Fennoscandian tree-ring anatomy shows a warmer modern than medieval climate

Earth system models and various climate proxy sources indicate global warming is unprecedented during at least the Common Era(1). However, tree-ring proxies often estimate temperatures during the Medieval Climate Anomaly (950-1250 ce) that are similar to, or exceed, those recorded for the past century(2,3), in contrast to simulation experiments at regional scales(4). This not only calls into question the reliability of models and proxies but also contributes to uncertainty in future climate projections(5). Here we show that the current climate of the Fennoscandian Peninsula is substantially warmer than that of the medieval period. This highlights the dominant role of anthropogenic forcing in climate warming even at the regional scale, thereby reconciling inconsistencies between reconstructions and model simulations. We used an annually resolved 1,170-year-long tree-ring record that relies exclusively on tracheid anatomical measurements from Pinus sylvestris trees, providing high-fidelity measurements of instrumental temperature variability during the warm season. We therefore call for the construction of more such millennia-long records to further improve our understanding and reduce uncertainties around historical and future climate change at inter-regional and eventually global scales.

Automated summary

Earth system models and climate proxies indicate unprecedented global warming. However, tree-ring proxies often show temperatures during the Medieval Climate Anomaly similar to or higher than the past century, creating inconsistencies with regional simulations. This study demonstrates that the current climate of the Fennoscandian Peninsula is significantly warmer than the medieval period, highlighting the dominant role of human activities in regional climate warming. More long-term records are needed to improve understanding and reduce uncertainties in historical and future climate change at global scales.