriculture, and human health. Many of these effects also reinforce each other, which is why climate change is difficult to slow once major trends are underway.

> **Key Term: global climate change**
> Long-term shifts in Earth’s climate, now driven mainly by human-caused increases in greenhouse gases.

### Sea-level rise and coastal change

Sea-level rise is one of the clearest major effects of climate change. For APES, remember the two main drivers: warming seawater expands, and land-based ice melts.

> **Key Term: thermal expansion**
> The increase in water volume as seawater warms.

This first cause is easy to miss because students often focus only on melting ice. But the ocean stores a huge amount of heat, and warmer water takes up more space. That means sea level can rise even before all possible ice-melt impacts are considered.

The second major cause is the melting of glaciers and ice sheets on land. This distinction matters:

- **Melting land ice** raises sea level
- **Melting floating sea ice** has much less direct effect on sea level

If a glacier in Greenland melts, that water enters the ocean and adds to total volume. If already-floating sea ice melts, it does not add much extra water level because it was already displacing water.

> **Key Term: sea-level rise**
> The long-term increase in average ocean height, caused mainly by thermal expansion and melting land ice.

Global mean sea level rose by about **0.20 m** from **1901 to 2018**, and the rate has accelerated. On the AP exam, you do not need every figure, but you do need the pattern: recent rise is faster than earlier background rates.

Why is sea-level rise so damaging? Because it changes the baseline from which all coastal hazards operate. A higher ocean means:

- more frequent coastal flooding
- more shoreline erosion
- greater saltwater intrusion into aquifers and soils
- more wetland and estuary loss
- stronger storm-surge damage
  > **Key Term: saltwater intrusion**
  > The movement of seawater into coastal aquifers or soils, reducing freshwater quality.

Saltwater intrusion harms both natural habitatss and people. Coastal wetlands can shift in salinity and lose species that cannot tolerate the change. Human communities may lose freshwater supplies for drinking or irrigation. Agricultural land can become less productive if salts build up in soils.

Sea-level rise also worsens storm damage. A storm surge starts from a higher baseline water level, so flooding reaches farther inland.

> **Test Tip:** If an FRQ asks why climate change increases coastal damage, do not stop at “sea level rises.” Add the next step: higher baseline sea level makes storm surge and flooding more destructive.

Some exam questions also test environmental justice. Low-income coastal communities, island populations, and some Indigenous communities often face the greatest risk because they have fewer resources for relocation, sea walls, insurance, or rebuilding.

### Extreme weather, heat, drought, and precipitation

Climate change does not mean every place simply gets hotter and wetter in the same way. Instead, warming shifts probabilities. Some extremes become more frequent, more intense, or both.

The most direct effect is more extreme heat. As average temperatures rise, heat waves become more common and more severe. This is dangerous because human bodies depend on cooling through sweating. High humidity makes that cooling less effective.

> **Key Term: wet-bulb temperature**
> A measure combining heat and humidity; high values are especially dangerous because sweating cools the body less effectively.

This helps explain why heat is not just uncomfortable but deadly. The very young, older adults, outdoor workers, and people without reliable cooling are especially vulnerable.

Warming also speeds up evaporation from soil and water. In some areas, this intensifies drought. Even if rainfall stays similar, hotter conditions can dry soils faster, stressing crops and natural vegetation.

> **Key Term: drought**
> A prolonged period of reduced water availability, often worsened by low precipitation and higher evaporation.

Drier vegetation then becomes fuel for wildfires. Climate change does not directly ignite every fire, but it can create hotter, drier, longer fire seasons in which fires spread more easily and burn more intensely.

At the same time, a warmer atmosphere can hold more water vapor. A useful rule is that air holds about **7% more water vapor per 1°C** of warming. This means storms and rainfall events can release more water in short periods.

That is why flood risk can increase even where annual rainfall changes little. More rain may come in intense bursts rather than in gentle, spread-out events.

> **Key Term: extreme precipitation**
> Very heavy rainfall events that increase flood and runoff risk.

The full chain looks like this:

- greenhouse gases rise
- air warms
- evaporation increases
- atmosphere holds more water vapor
- heavy downpours become more likely
- runoff, erosion, and flooding increase

This is much stronger than just saying “climate change causes floods.”

Storms need careful wording. APES examiners like precision here. Climate change does **not necessarily** mean more storms overall. It more reliably supports the idea of **stronger, wetter, and more damaging** storms in many cases, especially where warmer oceans and higher sea levels intensify impacts.

For tropical cyclones, warmer sea-surface temperatures provide more energy, warmer air can hold more water, and sea-level rise worsens storm surge. So the better phrasing is not “more hurricanes” but “heavier rainfall, greater intensity, and worse surge damage.”

> **Test Tip:** Avoid absolute claims such as “climate change causes all extreme weather.” Stronger exam phrasing is “climate change increases the frequency, intensity, or risk of some extreme events.”

### Oceans, acidification, and coral reef decline

Ocean change is a major climate topic because the ocean absorbs both heat and carbon dioxide from the atmosphere. That creates two different but linked problems: ocean warming and ocean acidification.

Ocean warming stresses marine organisms directly and contributes to sea-level rise through thermal expansion. It can also alter currents, habitats, and the geographic ranges of species.

Ocean acidification is a chemistry problem, not just a temperature problem.

> **Key Term: ocean acidification**
> The decrease in ocean pH when atmospheric CO₂ dissolves in seawater and forms carbonic acid.

The key chemical sequence is:

CO₂ + H₂O → H₂CO₃

Carbonic acid then releases hydrogen ions, which lower pH. Those hydrogen ions also reduce carbonate ion availability. That matters because many marine organisms need carbonate to build calcium carbonate shells or skeletons.

So the important biological chain is:

- atmospheric CO₂ rises
- more CO₂ dissolves into seawater
- carbonic acid forms
- H⁺ concentration rises
- pH falls
- carbonate ion availability drops
- shell-building becomes harder

Corals, mollusks, and some plankton are especially vulnerable. Since the preindustrial era, ocean pH has fallen by about **0.1 unit**, which corresponds to roughly a **26–30% increase in acidity**.

Students often confuse acidification with acid rain. They are different. Acid rain mainly involves atmospheric sulfur and nitrogen compounds falling onto land or water. Ocean acidification is driven mainly by dissolved atmospheric CO₂.

Coral reefs are threatened by both warming and acidification, but in different ways.

> **Key Term: coral bleaching**
> A stress response in which corals expel their symbiotic algae, usually because of unusually warm water.

Bleaching is mainly caused by heat stress. When corals lose their zooxanthellae, they lose much of their energy supply and turn pale. A bleached coral is stressed and starving, but not necessarily dead yet.

> **Key Term: zooxanthellae**
> Symbiotic photosynthetic algae that live in coral tissues and provide much of the coral’s energy.

Acidification, by contrast, weakens corals by making calcification harder. So warming causes bleaching, and acidification slows reef building and shell formation. Together, they are especially destructive.

This distinction is often tested:

- **Bleaching** = stress response, may be reversible
- **Coral death** = result if stress is severe or prolonged

At around **1.5°C** warming, warm-water coral reefs are projected to decline sharply. At **2°C**, most are projected to be lost. This is a strong example of how risks rise with each additional increment of warming.

Coral reef decline matters beyond coral itself. Reefs provide habitat, fisheries support, coastal protection, and tourism value. When reefs decline, biodiversity and human livelihoods decline too.

### Environments, agriculture, human health, and feedback loops

Climate change shifts where organisms can live and when they carry out seasonal life events.

> **Key Term: species range shifts**
> Changes in where species live, often poleward or upslope, as they track suitable climate conditions.

As temperatures rise, many species move toward cooler places: northward in the Northern Hemisphere, southward in the Southern Hemisphere, upslope on mountains, or into deeper/cooler waters. But movement is not always enough to save them. Species may face barriers, habitat fragmentation, competition, or simply no suitable place left to go.

Mountain-top and polar species are especially vulnerable because they can run out of space. A cold-adapted species already near the highest elevations or furthest poleward range may have nowhere cooler left to move.

Climate change also shifts **phenology**, the timing of biological events such as flowering, migration, or breeding. This can create mismatches. For example, plants may flower earlier, but pollinators may not arrive at the same time. That disrupts food webs.

Agriculture is affected through several pathways:

- heat stress reduces crop performance
- drought lowers soil moisture
- flooding damages fields and roots
- pests and pathogens shift their ranges
- changing rainfall disrupts planting and harvesting

Some colder regions may see short-term gains from longer growing seasons, but these gains are likely to be offset globally by losses elsewhere. On APES FRQs, a balanced answer should mention that climate impacts on crops are not identical everywhere.

Human health is affected in multiple ways:

- more heat illness and heat-related deaths
- respiratory harm from wildfire smoke and ozone
- injuries and deaths from flooding and storms
- food and water insecurity
- expanded risk from some vector-borne diseases
  > **Key Term: vector-borne disease**
  > A disease spread by another organism, such as mosquitoes or ticks.

Warmer conditions can lengthen breeding seasons and expand ranges for some disease vectors. That does not mean every disease spreads everywhere, but it does mean climate can increase risk in some regions.

Finally, climate systems include positive feedback loops that amplify change.

> **Key Term: positive feedback loop**
> A process in which an initial change causes further changes that amplify the original effect.

Two high-yield examples are:

1. **Ice-albedo feedback**
   - warming melts snow and ice
   - bright reflective surfaces are replaced by darker land or ocean
   - less sunlight is reflected
   - more heat is absorbed
   - more warming follows

2. **Permafrost feedback**
   - warming thaws permafrost
   - previously frozen organic matter decomposes
   - methane and CO₂ are released
   - greenhouse effect strengthens
   - more warming follows
     > **Key Term: permafrost**
     > Permanently frozen ground that can release CO₂ and methane when it thaws.
     >
     > **Key Term: albedo**
     > The reflectivity of a surface; snow and ice have high albedo, while darker land and water have lower albedo.

These feedbacks matter because they make climate change harder to slow. They do not start the original warming by themselves, but they intensify it once it begins.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Major climate change effects are linked through clear cause-and-effect chains, not isolated facts
- Sea-level rise is driven mainly by thermal expansion and melting land-based ice
- Melting floating sea ice has much less direct effect on sea level than melting land ice
- Sea-level rise increases coastal flooding, erosion, wetland loss, saltwater intrusion, and storm-surge damage
- Climate change increases the risk of extreme heat, drought, wildfire conditions, and heavy precipitation
- Warmer air can hold more water vapor, which helps explain more intense rainfall events
- Climate change can make some storms stronger and wetter, even if storm numbers do not always rise
- Ocean warming and ocean acidification are different processes with different biological impacts
- Ocean acidification occurs when CO₂ dissolves in seawater, forming carbonic acid and lowering pH
- Coral bleaching is mainly caused by heat stress and is not the same as coral death
- Climate change causes species range shifts, phenology changes, food-web disruption, and higher extinction risk
- Positive feedback loops such as ice-albedo feedback and permafrost methane release amplify warming

## Key Terms and Concepts

- global climate change
- thermal expansion
- sea-level rise
- saltwater intrusion
- wet-bulb temperature
- drought
- extreme precipitation
- ocean acidification
- coral bleaching
- zooxanthellae
- species range shifts
- vector-borne disease
- positive feedback loop
- permafrost
- albedo


Global Climate Change - Major climate change effects