Climate indicators – global radiation

SMHI has almost homogenous time series of global radiation since 1983. Since the middle of the 1980s up until around 2005-2006 the annual global radiation increased by almost 8% in Sweden. Similar tendencies have been observed in large areas of Europe. In more recent years the global radiation over Sweden has remained at this higher level.

Annual global radiation

Accumulated global radiation for each whole year since 1983 for eight stations in Sweden. The black curve shows an approximately 10-year moving average.
Accumulated global radiation for each whole year since 1983 for eight stations in Sweden. The black curve shows an approximately 10-year moving average.
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Changes - discussion

Since the mid-1980s up until around 2005 the annual global radiation has increased by almost 8% in Sweden. Similar tendencies have been observed in large areas of Europe. There are also measurements from before the 1980s and these data indicate that the global radiation was higher during the 1960s than during the 1980s.

The decline from the 1960s until the 1980s and the following increase up until the start of the 2000s are often referred to as global dimming and brightening by international researchers. Unfortunately the older measurements are not homogenous with current measurements so it is therefore not possible to compare the levels observed in the 2000s with those observed during the 1960s. Hopefully the older data can eventually be added to the measurement series but first a lot of work needs to be done to estimate the quality of older data and to carry out necessary corrections.

The two most important factors that affect global radiation are the sun altitude and the cloud cover. While changes in the sun’s altitude (and length of the daylight period) provide the large regular variation over a year, the cloud cover accounts for the variation in the monthly and annual means from year to year. When cloud cover is reduced the global radiation increases while the longwave radiation is reduced, and vice versa. If only the global radiation is studied it will not provide information about how the total positive contribution to the radiation balance on the earth’s surface is affected.

Winter global radiation

Accumulated global radiation during December, January and February.
Accumulated global radiation for Winter (Dec, Jan and Feb) since 1983 for eight stations in Sweden. The black curve shows an approximately 10-year moving average
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Spring global radiation

Accumulated global radiation during March, April and May
Accumulated global radiation for Spring (Mar, Apr and May) since 1983 for eight stations in Sweden. The black curve shows an approximately 10-year moving average
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Summer global radiation

Accumulated global radiation for Summer (Jun, Jul and Aug)
Accumulated global radiation for Summer (Jun, Jul and Aug) since 1983 for eight stations in Sweden. The black curve shows an approximately 10-year moving average
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Autumn global radiation

Accumulated global radiation for Autumn (Sep, Oct and Nov)
Accumulated global radiation for Autumn (Sep, Oct and Nov) since 1983 for eight stations in Sweden. The black curve shows an approximately 10-year moving average
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Background

The radiation balance on the earth’s surface significantly affects both the air temperature and evaporation. SMHI currently only observes the incoming components of the radiation balance, called global radiation and long-wave radiation. However, of these two, only the global radiation has been observed over a longer period of time and at several stations. Therefore this is the only parameter that can currently be used as a climate indicator.

The total amount of solar radiation that reaches a horizontal surface (on the ground) is called the global radiation. The global radiation therefore consists of the sum of the radiation direct from the sun and the diffuse radiation from the rest of the atmosphere, i.e. the solar radiation that is spread by the molecules and particles in the atmosphere or reflected by clouds. More specifically, the parameter which is measured by SMHI’s instruments is called global irradiance, which is the incident radiation effect per surface unit, given as W/m².

Integrating over time gives the total radiation energy per surface unit for a given time period, such as hour, day, month or year. This is called the global irradiation, with monthly and yearly values usually given in kWh/m² or MJ/m² (1 kWh/m² = 3.6 MJ/m²). However the term global radiation is often used as a general term for both global irradiance and global irradiation in which case it is necessary to clearly state whether it is an instantaneous or average value of the global radiation, or if it represents accumulated values.

SMHI has homogenised measurement series for global radiation since 1983. Compared with most other meteorological parameters the station network for radiation measurement is very sparse. The average annual and seasonal global radiation that is presented here is an average taken from only eight stations which are Kiruna, Luleå, Umeå, Östersund, Karlstad, Stockholm, Visby and Lund. The global radiation graphs show annual or seasonal accumulated values with the unit kWh/m².