Multi-Centennial internal variability in the North Atlantic will lead to additional warming over Europe in the next decades

Internal variations of climate can significantly influence global warming trends, especially at the continental scale, and could contribute to the recent abnormal observed warming over Europe. Model-based studies highlight that centennial variability of the North Atlantic can strongly affect this sector. However, a lack of high-resolution paleoclimate data does not allow a proper evaluation of the real existence of such a variability mode nor its amplitude. Here, we compile a series of annual proxy-based reconstructions over Europe from diverse sources to demonstrate and confirm the presence of such multi-centennial climate variability mode and quantify its amplitude. We show that this mode is closely tied to the internal variability of the Atlantic overturning circulation (AMOC) both in proxy-based reconstructions and climate models. When combined with instrumental observations, we show that the phase of this mode is crucial to be known. Indeed, results indicate that an internally-generated strengthening of the AMOC can explain a large part of the warming in the early 20^th century and the relative cooling in the second half of this last century. A change in phase of this mode since the early 2000s is able to explain the observed amplified warming over Europe, which is projected to persist until the 2050s. According to an observational-constraint approach, this mode of variability could amplify the forced projected warming in Northern Europe by more than 58% in the next three decades. These results underscore the importance of considering internal climate variability when assessing regional warming trends, in order to develop consistent adaptation strategies.