A new drought map of Europe and the Mediterranean has documented historically how the geographical changes evolved over the past 2,000 years based on tree rings.
Incorporating the previous atlases covering North America and Asia, the Old World Drought Atlas significantly adds to the historical picture of long-term climate variability over the Northern Hemisphere to help climate scientists pinpoint causes of drought and extreme rainfall in the past, and identify patterns that could lead to better climate model projections for the future.
In a research paper published in the journal Science Advances, scientists from 40 institutions. “The Old World Drought Atlas fills a major geographic gap in the data that’s important to determine patterns of climate variability back in time,” said Edward Cook, cofounder of the Tree Ring Lab at Columbia University’s Lamont-Doherty Earth Observatory, and leader of all three drought-atlas projects.
For example, if Europe had a wet year north of the Alps and a dry year to the south, that provides clues to circulation patterns and suggests influence from the North Atlantic Oscillation, one of the primary sources of climate variability affecting patterns in Europe.
The new atlas could also improve understanding of climate phenomena like the Atlantic Multi-decadal Oscillation, a variation in North Atlantic sea-surface temperatures that hasn’t been tracked long enough to tell if it is a transitory event, forced by human intervention in the climate system, or a natural long-term oscillation.
By combining the Old World Drought Atlas with the Asia and North America atlases, climatologists and climate modelers may also discover other sources of internal climate variability that are leading to drought and wetness across the Northern Hemisphere, Cook said.
In the Science Advances paper, Cook and his coauthors compare results from the new atlas and its counterparts across three time spans: the generally warm Medieval Climate Anomaly (1000-1200); the Little Ice Age (1550-1750); and the modern period (1850-2012).
The atlases together show persistently drier-than-average conditions across north-central Europe over the past 1,000 years, and a history of megadroughts in the Northern Hemisphere that lasted longer during the Medieval Climate Anomaly than they did during the 20th century.
But the authors also said it is difficult to reproduce megadroughts of the past, indicating something may be missing in their representation of the climate system, Cook said.
The drought atlases provide a much deeper understanding of natural climate processes than scientists have had to date, said Richard Seager, a coauthor of the paper and a climate modeler at Lamont-Doherty Earth Observatory. “By having tree ring-based hydroclimate reconstructions for three northern hemisphere continents, we can now easily see these patterns.”
In Europe and the Mediterranean, the new drought atlas expands scientists’ understanding of historic famines.
For instance, an unusually cold winter and spring are often blamed for a 1740-1741 famine in Ireland. The Old World Drought Atlas points to another contributor: rainfall well below normal during the spring and summer of 1741, wrote the authors.
The atlas also tracks the reach of the great European famine of 1315-1317 due to excessive precipitation made it difficult to grow food. The atlas shows yearly progressions from 1314 to 1317, including drier conditions in southern Italy, that had largely escaped the crisis.
The North America atlas, published in 2004, pointed out that a series of droughts starting around 900 years ago may have contributed to the eventual collapse of native cultures. Similarly, the Asia atlas, published in 2010, has led researchers to connect droughts to the fall of Cambodia’s Angkor culture in the 1300s, and China’s Ming dynasty in the 1600s.
The tree ring data was gleaned from both living trees and timbers found in ancient construction reaching back more than 2,000 years — 106 regional tree ring chronologies, each with dozens to thousands of trees, and were contributed to the project by the International Tree Ring Data Bank and European tree-ring scientists.