1. European summer temperatures since Roman times

Luterbacher et al., 2016 ERL

The spatial context is critical when assessing present-day climate anomalies, attributing them to potentialforcings and making statements regarding their frequency and severity in a long-term perspective. Recentinternational initiatives have expanded the number of high-quality proxy-records and developed newstatistical reconstruction methods. These advances allow more rigorous regional past temperaturereconstructions and, in turn, the possibility of evaluating climate models on policy-relevant, spatiotemporalscales. Here we provide a new proxy-based, annually-resolved, spatial reconstruction of theEuropean summer(June–August)temperature fields back to 755 CE based on Bayesian hierarchicalmodelling (BHM), together with estimates of the European mean temperature variation since 138 BCEbased on BHM and composite-plus-scaling (CPS). Our reconstructions compare well with independentinstrumental and proxy-based temperature estimates, but suggest a larger amplitude in summertemperature variability than previously reported. Both CPS and BHM reconstructions indicate that themean 20th century European summer temperature was not significantly differentfrom some earliercenturies, including the 1st, 2nd, 8th and 10th centuries CE. The 1st century (in BHM also the 10thcentury) may even have been slightly warmer than the 20th century, but the difference is not statisticallysignificant. Comparing each 50 yr period with the 1951–2000 period reveals a similar pattern. Recentsummers, however, have been unusually warm in the context of the last two millennia and there are no30 yr periods in either reconstruction that exceed the mean average European summer temperature of thelast 3 decades(1986–2015 CE). A comparison with an ensemble of climate model simulations suggeststhat the reconstructed European summer temperature variability over the period 850–2000 CE reflectschanges 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 simulationswith 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 mayindicate inflated variability of the reconstructions, a lack of sensitivity and processes to changes in externalforcing on the simulated European climate and/or an underestimation of internal variability oncentennial and longer time scales.