Long-term patterns, mechanisms, evidence, natural variation, human causes, impacts, and the tools for thinking clearly about climate risk and response
Weather changes from day to day. Climate shapes how species evolve, where civilizations form, and which crops grow in which valleys. When climate changes — slowly, over centuries, or rapidly, over decades — everything from sea level to wildfire season to coral reef health changes with it. This chapter explains what climate is, what controls it, how scientists reconstruct past climates, what causes climate to vary naturally, what is driving the current warming trend, and how societies can respond. The goal is not alarm and not dismissal. The goal is understanding: how evidence is gathered, how mechanisms work, and how decisions are made under uncertainty.
By the end of this chapter you should understand how climate differs from weather, what controls regional climate, how the greenhouse effect works, how carbon cycles through Earth's systems, how scientists read past climates from proxies, what causes natural climate variation, what the evidence shows about human-caused warming, what impacts are observed and projected, and how mitigation and adaptation differ as responses. You will also practice reading climate graphs, evaluating evidence claims, and reasoning about uncertainty.
A classmate says: "It was the coldest winter in twenty years, so climate change must be fake." How would you respond? By the end of this chapter, you will have the tools to explain exactly why a single cold winter cannot confirm or disprove a long-term climate trend — and to describe what evidence actually does and does not show.
Weather is what is happening in the atmosphere right now, or over the next few days. It is the temperature at noon on a Tuesday, the rainstorm that arrives Friday evening, the wind that makes November feel colder than the thermometer shows. Weather is measured, forecasted, and highly variable.
Climate is the long-term pattern of weather in a region. It is not any single day or storm. Climate describes what kinds of temperatures and precipitation a place typically receives across months, seasons, and years — averaged and characterized over decades, usually at least thirty years of data.
The classic distinction is this: weather is your mood; climate is your personality. One cold summer does not disprove warming, just as one warm winter does not prove it. Climate requires long records, wide geographic coverage, and careful statistical analysis. Scientists who study climate analyze trends — changes in averages and in the frequency or severity of extremes — across decades and centuries.
| Feature | Weather | Climate |
|---|---|---|
| Timescale | Hours to days | Decades to centuries |
| Spatial scale | Local to regional | Regional to global |
| Predictability | Days to roughly two weeks | Statistical patterns over long periods |
| Example | A blizzard in March | Average winter snowfall for a region |
Understanding climate requires thinking statistically. A single unusually warm year matters less than a consistent upward trend across thirty or forty years. Climate scientists look at distributions — how averages shift, how the frequency of extremes changes — not just single events. A cold month in a warming world is like a step down on a staircase that is still moving up overall.
Earth's surface is not uniformly warm or cold, wet or dry. Sunlight arrives at different angles at different latitudes. Continents and oceans store and release heat differently. Mountain ranges force air upward and block moisture. Ocean currents carry warm or cold water thousands of kilometers. These factors combine to create distinct climate zones. The most widely used classification system, developed by the German-Russian climatologist Wladimir Köppen, divides Earth into five major climate types based on temperature and precipitation patterns.
| Type | Name | Key Characteristics | Examples |
|---|---|---|---|
| A | Tropical | Warm year-round; abundant rainfall | Amazon basin, Congo, Southeast Asia |
| B | Dry | Evaporation exceeds precipitation | Sahara, Australian outback, Atacama |
| C | Temperate | Mild winters; warm to hot summers; ocean influence | Mediterranean coast, Pacific Northwest, SE United States |
| D | Continental | Cold winters; large seasonal swings; interior locations | Canada, Russia, northern United States |
| E | Polar / Highland | Cold year-round; high latitude or high elevation | Greenland, Antarctica, Tibetan Plateau |
Climate zones are not rigid boundaries. They shift over time as climate changes. During glacial periods, climate zones moved toward the equator. During warmer periods, they shifted toward the poles. Where climate zones sit — and how they may shift — has enormous implications for agriculture, biodiversity, and human settlement.
A world map showing climate zones color-coded by type: tropical (A) in dark green near the equator; dry (B) in tan/orange at subtropical latitudes; temperate (C) in light green at mid-latitudes on ocean-facing coasts; continental (D) in blue-grey in mid-latitude continental interiors; polar/highland (E) in white/pale blue at high latitudes and mountain peaks. Label major ocean currents (Gulf Stream, California Current, Peru Current) to show their influence on adjacent climate zones. Include a legend with zone letter, name, and color.
Climate does not depend on latitude alone. Four major factors interact to produce local and regional climate patterns.
The most powerful control on climate is latitude — the distance from the equator. Near the equator, sunlight strikes Earth's surface at a steep angle, concentrating solar energy in a small area. Near the poles, sunlight strikes at a shallow angle, spreading the same amount of energy over a larger area. The result is that tropical regions are far warmer than polar regions. Latitude also controls the length of days across seasons, producing large seasonal temperature swings at high latitudes.
Temperature decreases with altitude at an average rate of about 6.5°C per 1,000 meters — called the environmental lapse rate. A mountain at 3,000 meters elevation may be 15–20°C cooler than the surrounding lowlands. Elevation also affects precipitation: when air rises over mountains it cools and water vapor condenses, producing rain or snow on the windward side. On the leeward side — the rain shadow — descending air warms and dries, creating arid conditions.
Water heats and cools more slowly than land because of its higher heat capacity. Coastal (maritime) climates are moderated by the ocean: winters are milder and summers are cooler than inland locations at the same latitude. Interior continental climates are more extreme. Ocean currents amplify these effects: the Gulf Stream moderates western European climates; cold currents along western coastlines produce cool, foggy conditions and support productive fisheries.
Mountain ranges create rain shadows, channel winds, and trap cold air in valleys. Topography interacts with all other factors. Two locations at the same latitude and elevation can have dramatically different climates depending on their position relative to mountains and the ocean.
Latitude — controls angle of sunlight and seasonal day length. More poleward = colder and more seasonal.
Elevation — temperature drops ~6.5°C per 1,000 m. Mountains force precipitation on windward sides and create rain shadows on leeward sides.
Ocean proximity — oceans moderate temperature swings. Coastal = mild; interior = extreme seasons.
Topography — shapes airflow, precipitation, and local temperature patterns at finer scales.
Earth is warmed by the sun but also cooled by radiating energy back to space. The balance between incoming solar energy and outgoing heat energy determines Earth's average temperature. The greenhouse effect is the mechanism by which certain gases in the atmosphere slow the loss of heat to space, keeping Earth warmer than it would otherwise be.
The sun emits energy primarily as visible light and ultraviolet radiation — short-wavelength, high-energy radiation. Most of this passes through the atmosphere and reaches Earth's surface. Earth's surface absorbs solar radiation and warms. It then re-radiates energy as infrared radiation — long-wavelength, lower-energy heat radiation. Greenhouse gases absorb this outgoing infrared radiation and re-emit it in all directions, including back toward Earth's surface. The result is that the surface and lower atmosphere are warmer than they would be if all the outgoing infrared radiation escaped directly to space.
This process is analogous to a blanket. A blanket does not generate heat; it slows the loss of heat from your body. Greenhouse gases do not generate energy; they slow the loss of heat from Earth's surface to space.
Natural greenhouse effect — without any greenhouse gases, Earth's average surface temperature would be approximately −18°C instead of the actual +15°C. The natural greenhouse effect provides 33°C of warming, making Earth habitable.
Enhanced greenhouse effect — human activities have increased greenhouse gas concentrations, intensifying the effect and pushing the energy balance toward warming. Not a new mechanism — an intensification of a natural one.
| Greenhouse Gas | Formula | Main Natural Sources | Human Sources |
|---|---|---|---|
| Water vapor | H₂O | Evaporation; ~50% of natural greenhouse effect | Increases as temperature rises (feedback, not direct emission) |
| Carbon dioxide | CO₂ | Volcanoes, respiration, decomposition | Burning fossil fuels, deforestation, cement production |
| Methane | CH₄ | Wetlands, animal digestion, permafrost | Livestock, rice paddies, landfills, natural gas leaks |
| Nitrous oxide | N₂O | Soils, oceans | Fertilized agricultural soils |
A cross-section diagram showing: (1) the sun emitting short-wave radiation (yellow arrows) passing through the atmosphere to Earth's surface; (2) Earth's surface absorbing solar energy and emitting long-wave infrared radiation (red arrows) upward; (3) greenhouse gas molecules in the lower atmosphere absorbing upward infrared and re-emitting it in all directions — some escaping to space, some directed back toward the surface; (4) a label showing the surface temperature with and without the greenhouse effect (−18°C vs. +15°C). Use a warm-color scheme (yellow/orange) for incoming solar radiation and cooler red for outgoing infrared.
Carbon moves continuously through Earth's systems. It is stored in the atmosphere, oceans, land (soils, rocks, living organisms), and the deep Earth. The movement of carbon among these reservoirs is the carbon cycle, and it is deeply connected to climate.
| Reservoir | Approximate Size | Key Notes |
|---|---|---|
| Atmosphere | ~900 Gt C | Primarily as CO₂; concentration now over 420 ppm |
| Oceans | ~38,000 Gt C | Largest active reservoir; exchanges CO₂ at surface |
| Land biosphere | ~2,000–3,000 Gt C | Living plants, soils, organic matter |
| Fossil fuels | Largest accessible reserve | Stored over millions of years; removed from active cycle by burial |
| Deep Earth | Enormous but slow | Volcanoes release CO₂ from mantle over geologic time |
Gt C = gigatons of carbon; one gigaton = one billion metric tons.
Over millions of years, the carbon cycle has acted as a long-term climate thermostat. When CO₂ rises (from volcanic activity), temperatures rise, the water cycle accelerates, weathering rates increase, and more CO₂ is drawn down into carbonate rocks — eventually cooling the climate. This negative feedback has kept Earth's temperature within a range compatible with liquid water across billions of years. However, this thermostat operates on timescales of hundreds of thousands to millions of years. It cannot compensate for rapid human emissions over decades to centuries — the geological processes are far too slow.
Current human CO₂ emissions are approximately 36–40 gigatons per year — more than one hundred times the natural rate of volcanic outgassing. Atmospheric CO₂ has risen from about 280 ppm before industrialization to over 420 ppm today, a level not seen in at least 800,000 years.
A systems diagram showing the five major carbon reservoirs as labeled boxes (atmosphere, ocean, land biosphere, fossil fuels, deep Earth) connected by arrows representing flows. Label each flow with its process name and approximate rate in Gt C/yr: photosynthesis (land and ocean), respiration and decomposition, ocean-atmosphere exchange, fossil fuel burning (~37 Gt/yr), volcanic outgassing (~0.3 Gt/yr), weathering (slow). Distinguish the natural cycle arrows (thin, dark) from the human emission arrow (thick, red). Note that human emissions exceed natural volcanic outgassing by approximately 100:1.
How do scientists know about past climates? Instrumental records — thermometers, rain gauges, satellites — are accurate but only cover roughly the past 150–200 years. To understand climate variation over centuries, millennia, and millions of years, scientists use proxy records — natural archives that preserve information about past temperature, precipitation, and atmospheric composition.
Since the mid-nineteenth century, systematic temperature records have been collected from weather stations and from ships and buoys at sea. These records show a clear warming trend from the late nineteenth century to the present. Satellite records since 1979 confirm tropospheric warming consistent with surface records. When examining temperature records, scientists calculate anomalies — departures from a baseline average — to remove geographic and elevation differences between stations and show whether temperatures are above or below the historical norm.
Ice cores drilled from Greenland and Antarctica preserve a remarkable record of past climate. Each year's snowfall leaves a layer. As layers compact into ice, air bubbles are trapped — preserving a sample of the actual atmosphere at the time the ice formed. From ice cores scientists can measure atmospheric CO₂ and methane concentrations going back 800,000 years, temperature (from oxygen isotope ratios), volcanic eruptions (ash layers), dust, and sea salt. The Vostok and EPICA Dome C cores from Antarctica are among the most important paleoclimate records ever recovered, showing that CO₂ concentration and temperature have tracked each other closely through glacial-interglacial cycles.
Trees grow a new ring each year. Wide rings indicate favorable growing conditions; narrow rings indicate stress. By analyzing ring widths, scientists can reconstruct temperature or precipitation records going back thousands of years. Ocean sediment cores contain shells of foraminifera whose chemistry records the temperature and salinity of past seawater, extending climate reconstruction back millions of years. Coral annual bands, pollen in lake sediments, cave formations, and historical documents provide additional overlapping evidence. No single proxy is perfect; climate scientists use many overlapping proxies from different regions, cross-validating them against each other and against instrumental records.
The current warming trend is occurring faster than natural variability alone can explain, and the atmospheric CO₂ concentration is higher than at any time in the 800,000-year ice core record — and likely higher than at any time in several million years.
Earth's climate has never been static. Long before humans existed, climate shifted between glacial periods with ice sheets covering large parts of the continents and warmer interglacial periods. Understanding natural climate variation helps scientists distinguish it from human-caused changes.
Earth's orbit around the sun and the orientation of its axis are not perfectly constant. They vary over long cycles, changing the distribution of solar energy across the planet and seasons. These are called Milankovitch cycles, after the Serbian mathematician Milutin Milanković.
| Parameter | What Varies | Timescale | Primary Effect |
|---|---|---|---|
| Eccentricity | Shape of Earth's orbit (circular to elliptical) | ~100,000 years | Changes difference in solar energy at closest vs. farthest point from sun |
| Axial tilt (obliquity) | Tilt of axis from 22.1° to 24.5° | ~41,000 years | Greater tilt = stronger seasonal contrast |
| Precession | Wobble of Earth's axis | ~26,000 years | Changes which hemisphere points toward sun at perihelion |
Milankovitch cycles explain the pacing of ice ages over the past 800,000 years. However, the current orbital configuration would, if anything, produce slight cooling over the next several thousand years — the opposite of what is observed.
Solar output varies in an approximately 11-year cycle (the sunspot cycle) and shows longer-term changes. However, satellite measurements since 1978 show that solar output has not increased — in fact, it has very slightly decreased — while Earth has warmed significantly. Solar variation is not a driver of the current warming trend.
Volcanic eruptions inject sulfate aerosols into the stratosphere, reflecting sunlight and causing temporary cooling of 0.3–0.5°C lasting one to three years. The 1991 Pinatubo eruption cooled global temperatures for about two years. Volcanic cooling is real but temporary. It cannot explain long-term warming trends.
Internal variability such as El Niño–Southern Oscillation (ENSO) redistributes heat regionally and can raise or lower global average temperatures for a year or two. These patterns do not produce a long-term directional trend.
Natural climate variation is real and well documented. But knowing that past climate changes had natural causes does not tell you the cause of any specific change. Attribution science compares observed patterns against what models predict with natural forcings alone vs. with human forcings added. Natural forcings alone cannot match the observed warming record — human forcings are required.
The overwhelming scientific evidence is that the current warming of Earth's climate system is primarily caused by human activities, especially the burning of fossil fuels, which has substantially increased the concentration of greenhouse gases in the atmosphere.
| Line of Evidence | What It Shows |
|---|---|
| Rising CO₂ | From 280 ppm pre-industrial to 420+ ppm today — unprecedented in at least 800,000 years of ice core records |
| Isotopic fingerprint | Fossil-fuel carbon is depleted in ¹³C and contains no ¹⁴C; the declining ¹³C/¹²C ratio in atmospheric CO₂ confirms the added carbon comes from fossil sources, not volcanoes or ocean outgassing |
| Temperature trends | ~1.1–1.2°C warming above pre-industrial levels (2011–2020 vs. 1850–1900); clearest trend in the last 50 years |
| Asymmetric warming pattern | Nights warm faster than days; Arctic warms faster than tropics; troposphere warms while stratosphere cools — all signatures of greenhouse forcing, not solar forcing |
| Ocean warming | More than 90% of excess heat has been absorbed by the ocean; measurable throughout the water column |
| Physical changes | 20–25 cm of sea level rise since 1900; declining Arctic sea ice; global glacier mass loss; permafrost thaw; earlier spring timing |
An initial warming from greenhouse gases triggers several positive feedbacks that amplify the original effect.
Water vapor feedback — as temperature rises, more water evaporates and warmer air holds more water vapor. Water vapor is itself a greenhouse gas, trapping more heat and causing further warming. This is the most powerful amplifying feedback.
Ice-albedo feedback — as ice melts, darker land and ocean are exposed. They absorb more solar energy (lower albedo), causing further warming and further melting. This is why the Arctic is warming faster than the global average.
Permafrost feedback — thawing permafrost releases stored methane and CO₂ as organic matter decomposes, potentially accelerating warming.
A circular feedback loop diagram showing: Warming → Ice melts → Darker ocean/land exposed → More solar energy absorbed (lower albedo) → Further warming → More ice melts. Label this as a positive feedback. Include small inset illustrations: a highly reflective ice surface (white, ~85% albedo) vs. a dark ocean surface (~6% albedo). Add annotation: positive feedback amplifies initial change but does not produce unlimited runaway warming on its own.
Equilibrium climate sensitivity (ECS) is the amount of global average warming expected from a doubling of atmospheric CO₂, after the climate system fully adjusts. The current best estimate from the scientific community is likely in the range of 2.5°C to 4°C, with 3°C as a central estimate. The uncertainty comes primarily from the difficulty of quantifying cloud feedbacks. Even at the lower end, a doubling of CO₂ would produce substantial warming with serious consequences.
Warming does not affect all places equally or all aspects of Earth systems equally. Climate change involves a complex set of interacting impacts on physical systems, ecosystems, and human societies.
| Impact Category | Key Observations and Projections |
|---|---|
| Temperature extremes | Heat waves more frequent, intense, and longer-lasting; cold extremes less frequent; heat-related illness and death risks increase |
| Precipitation shifts | More intense precipitation events in many regions; longer dry periods between events in others; earlier snowpack peak reduces late-season water availability |
| Sea level rise | ~20–25 cm since 1900; driven by thermal expansion (~50%) and ice melt; projections of 0.3–1+ m by 2100 depending on emissions; amplifies storm surge damage |
| Ocean acidification | pH dropped ~0.1 units since pre-industrial (~26% more acidic); threatens shell-forming organisms (oysters, corals, foraminifera) and marine food webs |
| Ecosystem disruption | Species ranges shifting poleward and upslope; coral bleaching events increasing in frequency and severity; phenological mismatch between species |
| Food and water security | Agricultural productivity changes unevenly; snowpack and glacier retreat threatens meltwater-dependent communities |
| Human health | Heat illness; expanded range of mosquito- and tick-borne diseases; wildfire smoke; food and water security impacts |
Climate impacts are not evenly distributed. Low-income communities, small island nations, communities in dry or coastal regions, and communities that depend on climate-sensitive resources face greater risks with less capacity to adapt. This raises questions of fairness and environmental justice discussed further in Chapter 29.
Human societies can respond to climate change through two broad strategies: mitigation — reducing the emissions that cause climate change — and adaptation — adjusting to the changes that are already occurring or are expected. Both are necessary. Mitigation limits the severity of future change. Adaptation manages risks from change that has already occurred or is locked in by past emissions.
| Approach | Description |
|---|---|
| Transitioning energy systems | Solar, wind, hydropower, geothermal, and nuclear energy replace fossil fuels in electricity generation |
| Electrifying transportation | Electric vehicles powered by low-carbon electricity reduce transportation emissions |
| Energy efficiency | Better building insulation, efficient appliances and vehicles, improved industrial processes reduce demand |
| Protecting forests | Halting deforestation and replanting forests preserves and increases carbon storage |
| Reducing non-CO₂ emissions | Capturing methane from landfills and oil and gas operations; reducing fertilizer overuse |
| Carbon capture | Technologies to capture CO₂ from power plants or from the air; currently expensive and limited in scale |
Warming is largely proportional to the cumulative total of CO₂ emissions since industrialization — not just the current rate. Every ton of CO₂ emitted contributes to warming. Stopping emissions does not immediately stop warming, but it does stop warming from continuing to increase. The faster emissions fall, the less total warming occurs and the more time societies have to adapt.
Adaptation means adjusting systems to manage climate risks. Examples include: coastal protection (sea walls, managed retreat, wetland restoration); improved water management (drought-resistant crops, efficient irrigation, desalination); heat preparedness (cool spaces, urban tree canopy, early warning systems); infrastructure design that anticipates more extreme precipitation and heat; and ecosystem management that protects habitat corridors and reduces other stressors.
Both mitigation and adaptation involve costs and tradeoffs. Transitioning energy systems requires investment and can displace workers in fossil fuel industries. Adaptation costs fall unevenly across communities. Some risks — ecosystem extinctions, ice sheet loss, community displacement — have adaptation limits beyond which adjustment is no longer possible. Science provides information about what is physically possible. Decisions about which responses to pursue involve economics, politics, values, and ethics — not science alone.
Climate science is one of the most intensively studied topics in modern science. The core conclusions — that Earth is warming, that the primary cause is human greenhouse gas emissions, and that continued emissions will cause further warming — rest on multiple independent lines of evidence. But climate science is not free of uncertainty, and understanding what is uncertain — and what is not — matters for sound reasoning.
Earth's average surface temperature has risen approximately 1.1–1.2°C since pre-industrial times.
Atmospheric CO₂ has risen from 280 ppm to over 420 ppm — the highest level in at least 800,000 years.
The isotopic signature of the added CO₂ confirms it comes from fossil fuels.
Human greenhouse gas emissions are the dominant cause of the warming observed since the mid-twentieth century.
The ocean is warming, sea level is rising, glaciers and ice sheets are losing mass, and Arctic sea ice extent is declining.
Climate sensitivity — the precise amount of warming expected from a given CO₂ increase has a range (roughly 2.5°C to 4°C per CO₂ doubling), not a single value.
Regional projections — global warming is projected with greater confidence than regional changes in precipitation, drought, and extreme events.
Ice sheet dynamics — the stability of the Greenland and West Antarctic ice sheets, and therefore the upper end of sea level rise projections, involves processes not fully understood.
Tipping points — the levels of warming at which Earth systems cross thresholds producing large, rapid, irreversible changes are uncertain.
Future emissions — climate projections are scenarios based on different possible emission pathways — not predictions, but projections of what would happen under different assumptions.
Uncertainty does not mean "we don't know anything." It means there is a range of possible outcomes. When potential impacts are severe and irreversible, and when the cost of action is manageable relative to the cost of inaction, the reasonable response is to act rather than wait for perfect certainty. This is the same logic used in public health, engineering design, financial planning, and emergency preparedness — all fields where decisions are made under uncertainty. Uncertainty cuts both ways: if the outcome could be worse than central estimates, uncertainty does not support complacency.
15.0°C − 13.8°C = 1.2°C2020 − 1880 = 140 years = 14 decades1.2°C ÷ 14 decades ≈ 0.086°C per decadeIdentify the path of: (a) incoming solar radiation arriving at Earth's surface; (b) outgoing infrared radiation emitted by the surface; (c) the role of greenhouse gases in absorbing and re-emitting infrared radiation. Explain why the surface is warmer with greenhouse gases than without. Compare the temperature with and without the natural greenhouse effect (−18°C vs. +15°C).
Trace the path of a carbon atom through: (1) the atmosphere as CO₂; (2) into a land plant via photosynthesis; (3) back to the atmosphere via decomposition; (4) into the ocean at the surface; (5) into a shell of a marine organism. Identify which flow is most affected by human fossil fuel use and explain how this affects the atmospheric reservoir.
For each of the three orbital parameters (eccentricity, obliquity, precession), state: (a) what changes; (b) the approximate timescale; (c) the primary effect on solar energy distribution. Explain why Milankovitch cycles cannot explain the current warming trend.
Trace the feedback loop step by step: warming → ice loss → lower albedo → more solar absorption → further warming → further ice loss. State whether this is a positive or negative feedback. Explain why this feedback causes the Arctic to warm faster than lower latitudes.
Questions 11–13 refer to a hypothetical global temperature anomaly graph showing annual temperatures from 1880 to the present, with individual years shown and a smoothed trend line.
Identify two problems with how this statement represents climate projections. How would a more accurate statement be framed?
Evaluate this argument. What is it getting right? What is it getting wrong? What additional step is missing from the reasoning?
This reasoning misapplies ice core data in which temperature changes sometimes precede CO₂ changes at the start of glacial transitions. Explain what is actually happening during those transitions, and why this observation does not contradict the conclusion that human CO₂ drives current warming.
Identify at least two flaws in this argument. What additional data or context would be needed to evaluate whether a genuine slowdown occurred?
Chapter 22 turns from Earth's climate systems to Earth's place in the solar system. You will study the relationships among Earth, the Moon, and the Sun — how Earth's rotation produces day and night, how the axial tilt produces seasons (a concept that connects directly to the Milankovitch cycles you studied in this chapter), how the Moon's gravity drives tides, and how the positions of Earth, Moon, and Sun produce eclipses and the phases of the Moon. These astronomical relationships are not just interesting in themselves — they shape the daily and seasonal patterns of solar energy that drive the weather and climate systems you have just studied. The seasons you explored in Section 21.3 as a climate control will reappear in Chapter 22 as the consequence of a specific geometric relationship between Earth's tilted axis and its orbit around the sun.