Comment on the result
The two most important factors that affect global radiation are how high the sun is in the sky and cloudiness. While the change in solar altitude (and day length), gives the large regular variation over the year, the variation in cloudiness gives the variation in yearly and monthly values between different years. The content of particles and water vapor in the atmosphere is also important for how much solar radiation reaches the earth's surface.
Since the mid-80s, yearly global radiation has increased by around ten percent in Sweden. The rise was particularly clear until the first years of the 2000s. After that, 2018 and 2020 were important contributors to the continued, somewhat weaker, observed global radiation increase on a yearly basis.
A similar tendency with increasing global radiation is seen in large parts of Europe. Measurements taken before the 1980s indicate that solar radiation was higher in the 1960s than in the 1980s. The decline from the 1960s to the 1980s and the subsequent increase into the early 2000s is often referred to, in international climate research, as global dimming and brightening.
The trends in global radiation cannot be explained by variations in the sun's radiation. The main reasons for the observed increase since the 1980s are partly a reduction in cloudiness and partly a reduction in the amount of particles in the atmosphere thanks to greatly reduced emissions of air pollution both in Sweden and the rest of Europe. Satellite measurements of cloudiness show a reduction in total cloudiness over Sweden during the current period. It can be seen that the amount of atmospheric particles has decreased both in analyzes of SMHI's measurements of the so-called direct radiation and in the national air environment monitoring for which the Swedish Environmental Protection Agency is responsible.
The Swedish measurements that were carried out before 1983 are generally not homogeneous with today's measurements. It is therefore not currently possible to compare the levels observed so far in the 21st century with those observed during, for example, the 1960s. Hopefully, the presented measurement series will be able to be extended with previously measured data. Before that, extensive work needs to be done to produce the necessary corrections and estimate the quality of older data.
The longest series of measurements from Stockholm has already been homogenized in some parts, and the results are presented in this report.
Long-term global radiation in Stockholm
A more detailed look at the different seasons shows that it is mainly spring and summer that have contributed to the increasing trend in global radiation.
For example, the summer of 1998 was very cloudy, while especially May and July of 2018 were very sunny in Sweden. As the largest part of the annual insolation is received in late spring and summer, this also affects the annual solar insolation for 1998 and 2018.
For autumn and above all winter, it is more difficult to find any clear trend.
Spring means the three-month period March-May
Summer means the months June-August.
Autumn is the months September-November.
Winter is the three-month period December-February.
If winter is specified with only one year, it is the year for January and February. For example, winter 2020 refers to the period December 2019-February 2020. A clearer designation is winter 2019/2020.
How is the indicator global radiation defined?
The total amount of solar radiation that hits a horizontal (ground) surface is called global radiation in meteorology. The total global radiation is the sum of the radiation directly from the sun and the so-called diffuse radiation. Diffuse radiation is the solar radiation that has been scattered by the molecules and particles of the atmosphere or reflected by clouds, and therefore changed direction and is incident from the whole sky.
What is measured each time the instruments are read, is more specifically called global irradiance, which is the incident radiation effect per unit area, and is given in the unit W/m². By integrating over time, you get the total radiation energy per surface unit for a given time period, for example hour, day, month or year. This quantity is called global irradiation and for monthly and annual values it is usually given in kWh/m² or MJ/m² (1 kWh/m² = 3.6 MJ/m²). In meteorology, however, the term global radiation is often used as a general term for both global irradiance and global irradiance. In this case, however, you must clearly state whether it is instantaneous or average values that are intended or whether it applies to accumulated values.
Why is this indicator important?
The radiation balance at the ground surface is very important for, among other things, air temperature and evaporation. Global radiation also affects biological growth and is decisive for how much solar energy is available.
Worth noting is that by only studying global radiation, it is not possible to say how the total radiation balance at the ground surface has been affected. When cloudiness decreases leading to increased global radiation, the incoming long-wave radiation decreases instead and vice versa.
How has the indicator been calculated?
SMHI has had homogeneous measurement series of global radiation since 1983. Compared to most other meteorological parameters, the station network for radiation measurements is sparse. The mean annual and seasonal insolation presented here are averages calculated over only eight stations. The stations included in the analysis are Kiruna, Luleå, Umeå, Östersund, Karlstad, Stockholm, Visby and Lund. The global radiation diagrams show annual and seasonal accumulated values with the unit kWh/m².
For the winter of 1983, no value is presented as there is currently no correct value for December 1982.
Today, SMHI only observes the incoming components of the radiation balance, which are called global radiation and long-wave radiation (heat radiation from the atmosphere). Of these two, only the global radiation has been observed for a longer period of time and at several stations. Therefore, only global radiation can currently be used as a climate indicator.
Global radiation in the future
Future trends in solar irradiance are uncertain and results from model simulations vary. The available results indicate that the changes will be relatively small, even in high emission scenarios. It is most likely a matter of a slight reduction in solar radiation of a few percent towards the end of the century. The calculated changes in solar radiation can mainly be linked to changes in cloudiness.