Simulated and reconstructed European summer land temperature anomalies (with respect to 1500–1850 CE) for the last 1200 yr, smoothed with a 31 yr moving average filter. BHM (CPS) reconstructed temperatures are shown in blue (red) over the spread of model runs. Simulations are distinguished by solar forcing: stronger (SUNWIDE, purple; TSI change from the LMM to present >0.23%) and weaker (SUNNARROW, green; TSI change from the LMM to present <0.1%). The ensemble mean (heavy line) and the two bands accounting for 50% and 80% (shading) of the spread are shown for the model ensemble. Graphic: Luterbacher, et al., 2016 / Environmental Research Letters

1 February 2016 (IOP) – Most of Europe has experienced strong summer warming over the course of the past several decades, accompanied by severe heat waves in 2003, 2010 and 2015. New research now puts the current warmth in a 2100-year historical context using tree ring information and historical documentary evidence to derive a new European summer temperature reconstruction.

The work was published in the journal of Environmental Research Letters by a group of 45 scientists from 13 countries.

Warm summers were experienced during Roman times, up to the 3rd century, followed by generally cooler conditions from the 4th to the 7th centuries. A generally warm medieval period was followed by a mostly cold Little Ice Age from the 14th to the 19th centuries. The pronounced warming early in the 20th century and in recent decades is well captured by the tree ring data and historical evidence on which the new reconstruction is based.

The evidence suggests that past natural changes in summer temperature are larger than previously thought, implying that climate models may underestimate the full range of future extreme events, including heat waves. This past variability has been associated with large volcanic eruptions and changes in the amount of energy received from the sun.

The new research finding that temperatures over the past 30 years lie outside the range of these natural variations supports the conclusions reached by the International Panel on Climate Change (IPCC) that recent warming is mainly caused by anthropogenic activity.

“We now have a detailed picture of how summer temperatures have changed over Europe for more than two thousand years and we can use that to test the climate models that are used to predict the impacts of future global warming,” says the coordinator of the study, Professor Jürg Luterbacher from the University of Giessen in Germany.

The interdisciplinary study involved the collaboration of researchers from Past Global Changes’ (PAGES) original European 2k Network working group, Euro-Med2k. PAGES, a core project of Future Earth, is funded by the US and Swiss National Science Foundations and the US National Oceanic and Atmospheric Administration.

Recent summer temperatures in Europe are likely the warmest of the last two millennia


Simulated and reconstructed summer (June–August) temperature differences for three periods: (a), (b), (c) MCA (900–1200 CE) minus LIA (1250–1700 CE); (d), (e), (f) present (1950–2003 CE) minus MCA; and (g), (h), (i) present minus LIA. Model temperature differences (left and central columns) indicate average temperature changes in the ensemble of available model simulations (see table S13). Model simulations are grouped into SUNWIDE (TSI change from the LMM to present >0.23%; left column) and SUNNARROW (TSI change from the LMM to present <0.1%; middle column). Reconstructed temperature differences with the BHM method are shown in the right column. Simulations have been weighted by the number of experiments considered from each model. Dots indicate significant (p < 0.05) changes in the reconstruction; in the simulation ensemble a dot indicates at least 80% of agreement in depicting significant (p < 0.05) changes of the same sign. Graphic: Graphic: Luterbacher, et al., 2016 / Environmental Research Letters

ABSTRACT: The spatial context is critical when assessing present-day climate anomalies, attributing them to potential forcings and making statements regarding their frequency and severity in a long-term perspective. Recent international initiatives have expanded the number of high-quality proxy-records and developed new statistical reconstruction methods. These advances allow more rigorous regional past temperature reconstructions and, in turn, the possibility of evaluating climate models on policy-relevant, spatio-temporal scales. Here we provide a new proxy-based, annually-resolved, spatial reconstruction of the European summer (June–August) temperature fields back to 755 CE based on Bayesian hierarchical modelling (BHM), together with estimates of the European mean temperature variation since 138 BCE based on BHM and composite-plus-scaling (CPS). Our reconstructions compare well with independent instrumental and proxy-based temperature estimates, but suggest a larger amplitude in summer temperature variability than previously reported. Both CPS and BHM reconstructions indicate that the mean 20th century European summer temperature was not significantly different from some earlier centuries, including the 1st, 2nd, 8th and 10th centuries CE. The 1st century (in BHM also the 10th century) may even have been slightly warmer than the 20th century, but the difference is not statistically significant. Comparing each 50 yr period with the 1951–2000 period reveals a similar pattern. Recent summers, however, have been unusually warm in the context of the last two millennia and there are no 30 yr periods in either reconstruction that exceed the mean average European summer temperature of the last 3 decades (1986–2015 CE). A comparison with an ensemble of climate model simulations suggests that the reconstructed European summer temperature variability over the period 850–2000 CE reflects changes in both internal variability and external forcing on multi-decadal time-scales. For pan-European temperatures we find slightly better agreement between the reconstruction and the model simulations with high-end estimates for total solar irradiance. Temperature differences between the medieval period, the recent period and the Little Ice Age are larger in the reconstructions than the simulations. This may indicate inflated variability of the reconstructions, a lack of sensitivity and processes to changes in external forcing on the simulated European climate and/or an underestimation of internal variability on centennial and longer time scales.

European summer temperatures since Roman times

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