To the Editor:
In the Netherlands, annual cold-related mortality is higher than heat-related mortality. The projected change in climate will influence both. 1 We analyzed the mortality changes related to the gradual influences of anthropogenic climate changes on health risk. As an anthropogenic climate change is likely to change the frequency or severity of heat waves and cold spells, we specifically looked at the effects of climate change on marked short-term fluctuations in weather.
In an earlier paper, 2 we described the daily numbers of deaths due to all causes and to selected causes (malignant neoplasms, respiratory disease, and cardiovascular disease) in relation to daily average temperature in the Netherlands from 1979 to 1997. We found that for each degree Celsius increase above the optimum temperature, mortality increased by 0.5% for malignant neoplasms, 1.9% for cardiovascular disease, 12.8% for respiratory diseases and 2.7% for total mortality. (Optimum temperature was 15.5°C for malignant neoplasms and 16.5°C for the other causes of death.) For temperatures below the optimum, mortality in the same categories decreased by 0.2%, 1.7%, 5.2%, and 1.4% for each degree Celsius increase. 2
In addition, there were substantial increases in mortality during both the cold spells and heat waves in the study period. 2 Average total excess mortality during the heat waves was 389.8 per heat wave (or 39.8 deaths per heat wave day), with death rates in the elderly and due to respiratory disease most affected. The average excess mortality during the cold spells was 718.5 (or 46.6 deaths per cold spell day), which was mostly attributable to the increase in cardiovascular mortality among the elderly. The results concerning the forward displacement of deaths due to heat waves were not conclusive. We found no cold-induced forward displacement of deaths. 2
We have estimated future changes in mortality rates due to global warming by combining these observed mortality risks accompanying temperature change with the projected changes in temperature estimated by nine General Circulation Models (GCMs). GCMs are powerful tools that provide physically realistic estimates of future climate. 3 As expected, “cold” winters are estimated to become much rarer with global warming, and “hot” summers become much more frequent. Modifying the temperature in the study period according to the GCMs for the Intergovernmental Panel on Climate Change (IPCC) 550 ppm stabilization scenario (eg, approximate doubling of the pre-industrial level of greenhouse gases), the frequency of a cold spell would change from one in 4 years in the period 1979–1997 to one in 6 years in 2050. The number of heat waves would increase from one in every 3 years to one every 2 years. The extreme heat would also last for more consecutive days.
Looking at changes in temperatures toward the year 2050, we see that in the Netherlands a continuing decreasing trend in winter mortality is more pronounced than the increasing excess summer mortality rates. Net yearly decreases in mortality rates vary between 0.9 and 1.6% for cardiovascular mortality, between 1.6 and 3.9% for respiratory mortality, and between 0.6 and 1.1% for total mortality (Figure 1). Mortality changes due to malignant neoplasm are relatively insensitive to temperature changes. In absolute numbers, total mortality may decrease approximately 1100 per year (on a total population of about 16 million people). The same trend towards a reduction in deaths with rising temperature is seen in scenarios where the concentration of greenhouse gasses is stabilized around 350 ppm or 750 ppm.
These results shed light on possible changes in mortality related to the global warming predicted by several climate models. Although global climate change is likely to be accompanied by an increase in the frequency and intensity of heat waves, winters will likely be milder as well. Therefore, it is possible that in many temperate or cold regions, including the Netherlands, a decrease in cold-related mortality would counterbalance the increasing rates of heat-related mortality. However, the overall balance would also depend on adaptive responses (eg, air-conditioning, improved housing) and future health levels.
1. Intergovernmental Panel on Climate Change - IPCC. Climate Change 2001: Impacts, Adaptation and Vulnerability. Third Assessment Report Working Group II. Cambridge, UK: Cambridge University Press, 2001.
2. Huynen MMTE, Martens P, Schram D, Weijenberg MP, Kunst AE The impact of cold spells and heat waves on mortality rates in the Dutch population. Environ Health Perspect 2001; 109 (5): 463–470.
3. Williams LJ, Shaw D, Mendelsohn R. Evaluating GCM output with impact models. Climatic Change 1998; 39: 11–133.