What this lesson teaches
I can use evidence of past climates to explain the natural ways climate varies.
Syllabus K335, Topic 3.2, Key Question 1. What is the natural variability of climate?
Make a guess
Thermometer records only go back to about 1850. How can scientists know what climate was like 100,000 years ago?
- From old written diaries and records of the weather.
- By assuming it was much the same as the climate today.
- From natural clues, like shells in seafloor mud.
- They cannot know anything before 1850.
Show the answer
From natural clues, like shells in seafloor mud.
Proxies are indirect records linked to past climate. Shells in dated sediment layers hold chemistry that reflects past ocean conditions.
Proxies are indirect clues to past climate, such as shells in seafloor mud, which scientists date and compare.
Earth's climate has changed many times. There were cold glacial periods with more ice and warmer interglacial periods. Thermometer records go back only to about 1850. For older times, scientists use proxies: natural clues linked to past climate.
Seafloor mud builds up in layers. A sediment core is a long tube of these layers, oldest at the bottom. It holds the shells of tiny sea creatures. The type of shell and its chemistry show past ocean conditions. Scientists must date the layers and check for gaps or disturbed layers.
Oxygen comes in light and heavy forms, called isotopes. In cold glacial times, ice sheets lock up more light oxygen. So the sea and the shells in it have more heavy oxygen. Scientists must date and calibrate this clue. It is not a direct thermometer.
Temperature helps define climate zones. When Earth cools, cold zones spread towards the equator. When it warms, they shrink back towards the poles. Rainfall, oceans and mountains also affect where the boundaries lie, so a global conclusion needs evidence from several places.
- Proxy
- An indirect clue to past climate.
- Sediment core
- Layers of seafloor mud with shells, oldest at the bottom.
- More heavy oxygen in shells
- Points to a colder period with more ice.
Step by step
Find a record
Sediment layers hold shells from different times.
Date and read it
Shell chemistry shows past ocean conditions.
Compare records
Evidence from other places tests the reading.
Worked example: Evidence is not a cause
A sea core may show warm and cold periods taking turns. This shows climate changed. To explain why, scientists need more evidence, such as changes in Earth's orbit or volcanic eruptions.
Watch out for this
An oxygen isotope value always means the same temperature everywhere.
Shells and ice cores record isotopes differently, and ice volume and seawater also matter. Each record must be calibrated.
Check your understanding
Why compare a shell isotope record with other dated evidence?
- Several things affect a proxy, so other records test it.
- Sediment layers hold no climate information, so the shell record is needed instead.
- One shell directly measures global temperature, so the others confirm its date.
Show the answer
Several things affect a proxy, so other records test it.
Yes. Independent evidence makes the conclusion stronger.