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Global Warming and Mortality
Volume 8, Number 21: 25 May 2005

In a review article published in the Southern Medical Journal, Keatinge and Donaldson (2004) of Queen Mary's School of Medicine and Dentistry at the University of London begin the main body of their text with a clear declaration of the relative dangers of heat and cold when it comes to human mortality: "cold-related deaths are far more numerous than heat-related deaths in the United States, Europe, and almost all countries outside the tropics, and almost all of them are due to common illnesses that are increased by cold."  So what are the mechanisms by which cold kills?

Keatinge and Donaldson report that coronary and cerebral thrombosis account for about half of all cold-related deaths, and that respiratory diseases account for approximately half the rest.  With respect to the first of these sets of problems, they say that cold stress causes an increase in arterial thrombosis "because the blood becomes more concentrated, and so more liable to clot during exposure to cold."  The sequence of events, as they describe it, is that "the body's first adjustment to cold stress is to shut down blood flow to the skin to conserve body heat," which "produces an excess of blood in central parts of the body," and that to correct for this effect, "salt and water are moved out from the blood into tissue spaces," leaving behind "increased levels of red cells, white cells, platelets and fibrinogen" that lead to increased viscosity of the blood and a greater risk of clotting.

With respect to respiratory-related deaths, the British scientists report that the infections that cause them spread more readily in cold weather as people "crowd together in poorly ventilated spaces when it is cold."  In addition, they say that "breathing of cold air stimulates coughing and running of the nose, and this helps to spread respiratory viruses and bacteria."  The "train of events leading to respiratory deaths," as they continue, "often starts with a cold or some other minor infection of the upper airways," which "spreads to the bronchi and to the lungs," whereupon "secondary infection often follows and can lead to pneumonia."  They also note that cold stress "tends to suppress immune responses to infections," and that respiratory infections typically "increase the plasma level of fibrinogen, and this contributes to the rise in arterial thrombosis in winter."

Another interesting thing about cold-related deaths, as Keatinge and Donaldson describe it, is that "cold spells are closely associated with sharp increases in mortality rates," and that "deaths continue for many days after a cold spell ends."  On the other hand, they report that "increased deaths during a few days of hot weather are followed by a lower [our italics] than normal mortality rate," because "many of those dying in the heat are already seriously ill and even without heat stress would have died within the next 2 or 3 weeks."

So what are the implications of global warming for human mortality?  Keatinge and Donaldson state that "since heat-related deaths are generally much fewer than cold-related deaths" - and, we note, are comprised primarily of deaths that typically would have occurred a few weeks later even in the absence of excess heat - "the overall effect of global warming on health can be expected to be a beneficial one."  As an example, and even including the early heat-harvesting of naturally-expected deaths, they report that "the rise in temperature of 3.6F expected over the next 50 years would increase heat-related deaths in Britain by about 2,000 but reduce cold-related deaths by about 20,000."

In concluding their treatise, Keatinge and Donaldson report that "even in climates as warm as southern Europe or North Carolina [USA], cold weather causes more deaths than hot weather."  They note that "global warming will reduce this at first, but the improvement is not likely to continue without action to promote defenses against cold."  For example, they say that "people in regions with mild winters become careless about cold stress, protect themselves less effectively against cold, and generally have more winter deaths than people in colder regions [our italics]."  Hence, as they continue, "climatic warming therefore calls for action to control cold stress as well as heat stress," and they say that "if this is taken, rising temperatures could reduce overall mortality rates."

In closing, we consider one other health concern that was raised early in the global warming debate, and that is that diseases that are transmitted by insects might spread to cooler regions of the world in response to rising temperatures and become problems there.  However, in the words of Keatinge and Donaldson, "closer examination showed that this was unlikely to happen to a serious extent."  In the case of malaria, for example, this scourge of humanity - which they remind us "was once prevalent in most of Europe and even in Russia" - was largely eliminated from these regions in concert with earth's recovery from the cold temperatures of the Little Ice Age.  Why?  In the words of the two medical sleuths, "the main reason was that modern farming methods and changes in human living conditions had reduced the number of the mosquitoes that spread the disease and had reduced their access to people."

All things considered, therefore, it should be clear to most logically-thinking people that, in the words of Keatinge and Donaldson, "the overall effect of global warming on health can be expected to be a beneficial one."

Sherwood, Keith and Craig Idso

Keatinge, W.R. and Donaldson, G.C.  2004.  The impact of global warming on health and mortality.  Southern Medical Journal 97: 1093-1099.