12.8: Accessible Descriptions

Figure 12.1.1: Koppen Climate Classification System

World map showing global climate classifications using the Köppen system. Different regions are grouped into major climate types based on temperature and precipitation patterns. Tropical climates occur near the equator (approximately 0–15 degrees latitude), including rainforest regions in the Amazon, central Africa, and Southeast Asia, characterized by warm temperatures year-round (typically above 18°C) and high precipitation. Surrounding these are monsoon and savanna climates extending to about 10–25 degrees latitude, with seasonal rainfall patterns.

Dry climates dominate subtropical regions (approximately 15–35 degrees latitude), including deserts and semi-arid areas such as the Sahara, Arabian Peninsula, central Australia, and parts of the southwestern United States. These regions receive very low annual precipitation, often less than 250 mm for deserts and up to about 500 mm for steppe climates.

Temperate climates are found in mid-latitudes (about 30–50 degrees), including much of North America, Europe, East Asia, and parts of South America and Australia. These regions have moderate temperatures, with warm summers (often 10–25°C) and mild to cool winters. Subtypes include Mediterranean climates with dry summers, humid subtropical climates with hot summers and year-round precipitation, and marine west coast climates with mild, wet conditions.

Continental climates occur in higher mid-latitudes (about 40–70 degrees), especially in the interiors of North America and Eurasia. These regions experience large seasonal temperature ranges, with warm summers (above 10°C) and cold winters (often below −10°C).

Polar climates dominate regions above about 70 degrees latitude, including Antarctica and Greenland, where temperatures remain below 10°C year-round. Tundra regions have short, cool summers, while ice cap regions remain below freezing throughout the year.

Highland climates appear in mountainous regions such as the Andes, Rockies, Himalayas, and East African Highlands, where climate varies with elevation rather than latitude.

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Table 12.8.1 Detailed data table (Köppen climate classifications):

Climate group	Code examples	Temperature characteristics	Precipitation characteristics	Typical regions
Tropical	Af, Am, Aw	All months > 18°C	High rainfall (often >1500 mm/year)	Amazon, Congo Basin, Southeast Asia
Dry (arid/semi-arid)	BWh, BWk, BSh, BSk	Hot or cold, large daily range	Very low rainfall (<250 mm desert, <500 mm steppe)	Sahara, Arabian Peninsula, central Australia
Temperate	Csa, Csb, Cfa, Cfb, Cwa	Mild winters, warm summers (10–25°C)	Moderate rainfall, seasonal in some areas	Europe, eastern US, China
Continental	Dfa, Dfb, Dfc, Dfd, Dwa, Dwb	Large seasonal range, cold winters (<0°C)	Moderate precipitation, often summer maximum	Canada, Russia, northern US
Polar	ET, EF	Warmest month <10°C	Low precipitation (often <250 mm/year)	Antarctica, Greenland
Highland	H (varies)	Varies with elevation	Varies with elevation	Andes, Himalayas

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Table 12.8.2: Expanded classification table (selected subtypes):

Code	Description	Key characteristics
Af	Tropical rainforest	No dry season, heavy rainfall year-round
Am	Tropical monsoon	Short dry season, intense wet season
Aw	Tropical savanna	Distinct wet and dry seasons
BWh	Hot desert	Extremely dry, high temperatures
BWk	Cold desert	Dry with cooler winters
BSh	Hot semi-arid	Low rainfall, warm temperatures
BSk	Cold semi-arid	Low rainfall, cooler climate
Csa	Mediterranean (hot summer)	Dry summers, wet winters
Cfb	Marine west coast	Mild, wet year-round
Cfa	Humid subtropical	Hot summers, year-round precipitation
Dfa	Humid continental (hot summer)	Warm summers, cold winters
Dfc	Subarctic	Short cool summers, long cold winters
ET	Tundra	Short growing season
EF	Ice cap	Permanent ice, below freezing year-round

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Table 12.8.3: Regional summary table

Region	Dominant climate types	Key characteristics
Equatorial regions	Af, Am	Warm, wet year-round
Subtropics	BWh, BSh, Csa	Dry or seasonal rain
Mid-latitudes	Cfa, Cfb, Dfa	Moderate temperatures
High latitudes	Dfc, ET, EF	Cold climates
Mountain regions	Highland	Elevation-dependent

Figure 12.2.2: Layered Paleoclimate Archives

Composite image showing five types of natural climate proxies that record past environmental conditions through layered growth patterns. From top to bottom, the image includes tree rings, varves (lake sediment layers), speleothems (cave mineral deposits), corals, and ice cores. Each section displays repeating bands or layers that represent changes over time, often on annual or seasonal scales.

Tree rings show alternating wide and narrow growth bands, typically representing yearly growth, where wider rings indicate favorable growing conditions and narrower rings indicate stress such as drought or cold. Varves show alternating sediment layers deposited in lakes, usually one pair of layers per year, with thicker or darker layers indicating higher sediment input. Speleothems display layered mineral deposits formed by dripping water in caves, with variations reflecting changes in precipitation and chemistry over time. Corals show banded growth structures, often annual, influenced by water temperature and chemistry. Ice cores show layered ice accumulation, where each layer represents a year or season, preserving information about past temperature, atmospheric composition, and precipitation.

These proxies collectively provide records spanning different time scales, from decades to hundreds of thousands of years.

Figure 12.2.3: Reading Tree Rings

Diagram illustrating dendrochronology, the use of tree rings to reconstruct past events by matching patterns across different wood samples. Three horizontal wood sections are shown: a living tree, a dead tree, and a log from an older structure or ruins. Each section displays a sequence of growth rings, with alternating narrow and wide bands that represent yearly growth.

The living tree contains the most recent rings, including the outermost ring representing the current or most recent year. The dead tree shows an overlapping sequence of rings that match part of the living tree’s pattern, allowing the timeline to be extended further into the past. The log from ruins contains an even older sequence of rings that overlaps with the dead tree’s pattern, extending the record back further in time.

Distinctive ring patterns, such as unusually narrow or wide rings (shown as highlighted bands), align across the samples, demonstrating how overlapping sequences can be matched to create a continuous chronological record. This process allows scientists to build timelines that can extend hundreds to thousands of years into the past, with annual resolution (one ring per year).

Figure 12.3.2: Global Temperature Reconstructions for past 2000 years.

Graph showing global temperature anomalies relative to the 1961–1990 average over the past 2000 years. The horizontal axis represents year from 0 to 2000, and the vertical axis shows temperature anomaly in degrees Celsius, ranging approximately from −0.7°C to +0.5°C. Three datasets are displayed: a proxy-based reconstruction (blue line) with an uncertainty range (shaded band), a set of discrete proxy estimates (green points), and modern instrumental data (red line).

From year 0 to about 1000, temperatures fluctuate slightly below the reference average, generally around −0.2°C to 0°C. Between about 1000 and 1800, temperatures trend cooler, reaching a minimum near −0.3°C to −0.4°C during the period often associated with the Little Ice Age. The uncertainty range widens further back in time, indicating less precise estimates.

After about 1900, the instrumental record (red line) shows a rapid warming trend, rising from around −0.2°C to approximately +0.4 to +0.5°C by 2000. This recent increase is steeper and exceeds the range of variability seen in the earlier part of the record. The proxy data (green points) generally align with the reconstructed trend but show variability around it.

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Table 12.8.4: Detailed data table (approximate temperature anomalies by period):

Time period (year)	Temperature anomaly (°C)	Characteristics
0–500	~−0.2 to 0.0	Slightly below average, moderate variability
500–1000	~−0.2 to 0.0	Relatively stable, small fluctuations
1000–1400	~−0.2 to −0.3	Gradual cooling trend
1400–1800	~−0.3 to −0.4	Coolest period (Little Ice Age)
1800–1900	~−0.3 to −0.2	Beginning of warming
1900–2000	~−0.2 to +0.4 or +0.5	Rapid modern warming

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Table 12.8.5: Dataset comparison table:

Dataset	Time coverage	Data type	Key feature
Proxy reconstruction (blue)	0–2000	Smoothed reconstruction	Long-term trend with uncertainty band
Proxy data points (green)	0–2000	Individual estimates	Variability around reconstructed mean
Instrumental record (red)	~1850–2000	Direct measurements	Rapid recent warming trend

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Table 12.8.6: Uncertainty characteristics table:

Time period	Uncertainty range (°C)	Interpretation
0–500	±0.1 to ±0.2	Moderate confidence
500–1500	±0.2 to ±0.3	Increasing uncertainty
1500–1900	±0.3 to ±0.5	Lower confidence in exact values
1900–2000	±0.05 to ±0.1	High confidence (instrumental data)

Figure 12.3.3: Global Surface Temperature Reconstruction during Holocene.

Graph showing global temperature anomalies relative to the 1961–1990 average over approximately the past 10,000 years (Holocene). The horizontal axis represents year from about −9000 (9000 BCE) to 2000 CE, and the vertical axis shows temperature anomaly in degrees Celsius, ranging roughly from −0.7°C to +0.5°C. A smoothed reconstruction (blue line) is shown with an uncertainty range (shaded band), and modern instrumental data (red line) appears near the present.

From about −9000 to −6000, temperatures rise from approximately −0.3°C to about +0.2 to +0.3°C, indicating early Holocene warming. Between about −6000 and −3000, temperatures remain relatively high and stable, generally around +0.2°C to +0.3°C. After about −3000, a gradual cooling trend begins, with temperatures declining toward 0°C and then below average.

From about −1000 to 1800 CE, temperatures fluctuate mostly between −0.1°C and −0.4°C, reaching a minimum near −0.4°C during the late pre-industrial period. Around 1900, the instrumental record shows a rapid warming, rising from about −0.2°C to approximately +0.4 to +0.5°C by 2000. The uncertainty band is wider in earlier periods, indicating less precise estimates, and narrows toward the present.

Figure 12.3.5: Direct and Ice Core measurements of Carbon Dioxide.

Graph showing atmospheric carbon dioxide (CO2) concentration over the past 10,000 years, measured in parts per million (ppm). The horizontal axis represents time before 2005, from about 10,000 years ago to the present (0), and the vertical axis shows CO2 concentration ranging from about 250 ppm to over 380 ppm. Data points from multiple sources are plotted, with an inset zoom highlighting the most recent centuries.

From about 10,000 to 7,000 years ago, CO2 levels are relatively stable, fluctuating around 260–270 ppm. Between about 7,000 and 2,000 years ago, concentrations gradually increase to around 270–280 ppm. From about 2,000 years ago to the 18th century, CO2 remains fairly stable near 275–285 ppm with small variations.

Beginning around 1800, CO2 levels start to rise more rapidly, increasing from about 280 ppm to over 350 ppm by the late 20th century. By around 2005, concentrations reach approximately 380–390 ppm. The inset panel emphasizes this sharp modern increase, showing a steep upward trend compared to the relatively gradual changes over earlier millennia.

Figure 12.3.8: Proxy-based Reconstruction of Past 800,000 years.

Four-panel graph showing linked climate changes over the past 800,000 years, with time on the horizontal axis from 0 (present) to 800 thousand years ago. The figure presents four aligned records: (a) oxygen isotope ratios (delta-18-O) from deep-sea benthic foraminifera, (b) atmospheric carbon dioxide concentration, (c) Antarctic air temperature anomaly, and (d) sediment reflectance in the Southern Ocean near Antarctica. Vertical shaded bands mark repeated glacial (colder) and interglacial (warmer) periods occurring roughly every 100,000 years.

In panel (a), delta-18-O values range from about 3.0 to 5.5 per mil. Lower values (about 3.0 to 4.0) indicate warmer periods with reduced global ice volume, while higher values (about 4.5 to 5.5) indicate colder periods with expanded ice sheets. The curve shows repeated sharp decreases (rapid warming into interglacials) followed by gradual increases (slow cooling into glacials), forming a sawtooth pattern across approximately eight major cycles.

In panel (b), atmospheric CO2 varies between about 180 ppm and 280 ppm. During glacial periods, CO2 is consistently low (about 180 to 200 ppm). During interglacial periods, CO2 rises to about 260 to 280 ppm. The timing of CO2 increases closely matches the warming transitions seen in the isotope and temperature records, with relatively rapid rises at the start of interglacials and slower declines during cooling phases.

In panel (c), Antarctic temperature anomaly ranges from approximately −10°C during cold glacial maxima to near 0°C during warm interglacials. Temperature increases rapidly during transitions into interglacial periods (warming of roughly 8 to 10°C), then gradually decreases over tens of thousands of years. The temperature curve closely follows the CO2 record and inversely tracks the isotope record (higher temperatures correspond to lower delta-18-O values).

In panel (d), sediment reflectance ranges from about 10% to 40% and serves as a proxy for ocean conditions such as productivity and stratification. Higher reflectance values (around 25% to 40%) generally occur during warmer interglacial periods, while lower values (around 10% to 20%) occur during colder glacial periods. The pattern shows variability within each cycle but broadly aligns with temperature and CO2 changes.

Across all panels, the records demonstrate strong coupling: warm interglacial periods are characterized by lower delta-18-O values (~3–4), higher CO2 (~260–280 ppm), higher Antarctic temperatures (near 0°C anomaly), and higher sediment reflectance (~25–40%). Cold glacial periods show the opposite pattern: higher delta-18-O (~4.5–5.5), lower CO2 (~180–200 ppm), lower temperatures (~−8 to −10°C), and lower reflectance (~10–20%). The transitions into warm periods are relatively abrupt (on the order of thousands of years), while cooling into glacial conditions is more gradual (tens of thousands of years).

The figure illustrates about eight major glacial-interglacial cycles over 800,000 years, each lasting roughly 100,000 years, with consistent amplitude and timing relationships among ice volume, greenhouse gases, temperature, and ocean conditions.

Figure 12.4.1: Tectonic Plates

Global map showing the major tectonic plates of Earth and their boundaries. The continents and oceans are divided into large plates that move relative to one another. The largest plate is the Pacific Plate, which covers much of the Pacific Ocean basin. Other major plates include the North American, South American, African, Eurasian, Antarctic, Australian, and Indian plates. Several smaller plates are also shown, including the Nazca, Cocos, Caribbean, Philippine, Arabian, Somali, Scotia, and Juan de Fuca plates.

Plate boundaries are marked with different line types indicating interactions. Divergent boundaries (where plates move apart) are primarily located along mid-ocean ridges, such as the Mid-Atlantic Ridge between the North American and Eurasian plates in the north and between the South American and African plates in the south. Convergent boundaries (where plates move toward each other) occur along subduction zones, such as where the Nazca Plate meets the South American Plate along the west coast of South America, and where the Pacific Plate meets surrounding plates along the Pacific “Ring of Fire.” Transform boundaries (where plates slide past each other) occur in locations such as the boundary between the Pacific Plate and North American Plate along western North America.

The map shows that plate boundaries often align with major geological features: mountain ranges (for example, the Himalayas at the boundary of the Indian and Eurasian plates), ocean trenches (for example, along the western Pacific), and mid-ocean ridges. Plate sizes vary widely, with major plates spanning thousands of kilometers, while smaller plates occupy more localized regions.

Figure 12.4.2: Solar Cycle

Graph showing variations in solar activity and solar irradiance from about 1975 to 2005. The horizontal axis represents year, and the vertical axis shows solar irradiance in watts per square meter (W/m²), ranging approximately from 1365 to 1367 W/m². Multiple lines represent different indicators of solar activity, including total solar irradiance (daily and annual averages), sunspot observations, solar flare index, and 10.7 cm radio flux.

The data show repeating cycles approximately every 10 to 11 years. Solar irradiance varies slightly, with peaks around 1366.5 to 1367 W/m² during periods of high solar activity and lows near 1365.5 W/m² during quieter periods. These peaks occur around the early 1980s, around 1990, and around 2000.

Sunspot observations, solar flare index, and radio flux all rise and fall in sync with irradiance, indicating that higher solar activity corresponds to slightly increased solar energy output. The daily irradiance (shown with large variability) fluctuates rapidly, while the smoothed annual values show a clearer cyclic pattern. Overall, the amplitude of irradiance variation is small (about 1 to 1.5 W/m², or less than 0.1% of total output), despite larger variations in activity indicators.

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Table 12.8.7: Detailed data table (solar cycle characteristics):

Parameter	Approximate range	Behavior
Solar irradiance (W/m²)	~1365.5–1367	Small cyclic variation
Cycle length	~10–11 years	Repeating solar cycle
Irradiance variation	~1–1.5 W/m²	Less than 0.1% change
Peak years (approx.)	~1980, ~1990, ~2000	High solar activity
Minimum years (approx.)	~1976, ~1986, ~1996, ~2005	Low solar activity

Figure 12.4.3: Milankovitch Cycles

Diagram illustrating the three main components of Milankovitch cycles, which describe long-term variations in Earth’s orbit and orientation that influence climate. The image shows the Sun at the center and Earth orbiting along an elliptical path. Three labeled processes are indicated: eccentricity (E), tilt or obliquity (T), and precession (P).

Eccentricity (E) refers to changes in the shape of Earth’s orbit from more circular to more elliptical. The diagram shows an elongated orbit, indicating that the distance between Earth and the Sun varies over time. This cycle occurs on timescales of about 100,000 years (with additional longer cycles around 400,000 years) and affects the overall distribution of solar energy received by Earth.

Tilt (T), also called obliquity, refers to changes in the angle of Earth’s axis relative to its orbital plane. The diagram shows Earth’s axis tilted relative to vertical, with arrows indicating variation in this angle. The tilt varies between about 22.1° and 24.5° over a cycle of approximately 41,000 years. Greater tilt increases seasonal contrast (warmer summers and colder winters), especially at higher latitudes.

Precession (P) refers to the wobble of Earth’s rotational axis, changing the orientation of the axis over time. The diagram shows circular arrows around the axis, indicating this slow rotation. Precession operates on timescales of about 19,000 to 23,000 years and affects the timing of seasons relative to Earth’s position in its orbit (for example, whether summer occurs when Earth is closer to or farther from the Sun).

Together, these three cycles alter the distribution and intensity of solar radiation received at different latitudes and seasons, contributing to long-term climate patterns such as glacial and interglacial cycles.

Figure 12.4.4: Milankovitch Cycles and Past Glaciation

Multi-line graph showing variations in Earth’s orbital parameters and their relationship to climate over the past 1,000,000 years (1 million years). The horizontal axis represents time from 1,000 thousand years ago (kyr ago) to the present (“Now”). Five stacked panels display: precession, obliquity, eccentricity, solar forcing at 65°N during summer, and stages of glaciation (temperature state). Vertical shaded bands mark alternating colder (glacial) and warmer (interglacial) periods.

The top panel shows precession, with rapid oscillations occurring approximately every 19, 22, and 24 thousand years. The signal is highly variable, with frequent peaks and troughs indicating changes in the timing of seasons relative to Earth’s orbit.

The second panel shows obliquity (axial tilt), varying smoothly with a cycle of about 41,000 years. The amplitude remains relatively consistent, reflecting changes in Earth’s tilt between about 22° and 24.5°, which influences seasonal contrast.

The third panel shows eccentricity, with longer cycles of about 95, 125, and 400 thousand years. The variations are slower and lower frequency, indicating changes in the shape of Earth’s orbit from more circular to more elliptical.

The fourth panel shows solar forcing at 65°N during summer, which combines the effects of precession, obliquity, and eccentricity. This curve fluctuates on multiple timescales, with both high-frequency variations (tens of thousands of years) and lower-frequency modulation (hundreds of thousands of years). Peaks represent stronger summer solar input at high northern latitudes, while troughs represent weaker input.

The bottom panel shows stages of glaciation, with a temperature-related curve labeled from “cold” to “hot.” This curve shows repeated glacial-interglacial cycles, with cold periods lasting longer and warm periods appearing as shorter peaks. The timing of these climate changes broadly aligns with variations in solar forcing, especially the longer eccentricity-modulated cycles.

Overall, the figure illustrates how orbital variations influence solar energy distribution and are linked to the timing of ice ages, with dominant glacial cycles occurring roughly every 100,000 years.

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Table 12.8.8: Detailed data table (orbital cycles):

Parameter	Cycle lengths (kyr)	Frequency pattern	Primary effect
Precession	~19, 22, 24	High frequency	Seasonal timing changes
Obliquity	~41	Moderate frequency	Controls seasonal intensity
Eccentricity	~95, 125, 400	Low frequency	Modulates orbital shape

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Table 12.8.9: Solar forcing table:

Feature	Timescale influence	Description
High-frequency variation	~20–40 kyr	Driven by precession and obliquity
Low-frequency modulation	~100–400 kyr	Controlled by eccentricity
Peak forcing	—	Strong summer insolation at 65°N
Minimum forcing	—	Weak summer insolation

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Table 12.8.10: Glaciation characteristics table:

Feature	Approximate value	Description
Cycle duration	~100 kyr	Dominant glacial cycle
Cold period length	~80–90 kyr	Long glacial intervals
Warm period length	~10–20 kyr	Short interglacial peaks
Temperature pattern	Asymmetric	Gradual cooling, rapid warming

Figure 12.4.5c: Volcanic Eruptions and Solar Transmission

Graph showing the percentage of solar radiation transmitted through Earth’s atmosphere from about 1958 to 2008. The horizontal axis represents year, and the vertical axis shows solar radiation transmitted in percent, ranging from about 70% to 100%. A relatively steady baseline near 92–94% is visible, with several sharp decreases corresponding to major volcanic eruptions.

Most of the record shows small fluctuations around about 93%, indicating relatively stable atmospheric transmission. However, three major dips stand out. Around 1982, transmission drops sharply to about 78–80%, corresponding to the eruption of El Chichón (17.4°N, Volcanic Explosivity Index (VEI) 5). Around 1991, an even larger drop occurs to about 75–80%, associated with the eruption of Mount Pinatubo (15.1°N, VEI 6). Smaller dips are also visible around 1963 (Agung, 8.3°S, VEI 4) and around 1974–1975 (Fuego, 14.5°N, VEI 4), with transmission decreasing to roughly 90–92%.

After each eruption, solar transmission gradually recovers over several years as volcanic aerosols settle out of the atmosphere. The largest eruptions produce the deepest and longest-lasting reductions in transmitted solar radiation.

Figure 12.5.5: Trends in Sea Ice

Three-panel figure showing sea ice extent trends from 1979 to about 2020–2021 for the Northern Hemisphere (top), global total (middle), and Southern Hemisphere (bottom). The horizontal axes show year, and the vertical axes show sea ice extent in million square kilometers. Each panel includes annual bars, a long-term average line (1981–2010), and a linear trend line indicating change over time.

In the Northern Hemisphere panel (September, annual minimum), sea ice extent declines from about 7.5–7.8 million km² in the late 1970s to about 4.0–4.5 million km² by 2020. The 1981–2010 average is about 6.41 million km². The trend shows a decrease of about −12.79% per decade, equivalent to approximately −0.82 million km² per decade. Year-to-year variability is present, with some higher years in the 1980s–1990s and notably low values after 2007.

In the Global sea ice panel (monthly data, 1979–2021), total sea ice extent fluctuates seasonally between roughly 16 and 27 million km². The long-term average is about 23.25 million km². The overall trend is slightly negative, about −2.1% per decade (approximately −0.51 million km² per decade). Seasonal cycles dominate the variability, but a gradual decline is visible over time.

In the Southern Hemisphere panel (March, annual minimum), sea ice extent varies between about 3.0 and 5.2 million km². The 1981–2010 average is about 4.03 million km². The trend shows a small increase of about +1.74% per decade (approximately +0.07 million km² per decade), with noticeable variability and some higher values in the 2000s–2010s, followed by lower values in recent years.

Overall, the figure shows a strong decline in Arctic (Northern Hemisphere) sea ice, a slight increase in Antarctic (Southern Hemisphere) sea ice over much of the period, and a small net decline in global sea ice, with variability driven by seasonal cycles and regional differences.

Figure 12.5.7: Mountain Glacier Decline

Graph showing cumulative mass change of reference glaciers across multiple global regions from about 1950 to 2020. The horizontal axis represents year, and the vertical axis shows cumulative mass change in meters of water equivalent (m w.e.), ranging approximately from +5 to −35. Values are relative to a baseline around 1976, where most curves cross near zero. Multiple colored lines represent different regions, and a dashed black line shows the global mean across regions.

From 1950 to the mid-1970s, most regions show small fluctuations around zero, generally within about +2 to −2 m w.e., indicating relatively stable glacier mass. After about 1980, nearly all regions show a consistent downward trend, indicating cumulative mass loss.

By around 2000, most regions have lost between about −5 and −15 m w.e. After 2000, the rate of loss accelerates in many regions. By 2020, cumulative losses vary widely by region:

• Western Canada and USA show the largest loss, reaching approximately −30 to −35 m w.e.
• Central Europe also shows large losses, around −25 to −30 m w.e.
• Alaska, Asia Central, and Arctic Canada North show losses of roughly −15 to −25 m w.e.
• Scandinavia, Southern Andes, and Svalbard and Jan Mayen show somewhat smaller but still substantial losses, typically around −10 to −20 m w.e.
• Asia North and the Caucasus and Middle East show losses closer to −10 to −15 m w.e.

The dashed global mean declines from near 0 in 1976 to about −20 m w.e. by 2020, indicating widespread and consistent glacier mass loss across regions. The curves show increasing divergence after 1990, reflecting regional differences in climate and glacier response, but all trends are negative after the late 20th century.

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Table 12.8.11: Detailed data table (approximate cumulative mass loss by region, ~2020):

Region	Cumulative mass change (m w.e.)	Relative magnitude
Western Canada and USA	~−30 to −35	Very high loss
Central Europe	~−25 to −30	Very high loss
Alaska	~−20 to −25	High loss
Asia Central	~−18 to −25	High loss
Arctic Canada North	~−15 to −22	High loss
Svalbard and Jan Mayen	~−15 to −20	Moderate-high loss
Southern Andes	~−12 to −20	Moderate-high loss
Scandinavia	~−10 to −15	Moderate loss
Caucasus and Middle East	~−10 to −15	Moderate loss
Asia North	~−10 to −12	Lower (but still significant) loss

Figure 12.5.8: Greenland and Antarctica Ice Sheet Loss

Line graph showing cumulative ice mass change for Antarctica and Greenland from about 1992 to 2024. The horizontal axis represents year, and the vertical axis shows cumulative mass change in billion metric tons, ranging from about +500 to −6000. A dashed horizontal line at 0 marks no net change. Two main datasets are shown for each region: a combined estimate (bold lines) and a reference dataset (lighter lines), with shaded bands indicating uncertainty.

At the start of the record (early 1990s), both Antarctica and Greenland are near 0 change, indicating little net mass loss relative to the baseline. Over time, both regions show increasingly negative values, indicating cumulative ice loss.

Greenland (blue lines) shows a steep and accelerating decline. By about 2005, Greenland has lost roughly −500 to −1000 billion tons. Loss accelerates after 2010, reaching about −3000 billion tons by around 2015, and approximately −4500 to −5500 billion tons by 2024. The trend is strongly downward, with short-term variability but a clear long-term acceleration.

Antarctica (red lines) also shows a net loss, but at a slower rate. By about 2005, losses are around −200 to −400 billion tons. By 2015, cumulative loss reaches roughly −1000 to −1500 billion tons. By 2024, Antarctica has lost approximately −2000 to −2500 billion tons. The trend is downward but less steep than Greenland’s.

The uncertainty bands widen slightly over time, but both datasets consistently show negative trends. Greenland contributes the larger share of total ice mass loss over this period, with losses roughly two to three times greater than Antarctica by the end of the record.

Figure 12.5.9: Oceanic Heat Content

Graph showing global ocean heat content (0–2000 meters depth) from about 1960 to 2025. The horizontal axis represents year, and the vertical axis shows heat content in units of 10^22 joules, ranging from about −10 to +35. Multiple lines represent different averaging methods: a long-term smoothed trend (blue), yearly averages (black), and a recent 3-month average (red).

From about 1960 to the early 1970s, ocean heat content is relatively low, fluctuating between about −10 and −5 × 10^22 joules. Between the 1970s and 1990, values gradually rise to around 0 to +5 × 10^22 joules, with some short-term variability.

After about 1990, the increase becomes more pronounced. By around 2005, heat content reaches approximately +10 × 10^22 joules. The rate of increase accelerates further after 2010, with values rising to about +20 × 10^22 joules by the mid-2010s and continuing upward.

By 2024–2025, ocean heat content reaches approximately +30 to +33 × 10^22 joules, representing the highest values in the record. The red line (recent 3-month average) shows short-term variability but follows the same upward trend. The blue line (smoothed trend) highlights a steady and accelerating increase over time.

Overall, the graph shows a long-term increase in ocean heat content, with the majority of warming occurring after 1990 and especially after 2010.

Figure 12.5.10: Global Mean Sea Level Change

Graph showing global mean sea level change from about 1880 to 2020, with a secondary pie chart illustrating contributions to sea level rise from 2005 to 2015. The horizontal axis represents year, and the vertical axis shows global mean sea level in millimeters (mm), ranging from about −50 mm to +250 mm relative to a baseline in the late 19th century.

The main time series shows a steady increase in sea level over time. From about 1880 to 1920, sea level rises gradually from approximately −20 mm to about +40 mm. Between 1920 and 1970, the increase continues more steadily, reaching around +100 mm. From 1970 onward, the rate of rise accelerates. By about 2000, sea level reaches approximately +150 mm, and by 2020 it approaches about +240 to +250 mm.

Two data sources are shown: tide gauge measurements (blue line with shaded uncertainty band) covering the full record, and satellite altimetry (red line) from the early 1990s onward. The satellite data show a steeper rise compared to earlier decades, confirming an acceleration in sea level increase.

The inset pie chart shows contributions to sea level rise during 2005–2015. Thermal expansion of seawater accounts for about 40% of the total rise. Melting glaciers contribute about 17%, Greenland ice sheet loss contributes about 21%, Antarctic ice sheet loss contributes about 12%, and groundwater depletion contributes about 11%.

Overall, the figure shows a long-term rise in global sea level with clear acceleration in recent decades, driven by both ocean warming and increasing ice loss.

Figure 12.5.11: The Keeling Curve

Line graph showing atmospheric carbon dioxide (CO2) concentrations measured at Mauna Loa Observatory from about 1960 to 2020. The horizontal axis represents year, and the vertical axis shows CO2 concentration in parts per million (ppm), ranging from about 315 ppm to over 410 ppm.

The graph shows a clear long-term upward trend in CO2 levels. In 1960, concentrations are approximately 315–317 ppm. By 1980, levels rise to about 335–340 ppm. Around 2000, CO2 reaches roughly 370 ppm. By 2010, concentrations increase further to about 390 ppm, and by 2020 they exceed 410 ppm.

Superimposed on the long-term increase is a regular seasonal cycle, visible as a repeating sawtooth pattern each year. The seasonal variation is typically about 5–8 ppm, with peaks occurring in late spring and lows in early autumn, reflecting natural cycles of plant growth and decay in the Northern Hemisphere.

The overall trend shows an accelerating increase, with the rate of rise growing from about 1 ppm per year in the 1960s to about 2–3 ppm per year in recent decades. The data are attributed to long-term monitoring programs by scientific institutions, and the record is commonly known as the Keeling Curve.

Figure 12.6.4: U.S. Electricity Generation Sources

Chart showing sources of U.S. electricity generation in 2023, totaling about 4.18 trillion kilowatt-hours. The graphic divides electricity generation into major categories with percentage shares. Natural gas is the largest source at about 43.1% of total generation. Renewable energy accounts for about 21.4%, nuclear power contributes about 18.6%, coal provides about 16.2%, and petroleum and other sources account for about 0.8%.

Within the renewable category (21.4%), the breakdown includes wind at about 10.2%, hydropower at about 5.7%, solar at about 3.9%, biomass at about 1.1%, and geothermal at about 0.4%.

Overall, fossil fuels (natural gas, coal, and petroleum) together account for about 60.1% of electricity generation, while non-fossil sources (renewables plus nuclear) make up about 40.0%.

Figure 12.6.5b: Hydroelectric Dams

Diagram of a hydroelectric power plant showing how water stored in a reservoir is used to generate electricity. Water is held behind a dam in a reservoir at a higher elevation. It enters the system through an intake structure and flows into a large pipe called a penstock. The water travels downward through the penstock, gaining speed due to gravity.

At the lower end of the system, the flowing water turns a turbine. The turbine is connected to a generator located inside a powerhouse. As the turbine spins, it drives the generator to produce electricity. Electrical power is then transmitted away from the plant through power lines. After passing through the turbine, the water exits the system and flows into a downstream river.

The diagram shows the sequence: reservoir → intake → penstock → turbine → generator (in powerhouse) → transmission lines → river. The system converts gravitational potential energy of stored water into mechanical energy (turbine rotation) and then into electrical energy.