During his run for the White House in 2000, Al Gore was interviewed by Physics Today about his views on global warming. Among other things, he said that one of the likely consequences of the increase in global temperature predicted by the climate models then in vogue would be "expanded geographic ranges for diseases like malaria and dengue fever." We contended at that time that he was wrong on this point (see our Editorial of 11 Oct 2000), and subsequent research has continued to suggest the same.
In a major review of mosquito-borne diseases by one of the world's premier authorities on the subject, Reiter (2001) analyzed the history of malaria and dengue fever in an attempt to determine if the incidence and range of influence of these diseases would indeed increase in response to CO2-induced global warming, as claimed by Gore and others. This review indicated that the natural history of these vector-borne diseases is highly complex, and that the interplay of climate, ecology, vector biology and a number of other factors defies definition by the simplistic analyses utilized in models that generate predictions of future geographical changes in these diseases under various global warming scenarios.
That there has, in fact, been a resurgence of these diseases in some parts of the world is true; but, as Reiter notes, it is "facile to attribute this resurgence to climate change." This he shows to be the case via a number of independent analyses that clearly demonstrate that factors associated with politics, economics and human activity are the principal determinants of the spread of these diseases, which factors he describes as being "much more significant" than climate in promoting disease expansion. Consequently, as Reiter concludes, it is inappropriate to use climate-based models to predict the future prevalence of these diseases.
Two years later, Reiter took up the subject again, this time with 19 other scientists as coauthors (Reiter et al., 2003). They began by noting "it has frequently been stated that dengue, malaria, and other mosquito-borne diseases will become common in the United States as a result of global warming (Watson et al., 1996; Jetten and Focks, 1997; Patz et al., 1998; Watson et al., 1998)," undeterred by the fact that the Intergovernmental Panel on Climate Change had played a key role in promoting this claim (as per the two Watson et al. articles). And why were they undeterred? Because they had acquired solid evidence to prove that the IPCC was wrong on this point.
In the summer of 1999, toward the end of a significant dengue outbreak in "los dos Laredos" - Laredo, Texas, USA (population 200,000) and Nuevo Laredo, Tamaulipas, Mexico (population 290,000) - the team of scientists conducted a seroepidemiologic survey to examine factors affecting dengue transmission in the two cities, which lie adjacent to each other on opposite sides of the Rio Grande, but which experience, according to the team, "massive cross-border traffic across three multi-lane bridges." Their work revealed that "the incidence of recent cases, indicated by immunoglobulin M antibody serosurvey, was higher in Nuevo Laredo [16.0% vs. 1.3%], although the vector, Aedes aegypti, was more abundant in Laredo [91% vs. 37%]." Reiter et al. additionally determined that "environmental factors that affect contact with mosquitoes, such as air-conditioning and human behavior, appear to account for this paradox." They found, for example, that "the proportion of dengue infections attributable to lack of air-conditioning in Nuevo Laredo [where only 2% of the homes had central air-conditioning compared to 36% of the homes in Laredo] was 55%," which means that 55% of the cases of dengue in Nuevo Laredo would not have occurred if all households there had had air-conditioning.
Reiter et al. correctly conclude, therefore, that "if the current warming trend in world climates continues, air-conditioning may become even more prevalent in the United States, in which case, the probability of dengue transmission [there] will likely decrease." Likewise, if the economy of Mexico continues to grow (which it has the opportunity to do if the government does not unrealistically curtail the use of fossil fuels), the use of air-conditioners will likely gain momentum there as well, which would likely lead to even greater decreases in the occurrence of dengue fever in that country. It should be abundantly evident to all, therefore, that the development of wealth, which currently is heavily dependent on the availability of inexpensive fossil-fuel-derived energy, will lead to much greater decreases in mosquito-borne diseases such as dengue fever than any attempted manipulation of the world's climate ever would.
Another factor that could come into play in this regard is the CO2-induced potential for change in the concentrations of various "phytochemicals" found in certain plants that provide a degree of protection against diseases such as dengue fever. Idso et al. (2000), for example, grew spider lily plants out-of-doors at Phoenix, Arizona in clear-plastic-wall open-top enclosures that had their atmospheric CO2 concentrations continuously maintained at either 400 or 700 ppm for two consecutive two-year growth cycles. At the ends of each of these two-year periods, the underground bulbs of the plants were harvested and the amounts and concentrations of a number of disease-fighting substances found within them were measured. By this means it was learned that the 75% increase in the air's CO2 concentration resulted in a 56% increase in spider-lily bulb biomass, and that it increased the concentrations of five bulb constituents known to possess antiviral properties.
Mean percentage increases in these concentrations were, in the words of Idso et al., "6% for a two-constituent (1:1) mixture of 7-deoxynarciclasine and 7-deoxy-trans-dihydronarciclasine, 8% for pancratistatin, 8% for trans-dihydronarciclasine, and 28% for narciclasine, for a mean active-ingredient percentage concentration increase of 12%." Combined with the 56% increase in bulb biomass, these percentage concentration increases resulted in a mean active-ingredient increase of 75%. If other plants that produce these particular phytochemicals and still other antiviral agents are likewise stimulated by the ongoing rise in the air's CO2 content, it is possible that peoples' natural resistance to such diseases as dengue fever may be significantly enhanced in a high-CO2 world of the future, simply by eating more of its more health-promoting foods.
In light of these several observations, it would appear that Al Gore and all those who think like him are 180 degrees out of phase with reality on this issue. Their prescription for planetary health would directly attack that which actually enhances a number of health-promoting substances found in the foods we eat [see our Major Report Enhanced or Impaired? Human Health in a CO2-Enriched Warmer World], while producing the negative health consequences that would accompany the lowering of the standard of living that would result from the huge reductions in the usage of fossil fuels that they claim are required to attain their illusory dream of remaking earth's climate in the image of their equally illusory wisdom.
References
Idso, S.B., Kimball, B.A., Pettit III, G.R., Garner, L.C., Pettit, G.R. and Backhaus, R.A. 2000. Effects of atmospheric CO2 enrichment on the growth and development of Hymenocallis littoralis (Amaryllidaceae) and the concentrations of several antineoplastic and antiviral constituents of its bulbs. American Journal of Botany 87: 769-773.
Jetten, T.H. and Focks, D.A. 1997. Potential changes in the distribution of dengue transmission under climate warming. American Journal of Tropical Medicine and Hygiene 57: 285-297.
Patz, J.A., Martens, W.J.M., Focks, D.A. and Jetten, T.H. 1998. Dengue fever epidemic potential as projected by general circulation models of global climate change. Environmental Health Perspectives 106: 147-153.
Reiter, P. 2001. Climate change and mosquito-borne disease. Environmental Health Perspectives 109: 141-161.
Reiter, P., Lathrop, S., Bunning, M., Biggerstaff, B., Singer, D., Tiwari, T., Baber, L., Amador, M., Thirion, J., Hayes, J., Seca, C., Mendez, J., Ramirez, B., Robinson, J., Rawlings, J., Vorndam, V., Waterman, S., Gubier, D., Clark, G. and Hayes, E. 2003. Texas lifestyle limits transmission of Dengue virus. Emerging Infectious Diseases 9: 86-89.
Watson, R.T., Zinyowera, M.C. and Moss, R.H. (Eds.) 1996. Impacts, Adaptations and Mitigation of Climate Change: Scientific-Technical Analyses. Contribution of Working Group II to the Second Assessment of the Intergovernmental Panel on Climate Change (IPCC). Cambridge University Press, Cambridge, UK.
Watson, R.T., Zinyowera, M.C. and Moss, R.H. (Eds.) 1998. The Regional Impacts of Climate Change: An Assessment of Vulnerability. Special Report of the Intergovernmental Panel on Climate Change (IPCC) Working Group II. Cambridge University Press, Cambridge, UK.