How do we know that climate is changing?

How do scientists research the world's past climate to make comparisons with the present? What are greenhouse gases?

Reconstruction of a sediment core

Reconstruction of a sediment core
Photographer: Carl Bento © Australian Museum

'This is not a political issue. This is a moral issue. It affects the survival of human civilisation. It is not a question of left versus right; it is a question of right versus wrong. Put simply, it is wrong to destroy the habitability of our planet and ruin the prospects of every generation that follows ours'

Al Gore, American politician

Is the climate changing?

The world is getting warmer and weather patterns are changing. Humans are the primary cause. This is agreed by almost every climate scientist and world government. Climate change is not something for the future: it is happening right in front of our eyes.

Hasn't the climate always changed?

There have been ice ages and warmer periods in the Earth's history. The planet has survived, but not without major extinctions. This is the natural way of things. What is different now is that we are the main cause of the change: a change that could wipe out millions of species.

How are we changing the climate?

In the past 50 years, natural variations in the climate should have slightly cooled the world however average world temperatures have risen.

This is happening because humans are changing the atmosphere. Certain gases in the air, called greenhouse gases, act like a blanket for the Earth. Without them the temperature would be -18°C most days. However, you can have too much of a good thing. Many of the things we do in our modern world produce extra greenhouse gases which are making the world gradually warmer.

Where do most greenhouse gases come from?

Most greenhouse gases come from:

  • burning coal, oil and gas mainly to generate electricity
  • burning fuel in cars, trucks and planes
  • deforestation
  • industrial processes like making cement
  • the expulsion of internal wind by cows and sheep
  • fertilisers on farms
  • rotting manure and rubbish at landfill sites

The most important greenhouse gas is water vapour, but humans have very little effect on how much of it is in the air. The main gases we produce are carbon dioxide, methane, nitrous oxide and chlorofluorocarbons (CFCs).

Sediment Cores

Sediment Core: making things as clear as mud?

Scientists remove columns of mud from under the sea and use them to find out about the environment and climate in the past. By analysing the remains of dead animals and plants in the mud, they get information about when the sediment formed and the climate at that time.

Nature's history book

Layers of mud containing the remains of sea creatures are laid down on the sea bed and buried over time. Sometimes layers are formed quickly, so a long length of sediment core can correspond to a relatively short time. When this happens scientists can build up a detailed picture of past climate and how it changed year by year. One centimetre of mud usually represents somewhere between 300 and 2000 years.

What does the colour of mud tell us about the climate?

Pale mud is mainly made up of the shells of tiny sea animals that once lived in the surface waters. Pale layers like this were formed during ice ages. In warmer times the sediments were generally darker. With more land exposed as ice melted, dust blew into the sea and changed the colour of the mud.

Getting the big story from small fossils

Fossils of tiny shells are scattered throughout the mud, but are too small to see. Using a microscope, scientists can identify different species and use this information to reveal things about the climate when the animal lived. For example, they can work out the temperature of the ocean as different species prefer different temperatures.

Scientists can also extract the tiny fossil animals from the mud and analyse the chemicals in their shells. This information allows them to calculate the amount of ice on Earth, the sea temperature and salinity of the ocean when the animals were alive.

Other methods

Scientists also study columns of ice from the Antarctic, tree rings or fossilised coral. Combining the evidence from many different sources provides a more complete record of global climate in the past. By comparing the past with the present we can find out how the climate is changing.

Catherine Cooper and Fran Dorey
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