The Carbon Cycle - AP Environmental Science

A simplified carbon cycle includes these reservoirs: atmosphere, biosphere, ocean, lithosphere. Which reservoir generally stores carbon on the longest timescales (millions of years) in the form of fossil fuels and carbonate rocks?
Answer: Lithosphere The lithosphere reservoir stores carbon in fossil fuels and carbonates over millions of years, far longer than the atmosphere (short-term), biosphere (decades), or ocean (centuries for deep storage). Choice D is correct. This long-term storage is key to the geologic carbon cycle. It sequesters carbon from active cycling.
A simplified carbon budget for a region shows: photosynthesis removes 200 units from the atmosphere; respiration returns 180 units; combustion adds 30 units; ocean absorption removes 20 units. What is the net change in the biosphere carbon stock (biomass + detritus) over the period, ignoring transfers to lithosphere?
Answer: Increase by 20 units Biosphere reservoir includes biomass and detritus. Fluxes: photosynthesis adds 200 from atm, respiration removes 180 to atm. Net biosphere change: +200 -180 = +20 units increase. Combustion and ocean absorption affect atmosphere but not directly biosphere here. Ignoring lithosphere transfers, the net is positive storage. Option A correctly indicates increase by 20 units.
Permafrost in a high-latitude region thaws, and soil microbes decompose previously frozen organic matter. Which carbon-cycle outcome is most likely over the next few years?
Answer: A net transfer of carbon from the biosphere/soil to the atmosphere through increased respiration and decomposition Permafrost contains large amounts of frozen organic matter (dead plant and animal material) that has been preserved for thousands of years due to cold temperatures preventing decomposition. When permafrost thaws, this organic matter becomes available to soil microbes that decompose it through respiration, converting the organic carbon to CO2. This process transfers carbon from the biosphere/soil reservoir to the atmosphere reservoir. While some increased plant growth might occur in thawed areas, the decomposition of the vast stores of previously frozen organic matter typically far exceeds any carbon uptake through enhanced photosynthesis. The net result is a positive feedback to climate change, as more atmospheric CO2 leads to more warming, which causes more permafrost thaw and more CO2 release.
A carbon-cycle model shows net ocean absorption of 8 units/year (atmosphere → ocean). If atmospheric CO $_2$ rises due to combustion, which response is most consistent with the idea that the ocean can act as a partial buffer?
Answer: Ocean absorption may increase (more CO $_2$ dissolves), slowing but not stopping atmospheric increase The ocean reservoir buffers atmospheric CO2 by increasing absorption as concentrations rise, slowing accumulation. Choice A describes this partial buffering. Others misrepresent ocean dynamics. This highlights the ocean's role in the cycle. However, it leads to acidification.
A wildfire burns a forest. Which paired reservoir changes are most likely immediately after the fire (short-term), assuming no immediate regrowth?
Answer: Atmosphere increases; biosphere decreases Wildfires combust biosphere carbon (biomass) to atmosphere as CO2. Immediately, atmosphere gains carbon, biosphere loses it without regrowth. Ocean or lithosphere changes are secondary. Option B correctly pairs atmosphere increase with biosphere decrease. It shows short-term fire impacts on reservoirs.
A city plants trees and reports that annual photosynthesis removes 25 units of CO $_2$ from the atmosphere into the biosphere. However, annual respiration and decomposition return 20 units to the atmosphere, and local vehicle combustion adds 12 units from the lithosphere to the atmosphere. The nearby ocean absorbs 4 units from the atmosphere. What is the net change in atmospheric carbon?
Answer: Increase by 3 units Key carbon reservoirs are the atmosphere (CO2), biosphere (tree biomass), lithosphere (fossil fuels), and ocean (dissolved carbon). Fluxes include photosynthesis removing CO2 from the atmosphere to the biosphere, respiration and decomposition returning it from biosphere to atmosphere, combustion transferring from lithosphere to atmosphere, and ocean absorption moving from atmosphere to ocean. Here, photosynthesis removes 25 units, respiration/decomposition adds 20, combustion adds 12, and ocean absorbs 4. Net change: -25 + 20 + 12 - 4 = +3 units increase in atmosphere. This happens because inputs (respiration and combustion) exceed outputs (photosynthesis and absorption). Option B correctly reflects this increase by 3 units, showing how urban greening helps but doesn't fully offset emissions.
Which statement best explains why large-scale combustion of fossil fuels disrupts the carbon cycle balance?
Answer: It moves carbon rapidly from the lithosphere to the atmosphere faster than long‑term burial can remove it Fossil fuel combustion rapidly transfers ancient carbon from the lithosphere to the atmosphere as CO2. This flux outpaces slow removal processes like burial into the lithosphere via sedimentation. Photosynthesis and ocean uptake can't immediately offset it. Option A correctly explains the disruption due to rate imbalance. It underscores anthropogenic impacts on cycle dynamics.
A diagram of carbon reservoirs is described as follows: carbon can move among the atmosphere, biosphere, ocean, and lithosphere via photosynthesis, respiration, combustion, and ocean-atmosphere exchange. A student claims that because the ocean contains a large amount of carbon, adding fossil-fuel $CO_2$ to the atmosphere will not change atmospheric $CO_2$ much. Which statement best evaluates the student's claim?
Answer: The claim is incorrect because anthropogenic combustion adds carbon to the atmosphere faster than natural fluxes (including ocean absorption and photosynthesis) can remove it, increasing atmospheric $CO_2$ The carbon cycle operates on different timescales, with some fluxes much faster than others. While the ocean does contain a large carbon reservoir and absorbs some atmospheric CO₂, this absorption is limited by factors like surface saturation and mixing rates. Anthropogenic fossil fuel combustion adds CO₂ to the atmosphere at rates exceeding the combined capacity of natural sinks (ocean absorption, photosynthesis, rock weathering) to remove it on human timescales. This imbalance causes atmospheric CO₂ to accumulate despite the ocean's large size. The student's claim incorrectly assumes that reservoir size determines flux capacity. Option B correctly evaluates this: the claim is incorrect because combustion adds carbon faster than natural fluxes can remove it, increasing atmospheric CO₂.
A marine heatwave causes the surface ocean to outgas an extra 6 units of CO $_2$ to the atmosphere (ocean → atmosphere), while land photosynthesis and respiration are unchanged. What is the immediate effect on atmospheric carbon?
Answer: Atmospheric carbon increases by 6 units The ocean reservoir can release carbon to the atmosphere via outgassing, especially during heatwaves. An extra 6 units outgassed directly increases atmospheric carbon, with unchanged land fluxes. Choice B is correct; the atmosphere gains immediately. This shows how warming oceans can amplify CO2 levels. It's a feedback in climate change.
In a coastal region, a heat wave reduces phytoplankton growth for several weeks. During the same period, fish and microbial activity (respiration) continues at near-normal levels, and no major changes occur in fossil-fuel use locally. Which carbon-cycle flux change best explains why atmospheric $CO_2$ measured above the coast rises during the heat wave?
Answer: Decreased photosynthesis reduces the flux of carbon from the atmosphere into the biosphere/ocean surface, leaving more $CO_2$ in the atmosphere The carbon cycle involves reservoirs (atmosphere, biosphere, ocean, lithosphere) connected by fluxes like photosynthesis and respiration. Phytoplankton perform photosynthesis, removing CO₂ from the atmosphere and transferring it to the ocean/biosphere reservoir. When the heat wave reduces phytoplankton growth, photosynthesis decreases, meaning less CO₂ is removed from the atmosphere. Meanwhile, respiration by fish and microbes continues normally, still releasing CO₂ to the atmosphere. This imbalance—reduced CO₂ removal via photosynthesis but continued CO₂ addition via respiration—causes atmospheric CO₂ to rise. Option B correctly identifies that decreased photosynthesis reduces carbon flux from atmosphere to biosphere/ocean, leaving more CO₂ in the atmosphere.
A reforestation project converts abandoned farmland into a growing forest. For the first 20 years, tree growth is rapid, and the forest is not harvested or burned. Assuming other factors remain similar, what is the most likely net effect on carbon reservoirs during this period?
Answer: Biosphere carbon increases because photosynthesis stores more carbon in biomass than is returned by respiration The carbon cycle includes photosynthesis (atmosphere to biosphere flux) and respiration (biosphere to atmosphere flux). During reforestation, growing trees perform photosynthesis, removing CO₂ from the atmosphere and storing it as biomass in trunks, branches, roots, and leaves. Since the forest is not harvested or burned, this carbon remains stored in the biosphere reservoir. While trees do respire and release some CO₂, young growing forests typically have photosynthesis rates that exceed respiration rates, resulting in net carbon accumulation. Therefore, the biosphere carbon increases as more carbon is stored in biomass than is returned by respiration. The lithosphere is unaffected by tree growth on human timescales, and atmospheric carbon would decrease (not increase).
A forest has net ecosystem exchange such that photosynthesis removes 300 units of carbon from the atmosphere annually and respiration returns 290 units. If a wildfire combusts biomass and releases an additional 25 units to the atmosphere that year, and the ocean absorbs 5 units net from the atmosphere, what is the net atmospheric change for that year?
Answer: Increase by 10 units Forest fluxes: photosynthesis removes 300 from atmosphere to biosphere, respiration adds 290 back, wildfire adds 25 from biosphere to atmosphere, ocean absorbs 5 from atmosphere. Net: -300 +290 +25 -5 = +10 increase in atmosphere. Choice B is correct. This shows disturbances like fires can turn sinks into sources. Ocean absorption mitigates some impact.
A forest experiences drought that reduces photosynthesis by 30% while respiration remains about the same. Which outcome is most likely for carbon in the atmosphere over the drought period, assuming combustion and ocean fluxes are unchanged?
Answer: Atmospheric CO $_2$ increases because less CO $_2$ is removed by photosynthesis In forests, the biosphere reservoir stores carbon in plant biomass, while the atmosphere holds CO2. Photosynthesis fluxes carbon from atmosphere to biosphere, reducing atmospheric CO2, whereas respiration returns it from biosphere to atmosphere. A 30% reduction in photosynthesis means less CO2 removal, while unchanged respiration continues adding CO2 at the same rate. This imbalance leads to a net increase in atmospheric CO2. Combustion and ocean fluxes are assumed constant, so the effect stems from diminished photosynthetic uptake. Option B accurately predicts this outcome, emphasizing the role of photosynthesis as a key sink.
In a simplified model, the ocean is currently a net sink absorbing 2 gigatons of carbon per year from the atmosphere. If ocean warming reduces CO $_2$ solubility and the net flux changes to a release of 1 gigaton per year to the atmosphere (all else equal), what is the direction of change in atmospheric carbon due to this shift?
Answer: Atmospheric carbon increases because the ocean shifts from sink to source The ocean reservoir holds dissolved CO2, while the atmosphere contains gaseous CO2. Fluxes between them include absorption (atmosphere to ocean) when the ocean acts as a sink and outgassing (ocean to atmosphere) when it acts as a source. Currently, the ocean absorbs 2 Gt/year, removing carbon from the atmosphere. If warming reduces solubility, shifting to a release of 1 Gt/year, this adds carbon to the atmosphere instead of removing it. The net effect is an increase in atmospheric carbon due to the ocean changing from sink to source. All else equal, this shift disrupts the balance, leading to higher atmospheric levels. Option B correctly describes this directional change.
Which flux represents a transfer of carbon from the lithosphere to the atmosphere caused by humans?
Answer: Combustion of coal and oil The lithosphere reservoir includes fossil fuels like coal and oil, while the atmosphere holds CO2. Human-induced fluxes, such as combustion, transfer carbon from lithosphere to atmosphere by burning these fuels, releasing stored ancient carbon as CO2. Photosynthesis moves carbon from atmosphere to biosphere, respiration from biosphere to atmosphere, and ocean uptake from atmosphere to ocean. Combustion is uniquely anthropogenic and directly from lithosphere. Option C correctly identifies this flux, highlighting its role in modern carbon imbalances.
Which scenario would most likely cause the biosphere carbon reservoir to decrease over several years?
Answer: Deforestation and increased decomposition/respiration of dead biomass The biosphere reservoir stores carbon in biomass; deforestation reduces it by removing plants and increasing respiration/decomposition, releasing carbon to the atmosphere. Choice B correctly identifies this scenario. Others like A increase biosphere carbon, C affects ocean, D reduces atmospheric input but not biosphere directly. This shows land-use impacts on carbon storage. Reforestation could reverse such decreases.
A coastal wetland is restored, increasing plant growth and carbon burial in sediments. In carbon-cycle terms, this restoration most directly increases transfer of carbon from the biosphere toward which reservoir (longer-term storage)?
Answer: Lithosphere (via sediment burial) Wetland restoration boosts biosphere carbon via growth, with burial transferring it to lithosphere sediments for long-term storage. Choice B is correct; not atmosphere or direct ocean fluxes. This enhances carbon sequestration. It's a nature-based climate solution.
Consider a simplified yearly carbon budget for a forested region: photosynthesis removes $100$ units of carbon from the atmosphere into the biosphere, plant+microbial respiration returns $90$ units to the atmosphere, and a nearby factory's combustion adds $30$ units of carbon to the atmosphere. Ocean absorption in the adjacent coastal water removes $10$ units from the atmosphere. Assuming no other major fluxes, what is the net change in atmospheric carbon over the year for this system?
Answer: Atmospheric carbon increases by $10$ units To find net atmospheric change, we track all fluxes in and out of the atmosphere reservoir. Fluxes removing carbon from atmosphere: photosynthesis removes 100 units, ocean absorption removes 10 units (total removal = 110 units). Fluxes adding carbon to atmosphere: respiration returns 90 units, combustion adds 30 units (total addition = 120 units). Net change = total additions - total removals = 120 - 110 = +10 units. The atmosphere gains 10 units of carbon over the year because the combined additions exceed the combined removals. Option B correctly states that atmospheric carbon increases by 10 units, reflecting this imbalance between carbon sources and sinks in the system.
Atmospheric CO2 increases due to human fossil-fuel use. As a result, more CO2 dissolves into the surface ocean. Which additional outcome is most consistent with this increased ocean absorption of CO2?
Answer: An increase in ocean acidity because dissolved CO2 forms carbonic acid When atmospheric CO2 increases, more CO2 dissolves into the surface ocean following Henry's Law, which governs gas exchange between air and water. Once dissolved in seawater, CO2 reacts with water to form carbonic acid (H2CO3), which then dissociates to release hydrogen ions (H+) and bicarbonate ions. The increase in hydrogen ions lowers the pH of seawater, making it more acidic - a process known as ocean acidification. This represents a chemical change within the ocean reservoir as it absorbs more atmospheric carbon. Ocean acidification has significant ecological impacts, affecting marine organisms that build calcium carbonate shells or skeletons. The formation of carbonic acid and subsequent pH decrease is a direct and well-documented consequence of increased ocean CO2 absorption.
Which human activity most directly transfers carbon from the biosphere to the atmosphere while also reducing future photosynthetic uptake?
Answer: Deforestation followed by burning Deforestation burns biosphere carbon to atmosphere and reduces future photosynthesis by removing plants. This dual effect transfers carbon while diminishing uptake. Ocean absorption or shell formation remove from atm/ocean, burial from bio/ocean to litho. Option A correctly identifies the activity impacting biosphere-to-atm flux and future sinks.

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