What was done
The authors evaluated seasonal trends in deaths due to various diseases in Japan, using nationwide vital statistics from 1970 to 1999 and recorded weather data, specifically, mean monthly temperature.

What was learned
Japan, to quote Nakaji et al., has "bitterly cold winters," and their analysis indicates that the numbers of deaths due to infectious and parasitic diseases including tuberculosis, respiratory diseases including pneumonia and influenza, diabetes, digestive diseases and cerebrovascular and heart diseases rise to a maximum during that cold time of year.  Of the latter two categories, in fact, they found that peak mortality rates due to heart disease and stroke were 1.5 to 2 times greater in winter (January) than what they were at the time of their yearly minimums (August and September).

What it means
The team of nine scientists says that "to reduce the overall mortality rate and to prolong life expectancy in Japan, measures must be taken [our italics] to reduce those mortality rates associated with seasonal differences."  They also say "it has long been recognized that cold temperature acts as a trigger for coronary events," and that "major infectious diseases are epidemic in winter."  Hence, it is clear that to achieve the scientists' stated objectives, it is necessary to bring about a "reduction in exposure to cold environments," as they put it, which is precisely what global warming does, and what it does best when it warms more in winter than in summer, as Nakaji et al. have demonstrated to be the case in Japan, where winter warming over the past 30 years was twice as great as what it was during the rest of the year.