Because many wild organisms serve as vectors for human diseases, and these diseases are very well documented historically (with records going back hundreds of years), one might think of using the distribution and intensity of disease occurrence as an indicator of shifts in wild vector distributions or altered dynamics of pathogen transmission. Many disease vectors are known to be strongly influenced by climate (e.g., the anopheline mosquitoes that carry malaria).
The problem with using disease records is that the presence of the vector is necessary but not sufficient to cause disease transmission. Socioeconomic factorssuch as sanitation systems, vaccination programs, nutritional conditions, and so forthlargely determine whether the presence of the disease in wild vectors actually leads to outbreaks of disease in nearby human populations. In fact, transmission and virulence of disease are themselves directly affected by climate. Thus, although disease is a potentially important component of climate change impacts, it is not a useful indicator of the direct effects of climate change (see Chapter 9 and Section 19.2).
Crop plants, like plants in general, are more strongly affected by the direct effects of increased atmospheric CO2 than are animals. Increased CO2 alters the physical structures and the carbon/nitrogen balance in plantswhich in turn alters the plant's growth rate, yield, susceptibility to pest attack, and susceptibility to water stress. These effects interact with the effects of climate change itself in complex ways. In addition, the effects of climate change are buffered in agricultural systems as farming methods are altered to adjust to current climate conditions (e.g., irrigation practices, crop varieties used) (see Sections 5.3 and 19.2). A few selected attributes and systems may be possible indicators of climate change effects. Possible traits are leafing dates of grapevines in orchard with old stock, and planting dates of yearly crops in areas that have not changed seed variety over a given length of time.
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