Evidence for climate change

 What is climate change?

Earth’s atmosphere is made up of oxygen, a large amount of nitrogen and a small percentage of greenhouse gases, such as carbon dioxide and methane.

Greenhouse gases act like a blanket around the Earth. They trap warmth from the sun and make life on Earth possible. Without them, too much heat would escape and the surface of the planet would freeze. However, increasing the concentration of greenhouse gases in the atmosphere causes the Earth to heat more and the climate to change.

This process is often called global warming, but it is better to think of it as climate change. This is because it is likely to change other aspects of climate as well as temperature, and also bring about more extreme climate events such as floods, storms, cyclones and droughts.

The evidence show climate change is happening

There is lots of evidence that tells us the average temperatures of the world's atmosphere and oceans have increased over the past 150 years.

The evidence includes:

• direct temperature measurements on land
• changes in the dates when lakes and rivers freeze and their ice melts
• a reduction in the extent of snow cover in the Northern Hemisphere
• a reduction in glaciers
• extended growing seasons of plants
• changes in the heat stored in the ocean
• changes in rainfall patterns resulting in more floods, droughts and intense rain.

A number of biological changes have also been observed.

These include:

• shifts in the ranges of some plant and animal species
• earlier timing of spring events such as leaf-unfolding, bird migration and egg-laying for some species.

Together these indicators provide clear evidence that the climate is changing.

It is extremely likely that humans are the dominant cause of recent warming

It is true that climate change has been driven by natural causes in the past. Our climate has changed over millions of years — from ice ages to tropical heat and back again. Natural changes over the past 10,000 years have generally been gradual. This has enabled people, plants and animals to adapt or migrate. , However, some prehistoric climate changes may have been abrupt and are likely to have led to mass extinction of species.

Over the past 150 years there has been a marked and growing increase in greenhouse gas producing activities such as industry, agriculture and transportation. These human-induced activities are increasing the level of greenhouse gases in our atmosphere and causing the Earth to heat up at an unprecedented rate. This recent warming can only be explained by the influence of humans.

The effects of climate change will continue even after emissions are reduced

The climate system is very complex, and takes a long time to change. As a result, the effects of climate change will continue even if we reduce emissions now. For example, the deep oceans take centuries to heat up when the atmosphere above them warms. This means that oceans will continue to heat up, and therefore expand causing sea-levels to rise, even if greenhouse gas concentrations in the atmosphere are no longer increasing. Although we cannot avoid climate change entirely, reducing our emissions can limit its impact.

Future climate change

How the climate will change in the future largely depends on the total sum of greenhouse gases emitted since the start of the industrial revolution. Greenhouse gas emissions have continued to increase over past decades and limiting climate change will mean reversing this trend. Future climate change also depends on how the Earth responds to the increased heating. So we cannot be precise about future climate change. But we are generally sure of the direction of change (eg, the world will become warmer and global average sea-levels will rise).

It is predicted that without additional efforts to reduce greenhouse gas emissions, the Earth’s surface temperatures will increase between 3.7°C and 4.8°C by 2100, relative to the average temperature from 1850-1900. However, if we reduce greenhouse gas emissions by 40 to 70 per cent by 2050, the average warming is likely to stay below 2°C.

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