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Methods for Determining Past Climates

Methods for Determining Past Climates
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  • 0:01 What Is Paleoclimatology?
  • 1:05 Ice Core Samples
  • 2:46 Remnant Glacial Landforms
  • 4:01 Ocean Floor Sediment
  • 5:42 Fossils of Ancient Vegetation
  • 7:58 Lesson Summary
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Lesson Transcript
Instructor: Joanne Abramson

Joanne has taught middle school and high school science for more than ten years and has a master's degree in education.

You've probably heard that Earth has experienced an Ice Age. But if people weren't there to see it, how do we know? Discover how scientists use evidence collected today to deduce climates many thousands and millions of years in the past.

What is Paleoclimatology?

Just like you may occasionally update your wardrobe and hairstyle, our Earth's surface has gone through some pretty drastic overhauls during its lifespan. Approximately 600 million years ago, our planet was a giant ball of snow and ice, covered in frost over a kilometer thick from the poles to the equator.

During the dinosaur's time, around 120 million years ago, a multitude of erupting volcanoes dumped greenhouse gases into the atmosphere, creating a hot, humid rainforest. This was one of the warmest periods in Earth's history.

Obviously, though, there were no humans around to see this. So, how do we know that all these changes actually happened? Paleoclimatology is the study of the changes in Earth's climate throughout the history of our planet. Paleoclimatologists have several means of measuring the changes in climate, including taking ice core samples, observing remnant glacial land forms, surveying the sediment on the ocean floor and studying the fossils of ancient vegetation.

Ice Core Samples

Ice core samples are cylinders of ice drilled out of a glacier. Yearly snowfall collects on the surface of the glacier, burying the layers below. As the snow turns into ice, air bubbles are trapped, preserving samples of the Earth's atmosphere within.

Scientists drill deep into glaciers, removing portions for study. This allows them to take a peek at what our atmosphere was like many thousands of years ago. The oldest ice core samples we have taken so far contain ice that dates back 800,000 years.

Ice core samples allow paleoclimatologists to measure the Earth's past climate in a couple of ways. First, trapped samples of atmosphere allow scientists to measure the concentration of greenhouse gases. Higher concentrations of greenhouse gases in the atmosphere lead to higher temperatures on the surface of the planet.

Second, scientists can compare this information to another yard stick contained in the ice. Isotopes are variations of an atom that have a different atomic weight than the primary form. For example, 99% of oxygen atoms have an atomic weight of 16 atomic mass units (O^16) - this is the weight that you will see represented on the periodic table. However, we know many other versions of oxygen, all the way from O^12 to O^24.

The heavy oxygen isotope O^18 is found in liquid water molecules at higher concentrations when the temperature is colder. Knowing this, scientists can estimate past temperatures based on the amount of O^18 found in the glacier ice.

Remnant Glacial Landforms

Graph of Earth

As you may have noticed from the previous graph, Earth has gone through several glacial eras, also known as glaciations or ice ages. We can find evidence for these time periods in ice core samples. However, there are also indications of these ice ages visible to the naked eye. Remnant glacial land forms are geologic characteristics created by the movement of glaciers, or large sheets of ice. Yosemite National Park in California is famous for its breathtaking vistas created by the ebb and flow of glaciers.

As glaciers expand, the growing weight of the ice and snow erodes, crushes and cuts at the surrounding mountains, hillside and bedrocks. This action creates landforms such as plucked rocks, U-shaped valleys and arêtes. This animation shows how, as the glacier moves, it abrades, or cuts at the Earth's surface. It also plucks at loose bedrock, trapping the rocks and boulders and transporting them far from their point of origin. As glaciers recede, they leave behind their accumulated rocks and pebbles, creating land forms such as drumlins, moraines and kames.

Ocean Floor Sediment

Similar to how paleoclimatologists use samples of glaciers to look into the past, they also take cores drilled out of the ocean floor. Every day, dust, dirt and the remains of plants and animals fall to the bottom of the ocean floor. The skeletons of the deceased plants and animals settle on the bottom of the ocean, only to be covered by more dust, dirt and skeletons. Over time, these oceanic deposits build up, creating a record of the plants and animals that lived in the area. Digging down into these layers gives scientists a glimpse at the life and climate of a region dating back 170 million years.

One of the most useful fossils found in these sediment cores are foraminifera, a microscopic plankton found in all marine ecosystems. One species of foraminifera is only found in arctic environments. A discovery of this cold-water species is an indication that the climate of that region was once much colder, even glacial. Scientists can then date these fossils, creating a timeline of when that ocean zone experienced glaciations.

Further, oceanic deposits provide evidence of the biodiversity, or the amount and type of life in a particular area at a certain time. Warmer, tropical regions tend to have greater biodiversity than colder, arctic regions, as well as different types of plant and animal life. You would not, for example, find a polar bear living in a rain forest. Based on the degree of biodiversity found in a sample, as well as the adaptations represented in the fossils, scientists can deduce what the climate might have been.

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