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.
Predictions of unprecedented rainfall extremes in the 20th century driven by global warming turned out wrong, a study said Wednesday, casting doubt on methods used to project future trends. A massive trawl of Northern Hemisphere rainfall data for the last 1,200 years revealed there had been more dramatic wet-dry weather extremes in earlier, cooler centuries before humans set off fossil fuel-driven global warming.
This is problematic, said a study in the journal Nature, as the same data models used to anticipate that global warming would cause record rainfall extremes in the 1900s, are the basis for projections of things to come. “It might be more difficult than often assumed to project into the future,” the study’s lead author Fredrik Ljungqvist of Stockholm University told AFP of the findings.
“The truth can be much, much more complicated.”
The UN’s climate science panel, the consensus authority, contends that dry areas will become ever drier and wet ones wetter as the global temperature rises in response to greenhouse gas emissions.
But the new work said sky-high temperatures in the 20th century did not directly translate into record extremes between wet and dry weather, as many had expected. This meant that “much of the change is not only driven by temperature, but some internal, more random variability,” explained Ljungqvist.
“It’s therefore very, very hard also to predict (precipitation extremes) with models.”
Over the study period, drought was most severe during the 12th century, which was a warm one, and the 15th which was cold, said the scientist. For the study, a team of experts in history, climate, geology and mathematics, compiled drought and rainfall data for Europe, North Asia and North America, and reconstructed 12 centuries worth of “water history”.
They considered geologically preserved evidence of stream flow, lake levels, marine and lake sediments, tree rings and historical records. The team’s reconstruction for the 20th century differed vastly from climate models which had suggested wet areas should have been wetter, and dry ones drier, than ever before. “In the past, on a longer timescale, there have been even larger variabilities,” said Ljungqvist. This divergence “certainly adds fuel to the fiery debate” on the link between warming and rainfall extremes, Matthew Kirby of California State University’s Department of Geological Sciences wrote in a comment published by Nature.
“Do their results invalidate current predictive models? Certainly not. But they do highlight a big challenge for climate modellers, and present major research opportunities both for modellers and climate scientists…”
James Renwick of the Victoria University of Wellington said the predicted wet and dry extremes are “very likely” to materialise in the 21st, century. Extreme drought and downpours are among many risks that scientists warn about in a warmer world. Others include land-gobbling sea level rise, crop and water shortages, disease spread and wars over dwindling resources.
Large variations in precipitation over the past millennium
- April 6, 2016
- Stockholm University
- According to a new study, 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.
- Fredrik Charpentier Ljungqvist, Paul J. Krusic, Hanna S. Sundqvist, Eduardo Zorita, Gudrun Brattström, David Frank. Northern Hemisphere hydroclimate variability over the past twelve centuries. Nature, 2016; 532 (7597): 94 DOI: 10.1038/nature17418
Accurate modelling and prediction of the local to continental-scale hydroclimate response to global warming is essential given the strong impact of hydroclimate on ecosystem functioning, crop yields, water resources, and economic security1, 2, 3, 4. However, uncertainty in hydroclimate projections remains large5, 6, 7, in part due to the short length of instrumental measurements available with which to assess climate models. Here we present a spatial reconstruction of hydroclimate variability over the past twelve centuries across the Northern Hemisphere derived from a network of 196 at least millennium-long proxy records. We use this reconstruction to place recent hydrological changes8, 9 and future precipitation scenarios7, 10, 11 in a long-term context of spatially resolved and temporally persistent hydroclimate patterns. We find a larger percentage of land area with relatively wetter conditions in the ninth to eleventh and the twentieth centuries, whereas drier conditions are more widespread between the twelfth and nineteenth centuries. Our reconstruction reveals that prominent seesaw patterns of alternating moisture regimes observed in instrumental data12, 13, 14 across the Mediterranean, western USA, and China have operated consistently over the past twelve centuries. Using an updated compilation of 128 temperature proxy records15, we assess the relationship between the reconstructed centennial-scale Northern Hemisphere hydroclimate and temperature variability. Even though dry and wet conditions occurred over extensive areas under both warm and cold climate regimes, a statistically significant co-variability of hydroclimate and temperature is evident for particular regions. We compare the reconstructed hydroclimate anomalies with coupled atmosphere–ocean general circulation model simulations and find reasonable agreement during pre-industrial times. However, the intensification of the twentieth-century-mean hydroclimate anomalies in the simulations, as compared to previous centuries, is not supported by our new multi-proxy reconstruction. This finding suggests that much work remains before we can model hydroclimate variability accurately, and highlights the importance of using palaeoclimate data to place recent and predicted hydroclimate changes in a millennium-long context16, 17.
Added: LLNL found the same thing last year, see: https://www.llnl.gov/news/climate-models-overestimate-rainfall-increases