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

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Since June, scientists have been warning that Earth might experience an El Niño unlike any other. It now appears they were right. The periodic climate beast that emerged this year is stronger than any ever seen. And the really worrisome news: It likely hasn’t even reached its peak.

El Niños develop in the central and eastern equatorial Pacific Ocean. Over a short period, they release extraordinary amounts of ocean heat into the air. And by September 21, the daily sea-surface temperature where El Niños form was already 3.08 degrees Celsius (5.54 degrees Fahrenheit) warmer than the average from 1991 to 2020.

El Niños can fuel droughts, floods, heat waves and severe storms. The Oceanic Niño Index is a traditional measurement of their strength. Using daily sea-surface temperatures to calculate strength shows that the 2026 event is already more intense than any in recorded history.

“It has been a remarkable event,” says Nathaniel Johnson. What’s more, the climate scientist notes, this El Niño will probably strengthen even more. He expects it to peak “within the next few months.” Johnson works at the Geophysical Fluid Dynamics Laboratory in Princeton, N.J. It’s part of the National Oceanic and Atmospheric Administration (NOAA).

Ocean heat released by this El Niño could spike global land temperatures about 1.2 degrees C (2.16 degrees F) in the coming months. That may not sound like much. But by next February, it could lead to more than 450,000 additional heat-related deaths. That’s the finding of a new report out of the University of Chicago’s Climate Impact Lab.

NOAA has not yet formally called this El Niño a record-breaker. Why? It’s using a new measurement method: the Relative Oceanic Niño index. That should better capture how an El Niño’s heat will alter weather across the globe. And by this index, the El Niño in 1997 and 1998 topped the new one’s current strength.

But that record, too, is on track to be broken soon, based on a Sept. 10 forecast by NOAA’s Climate Prediction Center.

To understand what’s driving this historic event and its potential impacts, Science News spoke with two experts. One is Johnson, part of that U.S. team that produces El Niño forecasts. The other, Nathan Lenssen, is a climate scientist at the National Center of Atmospheric Research. It’s in Boulder, Colo. Their answers have been edited for length and clarity.

How much of this El Niño’s strength has been fueled by human-caused climate change?

Johnson: It’s really an unanswered question. If we look at the historical record, we have been seeing more rapid swings between El Niño and La Niña. [La Niña is the other — and opposite — extreme phase of a seesawing pair of climate patterns: Together they’re known as ENSO, or the El Niño Southern Oscillation.] There’s some evidence from computer models that increasing greenhouse-gas emissions lead to more of those extreme [ENSO] swings. But there’s no science consensus yet on how climate change is impacting El Niño. Especially an extreme one, like this.

Lenssen: There’s debate on how much we can directly attribute to climate change. But due to Earth’s warming, there is more energy in the oceans now. And El Niños release some of that ocean heat into the atmosphere.

What are the most predictable and unpredictable impacts of this El Niño?

Johnson: We see a lot of the most predictable impacts in the tropics. That’s where an El Niño develops. We’ve already seen a suppressed Atlantic hurricane season and enhanced Pacific hurricane season. We’ve also seen droughts in Central America. We tend to see droughts over northern South America and the Amazon. Meanwhile, the Indian monsoon can weaken.

By late fall, we typically see more impacts a bit farther from the equator. For instance, we tend to see more rain over the southern United States, but it’s typically drier than normal over the Pacific Northwest. And it’ll be warmer than average over the northern United States and Canada.

Lenssen: While this El Niño is quite powerful, that doesn’t necessarily mean we’re more confident about its impacts. A global warming of the oceans has made some historical changes seen at mid-latitudes less likely.

So there’s a kind of twist there. The El Niño in 2023 and 2024, for instance, didn’t lead to as much wet weather in California as expected. And some research has shown that was due to other warming ocean patterns.

How long could this El Niño last?

Johnson: El Niños typically last a year. So we’ll probably see this one weaken after winter and be over by spring or early summer. But when we have a big El Niño like this, we’re more likely to see a shift in the following year to La Niña [which also has global impacts on weather]. So that’s something we’ll be keeping our eye on.

Lenssen: It’s possible that this event could be followed by a double La Niña. Those occur back-to-back, separated by a brief period of neutral conditions. That happened in 2016 and 2017, after the strong El Niño of 2014 and 2015.

Aside from strength, what sets this event apart from past ones?

Johnson: We’ve seen such a dramatic change in the equatorial Pacific Ocean. The move has been very rapid, from a La Niña last winter to a very strong El Niño — all in less than a year. That might be the fastest transition that we’ve seen.

Is there any sort of silver lining here?

Lenssen: Though more work is needed to understand this, some research suggests future El Niños may actually become more predictable as the world continues to warm. That might give us months of additional lead time to prepare for their impacts. And that would be great.