Past 2,000 years of Iberian Peninsula climate reconstructed

Participating in the study were researchers from ICTJA-CSIC, ICTA-UAB, CREAF and the University of Barcelona (UB). The article, published in the journal Quaternary Science Reviews, represents "a significant advance in understanding the climatic mechanisms which have controlled part of our history", says lead author and ICTJA-CSIC researcher Guiomar Sánchez.

To be able to reconstruct the climate conditions, during the winter of 2012 the authors collected nine samples of sediments of the Cimera Lake in the Sierra de Gredos mountain range, a glacial lake located at 2,140 metres above sea level. According to Sánchez, "a lake such as ofof Cimera allows us to study the variations in climate of the past 2,000 years without the disturbances of human activities. The climate signal obtained was the least possibly altered one".

"We analysed the samples collected with a temporal resolution of ten years, thereby obtaining a series of highly precise data with which to characterise different climate periods and the main factors which regulated them", the researcher explains.

Marked Climatic Variations in the Centre of the Peninsula

The results obtained reveal that during the Roman period (200 BCE - 500 CE) there were alternating warm and cold periods in the centre of the Peninsula, with warm and humid winters and also warm summers. During the Early Middle Ages (500-900 CE) there was a change towards colder conditions, with cold and dry winters and warm summers.

During the Medieval Climatic Anomaly (900-1300 CE), climate in the Iberian central range was characterised by warm and dry winters and warm summers. In the following period, the Little Ice Age (1300-1850 CE), the opposite occurred: winters were humid and cold and summers were also cold.

The authors also characterised the industrial era period, ranging from 1850 to 2012. During this last period the effects of climate change have had an influence on the behaviour of the lake and this has been made evident in the sediments analysed. Guiomar Sánchez indicates that "the climate we know is much drier and warmer".

The Influence of NAO and EA in the Iberian climate

This study, integrated into earlier climatic reconstructions of other areas of the Peninsula, has allowed researchers to establish the precise influence of the main patterns of atmospheric circulation: the North Atlantic Oscillation (NAO) and the East Atlantic (EA), fundamental in understanding the dynamics of rainfall and peninsular temperatures. The NAO and EA show two phases, one positive and one negative.

The negative phase of NAO is characterised by more humid winters in southern Europe, while its positive phase causes drier winters. In contrast, the negative phase of the EA produces colder summers than those seen in its positive phase. The interaction of the two when in the same phase results in a more homogeneous climate in the peninsula, while the interaction of two opposite phases brings about more variability in the climate.

The gradients of humidity observed during the Roman period and the Early Middle Ages indicate an interaction between opposite phases of NAO and EA, while the absence of these gradients during the Medieval Climatic Anomaly and the Little Ice Age show that the two were acting in the same phase.

"Although the dominating climatic conditions during the winters and summers of these past 2,000 years are quite known, this study has allowed us to determine the role NAO and EA have had in establishing these conditions. Until now no one had determined this with such precision", adds Santiago Giralt, researcher at the ICTJA-CSIC and co-author of the study.

Conducting the study were scientists from the ICTJA-CSIC, the Faculty of Science in Lisbon, the CREAF, the Centre for Hydrographic Studies (CEDEX), the Sierra de Guadarrama National Park, the Faculty of Geology of the University of Barcelona and the ICTA-UAB, as well as researchers from the Edith Cowan University, Australia, and the University of Western Australia.

Original article: Sánchez-López, G., et al. (2016) "Climate Reconstruction for the Last Two MILLENNIA in Central Iberia: The Role of East Atlantic (EA), North Atlantic Oscillation (NAO) and their Interplay over the Iberian Peninsula." Quaternary Science Reviews, 149, 135-150, DOI: http://dx.doi.org/10.1016/j.quascirev.2016.07.021