Which description best defines an ecosystem?

A community of organisms interacting with one another and with the nonliving environment — B is correct because an ecosystem includes both interactions among organisms and interactions with nonliving surroundings. A describes a population, while C incorrectly excludes abiotic factors. D is incomplete because biomes and ecosystems are not identified by precipitation alone.

A forest floor has low light but high moisture, while the canopy has intense light and drying winds. Which conclusion is best supported by this pattern?

Abiotic conditions and resource distribution can create different opportunities and limits for species — B is correct because light and moisture vary across space, creating different conditions for organisms. A and D ignore spatial differences in resources. C is incorrect because species in different forest layers may still interact indirectly through shared resources and ecological relationships.

Two plant species live in the same forest. One uses dim understory light, while the other grows rapidly in bright canopy openings. What ecological process is illustrated when they use different light environments?

Resource partitioning — A is correct because the plants use different portions of the light resource, reducing direct competition. B is not the process described, C contradicts the variation in light availability, and D reverses the energy transformation explained in the article.

Which climate and vegetation combination most strongly identifies a savanna rather than a temperate seasonal forest?

Warm temperatures, a distinct rainy season, a distinct dry season, and grasses with scattered trees — B is correct because savannas are warm and have distinct rainy and dry seasons, with grasses and scattered trees. A, C, and D describe the year-round precipitation and four-season pattern associated with a temperate seasonal forest rather than a savanna.

A climate record shows four seasons, moderate temperatures, and precipitation in every month. Which biome is the best interpretation?

Temperate seasonal forest — B is correct because four distinct seasons combined with precipitation throughout the year are characteristic of a temperate seasonal forest. A would have warm conditions with a pronounced rainy season and dry season. The article does not provide the defining climate patterns for deserts or chaparral in this comparison.

Why should a biome not be identified from temperature alone?

Biome identification requires matching temperature, precipitation amount, precipitation timing, and characteristic vegetation and organisms — B is correct because temperature, precipitation amount, precipitation timing, and ecosystem structure must be considered together. A is false because temperature strongly affects which organisms can survive. C is false because precipitation patterns differ among biomes. D is incorrect because elevation, seasonal patterns, and local conditions also matter.

From the free course AP Environmental Science on Lykke

9.3 The Greenhouse Effect · 9.4 Increases in Greenhouse Gases

Key concepts: The greenhouse effect · Greenhouse gases · Increases in greenhouse-gas concentrations · Global climate change · Global warming potential (GWP) · Atmospheric residence time · Antarctic ice-core proxy data · Relationship between atmospheric carbon dioxide and global temperature · Climate-change feedbacks · Melting sea ice and thawing tundra

Earth’s atmosphere stays warm because certain gases absorb and re-emit outgoing infrared radiation, creating the greenhouse effect. Without this natural process, Earth’s surface would be too cold for life as we know it; the problem arises when human activities increase the concentration of greenhouse gases and…

9.3 The Greenhouse Effect · 9.4 Increases in Greenhouse Gases

Earth’s atmosphere stays warm because certain gases absorb and re-emit outgoing infrared radiation, creating the greenhouse effect. Without this natural process, Earth’s surface would be too cold for life as we know it; the problem arises when human activities increase the concentration of greenhouse gases and disturb the atmosphere’s energy balance.

The natural greenhouse effect

Sunlight passes through the atmosphere and warms Earth’s surface. The surface then releases energy as infrared radiation. Greenhouse gases absorb some of that outgoing energy and re-emit it in different directions, including back toward the surface.

STB-4.C.3: The greenhouse effect results in the surface temperature necessary for life on Earth to exist.

The greenhouse effect is therefore not itself pollution. A useful analogy is a blanket: a blanket is necessary to prevent excessive heat loss, but adding many more blankets makes the body too warm. Similarly, excess greenhouse gases strengthen the natural greenhouse effect and contribute to global climate change, meaning long-term changes in Earth’s climate patterns.

Principal greenhouse gases

STB-4.C requires identification of the greenhouse gases. The principal gases are carbon dioxide, methane, water vapor, nitrous oxide, and chlorofluorocarbons, or CFCs.

Greenhouse gas	Important human sources	Key climate concept
Carbon dioxide, $CO_2$	Fossil-fuel combustion, deforestation, cement production	Reference gas with GWP $=1$
Methane, $CH_4$	Livestock, landfills, rice cultivation, fossil-fuel extraction	High potency but short atmospheric residence time
Nitrous oxide, $N_2O$	Agricultural soils and nitrogen fertilizers	Higher GWP than methane
CFCs	Older refrigerants and industrial compounds	Highest GWP among the gases listed
Water vapor, $H_2O$	Evaporation and transpiration; mainly natural sources	Greenhouse gas with a short residence time

STB-4.C.1 identifies the principal greenhouse gases, including carbon dioxide, methane, water vapor, nitrous oxide, and CFCs. Water vapor is a greenhouse gas, but STB-4.C.2 emphasizes that its short atmospheric residence time means it does not contribute significantly to long-term global climate change compared with longer-lived increases in gases such as carbon dioxide.

Global warming potential and residence time

Global warming potential (GWP) compares how strongly a greenhouse gas contributes to warming relative to carbon dioxide over a specified time period. Carbon dioxide is assigned a GWP of $1$, making it the reference point.

STB-4.D.1 establishes the required potency ranking: CFCs have the highest GWP, followed by nitrous oxide, then methane, with carbon dioxide as the reference gas.

A gas’s atmospheric residence time is how long it remains in the atmosphere before chemical reactions, biological uptake, or other processes remove it. GWP and residence time are related but not identical: a gas can be extremely potent while present in relatively small quantities, whereas a less potent gas can have a major long-term effect if it accumulates and remains for a long time.

Misconception check — “The most powerful gas must cause the most total warming.” Not necessarily. Climate impact depends on potency, atmospheric concentration, residence time, and the amount released. Methane has a shorter residence time than carbon dioxide, but large methane emissions can still produce substantial warming.

Ice cores as evidence of past climate

Antarctic ice cores provide proxy evidence—indirect evidence used to reconstruct past conditions. As snow accumulates and becomes compressed into ice, tiny bubbles of ancient atmosphere remain trapped inside. Scientists can measure past carbon dioxide concentrations in those bubbles, while chemical properties of the surrounding ice provide information about past temperature conditions.

Across the historical record preserved in Antarctic ice, atmospheric carbon dioxide levels and global temperature show a general relationship: periods with higher atmospheric carbon dioxide tend to correspond to higher temperatures, while periods with lower carbon dioxide tend to correspond to lower temperatures. The correlation does not mean that carbon dioxide is the only climate influence, but it is important evidence that atmospheric composition and temperature are connected.

Feedback from thawing landscapes

Warming can trigger processes that add still more greenhouse gases to the atmosphere. Melting sea ice changes Earth’s reflective surface, while thawing tundra can expose previously frozen organic matter to decomposition. That decomposition can release carbon dioxide and methane, creating a reinforcing feedback: warming promotes greenhouse-gas release, which can promote additional warming.

A local activity can therefore produce a global consequence. For example, methane released from a regional landfill or fossil-fuel operation mixes through the atmosphere and contributes to climate change beyond the location where it was emitted. This is the meaning of the required connection that local and regional human activities can have impacts at the global level.

Worked reasoning: an evidence chain

Scenario: A region expands natural-gas extraction, and monitoring shows increased methane emissions. Explain why the activity can affect global climate.

The extraction activity increases atmospheric $CH_4$ concentration.
Methane is a greenhouse gas, so it absorbs outgoing infrared radiation.
Methane has a higher GWP than carbon dioxide, although its atmospheric residence time is relatively short.
The enhanced greenhouse effect contributes to global climate change.
Because methane mixes through the atmosphere, a regional emission can produce a global impact.

AP Science Practice 1.B — Concept Explanation: A strong explanation connects each cause to its mechanism and consequence. Do not merely write “methane causes warming”; identify methane as a greenhouse gas, explain its role in absorbing infrared radiation, and connect increased concentration to an enhanced greenhouse effect.

Retrieval check

Why can methane contribute strongly to climate change even though it has a short atmospheric residence time, and why is water vapor treated differently in this context? A complete answer should mention methane’s relatively high GWP, its emission-driven increase, and water vapor’s short residence time.