This study demonstrates that the intensity of the East Asian summer and winter monsoon is the primary driver of variations of summer and winter precipitation isotopes in central Japan. Japan lies in the northeast limits of the East Asian monsoon region. Understanding the past monsoon changes in Japan is important for determining whether the isotopic variability recorded in Chinese stalagmite reflects the East Asian summer monsoon intensity or rainfall variability in the Indian summer monsoon.

Observational data sets contain uncertainties, e.g. from the instrument accuracy, as well as from the fact that usually only a single method is used in processing. We have performed an assessment of the size of the uncertainties associated with choices in the method used. The largest effects come from changes which affect the station network or the gridding method used. However, for the temperature indices in places with many stations, these changes have little effect on the long-term behaviour.

We have developed HadISDH, a new gridded global land monthly mean climate montitoring product for humidity and temperature from 1973 to then end of 2013 (updated annually) based entirely on in situ observations. Uncertainty estimates are provided. Over the period of record significant warming and increases in water vapour have taken place. The specific humidity trends have slowed since a peak in 1998 concurrent with decreasing relative humidity from 2000 onwards.