Most of the climate change discussion has centred on global mean temperatures, but it is also important to consider minimum temperatures. The degree to which winters are properly cold has important effects: notably, on the distributions of pests and disease. The temperature a species can tolerate serves as a limit to its expansion, so warm winters can help undesirable creatures to spread into new areas. This is akin to how it is important for a course of antibiotics to kill 100% of the target bacteria. If it does not, a fullblown new infection is likely, once the drugs are discontinued.
The ranges of ants and bees have been extending northward in Europe and North America. Likewise, the populations of ticks carrying Lyme Disease and malarial mosquitoes have been shifting northward, along with those carrying Dengue Fever and Japanese encephalitis. This is part of a general trend in which species being displaced by climatic changes (See: Thomas Lovejoy notes). The fact that whole ecosystems do not move northwards and to higher altitudes at the same rate causes further problems, as predation relationships are disrupted.
These kinds of higher level effects are likely to become better understood as further research is carried out. The depth of information has already increased a great deal: the fourth IPCC report, which is in the process of being released, is based upon a review of more than 1,000 academic studies. The Third Assessment Report, in 2001, was based on about 100.
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