Climate model assumptions on global warming related changes in wet and dry extremes have been challenged by 1200 years of water balance data showing that the Northern Hemisphere has experienced considerably larger variations in precipitation during the past twelve centuries than in the twentieth century.
Here we publish news releases from the University of Stockholm and from the Swiss Federal Research Institute WSL describing the results of research published in Nature.
Large variations in precipitation over the past millennium
From Stockholm University
According to a new study in Nature, the Northern Hemisphere has experienced considerably larger variations in precipitation during the past twelve centuries than in the twentieth century. Researchers from Sweden, Germany, and Switzerland have found that climate models overestimated the increase in wet and dry extremes as temperatures increased during the twentieth century. The new results will enable us to improve the accuracy of climate models and to better predict future precipitation changes.
Variations in decadal to centennial-scale drought and pluvial episodes – across the Northern Hemisphere – have been reliably reconstructed back to the ninth century for the first time.
“Now we can compare precipitation changes in various parts of Europe, Asia, and North America for the past twelve centuries. Precipitation anomalies have been stronger and covered larger areas in some earlier centuries than during the twentieth century”, according to Fredrik Charpentier Ljungqvist, historian and climate researcher at Stockholm University and lead author of this study.
The researchers reconstructed changes in water availability by statistically analysing evidence for changes in precipitation and drought. To do this the researchers compiled hundreds of records of precipitation 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 precipitation variations, the researchers compared their reconstructed precipitation variations with a temperature reconstruction which also was developed by the team. They conclude that it is possible to see clear correlations between variations in temperature and precipitation only in a few specific regions. For instance, during both the relatively warm twelfth century, and the relatively cold fifteenth century, drought was observed to be most widespread in the Northern Hemisphere.
“The study shows the importance of placing recent precipitation changes in a millennium-long perspective. Actual measurements of precipitation are too short to tell if the observed changes today fall outside the range of natural variability. Instrumental measurements are also too short to test 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 was likely the warmest in at least the past millennium. However, unlike the climate model simulations, the new precipitation reconstruction does not show an increase of wet and dry anomalies in the twentieth century compared to the natural variations of the past millennium. The precipitation reconstruction contains some uncertainties. Nevertheless the difference between the simulated and the reconstructed precipitation in the twentieth century is a robust feature and the reconstruction also agrees with meteorological measurements.
“The climate models simulate pre-industrial precipitation variability reasonably well but simulate much stronger wet and dry anomalies during the twentieth century than those found in the reconstruction. This does not necessarily mean the mechanisms driving precipitation changes in climate models are wrong. The explanation for this could be that the global warming is not yet strong enough to trigger the changes in precipitation patterns that climate models simulate”, reports Charpentier Ljungqvist.
The temperature reconstruction presented, is in good agreement with the conclusions from the most recent report by the Intergovernmental Panel on Climate Change. Contrarily, the new precipitation reconstruction suggests that it is much harder to predict precipitation changes than previously thought. However, it is concluded that these new results can eventually improve the ability of climate models to better predict future precipitation changes.
End of Stockholm University news release.
1200 years of water balance data challenge climate models
From the Swiss Federal Research Institute WSL
Water availability in the Northern Hemisphere has seen much larger changes during the past twelve centuries than during twentieth century global warming, a new study in Nature reports. The team with participation of the Swiss Federal Research Institute WSL concludes that climate models overestimate wet and dry extremes as temperatures increased during the twentieth century. The new results can help to improve the ability of climate models to predict future hydroclimate changes.
The researchers from Sweden, Germany, and Switzerland have for the first time reconstructed the variations in water availability across the Northern Hemisphere seamless for the past twelve centuries. This allows for comparisons between various parts of Europe, Asia, and North America.
The study shows that hydroclimate extremes have been stronger and covered larger areas in some earlier centuries than during the twentieth century, explains lead author Fredrik Charpentier Ljungqvist from Stockholm University.
Water availability through the centuries
The researchers reconstructed changes in water availability – the hydroclimate – by statistically analysing evidence for changes in precipitation and drought, such as varying lake levels, soil humidity or river runoff. To do this the researchers compiled hundreds of records from various climate archives across the Northern Hemisphere: tree-rings, speleothems, lake sediments, and written historical records.
The scientists compared their reconstructed hydroclimate variations with a new temperature reconstruction they also developed, to understand links between the two. It turned out that only a few regions showed 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.
Test for climate models
According to Ljungqvist, the study shows the importance of a millennium-long perspective on hydroclimate changes. Meteorological measurements of precipitation and drought date back at most a few centuries – too short to tell if today’s changes fall outside the range of natural variability. “They are also too short to test if climate models are correct when predicting that drier regions get drier and wetter regions get wetter with global warming”, says Ljungqvist.
Both the new temperature reconstruction and the climate model simulations agree that the twentieth century likely was the warmest in at least the past millennium. “But unlike the climate models our reconstruction does not show any dramatic increase in hydroclimate extremes”, says Ljungqvist.
“The current models seem to simulate precipitation characteristics well over the pre-industrial period, but do not appear to capture as well the more recent anthropogenically driven changes in the climate system,” explaines co-author David Frank from WSL. More research is needed to find out why, he says.
This does not necessarily mean the mechanisms driving hydroclimate changes in climate models are wrong, points out Ljungqvist. “It could be that the global warming is not yet strong enough to trigger the changes in precipitation patterns that climate models simulate”. The authors conclude that while the reconstructions based on climate archive data also have their uncertainties, they are still a very valuable tool to test models used to predict future changes in water availability.
End of the Swiss Federal Research Institute WSL news release.
Ljungqvist, Fredrik Charpentier, Krusic, Paul J., Sundqvist, Hanna S., Zorita, Eduardo, Brattström, Gudrun & Frank, David; Northern Hemisphere hydroclimate variability over the past twelve centuries; Nature 532,94–98doi:10.1038/nature17418
University of Stockholm news release.