The most pronounced effects of climate change on vector-borne diseases such as malaria or dengue fever will undoubtedly occur where the diseases are newly introduced at the edges of the vector range and people have little resistance built up. In Africa, this will often be at higher elevations that were formerly too cold to support these diseases. Increasing numbers of malaria cases have already been reported in the highlands of Madagascar and Ethiopia as a result of warming; in Rwanda, record high temperatures and rainfall in 1987 brought malaria into the highlands where local residents had no immunity. These incidents have led public health officials to fear that relatively small increases in temperature from global warming could spread malaria into large urban centers such as Nairobi, Kenya, and Harare, Zimbabwe, that currently lie just outside of the malaria range [292] [293].
Under a similar scenario, malaria and dengue fever could spread into large swaths of the temperate zone where populations now lack resistance. Rough models of the spread of malaria affected by global warming show that malaria prevalence may increase by 50 million to 80 million cases per year with an associated 3° C rise in average global temperature by the year 2100 [294].
Other vector-borne diseases such as schistosomiasis, Chagas disease, sleeping sickness, river blindness, and various strains of encephalitis all could change their ranges and patterns of infection in the course of climate change. For example, recent modeling of the response of schistosomiasis to current global warming trends suggests that an additional 5 million cases will appear per year by 2050 [295]. Another recent study predicted that the population of black flies that carry river blindness could increase as much as 25 percent if temperature and precipitation patterns change in the manner predicted by some climate models [296]. Waterborne diseases including cholera and the suite of diarrheal diseases caused by organisms such as giardia, salmonella, and cryptosporidium could also be affected as precipitation patterns change, altering the dynamics of water courses and human access to water supplies and sanitation [297].
In addition, the changing temperature and rainfall patterns and the increasing CO2 levels projected to accompany climate change will undoubtedly have important effects on global agriculture, and thus on human nutrition. Determining how climate change will affect world agriculture is every bit as complex as determining its effects on infectious disease, and every bit as speculative. A variety of effects will inevitably occur, and these will vary greatly by region, resulting in more favorable agricultural conditions in some areas and less favorable conditions in others.
On the positive side, higher atmospheric CO2 levels are expected to have a “fertilizing” effect on some plants, increasing their growth rate and cutting transpiration rates, reducing their water demand. Increasing temperatures may bring longer growing seasons to some high-latitude farming regions, increasing yields and expanding the range of crops that can grow there. Higher rainfall in some areas might enable higher production from unirrigated land and more water for irrigation in these areas [298].
On the other hand, higher temperatures and diminished rainfall could reduce soil moisture in many areas, particularly in some tropical and midcontinental regions, reducing the water available for irrigation and impairing crop growth in nonirrigated regions. For example, drier summers and more frequent hot spells in the North American corn belt might reduce yields substantially, although it might extend the corn-growing region northward. Reduced rainfall in already arid regions like sub-Saharan Africa could have very negative consequences for agriculture in areas that can ill afford to lose production [299].
