KEY IDEAClimate has changed in

the past through natural causes, on timescales

ranging from millions to hundreds of years

AIMTo recognise that the world’s climate has

changed significantly over time

What do you already know about climate

change?

SNOWBALL!

http://www.bbc.co.uk/learningzone/clips/evidence-of-climate-change/1490.html

The distant pastAt times in the past, huge ice sheets stretched from the North Pole as far south as London. Scientists know that climate was different in the past. They use physical evidence such as:• fossilised animals, plants and pollen that no longer live in the UK• Landforms, like the U shaped valleys left by retreating glaciers• samples from the ice sheets of Greenland and Antarctica

• During the Quaternary (the last 2.6 million years of geological time), warm periods (interglacials) lasted for between 10,000 and 15,000 years. • Cold periods (glacials) lasted about 80,000-100,000 years. • During some glacial periods, it became so cold that the Earth plunged into an ice age. Huge ice sheets extended over the continents in the northern hemisphere. • There were also vast areas of floating sea ice. • The last time this happened was between 30,000 and 10,000 years ago, in the last ice age. • The ice sheets were 400-3000 metres thick, and so heavy that they made the Earth’s crust sag. • So much water was locked up in the ice sheets that sea levels fell by over 100 metres.

Ice sheets are like a time capsule. They contain layers of ice, oldest at the

bottom, youngest at the top. Each layer is one year of snowfall. Trapped in the

ice layers are air bubbles. These preserve air from the time the snow fell. Locked in

the air bubble is carbon dioxide. Climatologists can reconstruct past

temperatures by drilling a core through the ice and measuring trapped carbon

dioxide in ice layers.

http://www.youtube.com/watch?v=Jxi-OlkmxZ4

Al Gore explains global warming in 10

minsWatch the clip from An

Inconvenient Truth. Note 5 key facts that Al Gore

explains about our changing climate.

The recent pastThere is also evidence for climate change in more recent times. Evidence comes from:• old photographs, drawings and paintings of the landscape• written records, such as diaries, books and newspapers• the recorded dates of regular events, such as harvests, the arrival of migrating birds and tree blossom

These sources are often not very accurate, because they were not intended to record climate. However, they can still give us some idea of overall climate trends in the recent past. This type of evidence suggests that climate changes regularly – every few hundred years. Average temperatures over the past 2000 years or so have probably varied between 1-1.5ºC colder or warmer than average temperatures today.

McCarty Glacier in Alaska

Greenland Ice Sheet melting

Larsen B ice shelfLoss of

ice

Old photographs, paintings and drawings

Written records such as books and diaries

1400s

Sodium in ice, seas stormier, period of cooler temperatures

Use the information from ‘Discovering Antarctica’ to annotate the diagram.

Exam-style question

Describe some of the evidence that tell us that climate was different in

the past.

(4 marks)