From KlimaNachrichten

By Dr. Ludger Laurenz

So far, long-term forecasts over months and years have been more like “looking into a crystal ball”. No institution foresaw the drought in 2018/19 or the wetness in 2023/24. In the future, such extreme phases can be predicted years in advance, if the influence of cyclically fluctuating solar activity on the weather is taken into account.

The influence of cyclically fluctuating solar activity on weather and climate has been documented in thousands of scientific publications. Anyone can make sure of this with the help of Google Scholar. Against this background, I have searched for traces of the 22-year magnetic cycle or Hale cycle of the sun in historical weather data from Germany in recent years. The results are described in a publication in which I was involved ( Laurenz et al. 2019 ) and several contributions under Klimanachrichten.de. The focus is on precipitation, in which the solar traces are more clearly visible than in the temperature or sunshine duration.

With this article, I summarize my results and present the solar-based precipitation calendar for the next 6 years from 2025 to 2030. The data basis for the forecast is the historical monthly precipitation totals of the German Weather Service (DWD Time Series).

How the forecasts are derived from historical data will be described using the example of the precipitation total of October with the DWD data from 1903 onwards. The data series is composed of 6 Hale cycles, starting in 1903, 1926, 1946, 1968, 1988 and 2011. First, this data series is “cut up” at the beginning of the starting years of the Hale cycles. Then the six sections are superimposed. This is how the curve in Figure 1 is created.

The curve image may seem chaotic at first. On closer inspection, orderly structures become visible. In several cycle phases, the 6 curves run bundled in the same direction, down or up. This is evidence of solar influence.

The common trend of the curves is represented by the black mean line. This mean line provides the forecast data. The forecast value for the individual cycle years is converted into a percentage, the percentage deviation from the average since 1903.

The following table shows the forecast percentages of the individual months for the period 2025 to 2030 (see yellow box in Fig. 1). The forecast period must be limited to 2030 because it is not yet possible to estimate whether the current cycle will end in 2030 or 2033.

Notes on the interpretation of the table:

In the table, the forecast data is given as a percentage of the long-term average. Above-average precipitation totals are highlighted in blue, below-average precipitation totals in brown. The accumulation of the blue colour shows that above-average rainfall is to be expected for the second half of 2025. In the four years 2026 to 2029, precipitation totals are expected to be below average, with the lowest precipitation total in 2028. For 2030, exceptionally high amounts of precipitation are forecast for the entire year, which could result in flooding and significant problems in agriculture.

A second line describes the forecast quality in each case. In order to get an idea of how the division into three levels is carried out, the qualities for October are indicated in red lettering in Figure 1 above. The number 1 indicates a good forecast quality with a high probability of occurrence. Here, almost all curves are grouped closely around the mean value of the individual cycle years. Digit 2 with medium forecast quality is assigned if the precipitation total of the past 5 cycles varies moderately and shows a common trend. In the case of number 3, the forecast quality is low. This applies to individual cycle years with particularly strongly scattered curves or precipitation totals.

How reliable the forecasts have been in the past can also be seen in Figure 1. The red dotted line shows the trend of the precipitation total since 2011. In 12 of the 14 cycle years so far, the red-dashed line is relatively close to the forecast value of the black mean line, with a difference of less than 20 mm. In the cycle marked in blue, which begins in 1988, the hit rate is 18 out of 22 years, and in the cycle marked in brown, which begins in 1968, it is 13 out of 22 years. The examples show that the hit rate can fluctuate significantly between cycles.

Summary and outlook:

Precipitation activity in Germany is surprisingly strongly influenced by cyclically fluctuating solar activity. This can be used for a multi-year forecast. In this paper, a solar-based precipitation forecast for the next 6 years is presented for the first time, with a rain calendar for the period from 2025 to 2030. In the course of this year, it will become clear how reliable the forecasts are. Exceptionally high precipitation totals are predicted for July/August and October/November.

I informed the German Weather Service about my findings on the solar influence on the precipitation trend. They were noted there with interest. The German Weather Service has contacted a German university to further investigate the topic scientifically. If the results are positive, it is possible to test an application for the DWD seasonal forecasts.