Page 34 - 新思维科学学生用书9 样章

1.4 Climate change

Greenhouse gases

Carbon dioxide and methane are ‘greenhouse gases’. In Stage 8, you learnt how carbon dioxide helps
to keep the Earth warm. Without any carbon dioxide in the atmosphere at all, the Earth would be
a frozen place, unable to support life. But at the moment, we have too much carbon dioxide in the

atmosphere.

Look back at the carbon cycle diagram in Topic 1.3. You can see that some carbon from the
atmosphere ends up in fossil fuels. It takes a long time to form fossil fuels, and they can then stay
buried in the ground for millions of years. But if we extract them and burn them, we release the
carbon in them back into the air, in the form of carbon dioxide.

Carbon dioxide levels in the atmosphere are increasing. This is affecting the climate on Earth.

Climate change in the past

Climate is the long-term pattern of temperatures, wind and rainfall on Earth. The Earth’s climate has
been very different in the past compared to the climate today. Here are some examples of changes.

Ice ages

About 2 billion years ago, the Earth experienced the first ice age that we know about. Since then, the
Earth has cycled between relatively warm periods and relatively cold ones. In the warm periods, there

was no ice at all, even at the poles. In the colder periods, called ice ages, there was ice at the poles.

Snowball Earth

On at least one occasion, perhaps about 650 million years

ago, the whole Earth was covered with ice and snow.
Scientists call this ‘snowball Earth’, or sometimes
‘slushball Earth’, because they are not sure whether
everything was completely frozen. (Slush is

melting ice.) Scientists still do not completely
understand what caused this to happen.

This is what the Earth may have looked
like, 650 million years ago. At that time, the
continents were not in the same positions as

now.

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