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Pyrenean caves reveal a warmer past

Dr David Whitehouse, Science editor

The world’s high mountain regions are particularly sensitive to climate change. Straddling the border between France and Spain the Pyrenees occupy a crucial position in southern Europe, influenced by both Mediterranean and Atlantic climates. New research published in the journal ‘Climate of the Past,’ investigating climate proxy data based on stalagmites is revealing that past climates were warmer than our own.

This is the first climate reconstruction in the region based on speleothems over the past 2500 years. Previous reconstructions have been based on lake sediments, tree-rings and glaciers.

Global surface temperatures in the first two decades of the 21st century (2001–2020) were 0.84 to 1.10°C warmer than 1850–1900 AD (IPCC, 2021). According to the IPCC anthropogenic global warming is unprecedented in terms of absolute temperatures and spatial consistency over the past 2000 years. However, pre-industrial temperatures were less spatially coherent, and further work is needed to explain regional and natural climate change. Thus, according to the authors of the new study, “obtaining new and high-quality records in terms of resolution, dating and regional representativeness is thus critical for characterising natural climate variability on decadal to centennial scales.”

It is clear that the Pyrenees has followed the global trend. Their temperature has increased by more than 1.5 °C since 1882, as shown by the longest time series recorded at the Pic du Midi observatory. Recent studies confirm this warming trend, showing an increase of 0.1 °C per decade during the last century in the Central Pyrenees, or even 0.28°C per decade if only the 1959-2015 period is considered.

In addition, long-term snow depth observations (starting in 1955) show a statistically significant decline, especially at elevations above 2000 m. The glaciated area has decreased by 21.9% in the last decade from 2060 ha during the Little Ice Age (LIA) to 242 ha in 2016.

Stalagmites and stalactites record long-term climate data.

The new work has been carried out by researchers from seven nations and led by the Department of Geoenvironmental Processes and Global Change, Pyrenean Institute of Ecology in Zaragoza (Spain). It presents a composite record of oxygen isotope variations during last 2500 years based on eight stalagmites from four caves in the central Pyrenees dominated by temperature variations, with precipitation playing a minor role.

Main climate drivers: Solar and volcanoes

The scientists find that the Roman Period (especially 0-200 AD), the Medieval Climate Anomaly (MCA), and part of the Little Ice Age represent the warmest periods, while the coldest decades occurred during the Dark Ages and most of the LIA intervals (520-550 AD and 1800-1850 AD). Importantly, the LIA cooling or the MCA warming were not continuous or uniform and exhibited high decadal variability. The Industrial Era shows an overall warming trend although with marked cycles and partial stabilisation during the last two decades (1990-2010).

The researchers say that the strong coherence between the speleothem data, European temperature reconstructions and global tree-ring data means that the new data tells us about the nature of past climates importantly revealing that it is solar variability and major volcanic eruptions that appear to be the two main drivers of climate change in southwestern Europe during the past 2.5 millennia.

In summary: Over the past 2500 years it was the Roman Period that was the warmest. A cold period started around 300 AD with two particularly cold events in 500-650 and 750-850. The warm and dry Medieval Climatic Anomaly was well observed as well as the Little Ice Age. Cooling was observed during the Maunder Minimum and possibly the Dalton Minimum, both periods of low solar activity. Low temperatures started to increase around 1950 and the temperature increase since then is most notable in the past 2500 years.