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about Climate change and Global warming

Global temperature change

Many studies show that the global mean temperature has increased abnormally during the last 50-100 years. The rise in temperature has especially accelerated after around 1980. For a long time, climate sceptics had little confidence in these studies, but they have gradually accepted that the planet is getting warmer. The discussion now centres more on the reasons for the warming.


HadRUTC is one of the first series of temperature data published. This was done in a collaboration between Hadley Centre (UK Met Office) and the University of East Anglia [L15]. The following figure is based on the latest version of this series:

The figure shows how the temperature has developed compared to the 1961-1990 average. This average is chosen as the zero point because this period defines the "normal temperature" which meteorologists have traditionally used. The IPCC compares the global temperature with the temperature in pre-industrial times. Pre-industrial time is somewhat randomly defined as the period 1850 to 1900, though industrialisation was well under way at that time. The Paris Agreement aims to limit the temperature rise to below 2 °C, and preferably not above 1.5 °C, relative to pre-industrial times. According to the UK Met Office, the temperature increase compared to pre-industrial times is now 1.1 °C [L16]. The longer growing season, melting of glaciers and disappearing sea ice in the Arctic (see Animation of Arctic ice extent from 1979 to 2017) are also major proofs of the ongoing warming.

Most people now agree that this warming is mainly due to the greenhouse effect, although other processes (natural and man-made) can have a warming and also a cooling effect. In particular, the presence of so-called aerosols in the atmosphere are important (see the section below and Aerosols). But radiation from the sun has not affected the temperature development from 1980 to today, which is the period when warming has really accelerated.

Another temperature curve from the NASA Goddard Institute for Space Studies shows about the same trend as HadRUTC, and can be found here: [L21]

Interaction with the sea

The temperature development shows great variation from year to year. This can have many causes. One of the reasons is that the sea absorbs heat to varying degrees. The sea surface absorbs heat from the air above, solar radiation and other radiation from the atmosphere. The heat formed in the upper layers of the water masses spreads downwards in the depths. This is mainly due to ocean currents causing an upheaval of the water masses. As these ocean currents vary in intensity, the quantity of cold, deep water that rolls up to the surface also varies. With colder water at the surface, the heat transport from the air to the water surface increases. The air gets colder and the sea surface warmer.

One of the most important ocean currents that affects the climate in this way is the Humboldt Current in the Pacific Ocean. It flows from south to north along the western coast of South America and turns west along the equator towards Southeast Asia and Australia. This current is caused by wind blowing from east to west along the equator. Sometimes this wind stops, and the Humboldt Current that brings cold, deep water from the south is also affected. The water is getting warmer in the east and cooler in the west. Low- pressure systems form over the hot water in the east, and weaken westwards. Thus, the wind also weakens from east to west. This gives a reinforcing effect that further impairs water transport along the equator.

This phenomenon is called El Niño and causes rain and bad weather along the west coast of South and Central America. After an El Niño episode, the Humboldt Current will often intensify, causing an opposite effect. The western Pacific gets more rain than normal, which can cause flooding, and Australia is plagued with more frequent and powerful cyclones. Such years are called La Niña years. The alternation between El Niño and La Niña constitutes a cycle called the El Niño – Southern Oscillation (ENSO). See [L17] and [L18].

In El Niño years, weather systems around the world will be affected and the planet will heat up faster since less of the cold, deep water wells to the surface. Some El Niño episodes are more powerful than others and are often called super El Niño. We had such episodes in 1998 and 2015-2016, and the effect can be seen on the temperature curve showing global warming. In both 1998 and 2015-2016, all previous heat records were broken.

The ocean is, however, the part of the climate system that absorbs most (over 90 per cent) of the increased amount of heat due to global warming [L22]. The sea surface is the primary recipient of this heating. The heat spreads further down in a layer approximately 200 m thick, where the water mixes faster than further down in the depths. The deep water below this layer is affected more slowly by the heating. There are several estimates of how the temperature at the sea surface has developed over time. The following figure taken from [L43] shows 4 different estimates of the change in the global SST (Sea Surface Temperature) from 1854 to 2016:

The figure shows the deviation in the SST compared to the "normal period" 1961-1990. The measurement methods changed during World War II, so the curves in this period are uncertain [L105].


The HadRUTC figure shows an increasing temperature development starting already around 1910-1920. This rising trend was interrupted during World War II and replaced by a more or less flat curve until the early 1980s. One can speculate on the reasons for this development. An obvious thought is that the global warming caused by greenhouse gases started already around 1910-1920 and that other mechanisms which gave a cooling effect were effective after World War II. One such mechanism is the formation of aerosols in the atmosphere as a result of polluting emissions. Aerosols are particles or water droplets that have formed around particles. Aerosols can have both a warming and a cooling effect. Emissions of sulphur dioxide, which were a major problem after the war, form cooling aerosols by water vapour condensing around the sulphur particles, which gives increased cloud formation. Recent research shows that this mechanism is much more powerful than previously thought [L19].

Due to the problems caused by sulphur pollution (acid rain), industrial concerns in Europe and North America were forced to obey strict emission restrictions from around 1980-1990. As a result, there was a gradual reduction of global sulphur emissions between 1990 and 2015, totalling approximately 30% lower emissions [L20]. This may be one reason why global warming began to pick up speed from about the same time. Globally, sulphur pollution is still a big problem, and this has led to concerns that heating will increase further if more of the sulphur pollution is reduced ([L19]), which is necessary, especially due to the health damage caused by these particles.

Volcanic eruptions also lead to the formation of aerosols that can remain in the atmosphere for several years after the eruption. This also explains part of the uneven trend in the temperature development [L23].

See also a separate page on Aerosols.

Latest update: 2021-07-10