MEDICAL MATTERS: Scientists are almost unanimous in their consensus that the rise in atmospheric concentration of greenhouse gases will continue to cause warming and other climatic changes on the earth's surface, writes Muiris Houston.
For the past 20 years or more there has been a steady upward trend in average global temperatures.
The Intergovernmental Panel on Climate Change (IPCC) forecasts an increase in world average temperatures, by 2100, of between 1.4 and 5.8 degrees; the increase will be greater at higher latitudes and over land.
Such fundamental global environmental change will affect human health in many ways. But questions remain about the exact health effects and whether they might be beneficial as well as harmful.
A recent review article in the Lancet gave some interesting insights into the current and future risks for human health posed by climate change.
The health risks of extreme weather events such as periods of very high temperature, torrential rains, flooding and drought have been relatively well studied.When temperature is plotted against the number of daily deaths, a typical U-shaped relationship emerges: the middle trough represents a comfort zone where we are not stressed by heat or cold.
The right side of the curve shows the death rate increasing with hot temperatures, with a similar rise in deaths as temperatures drop to extreme cold.
Research shows that older people are most likely to die during extreme heatwaves. This was most strikingly illustrated during the heatwave of August 2003 in Europe, during which an additional 30,000 people died.
Others vulnerable to heatwaves are children and those with mental illnesses. And those with pre-existing illnesses such as heart attack, stroke and chronic breathing problems are most at risk.
Interestingly, there is an urban rural divide in the risk posed to health by extreme heat. The so-called urban heat island effect, whereby inner urban environments absorb and retain heat, means they magnify and extend the rise in temperatures compared with adjacent rural areas.
A recent reminder of how floods can affect human health was the hurricane-associated flooding of New Orleans. Between 1992 and 2001, the most frequent natural weather disaster was flooding; it is estimated floods killed almost 100,000 people and affected the health of a further 1.2 billion.
The immediate effects of flooding include injuries, infectious disease and exposure to toxic pollutants. Human sewage and animal wastes escape into drinking water, increasing the risk of water-borne diseases. And the massive dislocation caused by flooding leads to an increase in mental disorders and malnutrition in the medium term.
Infectious disease transmission is also sensitive to climate change. Both the infectious microbes themselves and the vector organisms such as ticks and mosquitoes are affected by alterations in rainfall and temperature.
The salmonella bacterium multiplies more rapidly in the guts of animals and in food at higher temperatures, leading to a greater risk of food poisoning. Cholera proliferates as the temperature of water increases.
Tick-borne viral encephalitis (inflammation of the brain tissue) has increased in Sweden in response to a succession of warmer winters over the past 20 years. Some reports suggest malaria has increased in the east African highlands in association with local warming, because of greater transmission by mosquitoes at high temperatures.
How will climate change affect our health in the future? One study has estimated that major cities in Europe and north America will have substantial rises in both the frequency and duration of severe heatwaves by 2090.
But even without substantial climate change, the trend towards increased urbanisation in all countries and a rise in the proportion of the population aged over 60 to 32 per cent by 2050 means a greater number of people will be at risk from heat extremes in the future.
Vector-borne diseases, including malaria, dengue fever and yellow fever, all of which are transmitted by mosquitoes, could become more common and affect a greater number of countries.
On the wider health front, the affect of climate change on cereal yields is projected to result in a 5-10 per cent increase in malnourishment worldwide.
The conflicts and the flows of migrants that result will typically increase infectious disease, mental health problems and injury and violent death.
Climate change is not all bad news for health. Milder winters should reduce the seasonal winter increase in death rates in northern latitudes.
And a drying effect in already hot regions may cut down on the number of mosquitoes with a reduction in diseases such as malaria and dengue fever in these areas.
Overall, however, it looks like climate change will add to, rather than lessen, the burden of ill health in the years to come.
Dr Muiris Houston is pleased to hear from readers, but regrets he cannot answer individual queries.