Human health effects

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The authors of the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) claimed to have “very high confidence” that “climate change currently contributes to the global burden of disease and premature deaths” (IPCC, 2007-II, p. 393, emphasis in the original). They also claim climate change will “increase malnutrition and consequent disorders … increase the number of people suffering from death, disease and injury from heatwaves, floods, storms, fires and droughts … continue to change the range of some infectious disease vectors … increase the burden of diarrhoeal diseases … increase cardio-respiratory morbidity and mortality associated with ground-level ozone … [and] increase the number of people at risk of dengue.” The IPCC admits warming weather would “bring some benefits to health, including fewer deaths from cold,” but says those benefits “will be outweighed by the negative effects of rising temperatures worldwide, especially in developing countries” (ibid.).

The 2009 report of the Nongovernmental International Panel on Climate Change (NIPCC) debunked many of those claims, starting with the simple fact that the modest warming that occurred in the twentieth century did not cause more “heatwaves, floods, storms, fires and droughts,” and consequently these imagined phenomena could not have harmed human health or well-being. Readers of the current report are referred to Chapter 5, where they will find more, and more recent, evidence to that effect.

Idso and Singer (2009) went on to examine research on the relationships between temperature and CO2 and diseases, heat-related mortality, nutrition, and human longevity, finding global warming is likely to improve rather than harm human health.

In the following pages we review new scientific research on these same matters, finding it supports the same conclusion. That analysis is followed by a brief discussion of viral and vector-borne diseases, after which we review papers that document CO2-induced changes in certain of the medicinal and nutritional properties of plants, which should bode well for future human health gains.


Temperature-Related Human Mortality

Christidis et al. (2010) report, “the IPCC AR4 states with very high confidence that climate change contributes to the global burden of disease and to increased mortality,” citing the contribution of Confalonieri et al. (2007) to that document. Idso and Singer (2009), however, reported that rising temperatures lead to a greater reduction in winter deaths than the increase they cause in summer deaths, resulting in a large net decrease in human mortality, based on findings described in the peer-reviewed scientific literature through 2007. In this interim report we review studies of the subject published after that time.

Christidis et al. extracted the numbers of daily deaths from all causes reported on death registration data supplied by the U.K. Office of National Statistics for men and women 50 years of age or older in England and Wales for the period 1976–2005, which they divided by daily population estimates they obtained by fitting a fifth-order polynomial to midyear population data, yielding deaths per million people. They compared the results with surface air temperature data that showed a warming trend during the three-decade period of 0.47°C per decade. In addition, they employed a technique called optimal detection, which can be used to estimate the role of human adaptation in the temperature-related changes in mortality they observed.

As expected, during the hottest months of the year, warming led to increases in death rates, while during the coldest months of the year warming led to decreases in death rates. The three scientists reported, for example, that if no adaptation had taken place, there would have been 1.6 additional deaths per million people per year due to warming in the hottest part of the year over the period 1976–2005, but there would have been 47 fewer deaths per million people per year due to warming in the coldest part of the year, for a lives-saved to life-lost ratio of 29.4. That, of course, represents a substantial net benefit from the warming experienced in England and Wales during the three-decade period. When adaptation was included in the analysis, they found there was an increase of only 0.7 deaths per million people per year due to warming in the hottest part of the year, but a decrease of fully 85 deaths per million people per year due to warming in the coldest part of the year, for a lives-saved to life-lost ratio of 121.4.

Working in the Castile-Leon region of Spain—a plateau in the northwestern part of the country which includes nine provinces with a low population density that can be considered as aging—Fernandez-Raga et al. (2010) obtained (from the country’s National Meteorological Institute) meteorological data from weather stations situated in eight of the provincial capitals for 1980–1998, and they obtained contemporary mortality data from the country’s National Institute for Statistics for deaths associated with cardiovascular, respiratory, and digestive-system diseases.

Various analyses of the monthly averaged data revealed a number of interesting results. First, for all three of the disease types studied, the three researchers found “the death rate is about 15% higher on a winter’s day than on a summer’s day,” which they describe as “a result often found in previous studies,” citing the work of Fleming et al. (2000), Verlato et al. (2002), Grech et al. (2002), Law et al. (2002), and Eccles (2002). Second, in a finding that helps to explain the first finding, the three researchers discovered that when monthly averaged human death rates were plotted against monthly averages of daily mean, maximum, and minimum air temperature, the results nearly always took the form of a U-shaped concave parabola, as shown in Figure 9.1.1.

For all three disease types, Fernandez-Raga et al. found all three temperatures (daily mean, maximum, and minimum) at which minimum death rates occurred—which they refer to as ideal or comfort temperatures—were all within about 1°–7°C of the maximum values typically reached by those three types of temperature, whereas they were anywhere from 14° to 24°C away from their minimum values. Consequently, the ideal or comfort temperatures were always very close to (and sometimes nearly identical to) the maximum values reached by the mean, maximum, and minimum temperatures experienced in the region, and they were much more removed from the minimum values of those three temperature parameters, as illustrated in the figure.

The data clearly demonstrate the people of the Castile-Leon region of Spain are much more likely to die from a cardiovascular disease in the extreme cold of winter than in the extreme heat of summer. The same was found to hold true with respect to respiratory and digestive system diseases. Cold has been found to be a much greater killer of people than heat almost everywhere in the world, and in conjunction with almost any type of illness.

In a more broad-based study, Analitis et al. (2008) wrote, “in recent years, the effects of meteorologic factors on health have attracted renewed interest because of the observed and predicted climate change, which is expected to result in a general rise in temperature.” This development, in their words, has led to a “recent focus on heat-wave episodes,” which may have fostered the perception that cold-related mortality is not as important a public health concern as is heat-related mortality.

To rectify this situation, the 14 researchers analyzed short-term effects of cold weather on mortality in 15 European cities (Athens, Greece; Barcelona, Spain; Budapest, Hungary; Dublin, Ireland; Helsinki, Finland; Ljubljana, Slovenia; London, United Kingdom; Milan, Italy; Paris, France; Prague, Czech Republic; Rome, Italy; Stockholm, Sweden; Turin, Italy; Valencia, Spain; and Zurich, Switzerland). Specifically, they assessed the effects of minimum apparent temperature on cause- and age-specific daily mortality over the cold half of the year (October–March), using data from 1990–2000 analyzed via “Poisson regression and distributed lag models, controlling for potential confounders.”

The international team of scientists—from Finland, Greece, Ireland, Italy, Slovenia, Spain, and Sweden—found “a 1°C decrease in temperature was associated with a 1.35% increase in the daily number of total natural deaths and a 1.72%, 3.30%, and 1.25% increase in cardiovascular, respiratory, and cerebrovascular deaths, respectively.” In addition, they reported “the increase was greater for the older age groups,” and the cold effect “persisted up to 23 days, with no evidence of mortality displacement.” The latter finding is extremely important because in the case of heat-related deaths there is such a displacement, and its impact is substantial.

In Germany, for example, Laschewski and Jendritzky (2002) analyzed daily mortality rates in Baden-Wurttemberg (10.5 million inhabitants) over the 30-year period 1958–97 to determine the sensitivity of the population of this moderate climatic zone to long- and short-term episodes of heat and cold. Their research indicated mortality showed “a marked seasonal pattern with a minimum in summer and a maximum in winter.” With respect to short-term exposure to heat and cold, however, they found “cold spells lead to excess mortality to a relatively small degree, which lasts for weeks,” and “the mortality increase during heat waves is more pronounced, but is followed by lower than average values in subsequent weeks.” Thissuggests, in their words, that people who died from short-term exposure to heat “would have died in the short term anyway.”

With respect to this short-term mortality displacement that occurs in conjunction with heat-related deaths, Laschewski and Jendritzky’s data demonstrate it is precisely that: merely a displacement of deaths and not an overall increase. They found, for example, that the mean duration of above-normal mortality for the 51 heat episodes that occurred from 1968 to 1997 was ten days, with a mean increase in mortality of 3.9 percent, after which there was a mean decrease in mortality of 2.3 percent for 19 days. The net effect of the two perturbations was an overall decrease in mortality of 0.2 percent over the full 29-day period. Analitis et al. concluded their paper by stating their results “add evidence that cold-related mortality is an important public health problem across Europe and should not be overlooked by public health authorities because of the recent focus on heat-wave episodes.”

In providing some background for another recent study of the subject, Young and Kakinen (2010) write, “Arctic populations, especially indigenous people, could be considered as ‘vulnerable,’ because their health status generally shows disparities when compared to the national or more southern populations,” and they state “it is not known if the harsh climate, and especially cold temperatures, could be a contributing or causative factor of the observed health inequalities.” To seek answers to this, the two researchers determined mean January and July temperatures for 27 Arctic regions, based on weather station data for the period 1961–1990, and their association with a variety of health outcomes assessed by correlation and multiple linear regression analyses.

The two researchers found mean January temperature correlated negatively with several health outcomes, including infant mortality rate, age-standardized mortality rates (all causes), perinatal mortality rate, and tuberculosis incidence rate, but it correlated positively with life expectancy. That is to say, as mean January temperature rose, the desirable metric of life expectancy at birth rose right along with it, while all of the undesirable health metrics (such as mortality and disease incidence) declined. For example, they report “for every 10°C increase in mean January temperature, the life expectancy at birth among males increased by about six years” and “infant mortality rate decreased by about four deaths per thousand live births.”

Young and Kakinen concluded the cold climate of the Arctic is “significantly associated with higher mortality” and “should be recognized in public health planning,” noting that “within a generally cold environment, colder climate results in worse health.” For people living in these regions, therefore, a little global warming could go a long way toward improving their quality of life, as well as the length of time they have to enjoy it.

In another impressive study, Deschenes and Moretti (2009) analyzed the relationship between weather and mortality, based on data that included the universe of deaths in the United States over the period 1972–1988, wherein they matched each death to weather conditions on the day of death and in the county of occurrence. These high-frequency data and the fine geographical detail allowed them to estimate with precision the effect of cold and hot temperature shocks on mortality, as well as the dynamics of such effects—most notably, the existence or absence of a “harvesting effect” whereby the temperature-induced deaths either are or are not subsequently followed by a drop in the normal death rate that could either partially or fully compensate for the prior extreme temperature-induced deaths.

The two researchers state their results “point to widely different impacts of cold and hot temperatures on mortality.” In the latter case, they discovered “hot temperature shocks are indeed associated with a large and immediate spike in mortality in the days of the heat wave,” but “almost all of this excess mortality is explained by near-term displacement,” so that “in the weeks that follow a heat wave, we find a marked decline in mortality hazard, which completely offsets the increase during the days of the heat wave,” such that “there is virtually no lasting impact of heat waves on mortality.”

In the case of cold temperature days, they also found “an immediate spike in mortality in the days of the cold wave,” but “there is no offsetting decline in the weeks that follow,” so “the cumulative effect of one day of extreme cold temperature during a thirty-day window is an increase in daily mortality by as much as 10%.” In addition, they write, “this impact of cold weather on mortality is significantly larger for females than for males,” but “for both genders, the effect is mostly attributable to increased mortality due to cardiovascular and respiratory diseases.”

In further discussing their findings, Deschenes and Moretti state, “the aggregate magnitude of the impact of extreme cold on mortality in the United States is large,” noting it “roughly corresponds to 0.8% of average annual deaths in the United States during the sample period.” They estimate “the average person who died because of cold temperature exposure lost in excess of ten years of potential life,” whereas the average person who died because of hot temperature exposure likely lost no more than a few days or weeks of life.

Interestingly, the two scientists additionally report many people in the United States have taken advantage of these obvious facts by moving “from cold northeastern states to warm southwestern states.” Based on their findings, they calculate “each year 4,600 deaths are delayed by the changing exposure to cold temperature due to mobility,” and “3% to 7% of the gains in longevity experienced by the U.S. population over the past three decades are due to the secular movement toward warmer states in the West and the South, away from the colder states in the North.”

Working in the Southern Hemisphere, Bi et al. (2008) used correlation and autoregressive integrated moving average regression analyses to derive relationships between various aspects of weather and mortality in the general population and elderly (65 years of age and older) of Brisbane, Australia—which they describe as having a subtropical climate—over the period 1986–1995. In doing so, they determined “death rates were around 50–80 per 100,000 in June, July, and August [winter], while they were around 30–50 per 100,000 in the rest of the year, including the summer.” They state “this finding applied both to the general population and to the elderly population, and to deaths from various causes.”

In discussing the fact that “more deaths occurred in the winter than during other seasons of the year, although winter in Brisbane is very mild,” the researchers noted “it is understandable that more deaths would occur in winters in cold or temperate regions, but even in a subtropical region, as indicated in this study, a decrease in temperatures (in winters) may increase human mortality.” Consequently, the evidence continues to grow that extremes of cold lead to the deaths of many more people than extremes of heat in both cold and warm climates.

In a study with a slightly different take on the subject, Tam et al. (2009) studied daily mortality data from 1997 to 2002, which they obtained from the Hong Kong Census and Statistics Department, examining the association between diurnal temperature range (DTR = daily maximum temperature minus daily minimum temperature), while focusing on cardiovascular disease among the elderly (people aged 65 and older). They discovered “a 1.7% increase in mortality for an increase of 1°C in DTR at lag days 0–3,” and they describe these results as being “similar to those reported in Shanghai.”

In discussing their findings, the four Hong Kong researchers stated, “a large fluctuation in the daily temperature—even in a tropical city like Hong Kong—has a significant impact on cardiovascular mortality among the elderly population.” In addition, we note it has long been known that the DTR has declined significantly over many parts of the world as mean global temperature has risen over the past several decades (Easterling et al., 1997), which is perhaps another reason why colder temperatures are a much greater risk to human life than are warmer temperatures: As the planet warms, local DTRs tend to decline, which leads to a corresponding decline in human death rates.

Turning to the Shanghai study mentioned by Tam et al., we find that Cao et al. (2009)—working within the nine urban districts of Shanghai, China—used time-series and case-crossover approaches to assess the relationship between DTR and coronary heart disease (CHD) deaths between 1 January 2001 and 31 December 2004, based on mortality data for elderly people (66 years of age or older), obtained from the Shanghai Municipal Center of Disease Control and Prevention, plus temperature data they obtained from a fixed-site station in the Xuhui District of Shanghai, which they adjusted to account for the mortality impacts of long-term and seasonal trends in CHD mortality, day of week, temperature, relative humidity, and concomitant atmospheric concentrations of PM10, SO2, NO2, and O3, which they obtained from the Shanghai Environmental Monitoring Center.

This work revealed, in Cao et al.’s words, that “a 1°C increase in DTR (lag = 2) corresponded to a 2.46% increase in CHD mortality on time-series analysis, a 3.21% increase on unidirectional case-crossover analysis, and a 2.13% increase on bidirectional case-crossover analysis,” and “the estimated effects of DTR on CHD mortality were similar in the warm and cool seasons.” Thus, the seven scientists concluded their data suggested even “a small increase in DTR is associated with a substantial increase in deaths due to CHD.” And since the DTR has declined significantly over most of the world as mean global air temperature has risen over the past several decades, it can be appreciated that the global warming with which this DTR decrease is associated (which is driven by the fact that global warming is predominantly caused by an increase in daily minimum temperature) has likely helped to significantly reduce the CHD-induced mortality of elderly people worldwide.

In one final study dealing with the heart and employing a generalized additive statistical model that blends the properties of generalized linear models with additive models, Bayentin et al. (2010) analyzed the standardized daily hospitalization rates for ischemic heart disease (IHD) and their relationship with climatic conditions up to two weeks prior to the day of admission—controlling for time trends, day of the season, and gender—in order to determine the short-term effects of climate conditions on the incidence of IHD over the 1989–2006 time period for 18 different regions of Quebec. Perhaps the most interesting and important finding of this study was, as they describe it, that “a decline in the effects of meteorological variables on IHD daily admission rates was observed over the period of 1989–2006.” This response, in their words, “can partly be explained by the changes in surface air temperature,” which they describe as warming “over the last few decades,” as is further described by Bonsal et al. (2001) and Zhang et al. (2000) for the twentieth-century portion of the study’s duration. In addition, they note “winters have been steadily warmer,” while “summers have yet to become hotter for most regions.” This is another beneficial characteristic of the warming that was experienced over most of the planet throughout the latter part of the twentieth century: a gradual reduction in DTR, as confirmed by the work of Easterling et al. (1997).

In summation, the material presented in this chapter represents overwhelming evidence for a positive effect of global warming on human health.

Viral and Vector-borne Diseases

With respect to viral and vector-borne diseases, in a review of the pertinent literature that describes “those mechanisms that have led to an increase of virus activity in recent years,” Zell et al. (2008) state “it is assumed that global warming is forced by the anthropogenic release of ‘greenhouse gases’,” and that a further “consistent assumption” has been a consequent “increased exposure of humans to tropical pathogens and their vectors.” However, they note “there is dissent about this hypothesis (Taubes, 1997; Reiter, 2001; Hay et al., 2002; Reiter et al., 2003; Randolph, 2004; Zell, 2004; Halstead, 2008),” and they thus go on to explore the subject in more detail, ultimately concluding “only very few examples point toward global warming as a cause of excess viral activity.” Instead, they find, “coupled ocean/atmosphere circulations and continuous anthropogenic disturbances (increased populations of humans and domestic animals, socioeconomic instability, armed conflicts, displaced populations, unbalanced ecosystems, dispersal of resistant pathogens etc.) appear to be the major drivers of disease variability,” and global warming “at best” merely “contributes.”

Similar sentiments were expressed that year by Wilder-Smith and Gubler (2008), who focused on the occurrence of dengue infections, reporting “climate has rarely been the principal determinant of [their] prevalence or range,” and “human activities and their impact on local ecology have generally been much more significant.” In this regard, they cite as contributing factors “urbanization, deforestation, new dams and irrigation systems, poor housing, sewage and waste management systems, and lack of reliable water systems that make it necessary to collect and store water.” They further note “disruption of vector control programs, be it for reasons of political and social unrest or scientific reservations about the safety of DDT, has contributed to the resurgence of dengue around the world.” In addition, they write, “large populations in which viruses circulate may also allow more co-infection of mosquitoes and humans with more than one serotype of virus,” which would appear to be borne out by the fact that “the number of dengue lineages has been increasing roughly in parallel with the size of the human population over the last two centuries.” Most important of all, perhaps, is “the impact of international travel.” Wilder-Smith and Gubler note “humans, whether troops, migrant workers, tourists, business travelers, refugees, or others, carry the virus into new geographic areas.” These movements, in their words, “can lead to epidemic waves.” Given such findings, the two researchers conclude “population dynamics and viral evolution offer the most parsimonious explanation for the observed epidemic cycles of the disease, far more than climatic factors.”

Also exploring this issue were Gage et al. (2008), who reviewed what was then known about it. The four researchers—all from the U.S. Centers for Disease Control’s National Center for Zoonotic, Vector-Borne, and Enteric Diseases—concluded “the precise impacts” of the various climatic changes that are typically claimed to occur in response to rising atmospheric CO2 concentrations “are difficult to predict.” Indeed, they write, “in some areas, climate change could increase outbreaks and the spread of some vector-borne diseases while having quite the opposite effect on other vector-borne diseases.”

In further discussing this complex situation, they note “the mere establishment of suitable vectors for a particular agent does not necessarily mean that spread to humans will commonly occur, as indicated by the limited transmission of dengue and malaria in the southern U.S.,” because, as they continue, “local transmission has been limited by factors unrelated to the climatic suitability of the areas for the relevant vector species.” In addition, they write, “in instances where a vector-borne disease is also zoonotic, the situation is even more complex, because not only must the vector and pathogen be present, but a competent vertebrate reservoir host other than humans must also be present.”

So what are some of the non-climatic factors that affect the spread of vector-borne diseases among humans? Gage et al. list “many other global changes concurrently transforming the world, including increased economic globalization, the high speed of international travel and transport of commercial goods, increased population growth, urbanization, civil unrest, displaced refugee populations, water availability and management, and deforestation and other land-use changes,” to which could be added the many different ways in which these phenomena are dealt with by different societies.

Kyle and Harris (2008) noted “dengue is a spectrum of disease caused by four serotypes of the most prevalent arthropod-borne virus affecting humans today,” and “its incidence has increased dramatically in the past 50 years,” such that “tens of millions of cases of dengue fever are estimated to occur annually, including up to 500,000 cases of the life-threatening dengue hemorrhagic fever/dengue shock syndrome.” The researchers conducted a review of the pertinent scientific literature, exploring “the human, mosquito, and viral factors that contribute to the global spread and persistence of dengue, as well as the interaction between the three spheres, in the context of ecological and climate change.”

The two researchers note “there has been a great deal of debate on the implications of global warming for human health,” but “at the moment, there is no consensus.” In the case of dengue, they write, “it is important to note that even if global warming does not cause the mosquito vectors to expand their geographic range, there could still be a significant impact on transmission in endemic regions,” as they report that “a 2°C increase in temperature would simultaneously lengthen the lifespan of the mosquito and shorten the extrinsic incubation period of the dengue virus, resulting in more infected mosquitoes for a longer period of time.” Nevertheless, they note there are “infrastructure and socioeconomic differences that exist today and already prevent the transmission of vector-borne diseases, including dengue, even in the continued presence of their vectors.” Consequently, it would appear that whatever advantages rising temperatures might confer upon the dengue virus vector, they can be overcome by proper implementation of modern vector-control techniques.

One year later, Russell (2009)—a professor in the Department of Medicine of the University of Sydney and founding director of its Department of Medical Entomology—reported, “during the past 10 years, there has been increasing concern for health impacts of global warming in Australia, and continuing projections and predictions for increasing mosquito-borne disease as a result of climate change.” However, he wrote, these claims “are relatively simplistic, and do not take adequate account of the current or historic situations of the vectors and pathogens, and the complex ecologies that might be involved.” He then went on to review the consequences of these several inadequacies for malaria, dengue fever, the arboviral arthritides (Ross River and Barmah Forest viruses) and the arboviral encephalitides (Murray Valley encephalitis and Kunjin viruses). He did this within the context of predictions of projected climate changes as proposed and modeled by Australia’s Commonwealth Scientific and Industrial Research Organization and the Intergovernmental Panel on Climate Change. He concluded “there might be some increases in mosquito-borne disease in Australia with a warming climate, but with which mosquitoes and which pathogens, and where and when, cannot be easily discerned.” The strongest statement he could make was that “of itself, climate change as currently projected, is not likely to provide great cause for public health concern with mosquito-borne disease in Australia.”

In another paper, Russell et al. (2009) wrote, “dengue has emerged as a leading cause of morbidity in many parts of the tropics,” noting “Australia has had dengue outbreaks in northern Queensland.” In addition, they reported, “substantial increases in distribution and incidence of the disease in Australia are projected with climate change,” or, more specifically, “with increasing temperatures.” They explored the soundness of these projections by reviewing pertinent facts about the history of dengue in Australia, determining that the dengue vector (the Aedes aegypti mosquito) “was previously common in parts of Queensland, the Northern Territory, Western Australia and New South Wales,” that it had “in the past, covered most of the climatic range theoretically available to it,” and that “the distribution of local dengue transmission has [historically] nearly matched the geographic limits of the vector.”

This being the case, the six scientists concluded the vector’s current absence from much of Australia, as Russell et al. described it, “is not because of a lack of a favorable climate.” Thus, they reasoned that “a temperature rise of a few degrees is not alone likely to be responsible for substantial increases in the southern distribution of A. aegypti or dengue, as has been recently proposed.” Instead, they reminded everyone that “dengue activity is increasing in many parts of the tropical and subtropical world as a result of rapid urbanization in developing countries and increased international travel, which distributes the viruses between countries.” Rather than attempts to limit dengue transmission by controlling the world’s climate, therefore, the medical researchers recommend that “well resourced and functioning surveillance programs, and effective public health intervention capabilities, are essential to counter threats from dengue and other mosquito-borne diseases.”

Studying dengue simultaneously in three other parts of the world, Johansson et al. (2009) wrote, “mosquito-borne dengue viruses are a major public health problem throughout the tropical and subtropical regions of the world,” and “changes in temperature and precipitation have well-defined roles in the transmission cycle and may thus play a role in changing incidence levels.” Therefore, as they continued, since “the El Niño Southern Oscillation (ENSO) is a multiyear climate driver of local temperature and precipitation world wide,” and since “previous studies have reported varying degrees of association between ENSO and dengue incidence,” as they describe it, they decided to analyze “the relationship between ENSO, local weather, and dengue incidence in Puerto Rico, Mexico, and Thailand.” They did so by searching for relationships between ENSO, local weather, and dengue incidence in Puerto Rico (1986–2006), Mexico (1985–2006), and Thailand (1983–2006), using wavelet analysis as a tool to identify time- and frequency-specific associations.

The three researchers reported they “did not find evidence of a strong, consistent relationship in any of the study areas,” and Rohani (2009), who wrote a Perspective piece on their study, stated they found “no systematic association between multi-annual dengue outbreaks and El Niño Southern Oscillation.” Thus, as included in the Editors’ Summary of Johansson et al.’s paper, their findings provided “little evidence for any relationship between ENSO, climate, and dengue incidence.”

In another review paper dealing with the possible impacts of climate change on the spread of infectious diseases, Randolph (2009) noted it is generally tacitly assumed—and even explicitly stated—that climate change will result only in a worsening of the situation, with the expansion of vector-borne diseases into higher latitudes and an increased disease incidence. In fact, she states that implicit in almost all of the literature on this subject—both popular and scientific—“is an assumption that environmental change is more likely to strengthen the transmission potential and expand the range, rather than to disrupt the delicate balance between pathogen, vector and host upon which these systems depend.”

The zoologist from the U.K.’s University of Oxford thus explores the evidence via an analysis of what the bulk of the accurately informed scientific literature on the subject seems to suggest. In doing so, she finds “the mercurial epidemiology of each vector-borne disease is the system-specific product of complex, commonly nonlinear, interactions between many disparate environmental factors.” These include “not only climate but also other abiotic conditions (e.g., land cover) and the physical structure of the environment (e.g., water sources), and further biotic factors such as host abundance and diversity.” She also indicates that a number of socioeconomic factors drive human living conditions and behaviors that determine the degree of exposure to the risk posed to them, and that nutritional status and concomitant immunity also determine the degree of resistance to infection.

In some interesting examples from the past, Randolph notes the upsurge of tick-borne diseases within preexisting endemic regions in central and Eastern Europe “appears to be an unforeseen consequence of the fall of the iron curtain and the end of the cold war,” which she describes as “a sort of political global warming.” Also noted is the fact that “the introduction of the mosquito Aedes aegypti to the Americas within water containers on board slave ships from Africa was repeated four centuries later by the dispersal of the Asian tiger mosquito, A. albopictus, from Japan to the United States within water trapped in used car tires (Hawley et al., 1987; Reiter and Sprenger, 1987).”

This phenomenon, according to Randolph, continues today, augmented by trade in other water-carrying goods such as Asian Luck Bamboo plants. Such activities have allowed this mosquito species “to establish itself in almost all New World countries, a dozen European countries, parts of West Africa, and the Middle East.” All of these disease expansions, in her words, have “nothing to do with climate change,” which also holds true for such chance events as “the introduction of West Nile virus into New York in 1999, most probably by air from Israel (Lanciotti et al., 1999),” and the introduction “of the BTV-8 strain of bluetongue virus into the Netherlands in 2006 from South Africa (Saegerman et al., 2008).”

Contemporaneously, Harvell et al. (2009) stated that “in temperate climates, we might expect the range and activity of mosquitoes and the pathogens they vector, such as malaria and dengue, to increase with warmer temperatures.” However, “from a later vantage point in 2009,” they indicated that “surprisingly, insect-vectored diseases resoundingly do not show a net expansion in range or increase in prevalence.” As for why this is so, the five scientists gave three explanations attributed to Lafferty (2009a): “(1) anthropogenic activities directly influence the distributions of vectors and infectious disease in ways unrelated to climate, (2) vectors and pathogens are limited by thermal maxima, so that temperature changes lead to shifts rather than expansions in distribution, and (3) other factors such as host acquired immunity and vector or parasite life history traits are linked to habitat suitability in addition to climate.” In addition, they noted the important role that may be played by “evolutionary changes in properties of the host or pathogen,” and in concluding their paper they therefore wrote, “ecologists need to consider how host biology, including movement behavior and acquired immunity, can mediate the impacts of global change on parasite/pathogen dynamics and disease severity,” because, as they concluded, “at present, many of these mechanisms are poorly known.”

Turning directly to the Lafferty (2009a) paper, we again read the projection that “global climate change will result in an expansion of tropical diseases, particularly vector-transmitted diseases, throughout temperate areas,” examples of which include “schistosomiasis (bilharzia or snail fever), onchocerciasis (river blindness), dengue fever, lymphatic filariasis (elephantiasis), African trypanosomiasis (sleeping sickness), leishmaniasis, American trypanosomiasis (Chagas disease), yellow fever, and many less common mosquito and tick-transmitted diseases of humans,” as well as many diseases of “nonhuman hosts.” In a critique of this point of view, based on his review of the scientific literature, he concludes, “while climate has affected and will continue to affect habitat suitability for infectious diseases, climate change seems more likely to shift than to expand the geographic ranges of infectious diseases,” and “many other factors affect the distribution of infectious disease, dampening the proposed role of climate.” In fact, he concludes, “shifts in climate suitability might actually reduce the geographic distribution of some infectious diseases.” And of perhaps even greater importance (because it is a real-world observation), he reports, “although the globe is significantly warmer than it was a century ago, there is little evidence that climate change has already favored infectious diseases.”

In a companion paper (Lafferty, 2009b), the U.S. researcher lists several ways in which ecologists “can contribute substantially to the general theory of climate and infectious disease,” some of the most important of which have to deal with “[1] multiple hosts and parasite species (Dobson, 2009), [2] nonhuman hosts (Harvell et al., 2009), [3] accounting for the effects of immunity (Dobson, 2009; Harvell et al., 2009; Ostfeld, 2009; Pascual and Bouma, 2009), [4] quality and details of [4a] climatic data and [4b] appropriate measures of disease response (Ostfeld, 2009; Pascual and Bouma, 2009; Randolph, 2009), [5] complex analyses to account for multiple, interdependent covariates (Dobson, 2009; Ostfeld, 2009; Pascual and Bouma, 2009; Randolph, 2009), [6] host movement in response to climate change (Harvell et al., 2009), and [7] geographic tools to account for distinctions between fundamental and realized niches (Ostfeld, 2009; Randolph, 2009).” These many and varied challenges confronting the scientific community in this emerging field of study show there is much unfinished business that must be conducted in researching the several potential relationships that may or may not exist between climatic change and the spread of infectious diseases.

In one additional study from 2009, Nabi and Qader (2009) analyze both sides of the global warming/malaria incidence debate, considering the climatic conditions that affect the spread of the disease (temperature, rainfall, and humidity), as well as the host of pertinent non-climatic factors that play important roles in its epidemiology (the presence or absence of mosquito control programs, the availability or non-availability of malaria-fighting drugs, changing resistances to drugs, the quality of vector control, changes in land use, the availability of good health services, human population growth, human migrations, international travel, and standard of living).

According to the two researchers, their results indicate “global warming alone will not be of a great significance in the upsurge of malaria unless it is accompanied by a deterioration in other parameters like public health facilities, resistance to anti-malarial drugs, decreased mosquito control measures,” etc. They write, “no accurate prediction about malaria can truly be made,” because “it is very difficult to estimate what the other factors will be like in the future.” The researchers do note, however, that mosquito-borne diseases were a major public health problem in the United States from the 1600s to the mid-1900s, “with occasional epidemics.” By the middle of the twentieth century, however, “malaria disappeared from the country along with the other mosquito borne diseases like Dengue and Yellow fever,” and “this decline was attributed to overall improvements in living conditions and better public health measures.” The continuance of both of these has kept these diseases at bay throughout the latter half of the twentieth century as well, even though that period experienced what some have characterized as “unprecedented global warming.”

In light of these several observations, plus many others from all around the world—which clearly establish the overriding importance of a country’s standard of living and concomitant level of health-promoting services—Nabi and Qader conclude, “as public health workers, it would be more justifiable for us to exert our efforts on these other [non-climatic] parameters for the eradication and control of malaria.”

Reiter (2010) notes the appearance of the West Nile virus in New York (USA) in 1999, plus the unprecedented panzootic that followed, “have stimulated a major research effort in the Western Hemisphere and a new interest in the presence of this virus in the Old World.” These developments have been driven in part by the fact that “a great deal of attention has been paid to the potential impact of climate change on the prevalence and incidence of mosquito-borne disease.”

Reiter reviews what researchers have learned about the subject and reports the worldwide implications for public health, summing things up in his final paragraph, where he states: “one point is clear: the importation and establishment of vector-borne pathogens that have a relatively low profile in their current habitat is a serious danger to Europe and throughout the world.” This state of affairs, in his view, “is a direct result of the revolution of transport technologies and increasing global trade that has taken place in the past three decades,” modern examples of which include “the global circulation of dengue virus serotypes (Gubler, 1998), the intercontinental dissemination of Aedes albopictus and other mosquitoes in used tires (Hawley et al., 1987; Reiter, 1998), the epidemic of chikungunya virus in Italy (Angelini et al., 2007), and the importation of bluetongue virus and trypanosomiasis into Europe (Meroc et al., 2008; Moretti, 1969).” In light of what his review reveals, he writes, “globalization is potentially a far greater challenge to public health in Europe than any future changes in climate (Tatem et al., 2006).”

In a study demonstrating the influence of globalization, Shang et al. (2010) used logistic and Poisson regression models to analyze bi-weekly, laboratory-confirmed dengue cases in Taiwan at their onset dates of illness from 1998 to 2007, in order to “identify correlations between indigenous dengue and imported dengue cases (in the context of local meteorological factors) across different time lags.” They found “the occurrence of indigenous dengue was significantly correlated with temporally-lagged cases of imported dengue (2–14 weeks), higher temperatures (6–14 weeks), and lower relative humidity (6–20 weeks),” and that “imported and indigenous dengue cases had a significant quantitative relationship in the onset of local epidemics.” Given these findings, the six Taiwanese researchers concluded, “imported dengue cases are able to initiate indigenous epidemics when appropriate weather conditions are present,” or as they stated in another place, “imported dengue are able to serve as an initial facilitator, or spark, for domestic epidemics.” Therefore, they suggest “early detection and case management of imported cases through timely surveillance and rapid laboratory-diagnosis may avert large scale epidemics of dengue/dengue hemorrhagic fever,” while noting “meteorology alone does not initiate an epidemic” and “an increase in viremic international travelers has caused global dengue hemorrhagic fever case numbers to surge in the past several decades.” This surge is often erroneously claimed to be due to global warming.

Gething et al. (2010) note that based on “model predictions,” it is “reported widely in global climate policy debates that climate change is adding to the present-day burden of malaria and will increase both the future range and intensity of the disease,” citing the IPCC (2007) and the U.S. Environmental Protection Agency (2010). Noting “it has long been known that the range of malaria has contracted through a century of economic development and disease control (Hay et al., 2009),” when “global temperature increases have been unequivocal,” they go on to explore this apparent incongruity “for the first time” in another data-based study by comparing “an evidence-based map of contemporary malaria endemicity (Hay et al., 209)” with “the most reliable equivalent for the pre-intervention era, around 1900 (Lysenko et al., 1968),” when malaria was “at its assumed historical peak.” This provides a comparison of “the magnitude of observed changes in range and endemicity to those proposed to occur in response to climate change.”

The six scientists—from the Spatial Ecology and Epidemiology Group, the Malaria Public Health and Epidemiology Group, and the Centre for Tropical Medicine of the U.K.’s University of Oxford, plus the Departments of Biology and Geography and the Emerging Pathogens Institute of the University of Florida (USA)—report “comparison of the historical and contemporary maps revealed that endemic/stable malaria is likely to have covered 58% of the world’s land surface around 1900 but only 30% by 2007,” and “even more marked has been the decrease in prevalence within this greatly reduced range, with endemicity falling by one or more classes in over two-thirds of the current range of stable transmission.” They state, “widespread claims that rising mean temperatures have already led to increases in worldwide malaria morbidity and mortality are largely at odds with observed decreasing global trends in both its endemicity and geographic extent.” In fact, they report, “the combined natural and anthropogenic forces acting on the disease throughout the twentieth century have resulted in the great majority of locations undergoing a net reduction in transmission between one and three orders of magnitude larger than the maximum future increases proposed under temperature-based climate change scenarios.”

Given such findings, Gething et al. conclude there has been “a decoupling of the geographical climate-malaria relationship over the twentieth century, indicating that non-climatic factors have profoundly confounded this relationship over time.” They state “non-climatic factors, primarily direct disease control and the indirect effects of a century of urbanization and economic development, although spatially and temporally variable, have exerted a substantially greater influence on the geographic extent and intensity of malaria worldwide during the twentieth century than have climatic factors.” As for the future, they write climate-induced effects “can be offset by moderate increases in coverage levels of currently available interventions.”

Writing that “pathogens cause roughly one in five human deaths, are responsible for 51% of years of life lost globally, and have long affected human demographics,” Dunn et al. (2010) note pathogens “have also been identified as drivers of human behavior, the politics and political stability of countries, human fertility, global economies, and more generally the course and dynamics of human history.” And, somewhat ominously, they report “researchers have linked the presence and prevalence of some pathogens to climate, as has been highlighted in recent discussions of climate change and disease.” They specifically mention malaria, plague, and dengue as examples. Thus, they conducted, as they describe it, “a global analysis of the relative influence of climate, alternative host diversity and spending on disease prevention on modern patterns in the richness and prevalence of human pathogens.”

The U.S., Canadian, and New Zealand researchers found that “pathogen richness (number of kinds) is largely explained by the number of birds and mammal species in a region,” and “the most diverse countries with respect to birds and mammals are also the most diverse with respect to pathogens.” Noting “we are unlikely to be able to change patterns of pathogen richness dramatically,” they observe that “pathogen richness, even when high, does not guarantee high prevalence, because of the potential impact of disease control effort.” In fact, they found “pathogen prevalence is much more sensitive to variation in health spending among regions,” and “importantly, for human health, the prevalence of key human pathogens is strongly influenced by disease control efforts.” Dunn et al. conclude, “even where disease richness is high, we might still control prevalence, particularly if we spend money in those regions where current spending is low, prevalence is high and populations are large.”

Finally, in a brief review of the roles played by various factors that may influence the spread of tick-borne diseases, Sarah Randolph (2010) of the University of Oxford’s Department of Zoology in the United Kingdom begins by noting many vector-borne diseases “have shown marked increases in both distribution and incidence during the past few decades, just as human-induced climate change is thought to have exceeded random fluctuations.” She writes, “this coincidence has led to the general perception that climate change has driven disease emergence.” However, after describing some of the outbreaks of tick-borne disease in Europe over the past couple of decades, Randolph states, “the inescapable conclusion is that the observed climate change alone cannot explain the full heterogeneity in the epidemiological change, either within the Baltic States or amongst Central and Eastern European countries,” citing the work of Sumilo et al. (2007). Instead, she writes, “a nexus of interrelated causal factors—abiotic, biotic and human—has been identified,” and “each factor appears to operate synergistically, but with differential force in space and time, which would inevitably generate the observed epidemiological heterogeneity.”

Many of these factors, she continues, “were the unintended consequences of the fall of Soviet rule and the subsequent socio-economic transition (Sumilo et al., 2008b).” among these factors she cites “agricultural reforms resulting in changed land cover and land use, and an increased reliance on subsistence farming; reduction in the use of pesticides, and also in the emission of atmospheric pollution as industries collapsed; increased unemployment and poverty, but also wealth and leisure time in other sectors of the population as market forces took hold.” In concluding, Randolph writes, “there is increasing evidence from detailed analyses that rapid changes in the incidence of tick-borne diseases are driven as much, if not more, by human behavior that determines exposure to infected ticks than by tick population biology that determines the abundance of infected ticks,” as per the findings of Sumilo et al. (2008a) and Randolph et al. (2008). She ends her brief analysis by stating, “while nobody would deny the sensitivity of ticks and tick-borne disease systems to climatic factors that largely determine their geographical distributions, the evidence is that climate change has not been the most significant factor driving the recent temporal patterns in the epidemiology of tick-borne diseases.”

The studies discussed above, coupled with numerous others referenced in the 2009 report of the Nongovernmental International Panel on Climate Change (Idso and Singer, 2009), suggest there is little of substance in the peer-reviewed scientific literature to support the contention that CO2-induced global warming will elevate human mortality due to an enhanced spreading of vector-borne diseases. In fact, the great bulk of that research tends to refute those claims.


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