Section 9.1Discovering Past Climates

Discovering Past Climates

People have been recording weather data for only a few hundred years. To learn about what Earth’s climate was like a few centuries ago, scientists must get creative.

People who study past climates are called paleoclimatologists.

Tree RingsTree growth is affected by temperature and rainfall. The amount that a tree grows each season is indicated by the size and colour of the annual rings, which you can see in the cross section of the tree trunk (below). Dendochronologists study details about the size, colour, and shape of each ring.

A wide tree ring indicates wet and cool weather, which allows trees to grow rapidly. A thin ring is produced in dry and hot conditions, when tree growth is slower. A dark ring marks growth during late summer, and a light-coloured ring indicates growth during spring.

Tree Rings

Living trees hold records of climate dating back no more than a few hundred years.

Tree trunks from archaeological sites allow scientists to determine what climate was like a few thousand years ago.

Ice Cores-Records of Past ClimatesLike tree rings, layers of snow and ice accumulate year after year. To uncover evidence of past climate conditions, scientists use a special drill, that penetrates deep into the layers to extract long, cylinder-shaped samples called ice cores as shown in the figure below.

Scientists study the thickness and composition of the ice in each layer in order to make inferences about past climates.

Evidence of Past Climates Obtained from Ice Cores

Ice cores hold four types of clues frozen in time:

1. Dissolved and particulate matter in ice – (dust, ash, salts, plant pollen, and other material) Frozen samples of these materials give clues about events and conditions, such as volcanic eruptions, meteorite impacts, forest fires, and vegetation cover.

2. Physical characteristics of the ice - (crystal size and shape) Indicate the conditions of temperature and humidity at the time the ice crystals formed.

3. The composition of trapped air bubbles - When water freezes, tiny air bubbles in the water may become trapped inside the ice. These pockets of air allow scientists to analyze the concentration of greenhouse gases over hundreds of thousands of years.

Evidence of Past Climates Obtained from Ice Cores

4. The composition of the ice - Water (H2O) contains varying proportions of hydrogen and oxygen isotopes. Isotopes are different forms of an atom. Water that contains oxygen-18 freezes at a higher temperature than water that contains oxygen-16. In addition, water that contains oxygen-16 evaporates more quickly than water that contains oxygen-18. Therefore, the relative concentration of isotopes in different layers of ice indicates the temperature at the time the ice formed.

Evidence of Past Climates Obtained from Ice Cores

Evidence of Past Climates from Sedimentary Rock

To obtain evidence of climates older than can be analyzed in ice cores, scientists examine rocks. Every year, billions of tons of sediment (fragments of rock) wash from the land and accumulate in thick layers on ocean floors and lake beds.

The hard parts of small sea creatures (such as diatoms, and algae) and pollen from flowering plants are preserved in these sediments. Over long periods of time, this deposited material becomes compressed and hardened into sedimentary rock.

What Sediment Cores from Lakes and Oceans Reveal

Data from the study of sediment cores gives insight into:global temperatures during a specific time (by knowing the common living thing)The temperature of the water in which an organism livedThe amount of rainfall and temperature levels (by analyzing the composition and amount of sediment in layers)

Evidence of Past Climates from Fossils and Preserved Organisms

The remains or traces of living things, called fossils, also provide valuable clues when paleoclimatologists reconstruct past climates.

Evidence of Past Climates from Fossils and Preserved Organisms

The types and abundance of fossilized remains in each rock layer help scientists reconstruct the environment at the time the layer formed – including the climate. Because plants and animals are uniquely adapted to the environment in which they live, studying these fossils gives scientists clues about what environments were like thousands of millions of years ago.

Scientists are still debating whether climate change is affected more by slow, gradual changes or by sudden, catastrophic changes. This question is at the centre of the controversy about whether humans can cause earth’s climate to change significantly.

However, most climatologists agree that humans are affecting the composition of Earth’s atmosphere. This influence may increase or decrease the rate at which climate change progresses.

The Rate of Climate Change