7.4: Accessible Descriptions

Figure 6.1.1: Global Carbon Cycle

Diagram illustrating the global carbon cycle, showing the major carbon reservoirs (stored carbon) and annual carbon exchanges among the atmosphere, land biosphere, oceans, soils, and fossil fuel reserves. Values in parentheses represent the amount of carbon stored in each reservoir in gigatons of carbon (GtC), while arrows show annual carbon fluxes in gigatons of carbon per year (GtC/yr). Red values indicate carbon added by human activities.

The atmosphere contains approximately 800 GtC. Each year, terrestrial plants remove about 120 GtC/yr from the atmosphere through photosynthesis, plus an additional 3 GtC/yr associated with increased uptake linked to human-caused carbon emissions. Plants return approximately 60 GtC/yr to the atmosphere through plant respiration. Soil microbes and decomposition return another 60 GtC/yr to the atmosphere. The land system as a whole has a net terrestrial carbon uptake of about 3 GtC/yr, meaning land ecosystems absorb slightly more carbon than they release.

The terrestrial carbon reservoirs include approximately 550 GtC stored in plant biomass and 2,300 GtC stored in soils. Beneath the surface, fossil fuel deposits contain approximately 10,000 GtC.

Human activities, including fossil fuel combustion, cement production, and land-use change, add approximately 9 GtC/yr to the atmosphere. Of this amount, roughly 4 GtC/yr remains in the atmosphere, shown as the net annual atmospheric carbon increase.

The ocean exchanges large amounts of carbon with the atmosphere. About 90 GtC/yr moves from the ocean to the atmosphere, while approximately 90 GtC/yr plus an additional 2 GtC/yr moves from the atmosphere into the ocean. This creates a net ocean carbon uptake of about 2 GtC/yr. Ocean biological processes include phytoplankton photosynthesis, respiration, decomposition, and transport of carbon into deeper waters.

The ocean carbon reservoirs include approximately 1,000 GtC in the surface ocean, 37,000 GtC in the deep ocean, and 6,000 GtC in reactive marine sediments. The deep ocean is the largest active carbon reservoir shown in the figure.

Overall, the diagram shows that human activities add approximately 9 GtC/yr to the atmosphere, while natural land and ocean sinks remove about 5 GtC/yr combined (3 GtC/yr on land and 2 GtC/yr in the ocean), leaving a net atmospheric increase of approximately 4 GtC/yr.

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Table 6.4.1: Carbon reservoirs (stored carbon)

Reservoir	Carbon stored (GtC)
Atmosphere	800
Plant biomass	550
Soil	2,300
Fossil fuel pool	10,000
Surface ocean	1,000
Deep ocean	37,000
Reactive sediments	6,000

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Table 6.4.2: Annual carbon fluxes

Process	Flux (GtC/yr)
Photosynthesis	120
Additional terrestrial uptake	+3
Plant respiration	60
Microbial respiration and decomposition	60
Fossil fuels, cement, and land-use change	+9
Ocean to atmosphere exchange	90
Atmosphere to ocean exchange	90
Additional ocean uptake	+2
Net terrestrial uptake	3
Net ocean uptake	2
Net atmospheric increase	4

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Table 6.4.3: Carbon budget summary

Component	Net change (GtC/yr)
Human emissions	+9
Land uptake	−3
Ocean uptake	−2
Atmospheric increase	+4

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Table 6.4.4: Relative size of carbon reservoirs

Rank	Reservoir	Carbon stored (GtC)
1	Deep ocean	37,000
2	Fossil fuel pool	10,000
3	Reactive sediments	6,000
4	Soil	2,300
5	Surface ocean	1,000
6	Atmosphere	800
7	Plant biomass	550

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Notes:

• Units are gigatons of carbon (GtC) and gigatons of carbon per year (GtC/yr).
• Numbers in parentheses indicate stored carbon pools.
• Red values represent carbon introduced by human activities.
• The largest annual natural carbon fluxes are photosynthesis, plant respiration, microbial decomposition, and air-sea gas exchange, each involving approximately 60–120 GtC/yr.
• The deep ocean contains nearly 46 times more carbon than the atmosphere and is the largest active carbon reservoir shown.

Figure 6.2.1: Ocean Acidification Equation

Diagram showing how dissolved carbon dioxide reacts with water to form carbonic acid, which releases hydrogen ions and forms bicarbonate and carbonate ions. Over time, increased hydrogen ion concentration lowers ocean pH, illustrating the process of ocean acidification.

The equation shown on Figure 6.2.1 is as follows:

\[ \mathrm{CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^- \rightleftharpoons 2H^+ + CO_3^{2-}} \]

Figure 6.2.2: Map of Global Change in Ocean pH.

Global map showing projected or observed changes in sea-surface pH across the world's oceans. The map uses a color scale representing pH change (Δ pH), with values ranging from approximately 0.00 (no change) to about −0.12 pH units (largest decrease). Negative values indicate ocean acidification, meaning surface ocean waters have become less alkaline due to the absorption of atmospheric carbon dioxide.

Most ocean regions show a decrease in pH between approximately −0.04 and −0.10. The largest decreases occur in parts of the North Atlantic, the Southern Ocean, and some high-latitude regions, where pH reductions approach −0.10 to −0.12. Much of the tropical and subtropical Atlantic, Pacific, and Indian Oceans show decreases of roughly −0.06 to −0.08. Smaller decreases, generally around −0.04 to −0.06, occur in parts of the equatorial Pacific, the western Pacific, and portions of the Indian Ocean.

The strongest acidification appears in the North Atlantic between eastern North America and western Europe, where pH reductions are commonly about −0.09 to −0.11. Large areas of the Southern Ocean surrounding Antarctica show decreases around −0.08 to −0.10. In contrast, some tropical and subtropical regions exhibit more moderate decreases near −0.05 to −0.07.

No major ocean region on the map shows an increase in pH; all displayed changes are neutral or negative, indicating widespread ocean acidification.

Figure 6.2.3: Time Series of Global Change in Ocean pH

Line graph showing mean seawater pH measured at the ALOHA ocean monitoring station near Hawaii from approximately 1988 to 2025. The horizontal axis represents time, spanning from October 1988 through late 2024 and early 2025. The vertical axis shows seawater pH, ranging from approximately 8.02 to 8.16 pH units. Two lines are displayed: a monthly average record with substantial short-term variability and a smoother annual average showing the long-term trend.

The monthly pH record fluctuates seasonally between approximately 8.03 and 8.15, with typical year-to-year variation of about 0.02 to 0.05 pH units. Despite this short-term variability, the annual average shows a clear long-term decline throughout the record.

In the late 1980s and early 1990s, annual average pH values are approximately 8.11 to 8.12. During the late 1990s and early 2000s, annual averages decline to approximately 8.09 to 8.10. By the late 2000s and early 2010s, annual averages are generally around 8.08 to 8.09. After 2015, the decline continues, with annual averages falling to approximately 8.05 to 8.07. By 2024–2025, annual average pH is approximately 8.04, the lowest value shown in the record.

Overall, the graph shows a decline in mean seawater pH of approximately 0.07 to 0.08 pH units over about 35 years, from roughly 8.11–8.12 to approximately 8.04. Because the pH scale is logarithmic, this represents a substantial increase in ocean acidity even though the numerical change appears small.

The graph illustrates ongoing ocean acidification at the Hawaii monitoring site, with a persistent downward trend superimposed on natural seasonal fluctuations.

Figure 6.2.4: Two-year Carbon Dioxide Concentrations at Mauna Loa Observatory

Line graph showing atmospheric carbon dioxide (CO2) concentrations measured at Mauna Loa Observatory, Hawaii, over approximately the most recent two-year period. The horizontal axis represents time across about 24 months, and the vertical axis shows CO2 concentration in parts per million (ppm). The graph displays a strong seasonal cycle superimposed on a continuing long-term increase in atmospheric CO2.

CO2 concentrations fluctuate annually by roughly 6–10 ppm, rising through autumn, winter, and spring before reaching a seasonal maximum in late spring (typically May), then declining through summer and early autumn. The seasonal decrease is driven primarily by Northern Hemisphere plant growth, which removes CO2 from the atmosphere through photosynthesis, while the increase during autumn and winter reflects reduced photosynthesis and continued respiration and decomposition.

Across the two-year period, the entire seasonal cycle shifts upward, indicating that atmospheric CO2 continues to increase from year to year. Typical values shown are in the range of approximately 420–430+ ppm, with the second year consistently higher than the first. Annual growth is approximately 2–4 ppm per year, consistent with recent observations from Mauna Loa.

The graph illustrates two simultaneous patterns:

1. A short-term annual oscillation caused by seasonal biological activity.
2. A long-term upward trend caused primarily by human emissions of carbon dioxide from fossil fuel combustion and land-use change.

Overall, the figure shows that atmospheric CO2 is not only varying seasonally but also steadily increasing, with each seasonal peak and minimum occurring at a higher concentration than the year before. This type of record is part of the Mauna Loa CO2 monitoring program and is a modern segment of the Keeling Curve.

Figure 6.3.1: Geologic Leg of the Carbon Cycle and Associated Feedbacks

Two-panel diagram illustrating the geologic carbonate-silicate cycle and its role in regulating atmospheric carbon dioxide (CO₂) and Earth's climate over millions of years.

The upper panel, titled The Geologic Carbonate-Silicate Cycle, shows the movement of carbon among the atmosphere, rocks, oceans, marine organisms, and volcanoes. Atmospheric CO₂ dissolves in rainwater and reacts with rocks through weathering. Two weathering reactions are shown:

• Carbonate rock weathering:
  CO₂ + H₂O + CaCO₃ → Ca²⁺ + 2HCO₃⁻
• Silicate rock weathering:
  2CO₂ + H₂O + CaSiO₃ → Ca²⁺ + 2HCO₃⁻ + SiO₂

The resulting calcium ions (Ca²⁺) and bicarbonate ions (HCO₃⁻) are transported by rivers to the oceans. Marine organisms use these materials to build calcium carbonate (CaCO₃) shells. The carbonate precipitation reaction is shown as:

• Carbonate precipitation:
  Ca²⁺ + 2HCO₃⁻ → CO₂ + H₂O + CaCO₃

The shells accumulate on the seafloor and are buried as carbonate sediments. Over geologic time, burial, metamorphism, and volcanism return carbon to the atmosphere. The metamorphic reaction shown is:

• Metamorphism:
  CaCO₃ + SiO₂ → CaSiO₃ + CO₂

Volcanoes then release CO₂ back into the atmosphere, completing the cycle. Large arrows indicate carbon movement among atmospheric CO₂, weathering, oceans, sediment burial, metamorphism, and volcanic degassing.

The lower panel, titled Feedback Cycles of the Carbonate-Silicate Cycle, illustrates a negative climate feedback. Atmospheric CO₂ increases the greenhouse effect, which raises surface temperature. Higher temperatures increase rainfall and accelerate silicate weathering. Increased weathering removes more CO₂ from the atmosphere, reducing greenhouse warming. The feedback loop shown is:

Atmospheric CO₂ → Greenhouse Effect → Surface Temperature → Rainfall and Weathering Rate → Reduced Atmospheric CO₂.

This feedback acts as a long-term climate stabilizer by counteracting changes in atmospheric CO₂ over geologic timescales.

Figure 6.3.2: Global Fossil Fuel Usage by Type since 1800

Stacked area chart showing global fossil fuel consumption from approximately 1800 to 2024, measured in terawatt-hours (TWh) of primary energy consumption. The chart separates total fossil fuel use into coal, oil, and natural gas, stacked to show both individual contributions and total consumption. The vertical axis ranges from 0 to about 140,000 TWh.

Consumption remains very low during the early 1800s and increases gradually through the nineteenth century, driven almost entirely by coal. Around 1900, total fossil fuel consumption is approximately 5,000–10,000 TWh. Growth accelerates during the twentieth century, particularly after World War II.

Coal dominates fossil fuel consumption until the mid-twentieth century. Oil consumption begins rising rapidly after about 1950, becoming the largest fossil fuel source for several decades. Natural gas consumption grows more slowly at first but accelerates strongly after about 1970 and continues increasing through the present.

By approximately 1950, total fossil fuel consumption reaches about 20,000 TWh. By 1975, it rises to roughly 60,000–70,000 TWh. Around 2000, total consumption approaches 90,000–100,000 TWh. Growth continues through the early twenty-first century, with total fossil fuel consumption exceeding 140,000 TWh by 2024, the highest value shown.

At the end of the record (2024), the approximate contributions are 45,000 TWh from coal, 55,000 TWh from oil, and 42,000 TWh from natural gas, totaling about 142,000 TWh.

The chart also includes source and explanatory notes. The data source is listed as the Energy Institute Statistical Review of World Energy (2025) and Smil (2017).

Two footnotes define key terms:

1. Watt-hour: A watt-hour is the amount of energy delivered by one watt of power over one hour. Since one watt equals one joule per second, one watt-hour equals 3,600 joules. Common multiples include:
   • Kilowatt-hour (kWh): 1,000 watt-hours
   • Megawatt-hour (MWh): 1 million watt-hours
   • Gigawatt-hour (GWh): 1 billion watt-hours
   • Terawatt-hour (TWh): 1 trillion watt-hours
2. Primary energy: Primary energy refers to energy available in natural resources before conversion into usable forms. Examples include coal before combustion, crude oil before refining, natural gas before use, or uranium before electricity generation. Primary energy accounting includes both the useful energy ultimately delivered and losses that occur during energy conversion and distribution.

Overall, the figure shows a dramatic increase in fossil fuel consumption since the Industrial Revolution, with especially rapid growth after 1950 and continued expansion through 2024.

Figure 6.3.3: Fossil Fuel Use by Country in 2024.

Choropleth world map showing fossil fuel consumption by country in 2024, measured in terawatt-hours (TWh) of primary energy consumption. Countries are shaded according to total fossil fuel use, with darker colors representing higher consumption. The scale ranges from 0 TWh to more than 20,000 TWh, with intermediate categories at approximately 200, 500, 1,000, 2,000, 5,000, and 10,000 TWh. Countries with no available data are shown with diagonal hatching.

The map indicates that fossil fuel consumption is concentrated in a small number of large economies. China is the darkest-shaded country and has the highest fossil fuel consumption, exceeding 20,000 TWh. The United States is also among the highest consumers, similarly exceeding 20,000 TWh. India is the next-largest consumer, generally falling within the 10,000–20,000 TWh range. Other major consumers include Russia and Japan.

Countries with consumption between approximately 5,000 and 10,000 TWh include Russia and some other large economies. Countries in the 2,000–5,000 TWh range include Canada, Saudi Arabia, Indonesia, South Korea, Germany, and several major energy-consuming nations. Much of Western Europe falls between 1,000 and 5,000 TWh, while many countries in South America, Africa, Central America, and Southeast Asia fall below 1,000 TWh.

Large portions of Africa show either relatively low fossil fuel consumption or missing data. Countries such as Brazil, South Africa, Mexico, Turkey, Iran, and Australia show moderate-to-high consumption, generally in the 1,000–5,000 TWh range. Small island nations and many lower-income countries are among the lowest-consumption regions shown on the map.

The figure includes a footnote defining watt-hours as a measure of energy:

• A watt-hour is the energy delivered by one watt of power for one hour.
• One watt-hour equals 3,600 joules.
• Common multiples include:
  • Kilowatt-hour (kWh) = 1,000 watt-hours
  • Megawatt-hour (MWh) = 1 million watt-hours
  • Gigawatt-hour (GWh) = 1 billion watt-hours
  • Terawatt-hour (TWh) = 1 trillion watt-hours

Figure 6.3.4: Fossil Fuel Use by Country: 1800-2024.

Multi-line graph showing fossil fuel consumption for selected countries and regions from approximately 1965 to 2024, measured in terawatt-hours (TWh) of primary energy consumption. The horizontal axis represents year, and the vertical axis ranges from 0 to 40,000 TWh. Lines are shown for China, United States, European Union (27), India, Japan, Australia, United Kingdom, and South Africa.

The most prominent trend is the rapid growth of fossil fuel consumption in China. China's consumption rises from roughly 1,000–2,000 TWh in the 1960s to about 10,000 TWh by the mid-1990s, then accelerates sharply after 2000. Around 2005–2007, China surpasses both the European Union and the United States. By 2024, China reaches approximately 38,000–39,000 TWh, the highest value shown on the chart.

The United States remains one of the largest consumers throughout the record. Consumption rises from about 14,000 TWh in 1965 to roughly 20,000 TWh by the late 1970s. It fluctuates around 18,000–24,000 TWh for several decades, peaks near 23,000–24,000 TWh in the mid-2000s, and then gradually declines. By 2024, U.S. consumption is approximately 21,000 TWh.

The European Union (27) increases from roughly 9,000 TWh in the 1960s to about 15,000 TWh around 1980–2005. After 2005, consumption steadily declines, reaching approximately 10,500–11,000 TWh by 2024.

India shows sustained growth throughout the record. Consumption increases from less than 500 TWh in the 1960s to about 2,000 TWh in 1990, around 5,000 TWh in 2010, and approximately 10,000 TWh by 2024. India nearly converges with the European Union by the end of the record.

Japan rises rapidly after 1965, reaching about 5,000 TWh around 2000–2010. Consumption then gradually declines to roughly 4,000 TWh by 2024.

Australia increases gradually from a few hundred TWh in the 1960s to about 1,400–1,600 TWh by 2024.

The United Kingdom peaks around 2,000–2,500 TWh in the 1970s and early 2000s before declining steadily to approximately 1,200–1,400 TWh by 2024.

South Africa shows moderate growth from a few hundred TWh in the 1960s to approximately 1,200–1,400 TWh by the 2010s, remaining relatively stable thereafter.

The chart illustrates diverging regional trends: strong growth in China and India, relative stability followed by decline in the United States, and long-term declines in Europe and the United Kingdom.

The figure includes a footnote defining watt-hours as a measure of energy:

• A watt-hour is the energy delivered by one watt of power for one hour.
• One watt-hour equals 3,600 joules.
• Common multiples include:
  • Kilowatt-hour (kWh) = 1,000 watt-hours
  • Megawatt-hour (MWh) = 1 million watt-hours
  • Gigawatt-hour (GWh) = 1 billion watt-hours
  • Terawatt-hour (TWh) = 1 trillion watt-hours

Figure 6.3.5: Per-Capita Fossil Fuel Usage by Country in 2024

Horizontal bar chart showing fossil fuel consumption per capita in 2024, measured as the average annual energy consumption from coal, oil, and natural gas in kilowatt-hours (kWh) per person. Countries and regions are ordered from highest to lowest consumption.

The United States has the highest per-capita fossil fuel consumption among the locations shown at 61,277 kWh per person, followed by Australia at 52,910 kWh per person. Both values are substantially higher than those of the remaining countries and regions.

A middle group includes Europe (28,621 kWh/person), Germany (28,391 kWh/person), and China (27,413 kWh/person), all clustered around 27,000–29,000 kWh per person.

The United Kingdom (20,390 kWh/person) and South Africa (20,184 kWh/person) form another group with similar per-capita consumption levels near 20,000 kWh.

France has a lower value of 17,201 kWh/person, while India has the lowest value shown at 6,969 kWh/person.

The chart highlights large differences in fossil fuel use per person. The United States consumes approximately 8.8 times more fossil-fuel energy per person than India, and about 2.2 times more than China. Australia's per-capita consumption is approximately 7.6 times higher than India's.

The figure includes a footnote defining watt-hours as a measure of energy:

• A watt-hour is the energy delivered by one watt of power for one hour.
• One watt-hour equals 3,600 joules.
• Common multiples include:
  • Kilowatt-hour (kWh) = 1,000 watt-hours
  • Megawatt-hour (MWh) = 1 million watt-hours
  • Gigawatt-hour (GWh) = 1 billion watt-hours
  • Terawatt-hour (TWh) = 1 trillion watt-hours