Lake sediments trace 16,000 years of Western Balkan summers

Sediments from a small mountain lake in Bosnia and Herzegovina preserve an approximately 16,000-year record of summer temperature change in the Western Balkans, revealing abrupt climatic shifts whose effects varied across Europe. Researchers at Eötvös Loránd University reconstructed past mean July temperatures from chironomid remains and other lake-sediment indicators, then used pollen, sediment particles, organic matter and geochemical elements to track associated changes in vegetation, erosion and lake conditions. Two peer-reviewed studies published in 2026 examine different periods in the same sequence, together covering the Late Glacial, the Holocene and more recent centuries.

The reconstructed record shows that a large cooling event associated with the Younger Dryas lowered estimated mean July temperatures at Lake Prokoško by approximately 2.3°C, a change intermediate between estimates from Alpine lakes and those farther east. The lake, which covers 4.8 hectares at an elevation of 1,670 metres in the Vranica Mountains of the Dinaric Alps, also records a later Middle Holocene interval in which estimated summer temperatures may have exceeded present-day levels. These findings emerge from a paleoclimate reconstruction based on biological, chemical and physical evidence preserved in lake sediments, rather than from direct instrumental measurements.

Lake Prokoško occupies a climatically significant position between Western, Central and Eastern Europe. Over thousands of years, remains from the surrounding landscape and the lake’s own ecosystem accumulated in bottom sediments, providing material for reconstructing regional environmental and climatic history. One study, published in Palaeogeography, Palaeoclimatology, Palaeoecology, focused on rapid changes between approximately 16,000 and 10,000 years ago during the Late Glacial and beginning of the Holocene. A separate study in Quaternary Science Reviews traced conditions through the Holocene to the present, extending the interpretation of the sediment sequence across the full period represented by the two investigations.

Chironomids, a group of aquatic insects also known as non-biting midges, supplied the principal temperature indicator. Their larvae occupy lake sediments and differ in their preferences for water temperature, oxygen levels and nutrient conditions. After moulting, larvae leave chitinous head capsules that sink to the lake bed, where they can be preserved for tens of thousands of years. The researchers identified the species represented in different sediment layers and compared the fossil assemblages with faunal and climate data from hundreds of modern European lakes. This comparison supported estimates of mean July temperature, while the accompanying pollen, sediment, organic-matter and chemical records provided evidence about forests, soil erosion, lake productivity and other environmental changes.

The oldest layers examined reflect cold conditions near the end of the last glacial period. At around 14,600 years ago, reconstructed mean July temperatures around Lake Prokoško were below 8°C. They subsequently rose by approximately 2.6°C within a comparatively short interval, reaching around 10.2°C. The reconstruction associates this warming with increased biological activity in the lake, spreading vegetation and more stable conditions in the catchment. The youngest sediments in the study cover the most recent millennium, where the chironomid community reflected not only temperature changes but also centuries of alpine grazing, greater nutrient input and the later expansion of nutrient-rich conditions.

The early warming ended abruptly when the Younger Dryas began around 12,900 years ago, the last major cooling episode at the close of the last glacial period. Estimated mean July temperatures at Lake Prokoško fell by approximately 2.3°C to about 7.9°C. Cold-water chironomid species became more abundant, while increased erosion transported more clastic material into the lake. Comparisons with reconstructions elsewhere in Europe revealed a marked spatial contrast: summer cooling of 3–4°C has been reconstructed at many Alpine lakes, whereas decreases in the Eastern and Southern Carpathians were generally less than 1°C. The Lake Prokoško estimate falls between those geographic extremes.

The researchers connect this west-to-east gradient, which becomes weaker toward the east, to oceanic and atmospheric changes in the North Atlantic region. For the Carpathian Basin, the contrast illustrates that the same large-scale climatic event can produce different responses in Western, Central and Eastern Europe. Lake Prokoško consequently provides a regional reference point for examining how the magnitude of a shared climate signal varied geographically. The researchers also emphasize that alpine lakes respond sensitively to relatively small temperature changes: variations in ice-cover duration, water stratification, oxygen content and biological activity can accompany those changes, making small mountain lakes useful environmental archives.

Warming resumed at the end of the Younger Dryas. Between approximately 11,800 and 11,400 years ago, reconstructed mean July temperatures increased by around 2.9°C. By about 10,500 years ago, they may have reached approximately 12.6°C. The lake became more productive, its ice-free season lengthened, and closed-canopy forests developed in the catchment, reducing erosion. Temperatures did not remain constant throughout the Holocene. Around 6,800 years ago, reconstructed mean July temperatures may have reached approximately 14°C, meaning that some summers around Lake Prokoško were warmer than those experienced today. The researchers distinguish this Middle Holocene warmth from present-day warming: the earlier conditions were mainly seasonal and driven by gradual changes in Earth’s orbital parameters that increased Northern Hemisphere summer insolation, whereas current warming is global, year-round, more rapid and primarily associated with human-caused greenhouse gas emissions.

A cooling trend began around 6,000 to 5,500 years ago during the Neoglacial transition toward a cooler Late Holocene. Reconstructed mean July temperatures fell from approximately 14°C to around 12.8°C, then to 11.8°C by about 5,670 years ago. Pollen evidence indicates that people were altering the landscape around the lake by the end of the Neolithic, approximately 6,000 years ago. Plants associated with grazing and open habitats became more common, followed later by more intensive forest use and the expansion of open areas. The same sediment archive therefore records both reconstructed climatic variability and the growing influence of human land use, although it does not by itself establish that human activity caused the temperature changes documented in the sequence.

Later Holocene temperatures continued to vary. Around 3,000 years ago, another significant cooling began, with mean summer temperatures falling from approximately 14.7°C to 12°C. Around 2,000 years ago, reconstructed temperatures rose during the Roman Warm Period, reaching approximately 13.3°C. Over the past millennium, centuries of alpine grazing opened the surrounding landscape and increased the nutrient load entering Lake Prokoško. The chironomid community changed significantly around 700 years ago, and species preferring more nutrient-rich waters became increasingly common over the following 500 years. Together, the two studies show that the lake’s fossil and sediment record captures environmental responses shaped by both regional climate change and local human transformation, while also demonstrating that estimates for individual periods remain reconstructions derived from sediment proxies and modern comparative data.

Subject of Research: Paleoclimate reconstruction of summer temperature variability in the Western Balkans

Article Title: Where three climate zones meet

Article References: Where three climate zones meet. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

DOI: Not provided

Keywords: Lake Prokoško, Western Balkans, summer temperatures, paleoclimate, chironomid fossils, Younger Dryas, Holocene, climate reconstruction, land use, alpine lake, three, climate