The detection and attribution of the early effects of climate change on the health of human populations is a priority. A range of anticipated health effects from climate change and depletion of stratospheric ozone have been described. Some of the direct-acting effects are likely to become evident within the coming decades. For example, an increase in heat wave-related deaths and an increase in ultraviolet radiation-induced skin cancer in some populations may occur soon or are already occurring.
There is good evidence that anthropogenic climate change is already affecting plant growth and distribution. There is also good evidence of climate-related changes in the distribution and behaviour of animal species both within Europe and elsewhere. A study of Edith's checkerspot butterfly in North America found that the species had extended its range north and reduced its range to the south
(164). This study also confirmed that the changes were consistent with observed shifts in climatic bands.
At the global level, patterns of changes in human disease are compatible with the advent of climate change. In particular, increases in vector-borne diseases have been observed in highland regions
(165). It is not possible, however, to attribute these increases directly to observed climate warming, because of the many other environmental changes that have occurred in these regions over recent decades, which affect malaria distribution and incidence (166). An analysis of recent historical malaria data in the highland region of Ethiopia demonstrated an increasing trend in malaria mortality and morbidity over the last two decades (167). Analysis of data indicates that increases in malaria outbreaks in Ethiopia during the past decade were mainly a result of the observed increase in night-time temperatures. Thus, regional climate change appeared to cause the extension of malaria transmission to higher altitudes. In Europe, changes in the northern limit of the European tick, Ixodes ricinus, in Sweden over the last two decades have recently been attributed to observed warming (136).
The time frame for the emergence of the health effects of climate change would depend on several factors. The "incubation" period (delay between environmental event and onset of ill health) ranges from almost zero (storm-induced injury, for example) to weeks or months (vector-borne infections) and to years and decades (ultraviolet radiation-related malignancies).
In addition, some factors influence the ability to detect whether change really is occurring. The extent and quality of information and variability in the background or pre-existing level of disease must be considered. The time when the health effects of climate change can first be detected will depend on two primary determinants:
• the sensitivity of response (the steepness of the rate of increase);
• whether there is a threshold.
The first detectable changes are likely to be in the geographical range (latitude and altitude) of certain vector-borne infectious diseases and/or in the seasonality of these diseases. For example, summertime foodborne infections (such as salmonellosis) may show longer-lasting annual peaks.
If extreme weather events become more frequent (such as heat waves, floods and droughts), then detectability will refer mainly to whether the frequency of such events or exposure has increased. If such events become more or less severe, then changes in the magnitude of the health effects associated with such events could be detected.
Any changes in the levels of nutrition and hunger will be difficult to attribute to climate change per se. Many complex factors influence food production. Temporal trends in production policies, soil degradation and variety of genotypes and phenotypes, along with trends in transport, storage, distribution and marketing, ensure that it remains difficult to discern any influence of climate change on food production.
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