Polski
Science, Issue 17/2025
By 2100, summer in Europe could extend by more than 40 days
Dec
Lake sediments hide valuable information about changes in climate over tens of millennia. Researchers at Royal Holloway University of London used this underwater database to study how summers in Europe changed during the Holocene era. The results became the basis for formulating a reliable forecast for the future.
Why does the temperature gradient determine the European summer?
Europe’s climate depends heavily on the temperature difference between the equator and the pole, the so-called latitudinal temperature gradient (LTG). It shapes the strength of westerly winds, the course of the jet stream and the frequency of blocking highs over Scandinavia. When the gradient weakens, winds become more twisty and prone to blocking, which promotes prolonged heat waves and extreme precipitation.
Recent decades have seen more frequent and persistent extreme weather events in Europe. However, meteorological instruments began operating only tens to hundreds of decades ago, making it difficult to discern long-term relationships. To supplement their indications, the study’s authors looked to climate records covering the last 10,000 years of the Holocene. The results were published November 19, 2025 in the journal Nature Communications.
Lake sediments as a natural record of the length of summer and winter
British scientists based their study on an analysis of annual sediment laminations in two of Europe’s best-preserved lakes: Nautajärvi in Finland (layers from 9,85,000 years BP to the present) and Diss Mere in England (layers from 10,300 to 2,100 years BP).
Layers are alternating layers of sediment – each layer corresponds to one year, and its structure reflects the two seasons: the summer part and the winter part. In summer, finer, often organic sediments enter the lake, while in winter, heavier, mineral sediments enter. In this way, the layers act as a natural climatic calendar, which records the proportions of sediments formed during the warm and cool parts of the year.
In order to translate the seasonal increase in sediment into actual summer length, the thickness of the sediment was compared with the number of summer days in Europe in 1900-1940. The correlations turned out to be very high, justifying the use of the proportion of the summer part in the annual layer as a reliable indicator of summer length in Europe during the Holocene era.
How has the European summer changed over 10,000 years?
The reconstruction showed that the number of summer days ranged from 164 to 202 per year. The researchers identified three characteristic stages:
- Early Holocene – shorter, more variable years;
- Middle Holocene – long and relatively stable summers, with the center of the distribution of values indicating about 195 days;
- Late Holocene – gradual shortening of summer and cooling, with values of about 189 days.
The results from the two lakes were highly correlated, indicating the regional – rather than local – nature of the changes.
When the length of the European summer was matched with temperature gradient (LTG) simulations from the TraCE-21ka model, a significant correlation of r = 0.6, (p < 0.001) was obtained. As a result, as much as 36 percent of the variability in summer length could be explained by LTG. In comparison, summer surface water temperatures explained only 20 percent of the variability. An in-depth analysis found that a 1°C change in LTG corresponded to an average of 5.6 days of change in summer length.
The Holocene gradient varied by as much as 5°C, which translated into differences in summer length. These could amount to up to one month. These changes were first mainly due to the disappearance of ice sheets and the inflow of meltwater, and after ca. 5,000. BP years – from slow orbital changes affecting solar input.
Contemporary changes in relation to the past
The data from 1960-1990 coincides well with the model’s simulations, which strengthens confidence in Holocene-contemporary comparisons. In the 20th century. LTG fluctuations remained within normal limits, generating variability in summer length up to about 20 days.
Since the 1980s. However, we are seeing a systematic weakening of the LTG. At the same time, summer in Europe is lengthening, accompanied by more frequent and more intense weather extremes.
Summer in Europe may extend even longer
According to CMIP6 projections, LTG will weaken by 2100:
- to -4.6°C in the low emissions scenario (SSP1-2.6);
- Up to -11.6°C in the high emissions scenario (SSP5-8.5).
The Holocene minimum (-2.3°C) will be exceeded by 2050 in all scenarios. If we apply a simple Holocene relationship (5.6 days per 1°C LTG), European summers could lengthen by 2100:
- 13 days at low emissions;
- 42 days at high emissions.
That’s up to 22 days more than previous forecasts based on temperature alone suggested.
Conclusion: the past warns of the future
A British study argues that LTG has a persistent and strong influence on the length of seasons in Europe, and that climate models may underestimate atmospheric dynamics leading to extremes. Analysis of lake sediments from the Holocene reveals that a weakening temperature gradient favors longer, warmer and more extreme years, and that these processes may take on proportions in the coming decades that, to our knowledge to date, have not been reached in the past 10,000. years.
pic. main: Greyerbaby/Pixabay
Source:
Martin-Puertas, C., Boyall, L., Hernandez, A. et al. Consistent response of European summers to the latitudinal temperature gradient over the Holocene. Nat Commun 16, 9969 (2025). https://doi.org/10.1038/s41467-025-65804-x
