Impacts of climate and climate change on health can be direct or indirect. Direct effects that are readily attributed to climate include heat stress and the consequences of natural disasters. However, the resulting burden of disease and injury may be less than that from indirect effects such as disrupted agriculture and reduced food security. Positive and negative effects can be anticipated, but there is insufficient evidence to state confidently what the balance will be (see Chapter 9). We have not attempted to estimate the overall economic costs of climate change impacts on health in the Australia-New Zealand region because there is considerable debate about the derivation and interpretation of monetary costs (see Chapter 19).

Guest et al. (1999) compared heat-related deaths in the five major Australian cities in the period 1977-1990 with those expected under different climate change scenarios (CSIRO, 1996a) for the year 2030. They estimate that greenhouse-induced climate change would increase climate-related deaths in the summer by a small amount, but this would be more than balanced by a reduction in climate-related deaths in the winter. Overall, this resulted in a decrease of 8-12% in climate-attributable mortality under the CSIRO "high" scenario compared to a scenario with no climate changes (but expected population changes).

The only study in New Zealand to date of elevated temperatures and mortality was conducted by Hales et al. (2000). Daily numbers of deaths in Christchurch were compared with measures of weather and ambient particulate pollution from June 1988 to October 1993. Above the third quartile (20.5°C) of summer maximum temperatures, an increase of 1°C was associated with a 1.3% increase (95% confidence interval, 0.4-2.3%) in all-cause mortality and a slightly greater increase in mortality from respiratory conditions. There was no evidence of interaction between the effects of temperature and particulate air pollution. Greater than expected numbers of deaths also occurred during winter days of low temperature, although this was not statistically significant. This suggests that cold-related deaths will be less common in a warmer climate. However, the mechanisms that explain excess winter mortality are not well understood.

The effects of solar ultraviolet (UV) radiation on skin cancer, skin aging, and cataracts of the eye are particularly important in New Zealand and Australia, which already have the highest skin cancer rates in the world (Marks et al., 1989). The etiology of skin cancer is not fully understood, and factors other than sun exposure undoubtedly are involved. However, UV-B is a key factor. Present levels of UV radiation in this region are relatively high and have been increasing in the past 20 years (McKenzie et al., 1999). It is expected with a high degree of confidence that if UV flux at ground level increases at a faster rate as a result of greenhouse-related cooling in the upper stratosphere and subsequent slowdown in the breakdown of ozone-depleting substances, the incidence of melanomas and other skin cancers will increase (Armstrong, 1994; Longstreth et al. 1998). Australians and New Zealanders of pale-skinned European descent will be particularly vulnerable to these effects. This topic is discussed in more detail in Chapter 9.

The numbers of notified cases of arbovirus infections (illnesses caused by insect-borne viruses) have increased in Australia in recent years (Russell, 1998), and exotic insect species such as Aedes albopictus and Aedes camptorhynchus that are competent vectors of viruses such as dengue and Ross River virus have been detected at New Zealand borders (Hearnden et al., 1999).

There is good evidence that the frequency of mosquito-borne infections in this region is sensitive to short-term variations in climate. For example, outbreaks of Ross River fever and Murray Valley encephalitis in southeast Australia tend to follow heavy rainfall upstream in the Murray-Darling catchment (Nicholls, 1993; Maelzer et al., 1999). In other parts of Australia where the predominant vector is the coastal mosquito A. camptorhynchus, variations in sea level also contribute to outbreaks of illness from Ross River virus (Mackenzie et al., 1994). No quantitative estimates have been made of the possible impact of long-term climate change on rates of vector-borne infections. However, present climate change scenarios suggest that parts of Australia and New Zealand will experience conditions that are more favorable to breeding and development of mosquitoes (Bryan et al. 1996). In these areas, warmer conditions will tend to extend the range of reservoir hosts, decrease the extrinsic incubation period of arboviruses, and encourage outdoor exposure of humans (Weinstein, 1997). Therefore, it is expected with a high degree of confidence that the potential for insect-borne illness will increase. Whether this potential is translated into actual occurrence of disease will depend on many other factors, including border security, surveillance, vector eradication programs, and effectiveness of primary health care.

Endemic malaria was present in North Queensland and the Northern Territory until early in the 20th century (Ford, 1950; Black, 1972). Vectors to transmit the disease still are present in that part of Australia, and climate change will favor the spread of these mosquitoes southward (Bryan et al., 1996). The disease-limiting factor at present is the effectiveness of local health services that ensure that parasitemic individuals are treated and removed from contact with mosquitoes. Therefore, climate change on its own is unlikely to cause the disease to return to Australia, unless services are overwhelmed. In New Zealand there currently are no mosquitoes that are capable of transmitting malaria; even under global warming scenarios, the possibility of an exotic vector becoming established is considered to be slight (Boyd and Weinstein, 1996).

Studies of the prevalence of asthma in New Zealand have shown an association with average temperature (Hales et al., 1998). Electorates with lower mean temperatures tend to have lower levels of asthma, after adjusting for confounding factors. The reason is not clear but may be related to exposure to insect allergens. If this were so, warming may tend to increase the frequency of asthma, but too little is known about the causes of the disease to forecast the impact of climate change on asthma in Australia and New Zealand.

Climate change may influence the levels of several outdoor air pollutants. Ozone and other photochemical oxidants are a concern in several major Australian cities and in Auckland, New Zealand (Woodward et al., 1995). In Brisbane, Australia, current levels of ozone and particulates have been associated with increased hospital admission rates (Petroeschevsky et al., 1999) and daily mortality in persons ages 65 and over (Simpson et al., 1999). Outdoor particulate pollution in the winter (largely generated by household fires) has been associated with increased daily mortality in Christchurch, New Zealand (Hales et al., 2000). Formation of photochemical smog is promoted in warmer conditions, although there are many other climatic factors—such as windspeed and cloud cover—that are at least as important as temperature but more difficult to anticipate. A rise in overnight minimum temperatures may reduce the use of fires and hence emissions of particulates, but it is not known how this might affect pollution and population exposures.

Toxic algal blooms may affect humans as a result of direct contact and indirectly through consumption of contaminated fish and other seafood. At present this is not a major public health threat in Australia or New Zealand, but it is an economic issue (because of effects on livestock and shellfish). It could affect very large numbers of people (Oshima et al., 1987; Sim and Wilson, 1997). No work has been carried out in Australia or New Zealand relating the health effects of algal blooms to climate. Elsewhere in the Pacific, it has been reported that the incidence of fish poisoning (resulting from ingestion of fish contaminated with ciguatoxins) is associated with ocean warming in some eastern islands, but not elsewhere (Hales et al., 1999). It is uncertain whether these conditions will become more common in Australia and New Zealand with projected climate change.

Since 1800, deaths specifically ascribed to climatic hazards have averaged about 50 yr^-1 in Australia (Pittock et al., 1999), of which 40% are estimated to be caused by heat waves and 20% each from tropical cyclones and floods. Although this is not necessarily representative of present conditions because of changing population, statistical accounting, and technologies, it is an order of magnitude estimate. This suggests that if heat waves, floods, storm surges, and tropical cyclones do become more intense, some commensurate increase in deaths and injuries is possible. Whether this will occur will depend on the adequacy of hazard warnings and prevention. Statistics from other developed countries with larger populations indicate a recent trend toward increasing damages but decreasing death and injury from climatic hazards. Thus, hazard mitigation is possible, although it must more than outweigh increased exposure resulting from larger populations in hazardous areas.

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