Long-term climate change can be observed as a signal standing out against a background of natural climate variability (Fig. 2). To help in determining the meaning of this signal, we need historical climate data to measure natural variability. Because instrument records are available only for the recent past (not quite 150 years), previous climates must be deduced from paleoclimatic records such as tree rings, pollen series, faunal and floral abundances in deep-sea cores, isotope analyses of coral and ice cores, and diaries and other documentary evidence. Results of these analyses show that average North American temperatures in the mid- to late twentieth century appear to have been warmer than during any similar period in the last five centuries and likely the highest in at least the past 1,300 years (Solomon et al. 2007). The increasing temperature trend is accelerating rapidly. From 1906 to 2005, global average temperature rose by 0.74 °C. According to the IPCC, by 2100 health-relevant weather extremes will be very likely (Table 1). The rate of change is faster now than in any period in the last 1,000 years.

Although warming is the average effect across the Earth’s surface, changing temperatures are only part of the story. Higher temperatures evaporate soil moisture more quickly (thus severe droughts), while warm air can hold more moisture and result in heavy rains; such “hydrologic extremes” (floods and droughts) are very much a part of climate change scenarios and therefore of substantial concern to public health professionals. In addition, Arctic and Antarctic ice are melting, thereby releasing vast amounts of water into the oceans, raising their levels (from melting land-based glaciers) and potentially altering the flow of their currents. The weather patterns that result from these and other changes vary greatly from place to place, and over short periods of time, emphasize the importance of climate variability. For these reasons, the term climate change is more accurate than global warming and is the accepted term for these phenomena.

Accordingly, the accelerating temperature changes noted above have been correlated with the Earth system changes. Since 1961, sea level has risen on average by approximately 2 mm per year (Solomon et al. 2007), and snow cover and glaciers have diminished in both hemispheres. Most striking is the extent to which the Arctic ice cap has melted in the past 30 years. These trends are forecast to continue. According to the IPCC, in 90 years sea level will rise between 18 and 59 cm. Extremes of the hydrologic cycle (e.g., floods and droughts) are also expected to accompany global warming trends.

The relationships between human health and environmental changes due to climate change have been reviewed extensively elsewhere (McMichael et al. 2006; Patz et al. 2000, 2005) and are summarized in Fig. 3. Direct effects from a warming climate have been shown in several studies on the correlation between heat waves and excess mortality (Curriero et al. 2002). Natural disasters such as floods, droughts, and intense storms have claimed millions of lives during the past two decades, and affected many more physically, mentally, or through the loss of property or livelihoods (International Federation of Red Cross 1998). Further, the IPCC’s midrange sea level rise projections (a 40-cm rise by the 2080s) will put 200 million people at risk for a range of health problems including displacement, salt water intrusion into fresh water aquifers, or disruption of stormwater drainage and sewage disposal (IPCC 2007). Warmer temperatures are likely to affect air quality through changes in ozone concentrations, a known pulmonary irritant associated with pneumonia, chronic obstructive pulmonary disease, asthma, and premature mortality (Ebi and McGregor 2008). In addition, extensive research suggests that climate change will influence aeroallergens and related human allergic disorders through changes in pollen season (Beggs 2004; Ziska et al. 2011). Nutrition and food security will also be affected through changes in crop yields, unreliability of supplies, and impacts on prices (Battisti and Naylor 2009; Schmidhuber and Tubiello 2007).

Food- and waterborne diseases are likely to become a greater problem as climate changes. For example, flooding can contaminate drinking or recreational water with pollution from sewage lines or agricultural fields (Lipp et al. 2001; Thomas et al. 2006). Heavy rainfall can overwhelm sewage systems and treatment plants, which then discharge excess wastewater directly into surface water bodies (Patz et al. 2008).

Disease outbreaks from most waterborne pathogens are distinctly seasonal and cluster in key watersheds (Curriero et al. 2001). There is strong evidence that links incidence of waterborne outbreaks from pathogens such as Cryptosporidium (MacKenzie et al. 1994), Escherichia coli 0157:H7 (Hrudey et al. 2003), and Campylobacter jejuni (Hrudey et al. 2003) following heavy rains. Storm events of >3 inches of rainfall within 24 h can overwhelm combined sewer systems and lead to an overflow that contaminates recreational and drinking water sources (Patz et al. 2008). For example, levels of E. coli in channels leading from Milwaukee to Lake Michigan can be up to ten times higher in areas where there are no sewage overflows (Fig. 4). Climate change is anticipated to increase the frequency of these occurrences. Regional climate models, for example for the Great Lakes area of the United States, show a 50–120 % increase in sewage overflow events by the end of this century (Patz et al. 2008). This will pose increased hazards to drinking and recreational water quality. In Peru, childhood diarrheal rates increased 200 % during the 1997–1998 El Nino episode, likely due to higher survival time of diarrhea-causing pathogens in combination with an increase in warm-weather behavior such as higher demand for water and less conscientious hygiene practices (Checkley et al. 2000) (Fig. 5). The worldwide average for diarrheal diseases in the future is projected to rise 20 % for the period 2040–2069 and 29 % for 2070–2099 (Kolstad and Johansson 2011).

The relationship between foodborne outbreaks and temperature has been shown for several pathogens and in a variety of geographic settings (Bentham and Langford 2001; Lake et al. 2009; Zhang et al. 2007). For example, (D’Souza et al. 2004) showed that increases in salmonellosis notifications in five Australian cities were related to a rise in the mean temperature in the previous month. Heat contributed to an estimated 30 % of cases of salmonellosis in much of continental Europe, especially when temperatures exceeded a threshold of 6 °C above average (Kovats et al. 2004). A recent re-evaluation of foodborne illness over time in England and Wales confirmed the correlation with temperature in the current and previous week. The study discusses the importance of lowering pathogen loads in livestock through vaccinations of chicken flocks, limits on antibiotic use in cattle to retard development of resistant strains, and improvement of hygiene practices in abattoirs as methods to help curtail foodborne outbreaks (Lake et al. 2009).

Research on climate change impacts on biological systems is a rich field of study with origins in the late eighteenth and early nineteenth centuries when (Bumpus 1899) documented the effects of an extreme winter storm on the selection of body size in house sparrows (Passer domesticus) and (Grinnell 1917) observed the role of temperature in defining the geographic range of many species (Parmesan 2006). More recent studies on geographic range shifts have benefited from long-term observational records of many species taken by dedicated naturalists, for example, Aldo Leopold’s observations on the timing of spring events on a Wisconsin farm (Bradley et al. 1999; Parmesan 2006). The IPCC has documented long-term studies of biologic responses to climate change on every continent (Fig. 6). Less well studied are the cascading relationships between climatic shifts, adaptation, alteration, or extinction of biota, and their resultant impacts on human health. One reason for this lack of continuity may be the dearth of multisystem, multidisciplinary research studies on climate change impacts, and the propensity to simplify complex issues by carving out small, more manageable pieces of the puzzle for investigation. Although highly detailed, system-specific information on biological impacts of climate change are necessary, integration of these biologic studies into a larger ecological web of effects will make them more relevant and useful than treating them in isolation. It is here that we begin to draw connections between species responses to climate change and human health and demonstrate areas in which biology, veterinary medicine, and public health overlap.