How does rising atmospheric CO2 affect marine organisms?

Click to locate material archived on our website by topic


Health Effects (Temperature - Stroke) -- Summary
One hears a lot about the consequences of rising temperatures for human health these days, especially at the hot end of the temperature spectrum, where periodic heat waves lead to more deaths than would be expected under long-term average thermal conditions.  Less heralded is the fact that much greater numbers of people die as a result of ailments triggered by unseasonable cold spells.  In this summary we discus this latter phenomenon within the context of human death due to stroke, as described in several articles we have reviewed on our website.

Feigin et al. (2000) conducted what they call "the first truly population-based study on the relationship between stroke occurrence and weather parameters in Russia," working within the city of Novosibirsk, Siberia, which has one of the highest stroke incidence rates in the world.  Based on analyses of 2208 patients with sex and age distributions similar to those of Russia as a whole, they found a statistically significant association between stroke occurrence and low ambient temperature over the period 1982-93.  In the case of ischemic stroke (IS), which accounted for 87% of all stroke types, they determined that "the risk of IS occurrence on days with low ambient temperature [was] 32% higher than that on days with high ambient temperature."  Given what they describe as "the highly significant association observed between low ambient temperature (< -2.0°C) and IS occurrence (P = 0.02), together with the proportion of days with such temperature in the region during a calendar year (41.3%)," they conclude that the "very high stroke incidence in Novosibirsk, Russia may partially be explained by the highly prevalent cold factor there," and they suggest the implementation of "preventive measures in [the] region, such as avoiding low temperature."

Hong et al. (2003) investigated the association between the onset of ischemic stroke and prior episodic decreases in temperature in 545 patients who suffered strokes in Incheon, Korea from January 1998 to December 2000.  They report that "decreased ambient temperature was associated with risk of acute ischemic stroke," with the strongest effect being seen on the day after exposure to cold weather, further noting that "even a moderate decrease in temperature can increase the risk of ischemic stroke."  They also found that "risk estimates associated with decreased temperature were greater in winter than in the summer," which suggests that "low temperatures as well as temperature changes are associated with the onset of ischemic stroke."  Last of all, they explain that the reason for the 24- to 48-hour lag between exposure to cold and the onset of stroke "might be that it takes some time for the decreasing temperature to affect blood viscosity or coagulation," which is also suggested by the work of Keatinge et al. (1984), who found that blood viscosity and the plasma fraction of platelets began to increase one hour after cold exposure and did not reach a peak until sometime beyond six hours later.

Nakaji et al. (2004) evaluated seasonal trends in deaths due to various diseases in Japan, using nationwide vital statistics from 1970 to 1999 together with mean monthly temperature data.  They report that Japan has "bitterly cold winters," and their analysis indicates that the number of deaths due to cerebrovascular disease rises to a maximum during that cold time of year.  In fact, they found that the peak mortality rate due to stroke was two times greater in winter (January) than it was at the time of its yearly minimum (August and September).  As a result, the team of nine scientists says it is necessary to bring about a "reduction in exposure to cold environments," as they put it, which is precisely what global warming does, and what it does best when it warms more in winter than in summer, as Nakaji et al. have demonstrated to be the case in Japan, where winter warming over the past 30 years was twice as great as what it was during the rest of the year.

Chang et al. (2004) analyzed data from the World Health Organization (WHO) Collaborative Study of Cardiovascular Disease and Steroid Hormone Contraception (WHO, 1995) to determine the effects of monthly mean temperature on rates of hospitalization for arterial stroke and acute myocardial infarction among women aged 15-49 from seventeen different countries in Africa, Asia, Europe, Latin America and the Caribbean.  They report that among these women, a 5°C reduction in mean air temperature was associated with a 7% increase in the expected hospitalization rate due to stroke; and they note that lagging the effect of temperature suggested that this effect was relatively acute, within a period of about a month.

Finally, in a review article that touches on multiple aspects of temperature-related deaths, Keatinge and Donaldson (2004) report that "cold-related deaths are far more numerous than heat-related deaths in the United States, Europe, and almost all countries outside the tropics," noting that coronary and cerebral thrombosis account for about half of all cold-related mortality.  In describing the mechanisms behind this cold temperature-death connection, 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.  In addition, they say that cold stress "tends to suppress immune responses to infections," and that respiratory infections typically "increase the plasma level of fibrinogen," which also "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 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 in general and deaths due to stroke more specifically?  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 heat-harvesting of naturally-expected deaths, they report that "the rise in temperature of 3.6°F 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."

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."  Consequently, if the warming of the 20th century continues into the future, we can expect a significant drop in deaths due to strokes nearly everywhere in the world.

References
Chang, C.L., Shipley, M., Marmot, M. and Poulter, N.  2004.  Lower ambient temperature was associated with an increased risk of hospitalization for stroke and acute myocardial infarction in young women.  Journal of Clinical Epidemiology 57: 749-757.

Feigin, V.L., Nikitin, Yu.P., Bots, M.L., Vinogradova, T.E. and Grobbee, D.E.  2000.  A population-based study of the associations of stroke occurrence with weather parameters in Siberia, Russia (1982-92).  European Journal of Neurology 7: 171-178.

Hong, Y-C., Rha, J-H., Lee, J-T., Ha, E-H., Kwon, H-J. and Kim, H.  2003.  Ischemic stroke associated with decrease in temperature.  Epidemiology 14: 473-478.

Keatinge, W.R., Coleshaw, S.R., Cotter, F., Mattock, M., Murphy, M. and Chelliah, R.  1984.  Increases in platelet and red cell counts, blood viscosity, and arterial pressure during mild surface cooling: factors in mortality from coronary and cerebral thrombosis in winter.  British Medical Journal (Clinical Research Education) 289: 1405-1408.

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

Nakaji, S., Parodi, S., Fontana, V., Umeda, T., Suzuki, K., Sakamoto, J., Fukuda, S., Wada, S. And Sugawara, K.  2004.  Seasonal changes in mortality rates from main causes of death in Japan (1970-1999).  European Journal of Epidemiology 19: 905-913.

WHO.  1995.  WHO Collaborative Study of Cardiovascular Disease and Steroid Hormone Contraception.  A multinational case-control study of cardiovascular disease and steroid hormone contraceptives: description and validation of methods.  Journal of Clinical Epidemiology 48: 1513-1547.

Last updated 20 July 2005