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This paper examines the characteristics of the heatwave that affected western and central Europe in June 2017. Using a novel algorithm, we show that its extension, intensity, and persistence were comparable to those of other European mega‐heatwaves, but it occurred earlier in the summer. The most affected area was Iberia, which experienced devastating forest fires with human casualties and the warmest temperatures of the reanalysis period from daily to seasonal scales. The peak of the mega‐heatwave displayed an unprecedented warm air intrusion due to a record‐breaking subtropical ridge with signatures closer to those of July and August. The atmospheric circulation was the main triggering factor of the event. However, thermodynamical changes of the last decades made a substantial contribution to the event, by increasing the likelihood of surpassing high‐temperature thresholds. This episode could be a good example of a coming future, with high‐summer mega‐heatwaves occurring earlier., This work was supported by the Ministerio de Economía y Competitividad through the PALEOSTRAT (CGL2015‐69699‐R) and ISIPEDIA (ERA4CS) (PCIN‐2017‐046) projects. Ricardo M. Trigo and Pedro M. Sousa are supported by project IMDROFLOOD: Improving Drought and Flood Early Warning, Forecast and Mitigation using real‐time hydroclimatic indicators, JPND‐WaterJPI/0004/2014, financed by FCT/MCTES (PIDDAC). A. Sánchez‐Benítez was funded by grant FPU15/03958 from the Ministerio de Educación, Cultura y Deporte (MECD).

Ships' logbooks have been preserved in archives of different European countries. This paper reviews how their records provide reliable information relevant to meteorology and climatology, extending the observational record back to at least the early 18th century. This allows describing weather during historical events, improving the knowledge on hurricanes or unveiling multidecadal variability previously unsuspected, such as the steady enhancement of the Australian monsoon, the high variability of the atmospheric circulation over the Euro‐Atlantic region during the Late Maunder Minimum or the relationship between the Western North Pacific Summer Monsoon and the El Niño—Southern Oscillation. Observations from ships can feed long‐term reanalysis projects and contribute to reduce their uncertainties over the oceans. The extended record of observations also aids the search of analogues before the human fingerprint, thus improving the detection and attribution of climate change. The integration with paleoclimate proxies is a complex task that requires merging heterogeneous records with a wide range of time resolutions, spatial density, and responses to the climate system. However, recent international efforts open the field to new opportunities. Summing up, logbooks are a consistent, but underexploited, source of relevant climatic data that will widen our knowledge of the past climate. This in turn provides a way to better understand present climatic variations and predict future changes., This research was funded by the Spanish Ministerio de Economía y Competitividad through the projects CGL2013‐44530‐P, CGL2014‐51721‐REDT, CGL2015‐69699‐R, and CGL2015‐72164‐EXP/AEI.