What was done
The authors "carried out time-series analyses in 12 U.S. cities to estimate both the acute effects and the lagged influence of weather on respiratory and cardiovascular disease (CVD) deaths."  The weather parameters studied were temperature and humidity.  The cities were Atlanta, Georgia; Birmingham, Alabama; Canton, Ohio; Chicago, Illinois; Colorado Springs, Colorado; Detroit, Michigan; Houston, Texas; Minneapolis-St. Paul, Minnesota; New Haven, Connecticut; Pittsburgh, Pennsylvania; and Seattle and Spokane, Washington.  These cities were divided into two groups : hot (Atlanta, Birmingham and Houston) and cold (all the rest).

What was learned
The authors could detect no clear evidence for a humidity effect on respiratory or CVD deaths.

In the hot cities, they report that "neither hot nor cold temperatures had much effect on CVD or pneumonia deaths," although for the sub-categories of chronic obstructive pulmonary disease and myocardial infarctions there were some lagged effects.

In the cold cities, on the other hand, the authors report that "both high and low temperatures were associated with increased CVD deaths," with the effect of cold temperatures persisting for days but the effect of high temperatures restricted to the day of the death or the day before.  Of particular interest was the finding that "for all CVD deaths the hot-day effect was five times smaller than the cold-day effect."  In addition, the hot-day effect included some "harvesting," where the authors "observed a deficit of deaths a few days later," which they say they "did not observe for the cold-day effect."

Another important finding of the study was "the substantial [respiratory] mortality increase in cities with more variable temperature."  This phenomenon, say the authors, "suggests that increased temperature variability is the most relevant change in climate for the direct effects of weather on respiratory mortality."

What it means
What was the impetus for the authors' study?  As they describe it, they were concerned about "the rapid buildup of greenhouse gases" that "is expected to increase both mean temperature and temperature variability around the world."

With respect to their concern about the expected increase in mean temperature, their findings clearly demonstrate that, in a warming world, the number of lives saved as a consequence of the alleviation of the cold-day killing effect likely will far exceed the number of lives lost as a consequence of the intensification of the hot-day killing effect.  With respect to their concern about weather becoming more variable and thereby leading to increased temperature related deaths, we note that this concern is based on a patently false assumption.  Robeson (2002), for example, has clearly demonstrated - from a 50-year study of daily temperatures at over 1,000 U.S. weather stations - that temperature variability declines with warming, and at a very substantial rate, so that this aspect of weather in a warmer world would lead to a great reduction in temperature related deaths at both the high and low end of the daily temperature spectrum.  Hence, as we have said so many times in reviewing scientific papers describing studies of this nature, warmer is better... much better ... than colder.

Reference
Robeson, S.M.  2002.  Relationships between mean and standard deviation of air temperature: implications for global warming.  Climate Research 22: 205-213.