The release of greenhouse gases—principally carbon dioxide—into Earth’s atmosphere since the late nineteenth century has contributed to a climate unfamiliar to our species, Homo sapiens. This new climate has already altered the epidemiology of infectious diseases. Continued accumulation of greenhouse gases in the atmosphere will further alter the planet’s climate and the incidence and severity of infections. In some cases, climate change may establish conditions favoring the emergence of infectious diseases, while in others it may render areas that are presently suitable for certain diseases unsuitable. This chapter presents the current state of knowledge regarding the known and prospective infectious-disease consequences of climate change.
The term climate change refers to multi-decadal alterations in temperature, precipitation, wind, humidity, and other components of weather. Over the past 2.5 million years, the earth has warmed and cooled, cycling between glacial and interglacial periods during which average global temperatures moved up and down by 4–7°C. During the last glacial period, which ended roughly 12,000 years ago, global temperatures were, on average, 5°C cooler than in the mid-twentieth century (Fig. 125-1).
Overview of the earth’s temperature and primary greenhouse gases over the last 600,000 years. Variations of deuterium (δD; black) serve as a proxy for temperature. Atmospheric concentrations of greenhouse gases—CO2 (red), CH4 (blue), and nitrous oxide (N2O; green)—were derived from air trapped within Antarctic ice cores and from recent atmospheric measurements. Shaded areas indicate interglacial periods. Benthic δ18O marine records (dark gray) are a proxy for global ice-volume fluctuations and can be compared to the ice core data. Downward trends in the benthic δ18O curve reflect increasing ice volumes on land. The stars and labels indicate atmospheric concentrations as of the year 2000. CO2 levels surpassed 400 ppm as of 2013 and are rising at a rate of 2–2.5 ppm per year. (From Intergovernmental Panel on Climate Change Fourth Assessment Report. Working Group I. Cambridge University Press, 2007, Fig. 6.3.)
The present climate period, known as the Holocene, is remarkable for its stability: temperatures have largely remained within a range of 2–3°C. This stability has enabled the successful population and cultivation of much of the earth’s landmass by humanity. Current climate change differs from that in the past not only because its primary cause is human activities but also because its pace is faster. The current rate of warming on Earth is unprecedented in the last 50 million years. The 5°C of warming that occurred at the end of the last ice age about 12,000 years ago took roughly 5000 years, whereas such a temperature increment may occur within the next 150 years unless the release of greenhouse gases is substantially reduced in coming decades. Climate science, although still ...