What was done
The authors note that both the number and intensity of heavy precipitation events are projected to increase in a warming world, according to the Intergovernmental Panel on Climate Change.  Hence, they investigate this climate-model-derived hypothesis with digitalized hourly precipitation data recorded at the Tokyo Observatory of the Japan Meteorological Agency for the period 1890-1999.  Whereas most prior studies of this subject have been based on daily totals of precipitation, however, Kanae et al. focus on hourly observations, saying "the hourly record is more valid in the investigation of precipitation over warm humid Asia where heavy precipitation in a short duration embarrasses the lives of the highly populated people mostly living in the cities on very narrow and fragile alluvial plains."

What was learned
Kanae et al. report that "many hourly heavy precipitation events (above 20 mm/hour) occurred in the 1990s compared with the 1970s and the 1980s," and that against that backdrop, "the 1990s seems to be unprecedented."  However, they note that "hourly heavy precipitation around the 1940s is even stronger/more frequent than in the 1990s."  In fact, their plots of both maximum hourly precipitation and the number of extreme hourly precipitation events rise fairly regularly from the 1890s to peak in the 1940s, after which declines set in that bottom out in the 1970s and then reverse to rise to endpoints in the 1990s that are not yet as high as the peaks of the 1940s.

What it means
The interpretation of these results depends strongly upon what one believes about the temperature history of the world.  If one accepts the IPCC-inspired climate-alarmist claim that the 1990s were the warmest decade of the past one to two millennia (Mann et al., 1998, 1999; Mann and Jones, 2003), one must conclude that the extreme precipitation predictions are seriously in error - at least for the site in question, although we note that many papers we have reviewed from all around the world also fail to support the climate-alarmist contention of unprecedented extreme rainfall during the 1990s [see Weather Extremes (Precipitation - Trends) in our Subject Index].  However, if one believes that global mean temperature has yet to eclipse the values it exhibited in the 1930s and 40s, as we do, the prediction of increasing extreme precipitation in response to rising temperature is right on the money ? which raises an important question: do the heavy precipitation data of this and the many other studies we have reviewed argue for the validity of our view of the subject, i.e., that the mean global temperature of today is no greater than it was back in the 1930s and 40s?  This intriguing "fingerprint" of non-human-induced, i.e., natural, climate change is clearly worthy of further investigation.

References
Mann, M.E., Bradley, R.S. and Hughes, M.K.  1998.  Global-scale temperature patterns and climate forcing over the past six centuries.  Nature 392: 779-787.

Mann, M.E., Bradley, R.S. and Hughes, M.K.  1999.  Northern Hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations.  Geophysical Research Letters 26: 759-762.

Mann, M.E. and Jones, P.D.  2003.  Global surface temperatures over the past two millennia.  Geophysical Research Letters 30: 10.1029/2003GL017814.