The world's climate varies naturally as a result of:
- The way the ocean and the atmosphere interact with each other
- Changes in the Earth's orbit
- Changes in energy received from the sun
However, there is now very strong evidence and almost universal agreement that significant recent global warming can’t be explained just by natural causes. The changes seen over recent years, and those predicted for the next century, are considered to be mainly the result of human behaviour. The Intergovernmental Panel on Climate Change (IPCC) is a scientific body set up by the UN to look at climate change. It says that human activity is the main cause of the changes seen in climate during recent decades. In the last 100 years, the Earth has warmed by 0.75 degrees Celsius. Global sea levels have gone up, glaciers and sea ice have melted, and extreme weather events, like floods and droughts, are likely to happen more often.
By examining rocks, pollen records, tree rings and ice cores, scientists can look back through time and see that our climate has changed many times before.
Over very long periods of time, a cycle has been observed in our climate – that of ice ages. This is a shift in the Earth's climate between glacial ages, when global temperature drops and large areas of the planet become covered in snow and ice, and interglacial ages, which brings less cold conditions, such as those we are experiencing now. The global temperature change between each is about 4 °C.
There's evidence to show these cycles are triggered by small changes in the Earth's orbit affecting the amount of energy the planet gets from the sun. The full cycle, from glacial age to glacial age, takes about 100,000 years. Evidence suggests the last ice-age ended about 11,000 years ago.
Over shorter periods of time, we know from many difference sources that there have been smaller changes in regional climates. There is evidence of a period of slightly warmer climate in Europe between the tenth century and thirteenth century known as the Medieval Warm Period. It was followed by a cooler period, known as the Little Ice Age. Most of the information on these periods originates from Europe, and there's not enough evidence to say this was a global climate phenomenon.
Today’s global climate
Since the last ice age ended about 11,000 years ago, global average temperatures have been largely steady, averaging about 14 °C. This stability has allowed complex ecosystems to thrive, supporting a wide range of life on Earth. In the last century, however, our climate has started to change and the evidence points to a change in our climate that's happening quickly, especially when compared to known historical climate cycles.
This video looks at climate change over a range of timescales. Long term changes include changes in the Sun's output, the orbit of the Earth, the tilt of the Earth's axis and wobble in the Earth's axis of rotation. Includes a look at how changes in albedo or reflectivity and increased urbanisation and industry have influenced global warming through increased pollution.
How do we know that Climate is changing?
The widespread deployment of scientific weather instruments over the last century means we can now reliably measure where, and how large, recent changes in climate have been.
- Increasing temperatures — global surface temperature records estimate that the Earth has warmed by about 0.75°C in the last century, mos t of this in the last four decades. Our oceans have also warmed up; in fact they have absorbed more than 80% of the heat added to the climate system.
- Changes in rainfall — evidence shows rainfall patterns are changing across the globe. Generally, wet places are becoming wetter and dry areas are becoming drier. There are also changes between seasons in different regions. However, such changes are harder to detect than those in temperature due to the large natural fluctuations in rainfall. •
- Changes in nature — in the UK, the growing season appears to have lengthened due to spring starting earlier and the delayed onset of autumn/winter. Wildlife experts have noted that many species are changing their behaviour, from butterflies appearing earlier in the year to birds starting to change their migration patterns.
- Sea-level rise — since 1900, sea-levels have risen by about 10 cm around the UK and about 17 cm globally, on average. Evidence shows the rate of sea-level rise has increased. • Melting glaciers — glaciers all over the world are retreating. This has been observed in the Alps, Rockies, Andes, Himalayas, Africa and Alaska.
- Reduction in Arctic sea-ice — Arctic sea-ice has been declining since the late 1970s, reducing by about 0.6 million km² per decade — an area about the size of Madagascar.
- Shrinking ice-sheets — the Greenland and West Antarctic ice-sheets, which between them store the majority of the world's fresh water, have both started to lose mass, causing sea-level rise of about 0.5 mm a year.
People often talk about sea-level rise being one of the major consequences of climate change. But how can increasing temperatures raise sea-levels around the world?
A warming climate raises sea-levels in two important ways:
- Thermal expansion — as water warms it expands, like liquid in a thermometer. A warming climate will heat the oceans, causing sea-levels to rise.
- Ice-melt — large amounts of water are locked in glaciers and ice-sheets around the world. Warmer weather is causing these to melt. Water from land-based ice flows into the oceans, raising sea-levels. Scientists estimate that if global temperatures continue to rise unchecked, the Greenland ice sheet could melt completely in a few thousand years, pushing up global sea-level by up to seven metres.
What happens on a global scale to sea-levels might differ from that which is experienced more locally. Land movement, such as subsidence or local uplift, and regional variations in ocean currents will mean that some areas could see a relatively lower rise in sea-level than the global average while some regions might face greater sea-level rises.
Sea-levels around the UK have already risen by 10 cm since 1900. If this continues, buildings, infrastructure and ecosystems in coastal areas could be at risk from increased flooding and damage from storms. Scientists are working hard to find out how much more sea-levels could rise.
KEY POINT: Does melting sea ice contribute to sea-level rise? When ice on the land melts it causes sea-levels to rise, but when floating sea ice melts there is no change to sea-level. This is because the ice ‘displaces’ almost the same volume of water whether it’s frozen or liquid.
Evidence for global warming - polar ice caps
This video reviews the changes in the seasonal sea ice cover in the Arctic over the past 30 years in relation to changes in the average annual temperatures of the area, using satellite images and temperature graphs. Looks at the evidence from oxygen isotopes in ice cores from Antarctica going back 11,500 to 135,000 years to illustrate global warming.
Impacts of future climate change around the world
Scientists have already observed changes in many natural and human environments that are more than likely due to a warming planet. As temperatures continue to rise and rainfall patterns change, the possible impacts of climate change will vary widely across the globe.
How people and natural systems are affected will depend not only on the nature and level of climate change, but also on their vulnerability to change and ability to adapt. A lot of research is being done around the world to better understand what the impacts of climate change could be, and some examples are presented below.
Impacts on ecosystems
Climate change may increase the risk of forest fire as temperatures rise and rainfall patterns change. In regions such as the Amazon, additional pressures from human activity, such as deforestation, could affect the ability of the forest to cope with future climate change. Other areas of high biodiversity, such as those in South Africa, may see losses of species as habitat conditions change too quickly for plants and animals to adapt. On the other hand, in certain locations, some animals and plants may benefit and flourish in a changing climate.
Impacts on the developing world
The environmental stresses of climate change will be felt across the globe, but it seems likely that poorer countries will see the most severe effects. This is particularly true for regions that are already more prone to extremes of flood and drought, with a large share of their economy held in climate sensitive sectors, such as farming. Food, water and energy – essential for human survival
– are already in short supply in many parts of the world and shortages may worsen as populations grow and weather patterns change.
Impacts on the Greenland Ice Sheet
The Greenland ice sheet covers an area of about 1.7 million km2 and is the second largest body of ice in the world. As our climate warms, the ice sheet will melt and reduce in size. Scientific research has shown that if global temperatures rise unchecked, then it could melt completely over several thousand years — pushing up global sea-levels by up to 7 metres. By 2100 it is more likely that the Greenland Ice Sheet melt could contribute 1 to 12 cm to global sea-level rise, although the exact amounts are quite uncertain and could be more.
Impacts on health
It is likely that climate change will have both positive and negative impacts on our health and wellbeing. The possible increases in heatwaves, floods, storms, wildfires and droughts could harm peoples’ health around the world, especially in the more vulnerable populations of the young, elderly and poor.
However, as temperatures rise, deaths from cold-related diseases might be expected to reduce, especially in countries like the UK. Patterns of disease may also vary, with some regions seeing a change in the number of outbreaks of diseases like malaria or dengue as the climate zones that support the disease life-cycle are altered by climate change.
Some specialists argue that efforts to reduce our greenhouse gas emissions could lead to healthier lifestyles and environments where we live, go to school and work; although we need to ensure there are no negative, unintended consequences of these efforts.
Projections of climate change in the UK
The Met Office Hadley Centre has modelled how the climate of the UK might change over the rest of the 21st century.
It’s important to remember that there are uncertainties in the climate modelling process, so when projections of climate change are presented, there is always a range of possible change rather than a single figure. These projections are based on the available evidence we currently have, so the projections will also change as our understanding improves.
- If global emissions continue on a similar path to today, the annual average temperature rise for the whole of the UK by the end of the century is very likely to be more than 2 °C and less than 5 °C.
- Temperatures are expected to rise across the UK with more warming in summer than in winter. The summer average temperature rise in the South East is very likely to be above 2 °C and below 6.4 °C.
- Climate change could increase the number of intense downpours of summer rainfall, which could lead to flash flooding. Scientists do not yet know whether our summers will be wetter or drier.
- The extreme heatwave of 2003, when average summer temperatures were 2 °C higher than normal, led to more than 2,000 additional deaths in the UK. Summers like 2003, which was an unprecedented event, could become a relatively normal summer by the 2040s.
- Heavier winter precipitation is expected to become more frequent, potentially causing more flooding. Sea-level across the UK is projected to rise between 11 and 76 cm by the end of the century. In the worst case, rises of up to 1.9 m are possible but highly unlikely.
Impacts of climate change in the UK
Many aspects of our lives and lifestyles here in the UK could be affected by climate change – both positively and negatively.
Impact on energy
Here in the UK our energy use is affected by the weather and our climate. Because we use more energy keeping warm in winter than we do in summer, the infrastructure we use to supply us with energy is designed to perform best in cold weather conditions. With warming summers, and increasing use of air conditioning to keep homes and offices comfortable in the summer months, there could be changes in the pattern of energy demand through the year in response to climate change. To minimise the risk of summer power black-outs in the future, energy companies are looking at potential risks to their infrastructure – for example power cables tend to underperform in hot weather – and the availability of river water for cooling power station turbines.
Impact on water
From extreme rainfall to heatwaves, weather has a significant impact on the UK’s water industry. With higher temperatures and more droughts a possibility for the future, climate change may increase the pressure on water demand, already very high in some areas of the UK. Water quality could also be affected by rising temperatures and changing rainfall patterns, with declining river flows in summer-time and an increased risk of toxic algal blooms. Flooding of sewerage systems in urban areas could increase due to more heavy rainfall.
Impact on agriculture
Higher year-round temperatures and longer growing seasons could mean that new crops flourish in the UK although warmer temperatures might make it easier for diseases and pests to survive milder UK winters.
An increase in the number or intensity of extreme events could have an impact on UK agriculture. Without irrigation or changes to the varieties grown, droughts could reduce the overall UK crop yield during some years. More heavy rain could lead to increased risk of flooding which can wash nutrients from the soil and lead to water-logging of fields.
Impact on the built environment
Climate change could have a dramatic effect on the way we build houses in the future. Those built today might need to cope with possible extreme summer heat, increased risk of damaging storms and the risk of summer and winter flooding. As temperatures rise there is more chance of soil drying out during spring and summer. Foundations in clay soils could be at particular risk. Designers must ensure that our homes are not just efficient, comfortable and safe in today's climate but remain so throughout their lifetime. This means that houses should be designed using weather information that represents the future climate and not just historical weather observations. Scientists are already working closely with industry experts to ensure our built environment is resilient to climate change.
Impact on transport
Extreme weather, including floods, heatwaves and snowstorms, has a major impact on the transport network of the UK. Today, road surfacing can melt and train rails buckle during spells of hot weather. Unless they are replaced with more resilient materials, this could happen more often in the future as heatwaves are projected become more common. On the other hand, heavy snowfall, which has caused significant disruption to all forms of transport around the UK over recent winters, should occur less frequently as temperatures rise. It is important to remember that despite climate change, we will still suffer occasional severe winters, so we will have to continue to prepare for the impacts snow and ice have on our transport network.
Climate change mitigation and adaptation
Many aspects of human society and the natural environment are sensitive to weather and climate. A lot of people, from scientists to politicians to the general public, want to find ways of reducing the negative impacts of climate change and enhancing the positive ones. Most of the possible ways of doing this fall into one of two groups – mitigation or adaptation.
Man-made greenhouse gas emissions are very likely to have been the main cause of the climate change we have seen over the past few decades. If our future emissions are reduced early and rapidly, the extent of future climate change will be lessened – this is often called mitigation. On the other hand, if greenhouse gas emissions continue at the same rate as today, or increase, it is very likely the climate change we will experience will be at a rate greater than we have had to cope with in the 20th Century.
Our climate will not react straight away to any changes in the emission of greenhouse gases – it will take time for the mitigation decisions we make now to have an effect on global climate. Even if all our emissions stopped today, it is likely that we would experience climate change for at least the next 30-40 years. Altering our behaviour to respond to this climate change is called adaptation. Adaptation means not only protecting against the possible negative impacts of climate change, but also taking advantage of any benefits. Examples of adaptation may be modifying our buildings so they remain cool during the hotter summers, managing flood risk or animals moving further north or to higher altitude, if they can, to cope with the rising temperatures. It is really important that governments, businesses and individuals understand that adaptation will be necessary over the next few decades, no matter how hard we try to reduce our emissions on greenhouse gases.