Climate change

What is meant by climate change?
Has climate changed in the past?
Has the climate changed recently?
Should we trust the evidence?
Old weather records
What do the weather and climate records tell us?
What is causing global warming?
Can anything be done about climate change?

What is meant by climate change?

We first need to understand that there is a difference between weather and climate. Weather is all around us. It is a description of what's happening with the air, sun, rain and wind when you go outside. This can be during a period as short as a few minutes or a few hours, or as long as a few days or even weeks.

Climate is the average weather conditions at a particular place over a long period of time (for example, more than 30 years).

There is an old saying which sums this up: "Climate is what you can expect, weather is what you get".

Climate is different all over the world. In the United Kingdom we have a temperate (mild) climate. It's neither very hot nor cold, and neither very dry nor very wet. At the the North Pole the climate is much colder than ours in the UK. In the Sahara Desert it is very hot and dry, and in the Amazon rainforest it is hot, but it rains a lot throughout the year.

Most scientists now believe that the changes we are seeing to the world's climate are partly due to the actions of humans. They talk about 'global warming', meaning that the climate of the world as a whole is getting hotter. This is what people call 'climate change'.

Has climate changed in the past?

The Earth was formed around five billion years ago and we know ever since it has had lots of different climates. These changes have included polar regions without ice, to ice sheets across much of the northern hemisphere, including the United Kingdom.

An example of tree rings

The last ice age which covered most of the United Kingdom melted away about 10,000 years ago. Since then, the UK's climate has sometimes been warmer and sometimes cooler than it is now.

We know all this from studying things like fossils, trees and glaciers. Fossils provide lots of useful information about the climate. Some animals can only live in warm places. So if you find their bones, you’ll know they lived in a warm place – even if that place is now in the cold (polar) regions.

By looking at the rings in a tree you can tell how old it is - every year a tree grows, it adds another ring – but also what the weather was like. The size of the tree ring is affected by the amount of rainfall as well as the local air temperatures in the growing season.

The retreat of the South Cascade Glacier, Washington State between 1928 and 2000. Images courtesy of USGS.

The size and positions of glaciers is a good indication of the climate on the Earth – the more ice coverage, the colder the planet.

In a recent study of glaciers for the period 1900–1980, it was discovered that 142 of 144 glaciers around the world were getting smaller. Warmer summer temperatures can result in the glacier losing more ice from the bottom than it is getting from snowfall at the top.

Has the climate changed recently?

Human weather records, tree rings and information from glaciers and fossils show that there has been a big change in the climate over the past few hundred years.

In Europe, we know there was a warmer period during the 14th century. This was followed by a quite sudden change to cooler weather in the 15th century. This cooling carried on until we had what some people call the 'Little Ice Age' in the 17th and 18th centuries.

Since then, during the 19th and 20th centuries, and especially this century, we have seen our climate warming up. The evidence for this mostly comes from measurements of temperature. The measurements show that the warming up is getting much faster than we have ever seen before.

The graph below shows what scientists know has happened to temperatures over the past 150 years. Although you can see small drops in temperature for a few years at a time, the line does clearly show that temperatures have risen over the past 150 years.

Should we trust the evidence?

Scientists and others have found lots of very useful information in books from the past few hundred years. These books include personal diaries written by people at the time, telling us how the weather affected them in their daily lives.

Farmers have also kept valuable records of when they planted out seeds and harvested their crops. These records are very useful to scientists, as they only do this when the weather is right. The dates when harvests were good and bad are usually written down. These records provide really useful information, although they are not actual weather records.

Historical weather records and the evidence from things like fossils and ice give us a much better idea of how climate has changed over many thousands of years.

As an example, in places like Siberia, in the north of Russia, the ground is frozen all year round, and has been for thousands of years. This deeply-frozen ground is called permafrost. Because of rising temperatures around the world, some of the permafrost is now thawing out which may cause further global warming, as scientists know that when permafrost thaws out a lot of methane, which is a powerful greenhouse gas, is released into the air.

Old weather records

There are many, very old weather records that scientists use to see what the weather was like a long time ago. But not all - especially those about 300 years old - contain all the information they should.

Also, in the past, the instruments that recorded the weather were too expensive for most people and did not work very well. But, fairly accurate measurements of temperature, rainfall and air pressure have now been possible for a long time.

All of these weather records are now being used to see what the weather and climate has been like for the past few hundred years.

What do the weather and climate records tell us?

The old weather records show that we are now seeing much higher temperatures. Also, temperatures during the past 30 years are getting much warmer, much faster than we have seen for a very long time.

However, some of these changes could be caused by other things.

• As villages become small towns and continue to grow, weather records (especially temperatures) are going to be affected by all the new houses.
• The instruments that measure the weather are sometimes moved to other places.
• The equipment used to record the weather now has become more accurate.

All of this can make a big difference to how scientists use the information.

What is causing global warming?

A major cause of global warming is known as the 'Greenhouse Effect'. In a greenhouse the temperature inside begins to rise above the temperature outside when the sun shines on it. This is because the sun's heat gets trapped inside.

In the air, there are several gases that do the same thing as the glass in the greenhouse. The three 'greenhouse' gases that do this the most are called water vapour, methane and carbon dioxide.

While humans don't create much water vapour in the air, we do make methane and carbon dioxide. Methane comes from several places, including agriculture (livestock), wetlands and landfill sites, and is a very powerful greenhouse gas.

When humans burn forests, oil and gas, it also makes huge amounts of carbon dioxide. In terms of global warming, carbon dioxide is the most important greenhouse gas.

Over the past 50 years, scientists have measured a steady increase in the amount of carbon dioxide in the air. This extra carbon dioxide has been shown to help warm up the whole Earth.

In fact, since the Industrial Revolution started around 200 years ago, the amount of carbon dioxide in the air has increased by 30% - mostly due to human activities.

The amounts of two other important greenhouse gases in the air, methane and nitrous oxide, are also increasing.

In fact, we now have more carbon dioxide and methane in the air than the earth has seen for at least 650,000 years.

Can anything be done about climate change?

There is more and more evidence that human beings are the main cause of the increase in two main greenhouse gases in the air; methane and carbon dioxide. Farming and wetlands are some of the biggest sources of methane. The burning of fossil fuels (like coal and gas), cement production and the burning of forests by humans is the biggest cause of the increase in carbon dioxide in the air.

On current scientific evidence, 'global warming' could be slowed if the amount of greenhouse gases in the air were reduced.

You can play your part. Perhaps use the bus or try to walk to school, instead of using a car. Changing to the energy saving light bulbs is also a good idea, as they use only 20% of the electricity used by a normal light bulb. And recycling as much as possible can save huge amounts of energy.