AP Environmental Science

## Learning Outcomes

- Explain how scientists reconstruct past climates using proxy records
- Identify major natural drivers of climate change over geologic time
- Distinguish between Milankovitch cycles, tectonic change, and volcanism
- Describe how glacial and interglacial periods affect sea level and habitatss
- Use named examples such as Snowball Earth, the Permian extinction, and the PETM
- Compare natural long-term climate change with rapid modern human-caused warming

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand climate change over geologic time as a natural disruption to the environments, with a focus on the following syllabus points:

- Know that Earth’s climate has changed repeatedly over geologic time
- Identify paleoclimate evidence from ice cores, tree rings, sediment cores, fossils, pollen, and isotope data
- Explain how Milankovitch cycles alter the distribution of incoming solar radiation and help drive glacial-interglacial cycles
- Understand how plate tectonics can shift continents, alter ocean circulation, and change long-term climate
- Explain the difference between short-term volcanic cooling and longer-term volcanic greenhouse warming
- Understand how climate shifts can change sea level, habitats, migration patterns, and extinction risk
- Distinguish natural climate drivers over thousands to millions of years from current rapid anthropogenic warming

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. What is a climate proxy, and why do scientists need proxies to study ancient climates?
2. How do Milankovitch cycles help drive glacial and interglacial periods?
3. Why can volcanic activity both cool and warm Earth, depending on timescale?
4. How can plate tectonics change biodiversity without directly killing organisms?
5. Name one geologic event linked to major life systems disruption and explain why.

## Introduction

Climate change is not only a modern issue. Earth’s climate has shifted many times over deep time, sometimes gradually over millions of years and sometimes quickly enough to disrupt natural systemss on a huge scale. For AP Environmental Science, this topic matters because climate change is a major natural disruption that can alter habitats, force migration, raise or lower sea level, and contribute to extinction.

Students often make two mistakes here. First, they confuse **weather** with **climate**. Second, they assume that because climate changed naturally in the past, the causes must be the same today. The exam expects you to understand both ideas clearly: Earth has always experienced natural climate change, but the present warming trend is unusually rapid and strongly linked to human greenhouse gas emissions.

> **Key Term: paleoclimatology**
> The study of climates of past geologic ages using indirect evidence from natural records rather than direct thermometer measurements.

### How scientists know past climates changed

Direct measurements of temperature, atmospheric carbon dioxide, and rainfall are very recent on a geologic timescale. Instrument records mostly begin in the 1800s. To understand climates thousands or millions of years ago, scientists use **proxy records** preserved in natural archives.

> **Key Term: proxy record**
> Indirect evidence of past environmental conditions preserved in natural materials such as ice, tree rings, sediments, fossils, or cave deposits.

Different proxies tell scientists different things.

- **Ice cores** preserve trapped air bubbles containing past atmospheric gases such as **CO2** and **CH4**. They also preserve dust, volcanic layers, and isotopic data linked to past temperature.
- **Tree rings** provide yearly records of growing conditions. Wider or denser rings may reflect favorable temperature and moisture conditions, though the exact meaning depends on species and location.
- **Sediment cores** from lakes and oceans preserve layered records of pollen, ash, shells, dust, and microfossils. These help reconstruct temperature, precipitation, vegetation, and volcanic events.
- **Fossils and pollen** show which organisms lived in an area. Because species have ecological tolerances, their presence can suggest warmer, colder, wetter, or drier conditions.
- **Speleothems** such as stalactites and stalagmites record changes in water chemistry and rainfall.
- **Corals** record past ocean conditions in their growth patterns and chemistry.

Why use many proxies rather than just one? Because each has strengths and limits. Tree rings have excellent annual detail but do not extend very far into deep time. Sediment cores cover much longer periods but with less fine resolution. Ice cores can directly show past atmospheric composition, but they are limited to places where long-term ice survives. Combining evidence makes climate reconstructions more reliable.

> **Key Term: ice core**
> A cylinder of ice drilled from glaciers or ice sheets that preserves trapped gases, dust, and isotopic evidence of past climate.

A detail students often miss is that isotope evidence must be interpreted carefully. Oxygen isotope ratios can reveal past temperature and ice volume, but the meaning depends on the material tested. In marine sediments, higher **δ18O** often indicates colder conditions and more global ice. In ice cores, lower **δ18O** usually indicates colder conditions. Mixing these up is a common error.

> **Test Tip:** If a question asks how scientists know ancient climates changed, do not just name one source like “ice cores.” Explain what the proxy preserves and how that evidence links to temperature, gases, or precipitation.

### The main natural drivers of geologic climate change

Earth’s climate has changed over geologic time for several major reasons. For APES, organize them into three core drivers: orbital change, plate tectonics, and volcanism. You may also see references to ocean-atmosphere circulation and natural changes in atmospheric carbon dioxide.

#### Orbital forcing and Milankovitch cycles

Milankovitch cycles are natural variations in Earth’s orbit and orientation that change how sunlight is distributed across latitudes and seasons.

> **Key Term: Milankovitch cycles**
> Natural changes in Earth’s orbit and axial orientation that alter the distribution of incoming solar radiation and help pace glacial and interglacial cycles.

There are three parts:

- **Eccentricity**: changes in the shape of Earth’s orbit, roughly every **100,000 years**
- **Obliquity**: changes in axial tilt, roughly every **41,000 years**
- **Precession**: wobble of Earth’s axis, roughly every **19,000 to 23,000 years**

The key idea is not that these cycles massively change total incoming solar energy. Instead, they change where and when solar energy is strongest. That especially matters at high latitudes.

Why does high-latitude summer sunlight matter so much? Because glaciers and ice sheets grow when winter snow survives the summer. If summers are cool, less snow melts, so ice builds up year after year. If summers are warmer, more ice melts, favoring interglacial conditions. Over long periods, these changes help drive the alternation between glacial and interglacial phases.

This is a cause-and-effect chain the exam likes:

orbital variation -> altered summer insolation at high latitudes -> more or less ice-sheet melting -> changing albedo -> further cooling or warming

Ice and snow have high albedo, so more ice means more sunlight reflected, which reinforces cooling. Less ice lowers albedo and allows more absorption of solar energy, reinforcing warming.

Students sometimes say Milankovitch cycles explain modern warming. That is wrong. These cycles operate over tens of thousands of years and do not match the speed or pattern of current warming.

#### Plate tectonics and long-term climate change

Plate tectonics affects climate over millions of years, not decades.

> **Key Term: plate tectonics**
> The movement of Earth’s lithospheric plates, which can shift continents, change ocean circulation, build mountains, and alter long-term climate.

Continents drifting to different latitudes changes the amount of solar heating they receive. A continent near the equator has a very different climate from one near the poles. But tectonics does more than move landmasses.

It also changes climate by:

- altering ocean circulation through opening or closing seaways
- building mountain ranges that create rain shadows
- increasing rock weathering, which can remove atmospheric carbon dioxide over long timescales
- changing surface albedo if more land is covered by snow or ice

For example, mountain building can cool climate indirectly. When mountains rise, more rock is exposed to weathering. Chemical weathering of some rocks removes **CO2** from the atmosphere over long periods. Less atmospheric CO2 weakens greenhouse warming, favoring cooling.

Tectonics can also alter biodiversity without direct killing. If continents shift, climates move with them. Habitats may become wetter, drier, hotter, colder, more isolated, or newly connected. Species then must adapt, migrate, or decline. This can create new evolutionary opportunities in some areas while causing extinctions in others.

#### Volcanism: short-term cooling and long-term warming

Volcanoes are a good example of how the same process can have different climate effects depending on timescale.

Large eruptions can inject sulfur-rich gases and ash into the atmosphere. Sulfate aerosols reflect sunlight, which can cool climate for months to a few years. This is why major eruptions can temporarily lower global temperatures.

But volcanism can also contribute carbon dioxide to the atmosphere. Over much longer geologic intervals, sustained volcanic activity can add enough CO2 to strengthen greenhouse warming.

This distinction matters:

- **short term**: aerosols -> more reflection -> cooling
- **long term**: volcanic CO2 -> stronger greenhouse effect -> warming
  > **Test Tip:** When a question mentions volcanism, always ask: are they testing short-term aerosol cooling or long-term greenhouse warming? State the timescale in your answer.

### Glacial cycles, sea level, and habitat disruption

Earth’s climate history includes repeated **glacial** and **interglacial** periods. During glacial periods, more water is locked up in land ice. During interglacial periods, more melts and returns to the oceans.

> **Key Term: glacial period**
> A colder interval when ice sheets and glaciers expand over large areas of Earth.

This matters because sea level changes along with ice volume. When more glacial ice is stored on land, sea level falls. When glaciers and ice sheets melt, sea level rises. Lower sea levels can expose continental shelves and connect land areas. Higher sea levels can flood coasts, reduce land area, and reshape estuaries and shorelines.

These shifts affect natural systemss in several linked ways:

- coastlines move
- wetlands appear or disappear
- islands connect to or separate from continents
- migration routes change
- habitats shrink, expand, or shift location
- species can become isolated, causing speciation or extinction risk

The geologic record shows that sea level has varied greatly across time. A lower sea level during glacial maxima changed the size and shape of landmasses. That in turn altered the distribution of species and habitatss.

This is why climate change is such a powerful natural disruption. It is not just “warmer” or “cooler.” It reorganizes the physical template on which living systemss depend.

### Major deep-time examples and what they show

Named examples help turn general principles into exam-ready evidence.

#### Snowball Earth

Snowball Earth refers to severe global glaciations during the Neoproterozoic, about **720 to 635 million years ago**. These events likely involved ice covering much or nearly all of Earth’s surface.

The key lesson is feedback. Once ice expanded far enough, Earth’s albedo increased sharply. More sunlight was reflected, causing more cooling and still more ice growth. This shows how climate systems can shift into extreme states when positive feedbacks become strong enough.

Ecologically, such severe cooling would have drastically reduced habitable environments and reshaped the course of evolution.

#### End-Permian mass extinction

The Permian mass extinction, about **252 million years ago**, was the largest known mass extinction. It is widely linked to massive volcanism, extreme warming, and major ocean disruption.

This example shows how climate change can combine with other stresses. Massive volcanism likely released huge amounts of greenhouse gases, warming the climate and contributing to ocean acidification and deoxygenation. Marine marine lifes were especially hard hit because ocean chemistry and oxygen levels changed.

The APES lesson is that biodiversity loss often results from chains of interacting effects, not one isolated cause.

#### Paleocene-Eocene Thermal Maximum

The PETM occurred about **56 million years ago** and involved geologically rapid warming associated with a large carbon release.

This event matters because it shows how carbon-cycle disruption can quickly alter climate, species distributions, and ocean chemistry. Organisms had to shift ranges, adapt, or disappear locally. It is often discussed as a deep-time example of carbon-driven warming, although it still occurreded more slowly than modern human-caused warming.

> **Key Term: mass extinction**
> A geologically brief event in which a very large proportion of Earth’s species go extinct.

### Why this matters for biodiversity today

Deep-time climate change teaches a repeated pattern: when climate changes, natural systemss change too. Species can respond in only a few ways:

- adapt
- migrate
- go extinct

Generalist species often cope better with environmental change than specialists. Species with narrow ecological tolerances, limited dispersal ability, or dependence on very specific habitats face the greatest risk when climate shifts rapidly.

Natural climate changes in the past often occurreded over very long periods, giving many organisms more time to shift ranges or evolve. Modern warming is different because it is happening much faster and is strongly tied to human emissions of greenhouse gases.

For APES, the safe comparison is this: natural geologic climate change is real and well documented, but it does not weaken the evidence for present human-caused warming. Instead, it shows how powerful climate can be as a driver of ecological systems disruption.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Paleoclimatology studies past climate using indirect evidence rather than direct measurements
- Proxy records include ice cores, tree rings, sediment cores, fossils, pollen, corals, and cave deposits
- Ice cores preserve trapped gases and isotopic evidence of past atmosphere and temperature
- Milankovitch cycles include eccentricity, obliquity, and precession
- Milankovitch cycles mainly change the distribution of solar radiation by season and latitude
- Cooler high-latitude summers favor ice-sheet growth, while warmer summers favor melting
- Plate tectonics changes climate by shifting continents, altering ocean circulation, and affecting weathering
- Volcanic eruptions can cause short-term cooling through aerosols and longer-term warming through CO2 release
- Glacial and interglacial cycles strongly affect sea level and habitat distribution
- Snowball Earth, the Permian extinction, and the PETM are major examples of climate-driven biosphere disruption
- Climate change can trigger migration, habitat shifts, bottlenecks, and extinction
- Natural climate change over geologic time is different in rate and cause from rapid modern anthropogenic warming

## Key Terms and Concepts

- paleoclimatology
- proxy record
- ice core
- Milankovitch cycles
- plate tectonics
- glacial period
- mass extinction


Natural Disruptions to Ecosystems - Climate change over geologic time