Climate conditions in the westernmost Mediterranean over the last millennium
Global warming is one of the most challenging threats faced by humanity nowadays, supported by an overwhelming body of scientific evidences. As an example, 2012 was the ninth-warmest year since temperatures are recorded (1880), Greenland ice sheet melted at unprecedented rate during July 2012, and CO2 daily mean concentrations reached the threshold of 400 ppm at Mauna Loa last May 2013 for the first time during the last 3 millions of years. Within this context, understanding past climate variability is essential to increase the knowledge on current climate change and to predict the Earth system responses and potential future climate scenarios.
In this regard, several time phases within the last millennium have been defined as reference for evaluating the future climate change since they are characterized by significant climatic shifts attributable to natural forcing and to some extent, to human activities. Among them, the Medieval Climate Anomaly was a warm period in the Northern Hemisphere lasting a few centuries (800-1300 yr AD) that accompanied by simultaneous prolonged droughts in North America and the Mediterranean region, and it is considered a recent “natural” analogue of the 20th century greenhouse warming. In contrast, the so called Little Ice Age (1300-1850 yr AD) was characterized by cooler conditions, when the lowest temperatures and the greatest extensions of continental and oceanic ice sheets since the last ice age were achieved.
With the aim of recognizing these climate variations, marine sediment records from the Alboran Sea basin in the westernmost Mediterranean Sea have been studied. This location is of high scientific interest due to its unique geographical, morphological, historical and climatic characteristics, that make this region particularly sensitive to climatic and anthropogenic forcing. These marine records enabled a comprehensive and high resolution reconstruction of climate responses during the last millennium. The obtained results have resulted from the combination of diverse analysis, including organic molecule proxies (called biomarkers), the geochemical and mineralogical composition of the sediments, and also sedimentological analyses.
In particular, the biomarker record revealed a gradual decline of sea surface temperatures of about 1.5-2ºC from the Medieval Climate Anomaly period to the Little Ice Age. This temperature drop was rapidly recovered during the second half of the 20th century, when sea surface temperature rose between 0.5-1.8°C.
The inorganic record revealed the Medieval Climate Anomaly and the second half of the 20th century as predominant dry periods, while humid conditions prevailed during the Little Ice Age and industrial times. These changing conditions at centennial scales were promoted by solar irradiance variations (higher during medieval times and lower during the Little Ice Age) and the modulation of the NAO in the Mediterranean region.
The NAO alternated between a ‘‘high-index’’ pattern, characterized by stronger westerlies transporting storms farther to the north and resulting in dry winters in the Mediterranean and northern Africa during the Medieval Climate Anomaly, and a “low-index” linked to opposite trends during the Little Ice Age. Anthropic contribution has been also evidenced in this region by the unprecedented temperatures achieved during the second half of the 20th century.
Biodiversity and Climate Research Centre (Frankfurt am Main, Germany), BiK-F.
Climate conditions in the westernmost Mediterranean over the last two millennia: an integrated biomarker approach. V. Nieto-Moreno, F. Martínez-Ruiz, V. Willmott, J. García-Orellana, P. Masqué, and J.S. Sinninghe Damsté. Organic Geochemistry, 55:1-10, 2013.
Climate imprints during the Medieval Climate Anomaly and the Little Ice Age in marine records from the Alboran Sea basin. V. Nieto-Moreno, F. Martínez-Ruiz, S. Giralt, D. Gallego-Torres, J. García-Orellana, P. Masqué, and M. Ortega-Huertas. The Holocene, 2013 (in press).