Growing season temperatures in Europe and climate forcings over the past 1400 years.
BACKGROUND
The lack of instrumental data before the mid-19th-century limits our understanding of present warming trends. In the absence of direct measurements, we used proxies that are natural or historical archives recording past climatic changes. A gridded reconstruction of spring-summer temperature was produced for Europe based on tree-rings, documentaries, pollen assemblages and ice cores. The majority of proxy series have an annual resolution. For a better inference of long-term climate variation, they were completed by low-resolution data (decadal or more), mostly on pollen and ice-core data.


RESULTS
An original spectral analog method was devised to deal with this heterogeneous dataset, and to preserve long-term variations and the variability of temperature series. So we can replace the recent climate changes in a broader context of the past 1400 years. This preservation is possible because the method is not based on a calibration (regression) but on similarities between assemblages of proxies. The reconstruction of the April-September temperatures was validated with a Jack-knife technique. It was also compared to other spatially gridded temperature reconstructions, literature data, and glacier advance and retreat curves. We also attempted to relate the spatial distribution of European temperature anomalies to known solar and volcanic forcings.


CONCLUSIONS
We found that our results were accurate back to 750. Cold periods prior to the 20(th) century can be explained partly by low solar activity and/or high volcanic activity. The Medieval Warm Period (MWP) could be correlated to higher solar activity. During the 20(th) century, however only anthropogenic forcing can explain the exceptionally high temperature rise. Warm periods of the Middle Age were spatially more heterogeneous than last decades, and then locally it could have been warmer. However, at the continental scale, the last decades were clearly warmer than any period of the last 1400 years. The heterogeneity of MWP versus the homogeneity of the last decades is likely an argument that different forcings could have operated. These results support the fact that we are living a climate change in Europe never seen in the past 1400 years.
Guiot
Joel
J
European Centre of Research and Teaching in Geosciences of Environment, UMR 6635 Aix-Marseille University/CNRS, Aix-en-Provence, France. guiot@cerege.fr
Corona
Christophe
C
D002980:Climate; D057231:Climate Change; D003625:Data Collection; D005060:Europe; D049668:History, 15th Century; D049669:History, 16th Century; D049670:History, 17th Century; D049671:History, 18th Century; D049672:History, 19th Century; D049674:History, 21st Century; D049691:History, Medieval; D007053:Ice; D011058:Pollen; D011996:Records; D012621:Seasons; D019445:Solar Activity; D013696:Temperature; D014197:Trees
D007053:Ice