New study in Nature: Large variations in hydroclimate over the past millennium

A new study in Nature finds that the Northern Hemisphere has experienced considerably larger hydroclimate changes earlier during the past twelve centuries than in the twentieth century. The researchers from Sweden, Germany, and Switzerland find that climate models overestimate the increase in wet and dry extremes as temperatures increased during the twentieth century. The new results will help improve the ability of climate models to predict future hydroclimate changes.

For the first time variations in decadal to centennial-scale drought and pluvial episodes, across the Northern Hemisphere, have been reliably reconstructed back to the ninth century.

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Figure 1. At the top the trend in annual measured precipitation between 1951 and 2010 is shown from the last scientific report of the Intergovernmental Panel on Climate Change (IPCC). Below the relative hydroclimate changes for four different centuries are shown from the new hydroclimate reconstruction covering the past twelve centuries. The stronger the green colour is the wetter it was and the stronger the brown colour is the drier it was. The anomalies are shown with regard to the long-term mean between AD 1000 and 1899.

“Now we can compare hydroclimate changes in various parts of Europe, Asia, and North America for twelve centuries. Hydroclimate anomalies have been stronger and covered larger areas in some earlier centuries than during the twentieth century”, says Fredrik Charpentier Ljungqvist, historian and climate researcher at Stockholm University and lead author of the article.

The researchers reconstructed changes in water availability by statistically analysing evidence for changes in precipitation and drought. To do this they compiled hundreds of records of hydroclimate change across the Northern Hemisphere from archives including tree-rings, speleothems, lake sediments, and historical records. This is the first hemispheric-scale assessment of how a key societal resource, water availability, has fluctuated over the past twelve centuries.

To investigate the links between temperature and hydroclimate variation, the scientists compared their reconstructed hydroclimate variations with a new temperature reconstruction that they also developed. The researchers conclude that only in a few regions is it possible to see clear correlations between changes in temperature and hydroclimate. For instance, drought was most widespread during both the relatively warm twelfth century and the relatively cold fifteenth century.

“The study shows the importance of placing recent hydroclimate changes in a millennium-long perspective. Meteorological measurements of precipitation and drought are too short to tell if the observed changes today fall outside the range of natural variability. Instrumental measurements are also too short for testing the ability of state-of-the-art climate models to predict which regions of the hemisphere will get drier or wetter with global warming”, says Charpentier Ljungqvist.

Both the climate model simulations and the updated temperature reconstructions agree that the twentieth century likely was the warmest in at least the past millennium. But unlike the climate model simulations, the new hydroclimate reconstruction does not show an increase of wet and dry anomalies in the twentieth century compared to the long-term natural variations. The hydroclimate reconstruction contains uncertainties but the difference between the simulated and the reconstructed hydroclimate in the twentieth century is a robust feature and the reconstruction also agrees with meteorological measurements.

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Figure 2. The percentage of the Northern Hemisphere land area with wet and dry anomalies, respectively, during the past the twelve centuries. The stronger the green colour is the wetter it was and the stronger the brown colour is the drier it was. The anomalies are shown with regard to the long-term mean between AD 1000 and 1899.

“The climate models simulate pre-industrial hydroclimate variability reasonably well but simulate much stronger wet and dry anomalies during the twentieth century than found in the reconstruction. This does not necessarily mean the mechanisms driving hydroclimate changes in climate models are wrong. It could be that the global warming is not yet strong enough to trigger the changes in precipitation patterns that climate models simulate”, says Charpentier Ljungqvist.

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Figure 3. Drought sensitive 1000-year old trees from the mountains of Greece. Indirect recorders of past precipitation and drought variability such as tree-ring width data were used by the researchers to reconstruct twelve centuries of Northern Hemisphere hydroclimate variability. Photo: Paul J. Krusic.

The temperature reconstruction, the scientists present, is consistent with conclusions in the most recent report by the Intergovernmental Panel on Climate Change. But the new hydroclimate reconstruction suggests that it may be more challenging to predict hydroclimate changes than previously thought because of the large natural variability the researchers found. These results can, however, contribute to eventually improve the ability of climate models to better simulate future hydroclimate changes.

About the study:

The article “Northern Hemisphere hydroclimate variability over the past twelve centuries” has been published in Nature, vol. 532, pp. 94–98, doi:10.1038/nature17418.

Authors: Fredrik Charpentier Ljungqvist, Paul J. Krusic, Hanna S. Sundqvist, Eduardo Zorita, Gudrun Brattström, and David Frank

Additional information:

Fredrik Charpentier Ljungqvist, Department of History and Bolin Centre for Climate Research, Stockholm University, mobile phone: +4670-662 07 28, e-mail fredrik.c.l@historia.su.se