Are the last warm summers primarily based on natural cycles?

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By Kalte Sonne

By Dr. Ludger Laurenz

In short:

  • The high temperature level of the last 20 years in Germany is primarily a consequence of the current warm phase of the Atlantic Multidecadal Oscillation (AMO). The increased sunshine duration in recent decades with the warmer AMO acts as a temperature amplifier of the AMO, with up to 4 °C higher high summer temperature in the current AMO warm phase.
  • The extreme heat of 2018 could be the result of the addition of warm phase of amO and influence of the Sun’s Hale cycle. Scientific evidence for the influence of the Sun’s Hale cycle is still pending. The author’s previous contributions on the influence of the Hale cycle on our weather are to be understood as a stimulus for climate and atmospheric researchers to check the Influence of the Hale Cycle.

Is the high temperature of the last 20 years an expression of an anthropogenic climate crisis or is the high temperature level primarily based on natural cycles? This question arises not only globally, but also in Germany, where the temperature level has risen even more than the global average. An answer is provided by the analysis of the trend of sunshine duration.

Sunshine duration as a temperature amplifier of the AMO

A natural influencing factor on the climate in Central Europe is the cyclically fluctuating water temperature in the northern Atlantic, which is referred to in climate science as the Atlantic Multidecadal Oscillation (AMO). The AMO is an ongoing series of long-lasting changes in the sea surface temperature of the North Atlantic with cool and warm phases, each lasting 20-40 years, and a difference of about 0.5 °C between the extremes. (

Changes in the last 20 years with the current warm phase are within the natural variability. The 114th Climate Show provides information on recent scientific publications on the Gulf Stream and water temperature in the North Atlantic. (

The connection between AMO and the temperature trend in Europe is well known and scientifically documented many times. The relationship with sunshine duration has only been studied more intensively in recent years (1), with the following result:

“A significant proportion of the variance in the annual sunshine duration over Central Europe can be explained by changes in water temperature conditions over the North Atlantic.”

A link between the AMO and sunshine duration in Central Europe is also shown by the comparison of the AMO index and the annual sunshine duration of Potsdam in the following figure. Here, the data from Potsdam were chosen because they go back to 1893, while in the area average of Germany data from 1951 onwards are only provided by the German Weather Service. The 11-year moving average is used to express the long-term trend undisturbed by individual-year effects and the influence of the 11-year Schwabe cycle.

Figure 1: AMO index and annual sunshine duration in Potsdam since 1893

The trend of the AMO index and the duration of sunshine are similar. The relationship between the water temperature in the Atlantic and the duration of sunshine in Potsdam is obvious. To highlight the trend of recent years, the 11-year moving average is gradually shortened to 6 years at the end and the curve is extended until 2021. By 2020, the level of the AMO index as well as the duration of sunshine has reached the highest values of the last 130 years.

The following figure with data of the area average of Germany, available from 1951, shows a close interlocking between sunshine duration and temperature.

Figure 2: Annual sunshine duration and average annual temperature in the area average of Germany since 1893

Since about 1990, both the duration of sunshine and the temperature have risen to today’s maximum level. Here, too, the curve is continuous until 2021 and the 11-year moving average is gradually shortened to 6 years.

The influence of AMO and solar activity on sunshine duration is most evident in midsummer. In the following figure, therefore, the sunshine duration is compared with the average temperature of July and August for the area average of Germany.

Figure 3: Correlation between sunshine duration and mean temperature July/August in the area average of Germany since 1951

In midsummer, there is a highly significant correlation between sunshine duration and temperature. Depending on the year, the duration of sunshine since 1951 has fluctuated between about 260 and 550 hours. This fluctuation causes a variation in the midsummer temperature of more than 4 °C.

The points of the last four years are marked in red and – influenced by AMO – are at a high level in the trend that has been set since 1951. Noteworthy is the extreme position of sunshine duration and temperature in 2018. In addition to the AMO, a second natural cycle, the Sun’s 22-year Hale cycle, could also be responsible for this extreme position, which will be shown in the following section.

Extreme heat 2018 – a product of AMO and Hale cycle of the sun?

In about 22 years, the sun goes through a cycle of activity called the Hale cycle. Each individual year correlates with weather trends (monthly or annual values) and ensures that certain trends repeat themselves after about 22 years. If you search with Google Scholar, you will get numerous proofs worldwide for the influence of the Hale cycle of the sun on weather trends.

Only since last year, thanks to the release of Leamon (2), the early years of the Hale cycles are known. With the help of the early years, the influence on the weather can be depicted even more vividly than before. The influence of the Sun’s Hale cycle on our weather can also be proven on the basis of the data on the sunshine duration of Potsdam available since 1893, as the following figure shows.

Figure 4: Sunshine duration July + August in Potsdam with Hale cycles of the sun

At the beginning of the Hale cycles, the duration of sunshine regularly reaches the maximum, only to fall to a minimum in the middle of the cycle. An exception is the cycle beginning in 1926, marked with (a). During this cycle, the AMO index has risen sharply. The sunshine duration has increased so much that in the middle of this Hale cycle, the expected drop was probably more than compensated for by the extremely rapid increase in the AMO index and the associated steep increase in sunshine duration.

Overall, an increasing trend in sunshine duration has been observed since 1893. The rashes within the Hale cycles gradually become smaller. The maximum peaks remain at the same level, while the minima increase continuously. In the current cycle, which begins in 2011, in the middle of 2022 due to the curve, a decrease in the duration of sunshine as in the Hale cycles before is hardly to be expected. The exceptionally high sunshine duration in July/August measured in Germany so far this year 2022 confirms the assumption from the curve.

The realization that the drop in sunshine duration in the middle of Hale cycles decreases from cycle to cycle is new. If all previous cycles had been averaged, a decrease in sunshine duration would have been expected for 2022. With the new insight, it becomes comprehensible why the sunshine duration remains at such a high level in 2022.

The previous explanations make it clear how strongly AMO and Hale cycle of the sun influence climate changes in Central Europe and the interplay between AMO and solar activity still offers a lot of material for research activities. In recent years, the author has been able to provide numerous proofs of the influence of the Sun’s Hale cycle on weather trends across Europe (345). In the hope that the observations will provide reason for further scientific analyses, he has turned to climate and atmospheric researchers in Berlin, Hamburg, Bremerhaven and Kiel in recent months, without having received an answer that has shown interest so far. Obviously, there is currently a certain reluctance to blame natural cycles for the high temperature level of recent years.

Basically, the duration of sunshine correlates not only with temperature, but also with precipitation activity. The cause of the drought of recent years, the current drought and the possible subsequent drought summers, as shown in this article, is more likely to lie in the interplay between ocean and solar cycles than in other factors. This assumption is supported by a 2021 publication by M. Ionita of the AWI in Bremerhaven, who has investigated the causes of droughts of the last 1000 years with her team and warns of an increased occurrence of drought in the next two decades (6).


(1)   Andrzej A. Marsz et al. 2021: The thermal state of the North Atlantic and macro-circulation conditions in the Atlantic-European sector, and changes in sunshine duration in Central Europe,

(2) Robert J. Leamon et al. (Feb. 2021), Termination of Solar Cycles and Correlated Tropospheric Variability, https://

(3) Laurenz, L. 2021: Significant influence of solar activity on the drought in 2018 and 2019,

(4) Laurenz, L. 2021: How solar activity in Germany over the past four years has shaped sunshine duration and temperature levels in midsummer

(5) Laurenz, L. 2021: Significant influence of solar activity on winter temperature in the polar night of Scandinavia,

(6) Ionita, M., Dima, M., Nagavciuc, V. et al. 2021: Past megadroughts in central Europe were longer, more severe and less warm than modern droughts.