Climate change is a large-scale, long-term shift in the planet's weather patterns or average temperatures. Earth has had tropical climates and ice ages many times in its 4.5 billion years. So what's happening now?
Since the last ice age, which ended about 11,000 years ago, Earth's climate has been relatively stable at about 14 °C. However, in recent years, the average temperature has been increasing.
The information below details the seven main sources of evidence for climate change. You can find out more about the difference between weather and climate, what drives our climate and how our climate is changing.
Scientific research shows that the climate - that is, the average temperature of the planet's surface - has risen by 0.89 °C from 1901 to 2012. Compared with climate change patterns throughout Earth's history, the rate of temperature rise since the Industrial Revolution is extremely high.
There have been observed changes in precipitation, but not all areas have data over long periods. Rainfall has increased in the mid-latitudes of the northern hemisphere since the beginning of the 20th century. There are also changes between seasons in different regions. For example, the UK's summer rainfall is decreasing on average, while winter rainfall is increasing. There is also evidence that heavy rainfall events have become more intensive, especially over North America.
Changes in nature
Changes in the seasons (such as the UK spring starting earlier, autumn starting later) are bringing changes in the behaviour of species, for example, butterflies appearing earlier in the year and birds shifting their migration patterns.
Sea level rises
Since 1900, sea levels have risen by about 10 cm around the UK and about 19 cm globally, on average. The rate of sea-level rise has increased in recent decades.
Glaciers all over the world - in the Alps, Rockies, Andes, Himalayas, Africa and Alaska - are melting and the rate of shrinkage has increased in recent decades.
Arctic sea-ice has been declining since the late 1970s, reducing by about 4%, or 0.6 million square kilometres (an area about the size of Madagascar) per decade. At the same time Antarctic sea-ice has increased, but at a slower rate of about 1.5% per decade.
The Greenland and Antarctic ice sheets, which between them store the majority of the world's fresh water, are both shrinking at an accelerating rate.
Why is our climate changing?
There are many factors that could cause a change in our climate.
Anything that affects the amount of energy being absorbed from the Sun, or the amount being radiated by the Earth - the planet's energy balance - may produce long- or short-term cooling or warming.
Natural or 'forcing'
An imbalance in the planet's 'energy account' can be caused by changes in the energy radiated by the Sun, changes in greenhouse gases, particles or clouds, or changes in the reflectivity of the Earth's surface. Imbalances caused by these changes are often called 'forcings'. A positive climate forcing will tend to cause a warming, and a negative forcing a cooling.
Changes in climate can also arise from variations within the climate system. For example, the El Niño/La Niña system, in which interactions between the oceans and the atmosphere cause global temperature changes lasting a number of years. This is natural variability.
Relatively small changes in the Earth's energy account can lead to further changes, and these can further modify, for example, the reflectivity of the Earth or the amount of water vapour. The climate system is therefore highly sensitive to small changes, as these often 'feedback', and have large, long-term effects on the climate.
Increased solar energy
Scientific research into the energy we receive from the Sun has found that it is not the main cause of the current warming trend. However, solar radiation is thought to have been responsible for increased warming early in the 20th century.
Greenhouse gas increase
There's overwhelming and growing evidence that the warming is due to vastly increased - and still increasing - quantities of greenhouse gas in the atmosphere.
The most important greenhouse gas, in that it has the strongest greenhouse effect, is water vapour. It increases in concentration as the atmosphere warms. The amount of water vapour in the atmosphere has increased, but there's no reason for this scale of change other than the increase in temperature.
Carbon dioxide (CO2) and methane are both important greenhouse gases, which have a 'forcing' effect (they increase the effect of warming). Their increase in concentration is mainly caused by emissions from human activity. However, there are also potentially large secondary effects, for example decreased carbon storage due to reduced forest growth or the potential release of large amounts of methane from permafrost, caused by raised temperatures.
The amount of CO2 in the atmosphere has increased dramatically - by about 38% (as of 2012) - since the Industrial Revolution. As we continue burning fossil fuels and other activities, the amount of CO2 will continue to rise. This means the extra CO2 will absorb and emit more and more of the Earth's outgoing radiation, and this will further warm our climate. As the atmosphere warms, the amount of water vapour it holds also increases - which further adds to the warming effect.
Methane has a strong greenhouse effect, but it doesn't stay in the atmosphere for more than about a decade. CO2 lasts for about 100 years or more, meaning it has a very long time to build up and affect our climate. Some of the CO2 in our atmosphere was emitted before World War I.
Cutting down forests, one of the major natural storage 'sinks' for carbon, is further increasing the imbalance between the CO2 we emit and the planet's capacity to re-absorb it.
Is climate change really caused by human activity?
Rising sea level text.
UK Temperature Tracker (CET)
CET has been considered the most effective way of measuring how the UK's temperature averages across long timescales. The area that weather stations are situated within for this measurement cover as far north as Lancashire, then down southwest toward Bristol and across to London.
The official CET figure is monitored and recorded by the UK Met Office, the Island's Weather Centre Uk temperature tracker, whilst not using the official data, is using a series of weather stations from a very similar area so should provide a good guide every month.
© Crown Copyright 2018, Met Office