The following is an abridged version of the ‘Introduction to climate change’ briefing given to assembly members. You can read the full briefing here.
What is global warming?
The atmosphere is a layer of gases around the Earth made up mostly of nitrogen and oxygen. It also contains smaller amounts of gases commonly referred to as ‘greenhouse gases’. These include carbon dioxide (CO2), methane and water vapour, among others. The sun’s rays shine on the Earth’s surface and warm it.Heat then radiates from the surface. Scientists have shown conclusively that greenhouse gases trap some of this heat in the atmosphere. Over the history of the Earth, this ‘greenhouse effect’ has helped keep the planet warm enough for life to flourish.
In recent years, however, concentrations of greenhouse gases in the atmosphere have increased at a rapid rate. This is particularly true of carbon dioxide. This is caused mainly by the burning of coal, oil and natural gas (known as ‘fossil fuels’). Around the world, fossil fuels are used as a fuel for transport, electricity generation, heating our homes, and in many industrial processes. The clearing of forest land around the world has also contributed: trees absorb carbon dioxide when they grow and release greenhouse gases back into the atmosphere if they are cut down, burned or left to rot.
Carbon dioxide is now at much higher concentrations in the atmosphere than it used to be. The more carbon dioxide there is in the atmosphere, the more heat is trapped, and the hotter the Earth becomes. This process is known as ‘global warming’ – the rising global temperatures across the Earth’s lands and oceans. In recent years the global average surface temperature has reached around 1°C above what it was in the second half of the 19th century (1850-1900).
Scientists have considered a host of factors that can affect the global temperature, including changes in the strength of the sun. They have high confidence that practically all of the global warming that we have seen recently is due to human actions.
Global warming is the main cause of what today we call ‘climate change’.
What is climate change?
Climate change refers to a shift in average weather conditions, including measures such as temperature, humidity, rainfall, cloudiness and wind patterns - and changes in the frequency or severity of these conditions. The Earth’s climate has changed throughout its history, in cycles that occur over very long periods of time.
Today we tend to use the phrase ‘climate change’ to refer to the very rapid changes in the climate that we have seen over the past 50 years or so. The scientific evidence is clear that these changes are not being driven by long-term natural climate cycles. Instead their main cause is global warming and the human activities that cause it.
What impacts of climate change are we already seeing?
Climate change has profound implications for people and the natural world. Impacts of climate change that we are already seeing include:
- Changes in extreme heat: Higher average temperatures mean heatwaves are now more frequent – and tend to be hotter when they occur. The increased temperatures also help make forest fires more likely and more intense.
- Increased rainfall: Warmer air holds more water, making heavier downpours more likely as temperatures have increased. This increased heavy rainfall can lead to increased flooding, damaging property and threatening lives.
- Changes in the availability of food and freshwater: Changing weather patterns have affected crop yields – the quantity of crops that can be produced from an area of land. The availability of freshwater for drinking and agriculture in some places has also been affected.
- Rising sea levels: Higher temperatures are causing the increased melting of huge ice sheets on land in Antarctica and Greenland, and an expansion of sea water, which increases its volume. Both of these factors are driving an increase in global sea-levels. The global sea-level has risen by around 20 cm since the start of the 20th century. This has made storm surges – the rise in sea level that occurs during intense storms – more likely to exceed existing sea defences and cause flooding. As many densely-packed cities are in low-lying coastal regions around the world, this hazard can affect large numbers of people. This is particularly true in developing countries such as Bangladesh, but cities like Venice and Miami are also low lying and will be affected too.
- Loss of biodiversity and nature: In the ocean, the increase in water temperature is putting pressure on ocean life. The Great Barrier Reef, where the coral population is in shallow water, has recently declined by up to 50%. Current ocean conditions haven’t existed in at least the last 65 million years. There is also evidence of climate change affecting nature on land, with many species of plants and animals shifting to new areas due to warming.
Much of the carbon dioxide we have already emitted will remain in the atmosphere for centuries – some even for thousands of years. As we continue to add to it, the concentration of carbon dioxide and other greenhouse gases will increase and the planet will become even hotter.
As the world warms, the impacts of climate change are becoming stronger and clearer: more frequent heatwaves, the declining availability of water in regions that are already dry today, substantial risks to the diversity of animals and plants around the world today.
The consequences of these impacts, and the possibility of higher migration of people around the world to escape them, have led to efforts to slow and eventually halt global warming by tackling its causes. Read more here about what is meant by ‘net zero’ and the questions Climate Assembly UK will consider.