AP Environmental Science

## Learning Outcomes

- Distinguish the natural greenhouse effect from the enhanced greenhouse effect
- Explain how increasing greenhouse gases cause warming through greater heat trapping
- Identify major physical impacts, including sea-level rise, ice melt, and changing precipitation
- Understand how ocean warming and ocean acidification differ and why both matter
- Describe positive feedback loops such as ice-albedo, water-vapor, and permafrost thaw
- Apply cause-and-effect chains to environmental systems and human impacts likely to appear on the AP Environmental Science exam

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand the impacts of increasing greenhouse gases, with a focus on the following syllabus points:

- Know that increasing atmospheric greenhouse gas concentrations strengthen the greenhouse effect and increase global average temperature
- Identify major impacts of warming, including heat waves, altered precipitation, melting glaciers and ice sheets, and sea-level rise
- Explain that sea-level rise is driven mainly by **thermal expansion** and melting of **land-based ice**
- Understand that ocean acidification is caused by atmospheric CO2 dissolving into seawater and forming carbonic acid
- Explain how warming affects natural systemss through range shifts, coral bleaching, biodiversity loss, and changing food webs
- Identify human impacts such as coastal flooding, agricultural stress, heat-related illness, and spread of disease vectors
- Understand positive feedback mechanisms including ice-albedo feedback, water-vapor feedback, and permafrost-thaw feedback

## 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 is the difference between the greenhouse effect and the enhanced greenhouse effect?
2. What are the two main causes of sea-level rise?
3. How does ocean acidification happen, and why does it harm corals and shell-forming organisms?
4. Why does melting land ice raise sea level more directly than melting sea ice?
5. Name two positive feedback loops that can make climate warming stronger.

## Introduction

Increasing greenhouse gases is one of the most tested parts of AP Environmental Science because it links chemistry, energy transfer, climate systems, ecologys, and human impacts. You need more than a list of gases. The exam often asks for the full chain: human activities increase greenhouse gases, greenhouse gases trap more outgoing infrared radiation, the planet warms, and that warming changes oceans, weather, biodiversity, and human systems.

This topic also includes some very common exam traps. Students often confuse the natural greenhouse effect with the human-enhanced one, mix up ocean warming and ocean acidification, or say that melting sea ice is the main direct cause of sea-level rise. Strong answers avoid those mistakes and explain each step clearly.

> **Key Term: enhanced greenhouse effect**
> The extra warming caused when human activities increase greenhouse gas concentrations, causing more outgoing infrared radiation to be trapped in the lower atmosphere.

### From more greenhouse gases to global warming

The greenhouse effect is natural and necessary. Without it, Earth would be too cold for most life as we know it. Greenhouse gases such as carbon dioxide, methane, nitrous oxide, and water vapor absorb outgoing infrared radiation and reradiate some of that heat back toward the surface. The problem is not the existence of the greenhouse effect. The problem is that human activity has intensified it.

Burning coal, oil, and natural gas adds large amounts of carbon dioxide to the atmosphere. Deforestation adds more because trees that once stored carbon are removed or burned, and fewer plants remain to absorb CO2 through photosynthesis. Agriculture adds methane from livestock and rice cultivation, plus nitrous oxide from fertilizer use. Waste systems add methane from landfills and wastewater.

> **Key Term: radiative forcing**
> A change in Earth’s energy balance. Increasing greenhouse gases create positive radiative forcing because they reduce the amount of heat escaping to space.

Why does this matter? Because climate is an energy system. If more energy enters and stays in the Earth system than leaves it, the system warms. That warming is measured as **global warming**, the rise in average surface temperature. **Climate change** is broader: it includes warming, but also changes in rainfall, storms, ocean conditions, and natural systemss.

A good APES answer often uses the chain explicitly:

- More greenhouse gases
- More infrared absorption
- More heat retained
- Higher global temperature
- Physical, biological, and human impacts
  > **Test Tip:** On FRQs, do not stop at “more greenhouse gases cause warming.” Add at least one linked consequence, such as “warming causes thermal expansion of seawater, which raises sea level.”

### Physical impacts: warming, ice melt, and sea-level rise

One of the clearest impacts of increasing greenhouse gases is rising global temperature. Warmer air increases the frequency of hot extremes and heat waves. It also changes the water cycle. Warm air can hold more water vapor, so some places experience heavier rainfall, while others face stronger drying because evaporation rises and rainfall patterns shift.

Glaciers and ice sheets are also affected. Mountain glaciers have been shrinking for more than a century, and global glacier area has dropped greatly since the late nineteenth century. Land ice melting matters because it adds water to the oceans.

> **Key Term: sea-level rise**
> The long-term increase in average ocean level, driven mainly by warming seawater expanding and by melting land-based ice.

A very common exam distinction is this:

- **Melting land ice** raises sea level because it adds water that was stored on land to the ocean.
- **Melting sea ice** has little direct effect on sea level because it is already floating.

Students often get this wrong by saying “melting polar ice” without distinguishing sea ice from land ice. Greenland and Antarctica matter especially because they contain huge stores of land-based ice.

The second main cause of sea-level rise is **thermal expansion**. Water expands when it warms, so even without adding more water, warmer oceans take up more space.

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

This is why sea-level rise is not just an ice story. It is also an ocean heat story. Oceans absorb more than 90% of the excess heat trapped by greenhouse gases. That stored heat makes sea-level rise a long-lasting process.

Sea-level rise has major consequences:

- coastal flooding
- shoreline erosion
- saltwater intrusion into freshwater aquifers
- wetland loss
- infrastructure damage
- displacement of people

Low-lying coasts, deltas, and island communities are especially exposed. Even relatively small increases in average sea level can make storm surges more destructive because the starting water level is already higher.

> **Test Tip:** If asked to explain sea-level rise, name both **thermal expansion** and **melting land ice**. You will often earn more than one scoring point by doing both clearly.

### Ocean impacts: warming, acidification, and marine food webs

Ocean warming and ocean acidification are linked to increasing greenhouse gases, but they are not the same process.

Ocean warming happens because the oceans absorb excess heat from the atmosphere. This changes sea surface temperatures, strengthens marine heat stress, and can alter currents and mixing. Warm water also holds less dissolved oxygen than cooler water, which stresses marine organisms.

Ocean acidification is a chemistry problem caused mainly by atmospheric CO2 dissolving into seawater.

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

The basic reactions are:

- CO2 + H2O ⇌ H2CO3
- H2CO3 ⇌ H+ + HCO3-

The key point is that more dissolved CO2 means more hydrogen ions. More H+ lowers pH and also reduces the availability of carbonate ions that many organisms need to build calcium carbonate shells and skeletons.

This is why corals, mollusks, and some plankton are vulnerable. Corals already face heat stress from warming water. Add acidification, and reef building becomes even harder.

Coral bleaching is a major example of combined stress. When corals are exposed to unusually warm water, they expel their symbiotic algae. Without these algae, corals lose much of their energy source and turn white. Some recover, but many do not.

The exam may ask you to separate these processes:

- **Ocean warming**: driven by excess heat
- **Ocean acidification**: driven by CO2 dissolution and changed seawater chemistry

Do not say acidification happens because the ocean is warmer. Warmer water can affect gas exchange, but acidification itself is mainly about more atmospheric CO2 entering the ocean.

Marine food webs can be affected in several ways:

- reduced calcification in shell-forming organisms
- coral reef decline
- lower oxygen availability
- changes in species ranges
- altered breeding and migration patterns

Because plankton and small marine organisms form the base of many food webs, changes at these levels can affect fish, seabirds, and marine mammals higher up.

### Environment and human impacts on land

As temperatures rise, many species shift their ranges poleward or upslope. This happens because climate zones move, and organisms try to stay within their tolerance ranges. Generalist species often adjust more easily than specialists. Species already living near mountaintops, polar regions, or narrow habitat limits may have nowhere left to go.

> **Key Term: range shift**
> A movement of a species’ geographic distribution, often toward cooler latitudes or higher elevations as climate conditions change.

Changes in timing are also important. Flowering, migration, breeding, and insect emergence may happen earlier in warmer conditions. These shifts can break ecological relationships. For example, if pollinators and flowering plants no longer peak at the same time, reproduction can suffer.

Agriculture is affected in mixed ways. Some cold regions may initially see longer growing seasons, but this benefit is often limited. Heat stress, drought, flooding, pest expansion, and soil degradation can reduce yields. CO2 fertilization is sometimes mentioned, but it is not a guarantee of higher crop production because plants still need nutrients, water, and tolerable temperatures.

Human health impacts include:

- more heat-related illness and death
- more exposure to wildfire smoke in some regions
- increased flood risk
- spread of some insect-borne diseases
- some increases in water-borne disease risk where warm or stagnant water expands pathogen and vector habitat

Warmer and wetter conditions can favor mosquitoes in some places, extending breeding seasons and geographic ranges. Warmer water can also increase bacterial activity in water supplies. However, do not claim climate change directly causes every outbreak. Better wording is that it can increase conditions favorable for transmission.

There are also economic and social effects:

- damage to transport and coastal infrastructure
- higher disaster recovery costs
- pressure on water supplies
- losses in fisheries and tourism
- displacement of vulnerable communities

These impacts are often unequal. Communities with fewer resources to adapt may be hit hardest even when they contributed less to greenhouse gas emissions.

### Positive feedbacks that make warming stronger

Feedbacks are crucial because they explain why warming can accelerate.

> **Key Term: positive feedback**
> A process that amplifies an initial change, making that change stronger.

One major example is the **ice-albedo feedback**. Snow and ice reflect a lot of sunlight. When they melt, darker surfaces such as ocean water or land are exposed. Darker surfaces absorb more energy, which causes more warming, which melts more ice.

A second example is the **water-vapor feedback**. Warmer air can hold more water vapor, and water vapor is itself a greenhouse gas. So warming can increase atmospheric water vapor, which can then trap more heat. For APES, remember that water vapor is mainly a feedback, not the main initial human forcing.

A third example is **permafrost thaw**. Frozen soils in high latitudes contain large stores of organic matter and methane. As permafrost thaws, decomposition and gas release increase, adding more greenhouse gases to the atmosphere.

There is also an ocean-related feedback. Warmer water holds gases less effectively, so warming can reduce the ocean’s capacity as a carbon sink. If oceans absorb a smaller share of atmospheric CO2, more remains in the atmosphere, which adds more warming.

These feedbacks matter especially in polar regions, where warming has been especially strong. Polar amplification is one reason climate change effects are often most dramatic there.

> **Test Tip:** If an FRQ asks for a feedback loop, write it as a sequence with arrows, such as “warming → ice melt → lower albedo → more absorption of solar energy → more warming.”

## Key Point Checklist

_This article has covered the following key knowledge points:_

- The natural greenhouse effect keeps Earth warm enough for life, but the enhanced greenhouse effect is human-caused extra warming
- Increasing greenhouse gases create positive radiative forcing by trapping more outgoing infrared radiation
- Global warming is the temperature rise itself, while climate change includes wider long-term changes in weather patterns, oceans, and natural systemss
- Major greenhouse gas impacts include rising temperatures, stronger heat extremes, shifting precipitation, and some increases in drought risk
- Sea-level rise is caused mainly by thermal expansion and melting land-based ice
- Melting sea ice has little direct effect on sea level, unlike melting glaciers and ice sheets on land
- Ocean warming and ocean acidification are linked but distinct: warming is a heat process, acidification is a CO2 chemistry process
- Ocean acidification lowers pH and reduces carbonate availability, making shell building harder for corals, mollusks, and some plankton
- Ecological system impacts include coral bleaching, range shifts, altered seasonal timing, and biodiversity loss
- Human impacts include coastal flooding, infrastructure damage, crop stress, heat-related illness, and disease-vector expansion
- Positive feedback loops include ice-albedo feedback, water-vapor feedback, permafrost-thaw feedback, and reduced ocean carbon uptake
- Strong APES answers use clear cause-and-effect chains rather than isolated facts

## Key Terms and Concepts

- enhanced greenhouse effect
- radiative forcing
- sea-level rise
- thermal expansion
- ocean acidification
- range shift
- positive feedback


Increases in the Greenhouse Gases - Impacts of increasing greenhouse gases