9.4 Increases in the Greenhouse Gases

When greenhouse gases build up in the atmosphere, more thermal energy gets trapped near Earth's surface and average global temperature rises. That warming drives problems like rising sea levels from melting ice and ocean expansion, plus disease vectors spreading from the tropics toward the poles, which can shift where and how human and animal populations live.

Why This Matters for the AP Environmental Science Exam

This topic connects greenhouse gas increases to real consequences for human health and the environment, which is exactly the kind of cause-and-effect reasoning the AP Environmental Science exam rewards. You should be able to identify a threat from rising greenhouse gases and then explain the mechanism behind it, not just name it.

Multiple-choice questions may ask you to match a cause (more CO2, melting ice, warmer oceans) to an effect (higher seas, flooding, shifting disease ranges). On free-response questions, you may need to describe an environmental problem caused by climate change and explain how it happens step by step. Linking each effect back to greenhouse gases and global warming is what earns points.

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Key Takeaways

• Excess greenhouse gases trap more thermal energy in the troposphere, raising average global temperature; even about 1 degree of warming can cause widespread change.
• Climate means long-term (30+ year) weather patterns, including temperature and average precipitation, not a single day's weather.
• Sea levels rise from two main causes: melting land ice (ice sheets, glaciers, permafrost) and thermal expansion of warming ocean water.
• Warming lets disease vectors like mosquitoes and ticks expand from the tropics toward the poles, spreading the diseases they carry.
• These changes can shift population dynamics and force population movements as habitats become less livable.
• Be ready to explain the mechanism behind each effect, not just list it.

Why Greenhouse Gases Are Increasing

Climate is the long-term (30+ years) weather pattern in an area, including temperature and average precipitation. When greenhouse gases increase, more thermal energy is trapped in the troposphere, which raises average global temperature. A rise of even one degree can make environments harder to live in for human and animal populations, affecting where they go and how their numbers change.

Industrialization drives much of this. As nations industrialize, they demand more energy, and most of that energy comes from burning fossil fuels, which releases CO2 and other greenhouse gases. The result is a sharp increase in atmospheric greenhouse gas concentrations over time.

The Intergovernmental Panel on Climate Change (IPCC) assesses climate change and its environmental and economic impacts. Its assessments help connect rising carbon dioxide to temperature increases and the consequences that follow.

Sources of the Main Greenhouse Gases

You do not need to memorize every source, but understanding where each gas comes from helps you explain why concentrations are climbing.

Carbon dioxide (CO2)

• Natural sources: cellular respiration, volcanic eruptions, and decay of organic matter.
• Human sources: burning fossil fuels (coal, oil, natural gas) for electricity, transportation, and industry; deforestation, which removes trees that absorb CO2; land-use changes like converting forests to farmland; and industrial processes such as cement production.

Methane (CH4)

• Natural sources: wetlands, the guts of animals, and wildfires.
• Human sources: fossil fuel extraction and transport; landfills, where organic waste breaks down with little oxygen; and agriculture, especially raising livestock and decomposing manure.

Water vapor

• Natural sources: evaporation, plant transpiration, and animal respiration.
• Human sources: some industrial processes and agricultural activities like irrigation.
• Note: water vapor is a greenhouse gas, but it has a short residence time in the atmosphere, so it does not contribute significantly to long-term climate change.

Nitrous oxide (N2O)

• Natural sources: soil decomposition and denitrification in the nitrogen cycle, plus lightning.
• Human sources: nitrogen-based fertilizers and manure, industrial production of nitric acid, fossil fuel combustion, and decomposition in landfills and sewage treatment.

Chlorofluorocarbons (CFCs)

• All main sources are human-made: refrigerants, solvents, and foam-blowing agents.
• CFCs have an extremely high global warming potential and are no longer widely produced, but existing CFCs in the atmosphere still contribute to warming.

Threats to Human Health and the Environment

Rising global temperatures cause a wide range of effects. The two that connect most directly to this topic are rising sea levels and the spread of disease vectors, but warming triggers several linked problems.

Rising Sea Levels

Melting polar ice caps, ice sheets, permafrost, and glaciers add water to the oceans, contributing to higher sea levels and coastal flooding. Rising temperatures speed up how fast meltwater flows from land into the sea, and this can cause erosion and damage to coastal communities and ecosystems.

A second cause is thermal expansion. As ocean water warms, its molecules move and vibrate more, so the water takes up more space. Warmer oceans expand, which raises sea levels even without added meltwater.

Spread of Disease Vectors

As Earth warms, diseases once confined to the tropics can spread toward the poles. Disease vectors such as mosquitoes and ticks can survive and reproduce in areas that were previously too cold, expanding their range and carrying the diseases they transmit into new regions.

Warming also shifts where host plants and animals live. When host ranges move, they can bring hosts into contact with new vector populations, raising the risk of disease transmission and changing the balance of ecosystems.

Ocean Acidification

Climate change and ocean acidification share a cause: rising atmospheric carbon dioxide. When CO2 from burning fossil fuels enters the air, the oceans absorb a large part of it. Dissolved CO2 lowers the pH of seawater, making it more acidic.

Acidification makes it harder for organisms like corals and shellfish to build and maintain shells and skeletons because it reduces available calcium carbonate. This can lower their populations and indirectly affect other species by altering the food chain and the structure of marine communities.

Extreme Weather and Changing Patterns

You cannot tie any single weather event directly to global warming, but the overall trend points to more frequent and intense extreme events, including droughts, heatwaves, flooding, and hurricanes. Higher temperatures can change atmospheric and oceanic circulation. More heat means more evaporation, which can worsen droughts, while more water vapor in the air can change precipitation patterns and fuel heavier storms.

Loss of Biodiversity

As temperatures rise, many species struggle to survive in their current habitats and may be displaced toward more suitable areas. If no suitable habitat is available, or if species cannot migrate or adapt fast enough, they may go extinct.

Warming also harms biodiversity indirectly. Droughts and heat waves reduce water availability and raise competition for resources, which can drive declines in certain species. More frequent floods and hurricanes can disrupt ecosystems and reduce small populations to the point where genetic diversity suffers.

How to Use This on the AP Environmental Science Exam

MCQ

Expect questions that pair a cause with an effect. Practice tracing the chain: more greenhouse gases trap more thermal energy, which raises temperature, which melts ice and expands ocean water, which raises sea level. Watch for answer choices that name a real effect but pair it with the wrong cause.

Free Response

When asked to describe an environmental problem from climate change, name the problem and then explain the mechanism. For sea-level rise, mention both melting land ice and thermal expansion. For disease spread, explain that warming lets vectors survive in new areas farther from the tropics. Always connect the effect back to greenhouse gases and warming so the cause-and-effect reasoning is clear.

Common Trap

Identifying an effect is not the same as explaining it. If a prompt asks how or why, give the step-by-step process, not just a label.

Common Misconceptions

• Weather is not climate. A cold day does not disprove warming. Climate is the long-term (30+ year) average pattern of temperature and precipitation.
• Sea-level rise is not only from melting ice. Thermal expansion of warming ocean water is a major cause on its own.
• Water vapor does not drive long-term climate change. It is a greenhouse gas, but its short residence time in the atmosphere keeps it from contributing significantly to lasting climate change.
• Disease vectors spread toward the poles, not away from them. Warming lets mosquitoes and ticks survive in regions that used to be too cold.
• You cannot blame one storm on climate change. Single events cannot be tied directly to warming, but the overall frequency and intensity of extreme events trends upward.
• A small population is not automatically a threatened one. Population size alone does not determine extinction risk; the ability to adapt or relocate matters more.

Related AP Environmental Science Guides

• 9.1 Stratospheric Ozone Depletion
• 9.3 The Greenhouse Effect
• Major Environmental Disasters
• 9.8 Invasive Species
• Unit 9 Overview: Global Change
• 9.2 Reducing Ozone Depletion