Global Effects of Increasing Greenhouse Gases
Climate is defined as the long-term (30+ years) weather patterns in a given area. This includes temperature and average precipitation. An increase in the amount of greenhouse gases can cause more thermal energy to be trapped in the troposphere. This will result in an increase in the average global temperature. If the average global temperature increases by even one degree, it can cause widespread environmental changes. These changes can make the environment uninhabitable for human and animal populations, affecting population movements and dynamics.
The Intergovernmental Panel on Climate Change (IPCC) was formed in 1988 as a group of 3,000 scientists with the goal of working together to assess climate change. IPCC’s goal is to determine the environmental and economical impact potential of climate change. IPCC scientists have developed an in-depth understanding of how increasing carbon dioxide dictates temperature increases and the environmental consequences of these changes.
The chart above depicts the increase in CO2 production by countries individually and collectively. Overall, the industrialization of nations creates a demand for energy. This energy is primarily generated from the burning of fossil fuels, which results in a sharp increase in CO2 and other greenhouse gases.
Changes to The Environment
Increasing global temperatures are expected to have a large range of impacts on the environment. Some of these effects include:
Rising Sea Levels 🌊
The melting of polar ice caps, ice sheets, permafrost, and glaciers can contribute to rising sea levels and flooding, which can have serious impacts, like erosion, on coastal communities and ecosystems. Rising temperatures increase the rate at which melted water flows from the land into the oceans.
Additionally, as the temperature of the Earth's oceans increases, the water expands. This is a result of warmer water taking up more space than colder water, due to the increased movement and vibration of the water molecules. As the oceans warm, they expand.
Spread of Disease Vectors 🦠🦟
Diseases that were previously confined to the tropics will begin to spread toward the poles. As the Earth's temperature increases, some disease vectors, such as mosquitoes and ticks, are able to survive and reproduce in areas where they previously could not. This can lead to the expansion of the range of these vectors and the diseases they transmit.
Since warming can also affect the distribution and abundance of the animals and plants that serve as hosts for disease vectors. As the climate changes, the ranges of host species may shift, bringing them into contact with new populations of vectors and increasing the risk of disease transmission. Or warmer temperatures may lead to changes in the distribution and abundance of certain species, which can disrupt the balance of ecosystems and increase the risk of disease outbreaks due to population density impacts.
Ocean Acidification 🐚
Climate change and ocean acidification are both caused by the increasing levels of carbon dioxide in the Earth's atmosphere. When excess amounts of carbon dioxide are released into the air through the burning of fossil fuels, some of it is absorbed by the oceans. This process increases the concentration of dissolved carbon dioxide in the water, which in turn decreases the pH of the ocean, making it more acidic.
The process of ocean acidification can affect the ability of some marine organisms, such as corals and shellfish, to build and maintain their shells and skeletons, by reducing access to calcium—an effect that can lead to declines in their populations. Acidification can also have indirect effects on other species by altering the food chain and the overall structure of marine communities.
Extreme Weather Events and Changing Weather Patterns 🌧️
It is important to note that the relationship between warming trends and extreme weather events is complex, and it is not possible to attribute any single extreme weather event directly to global warming. However, the overall trend is for an increase in the frequency and intensity of extreme weather events, including droughts, heatwaves, flooding, and hurricanes, due to climate change.
The increase in global temperatures caused by rising levels of greenhouse gases in the atmosphere can lead to changes in atmospheric and oceanic circulation patterns, which can in turn affect the occurrence of extreme weather events. For example, higher temperatures can lead to more evaporation, which can contribute to drought severity. Or an increase in the amount of water vapor in the atmosphere may change precipitation patterns and fuel more intense storms, heavy rainfall, and hurricanes.
Loss of Biodiversity 🐘
Changes in climate can have a variety of impacts on ecosystems and species, leading to the loss of biodiversity.
As temperatures rise, many species may find it increasingly difficult to survive in their current habitats. This can lead to the displacement of species, as they are forced to move to new areas in search of more suitable conditions. However, if suitable habitats are not available, or if species are unable to migrate or evolve fast enough to keep up with the changing climate, they may become extinct.
In addition to the direct impacts of rising temperatures, warming can also indirectly contribute to the loss of biodiversity through its effects on other environmental factors, such as the availability of water, the severity of natural disasters, and the spread of diseases. For example, when droughts and heat waves caused by increased global temperatures reduce the availability of water, rising resource competition can contribute to declines in certain species. Similarly, more frequent and severe natural disasters, such as floods and hurricanes, can create genetic drift and disrupt ecosystems to cause species loss.
Review: Why are Greenhouse Gases Increasing?
Carbon Dioxide
Natural sources of carbon dioxide emissions:
- Carbon dioxide naturally occurs as a byproduct of the metabolic reaction of cellular respiration.
- Volcanic eruptions release a large amount of CO2 and ash, which upon release can have significant short-term effects on climate. Historic large-scale eruptions have been at fault for reducing the global average temperature by blocking solar radiation.
- The decay of organic matter is another natural source of CO2. As plants and animals die, they decompose, releasing CO2 into the atmosphere.
Human-induced sources of increased CO2 emissions include:
- Carbon dioxide is produced during the combustion of fossil fuels. Burning of fossil fuels, including the use of coal, oil, and natural gas for electricity generation, transportation, and industrial processes, is the largest source of CO2 emissions into the atmosphere.
- Deforestation is another human-induced source of CO2 emission issues. Trees absorb CO2 from the atmosphere as they grow and photosynthesize. When trees are cut down, the carbon dioxide intake process is disrupted, leading to an increase in atmospheric CO2. An example of this is mass deforestation in the Amazon Rainforest, which is a terrestrial carbon dioxide sink.
- Land use changes feed into deforestation and CO2 emissions. Changes in land use, such as the conversion of forests to agricultural land can increase the release of CO2 into the atmosphere.
- Industrial manufacturing and processes also emit carbon dioxide. From cement production to waste decomposition, industries also contribute to increased carbon dioxide.
Methane
Natural sources of methane emissions include:
- Wetlands, such as swamps and marshes, produce methane through the decomposition of organic matter.
- In the guts of animals, methane can be produced, meaning animals can create methane through the digestion of their food.
- Wildfires can also release methane into the atmosphere.
Human-induced sources of methane include:
- Fossil fuel extraction and transportation can contribute to methane emissions. The extraction and transportation of fossil fuels, including coal, oil, and natural gas, can release methane into the atmosphere.
- Landfills are a significant source of methane emissions, as organic waste breaks down in low-oxygen conditions and releases methane gas into the atmosphere.
- Agricultural activities are a significant source of increased atmospheric methane. The raising of livestock, including cows, sheep, and goats, is a major source of methane emissions due to the digestion of their food and the decomposition of manure. Increased cattle farming created a large-scale methane increase.
- Some industrial processes, such as the production of iron and steel, also emit methane.
Water Vapor
Natural sources of water vapor include:
- Water vapor is released into the atmosphere through the process of evaporation, which occurs when the sun's energy heats the surface of oceans, lakes, and other bodies of water.
- Plants release water vapor into the atmosphere through a process called transpiration, which occurs when water is absorbed by the roots and transported to the leaves, where it is released into the air through small pores called stomata.
- Animals release water vapor into the atmosphere through the process of cellular respiration, which occurs when they breathe out air that contains water vapor.
Human-induced sources of increased water vapor include:
- Some industrial processes, such as the production of electricity, can release water vapor into the atmosphere as a byproduct of burning fossil fuels.
- Agricultural activities, such as irrigation and the application of fertilizers, can also release water vapor into the atmosphere.
Nitrous Oxide
Natural sources of nitrous oxide include:
- Nitrous oxide is naturally produced in soil through the decomposition of organic matter and denitrification during the nitrogen cycle, especially in low-oxygen environments like wetlands and the oceans.
- Lightning also can break down nitrogen molecules and produce nitrous oxide in the atmosphere.
Human-induced sources of increased atmospheric nitrous oxide:
- Agricultural activities are the main source of increased nitrous oxide. Nitrous oxide is produced in agriculture through the use of nitrogen-based fertilizers and the application of animal manure to fields.
- The industrial production of nitric acid and the burning of fossil fuels, especially the burning of coal for electric power generation, can also release a significant amount of nitrous oxide into the atmosphere.
- The decomposition of organic waste in landfills and sewage treatment plants can also release nitrous oxide into the atmosphere.
Chlorofluorocarbons (CFCs)
All of the main sources of CFCs are human-induced:
- CFCs were widely used as refrigerants in refrigerators, air conditioners, and other cooling systems.
- Solvents to clean electronic equipment and manufacture metal.
- CFCs were used as foam-blowing agents in the production of foam products, such as insulation and packaging materials.
CFCs are no longer produced or used in many countries due to the negative impacts they have on the ozone layer and their exceptionally high GWP, but existing CFCs in the atmosphere continue to have detrimental effects on warming.
🎥 Watch: AP Environmental Science Streams
Frequently Asked Questions
What are greenhouse gases and why are they bad for the environment?
Greenhouse gases (GHGs) are atmospheric gases—mainly CO2, CH4 (methane), N2O, and some fluorinated gases—that trap outgoing infrared radiation and warm Earth (the greenhouse effect). More GHGs means more warming: global climate change. That’s bad because it causes many linked environmental and human-health problems listed in the CED: sea level rise from melting ice sheets and ocean thermal expansion, permafrost thaw and methane release, ocean acidification (more CO2 dissolved lowers pH), coral bleaching and harmful algal blooms, shifts in species ranges (vector-borne diseases like malaria and dengue move poleward), heat-related mortality, coastal flooding and climate refugees, and declines in crop yields (CED Topic 9.4, EK STB-4.E.1). On the AP exam you should be able to name GHGs, link them to these impacts, and give examples of mitigation (e.g., switching to renewables). For a focused review, see the Topic 9.4 study guide (https://library.fiveable.me/ap-environmental-science/unit-9/increases-greenhouse-gases/study-guide/xy8sIvBYMxVP54em0thk), the Unit 9 overview (https://library.fiveable.me/ap-environmental-science/unit-9), and practice questions (https://library.fiveable.me/practice/ap-environmental-science).
How do greenhouse gases actually cause global warming?
Greenhouse gases cause global warming by trapping heat that would otherwise escape to space. Sunlight passes through the atmosphere and warms Earth’s surface; the surface then emits longwave (infrared) radiation. Gases like CO2, CH4, N2O and HFCs absorb that outgoing infrared and re-emit it in all directions, including back toward the surface, increasing net energy in the lower atmosphere and raising temperatures (the greenhouse effect). More greenhouse gas = stronger trapping, so Earth warms. Warming then triggers feedbacks from the CED keywords: melting ice lowers albedo, permafrost thaw releases methane, and warmer oceans absorb more CO2, which lowers pH (ocean acidification) and harms marine life. This mechanism and its impacts are central to Topic 9.4 (STB-4.E) and often show up on the free-response section—practice explaining cause → effect → feedback in your answers. For the topic study guide see (https://library.fiveable.me/ap-environmental-science/unit-9/increases-greenhouse-gases/study-guide/xy8sIvBYMxVP54em0thk). More practice problems: (https://library.fiveable.me/practice/ap-environmental-science).
What's the difference between the greenhouse effect and global climate change?
The greenhouse effect is the natural process where greenhouse gases (like CO2, CH4, N2O, water vapor) trap some outgoing infrared radiation and keep Earth warm enough to support life. “Enhanced” greenhouse effect refers to extra warming from increased atmospheric greenhouse gases due to human activities. Global climate change (or global warming) is the broad result of that enhanced greenhouse effect—the measurable shifts in average temperatures and climate patterns over time. On the AP CED this connects directly: excess greenhouse gases cause problems like sea level rise (melting ice + thermal expansion), ocean acidification, coral bleaching, and shifts in vector-borne disease ranges (EK STB-4.E.1). For exam prep, focus on mechanism (CO2 absorption → more trapped heat) and specific impacts listed in the CED. If you want a quick topic review, check the Topic 9.4 study guide (https://library.fiveable.me/ap-environmental-science/unit-9/increases-greenhouse-gases/study-guide/xy8sIvBYMxVP54em0thk), the Unit 9 overview (https://library.fiveable.me/ap-environmental-science/unit-9), and practice questions (https://library.fiveable.me/practice/ap-environmental-science).
Why do rising sea levels happen when greenhouse gases increase?
When greenhouse gases (GHGs) like CO2 and methane increase, they trap more outgoing heat in Earth’s atmosphere—that raises global temperatures. Two main processes then cause sea level rise: 1) melting land ice (glaciers and ice sheets) adds freshwater to the oceans, directly increasing volume; and 2) thermal expansion—as ocean water warms it expands, taking up more space. Together these explain most observed sea level rise in the CED (EK STB-4.E.1). Warmer oceans also speed ice-sheet loss and can create feedbacks that accelerate warming. This is a key APES idea on Unit 9 (global change) and shows up on both multiple-choice and free-response questions. For a focused review see the Topic 9.4 study guide (https://library.fiveable.me/ap-environmental-science/unit-9/increases-greenhouse-gases/study-guide/xy8sIvBYMxVP54em0thk). For extra practice, check unit resources and practice problems on Fiveable (https://library.fiveable.me/ap-environmental-science/unit-9 and https://library.fiveable.me/practice/ap-environmental-science).
I'm confused about how melting ice sheets relate to greenhouse gas increases - can someone explain?
Melting ice sheets are mostly an effect of increased greenhouse gases, and they also make warming worse—so they’re both a symptom and a feedback. Higher atmospheric CO2 and other GHGs trap more heat, raising global temps. That warms polar regions, melting land ice (Greenland, Antarctica). Melted land ice adds water to oceans → sea level rise and more coastal flooding (CED: sea level rise from melting ice + thermal expansion). Less sea ice and ice sheet area also lowers albedo (darker ocean absorbs more heat), creating a positive feedback that accelerates warming. Melting permafrost can release trapped carbon and methane (clathrates), increasing GHGs further. Those changes cause threats listed in the CED: coastal flooding, climate refugees, shifting disease vectors (malaria, dengue moving poleward), and impacts to marine life (acidification, coral bleaching). For AP review, focus on cause → effect → feedback links and health/environmental threats (Topic 9.4 study guide: https://library.fiveable.me/ap-environmental-science/unit-9/increases-greenhouse-gases/study-guide/xy8sIvBYMxVP54em0thk). For extra practice, try the Unit 9 problems (https://library.fiveable.me/ap-environmental-science/unit-9) or 1000+ practice questions (https://library.fiveable.me/practice/ap-environmental-science).
What are disease vectors and how do they spread because of climate change?
Disease vectors are organisms (usually mosquitoes, ticks, sandflies) that carry and transmit pathogens to people. Climate change spreads them by changing habitat suitability: warmer temperatures and milder winters let vectors survive farther from the tropics (poleward and to higher elevations), lengthen transmission seasons, and speed pathogen development inside vectors. Changes in precipitation and more standing water boost mosquito breeding; warmer, drier conditions can expand tick activity. The result: range expansion of malaria, dengue, and tick-borne diseases, plus more people exposed and higher disease risk (CED: “vector-borne disease range expansion,” STB-4.E). For AP prep: be ready to explain mechanisms (temperature, precipitation, season length) and name examples (malaria, dengue, tick-borne diseases). Review Topic 9.4 for concise examples and exam-style framing (study guide: https://library.fiveable.me/ap-environmental-science/unit-9/increases-greenhouse-gases/study-guide/xy8sIvBYMxVP54em0thk). For extra practice, try Fiveable’s practice problems (https://library.fiveable.me/practice/ap-environmental-science).
How does ocean water expansion work when the planet gets warmer?
When the planet warms, ocean water expands through a process called ocean thermal expansion. Warmer water molecules move faster, take up more space, and become less dense, so the same mass of water occupies a larger volume—raising sea level. This effect is separate from melting ice (like ice sheets) but adds to overall sea level rise, which is an important threat listed in the CED (STB-4.E: sea level rise from melting ice and ocean water expansion). Thermal expansion also changes ocean circulation and can amplify coastal flooding. For AP exam prep, know the term “ocean thermal expansion,” how temperature affects water density, and that it’s a key mechanism for sea level rise (Unit 9 content). For a deeper review, check the Topic 9.4 study guide (https://library.fiveable.me/ap-environmental-science/unit-9/increases-greenhouse-gases/study-guide/xy8sIvBYMxVP54em0thk) and practice questions (https://library.fiveable.me/practice/ap-environmental-science).
What specific health threats do humans face from increased greenhouse gases?
Higher greenhouse gas concentrations threaten human health in several concrete ways. Warming increases heat-related illness and mortality (more heat waves), worsens air quality (higher ozone and particulate pollution) leading to more respiratory and cardiovascular disease, and expands the range of vector-borne diseases (malaria, dengue, tick-borne illnesses) as vectors move poleward. Climate-driven sea level rise and coastal flooding increase injury, displacement, and water contamination, raising risks of waterborne disease and reduced sanitation. Warmer waters also boost harmful algal blooms that contaminate seafood and drinking water. These impacts and keywords (heat-related mortality, vector-borne disease range expansion, coastal flooding, harmful algal blooms) line up with CED STB-4.E and are likely to appear in free-response questions about human health impacts (Unit 9). For a focused review see the Topic 9.4 study guide (https://library.fiveable.me/ap-environmental-science/unit-9/increases-greenhouse-gases/study-guide/xy8sIvBYMxVP54em0thk), the Unit 9 overview (https://library.fiveable.me/ap-environmental-science/unit-9), and practice problems (https://library.fiveable.me/practice/ap-environmental-science).
Why would people have to move because of climate change?
Because rising greenhouse gases warm the planet, they trigger physical changes that can make places unsafe or unlivable—so people have to move. Sea level rise (from melting ice sheets and ocean thermal expansion) floods coasts and low-lying islands, wiping out homes and freshwater supplies. Permafrost thaw damages buildings and releases methane, destabilizing Arctic communities. Climate-driven droughts and heat waves reduce crop yields and freshwater, forcing rural-to-urban migration. Increased coastal/storm flooding and more frequent extreme heat raise heat-related mortality and harm infrastructure. Disease vectors (mosquitoes, ticks) expand toward the poles, increasing human health risks and straining local systems. These shifts create “climate refugees” and change population dynamics—exactly what the CED lists under STB-4.E (sea level rise, vector-borne disease expansion, crop yield decline). For a focused review, see the Topic 9.4 study guide (https://library.fiveable.me/ap-environmental-science/unit-9/increases-greenhouse-gases/study-guide/xy8sIvBYMxVP54em0thk). Practice questions on Fiveable can help you apply these ideas to AP-style prompts (https://library.fiveable.me/practice/ap-environmental-science).
Can you explain how greenhouse gases affect population dynamics?
Greenhouse gases change climate in ways that shift population dynamics for people and other species. Warming and sea level rise (from melting ice sheets and thermal expansion) force human migration out of coastal areas (climate refugees) and reduce habitable land. Higher temps expand ranges of disease vectors (mosquitoes, ticks), increasing malaria, dengue, and tick-borne disease in new regions, which raises morbidity and can lower local population growth. Crop-yield declines from heat stress, altered precipitation, and more extreme events decrease food security and can cause population movement or higher mortality. For marine life, increased CO2 causes ocean acidification, coral bleaching, and harmful algal blooms, which reduce habitat and food web support, forcing species to shift ranges or decline. On the AP exam, link these effects to human health and environment threats (STB-4.E) and cite concrete examples (sea level rise, vector expansion, crop loss, coral bleaching). For a focused review, see the Topic 9.4 study guide (https://library.fiveable.me/ap-environmental-science/unit-9/increases-greenhouse-gases/study-guide/xy8sIvBYMxVP54em0thk) and try practice questions (https://library.fiveable.me/practice/ap-environmental-science).
What happens to animal populations when greenhouse gases increase?
When greenhouse gases rise, animal populations respond in several predictable ways. Many species shift their ranges toward the poles or to higher elevations as temperatures rise, causing local population declines and new community interactions. Reproduction and phenology change—breeding or migration times can get out of sync with food availability, lowering survival and recruitment. Marine impacts include ocean warming and acidification that cause coral bleaching, reduced calcification, shifts in fish distributions, and local extinctions. Sea level rise floods coastal habitats, reducing breeding areas for shorebirds and coastal mammals. Warmer climates also expand disease-vector ranges (mosquitoes, ticks), increasing disease pressure on wildlife. All of this leads to altered population dynamics, increased local extinctions, and more climate refugees among species. For AP review, focus on examples in the CED (sea level rise, coral bleaching, vector-borne disease) and practice applying these to population-change questions (see the Topic 9.4 study guide and Unit 9 overview on Fiveable: https://library.fiveable.me/ap-environmental-science/unit-9/increases-greenhouse-gases/study-guide/xy8sIvBYMxVP54em0thk and https://library.fiveable.me/ap-environmental-science/unit-9).
How do greenhouse gases cause diseases to spread from tropical areas?
Greenhouse gases warm the planet, and that warmer climate lets tropical disease vectors (mosquitoes, ticks) move into higher latitudes and higher elevations. As temperatures rise, areas that were too cold become suitable for vector survival, reproduction, and faster pathogen development (for example, mosquitoes transmit malaria and dengue more effectively when it’s warmer). Changes in precipitation and more extreme weather also create new breeding sites (standing water after heavy rains) and longer transmission seasons, so more people are exposed for longer. Human responses—like migration from flooded/coastal areas—can further spread pathogens. This topic is covered in the CED under EK STB-4.E (vector-borne disease range expansion—malaria, dengue, tick-borne diseases). For AP review, see the Topic 9.4 study guide (https://library.fiveable.me/ap-environmental-science/unit-9/increases-greenhouse-gases/study-guide/xy8sIvBYMxVP54em0thk) and practice questions (https://library.fiveable.me/practice/ap-environmental-science) to prep for exam items on health impacts of climate change.
I don't understand the connection between CO2 levels and sea level rise - help?
Think of CO2 as a heat-trapping blanket: higher atmospheric CO2 raises global temperatures (the greenhouse effect), and warmer temperatures cause two main processes that raise sea level. First, melting land ice—ice sheets and glaciers on Greenland and Antarctica—adds extra water to the ocean. Second, ocean thermal expansion: warm water takes up more volume, so the same mass of water occupies more space. Both are listed in the CED (EK STB-4.E: “melting ice sheets and ocean water expansion”). The Arctic and polar regions warm faster, so ice loss there accelerates sea level rise and increases risks like coastal flooding and climate-driven migration. For exam prep, be ready to explain both mechanisms and link them to human/environmental threats (coastal flooding, climate refugees) as the CED requires. See the Topic 9.4 study guide for a focused review (https://library.fiveable.me/ap-environmental-science/unit-9/increases-greenhouse-gases/study-guide/xy8sIvBYMxVP54em0thk) and more practice questions at (https://library.fiveable.me/practice/ap-environmental-science).
What are some real examples of environmental problems caused by too many greenhouse gases?
Too many greenhouse gases drive global climate change, which causes concrete environmental problems you should know for APES (STB-4.E): - Sea level rise: melting ice sheets + ocean thermal expansion flood coasts and create climate refugees. - Permafrost thaw: releases methane, a potent GHG, creating a positive feedback loop. - Ocean acidification: more CO2 lowers ocean pH, harming shell-formers and causing coral bleaching. - Ocean warming: habitat loss, altered metabolism, poleward shifts of marine species. - Harmful algal blooms and reduced water quality from warmer waters. - Vector-range expansion: mosquitoes and ticks move poleward—increased malaria, dengue, and tick-borne diseases. - Heat-related morbidity/mortality and reduced crop yields from altered precipitation and extreme heat. These are exactly the kinds of impacts tested in Unit 9 (15–20% of the exam)—review the Topic 9.4 study guide (https://library.fiveable.me/ap-environmental-science/unit-9/increases-greenhouse-gases/study-guide/xy8sIvBYMxVP54em0thk) and practice problems (https://library.fiveable.me/practice/ap-environmental-science).
How fast are greenhouse gas levels actually increasing and what does that mean for us?
Since preindustrial times (about 1750) atmospheric CO2 has risen from ~280 ppm to roughly 415–420 ppm today—about a 45–50% increase. That rise has accelerated: CO2 is now climbing ~2–3 ppm per year (decadal averages have increased recently). Methane (CH4) has roughly tripled since preindustrial levels (hundreds of ppb → ~1900 ppb) and nitrous oxide (N2O) is up ~20–25%. What that means for you (APES-relevant): higher greenhouse gases drive faster global warming, which causes sea level rise (melting ice + thermal expansion), ocean acidification (more CO2 dissolved → lower pH), more frequent heat-related illness and expanded ranges for vector-borne diseases (malaria, dengue, ticks), coral bleaching, harmful algal blooms, and climate-driven migration. These are exactly the threats named in EK STB-4.E. For review, see the Topic 9.4 study guide (https://library.fiveable.me/ap-environmental-science/unit-9/increases-greenhouse-gases/study-guide/xy8sIvBYMxVP54em0thk). Unit 9 shows up a lot on the exam (15–20% weight), and Fiveable’s practice problems (https://library.fiveable.me/practice/ap-environmental-science) are great for drilling these concepts.