Monday, 23 April 2012

Does cold kill in the 21st century?

The reason I ask is that when I was clearing out old bills, I found this in the corner of a bill dated November 2000:

The thought occurred to me that global warming might save lives.

We know that heat waves kill. We might guess that cold weather kills also – but how do the two factors balance out? If under global warming we have milder winters, would the lives saved balance out the extra deaths due to hotter summers?
This graph is rather cool. I didn’t draw it myself. Can’t find the data, or I would.
Figure 2 from Brown et al, Exploratory analysis of seasonal mortality in England and Wales, 1998 to 2007, Health Statistics Quarterly 48, Winter 2010. (Available here)
At first sight, it appears that cold kills more than hot. There is evidence of a slight increase in mortality above about 25°C, and a larger increase below about 13°C. There is a lot of scatter for the highest values (above 2000 deaths/day) – more people dying in generally cold times, but apparently independently of the temperature within that range. We might surmise that this is more to do with general winter ailments like flu rather than people freezing to death.

Data here

There has been a general decline in excess winter mortality (definition: numbers dying in December to March in excess of numbers dying in April-July and August-November). In the same period there has been a smidgeon of an increase in average central England temperatures in the four winter months (actually c.1.3°C over the 61 years). The point is, there’s a lot more going on than temperature in the winter. There’s home insulation, and social and health care, all of which may contribute saving lives to different degrees.
If you plot excess deaths against the winter temperature, you get a fairly diffuse relationship (the slope is significant, just barely, but we must be careful not to assign causality too readily - the rise in winter temperatures over the last 60 years might have a small part to play in this compared to health and social care, insulation and clean air).