Widespread changes in phenology (particularly leaf canopy development) have been observed both from ground-based networks and from satellites for over two decades, indicating earlier onset of spring, later autumn, and a lengthening ofthe vegetative season in most northern areas - trends that are well correlated with observed warming. Many animal species (e.g., birds, butterflies) have adapted their life cycle (migration, breeding, etc.) in similar ways, either as a consequence of earlier vegetation development or triggered by temperature directly. Several meta-analyses of these observations demonstrate that, in the Northern Hemisphere, spring has arrived between 2.3 and 5.2 days earlier for every decade during the last 30 years. For example, swallows have arrived 2-3 days earlier for an increase in March temperature of 1 °C in England. Plants and birds adapted to early arrival generally show stronger reactions to warming than others - probably because they are more strictly limited by temperature than by other factors. Autumn arrives somewhat later, although the trend is less clear. Phenological indicators have a particular significance due to the wide range of organisms they cover and also because they are largely unbiased, that is, numerous species are observed irrespective of their expected rate and direction of change.
There are some counterexamples to these trends (lack of response, or response in a direction from climate change) in species of birds and small mammals, demonstrating that the linkage between environment and phenology is complex and involves many confounding factors. One such complication arises when the potential climatic range of a species shifts due to an atmospheric warming trend, but where no suitable habitat exists for whatever reason. In such cases, the low-latitude boundary may shift as expected, but similar area is not gained at higher latitude.
In some mountainous areas, shifts toward higher altitudes have been observed for some species and communities. These have mostly been detrimental for high-alpine species for which no further habitat exists. At lower altitudes, the effect is less clear, mostly because trends in tourism, agriculture, and forestry affect ecosystems as well.
The attribution of individual species' extinctions to climate change has been made for some amphibians and butterflies - in a statistical sense, it is very likely, however, that climate change has increased the extinction risk (and the extinction rate) for many species worldwide. A proportion of current extinctions is due to invasive species. While most invasions occur due to intended or unintended introduction by people, some cases of invasion have likely been enhanced by climate change.
Attribution of changes in agricultural and forestry systems to climate change during recent decades is difficult due to the intensive transformations of most systems due to improved technology and management during recent decades. Studies in Europe have nevertheless documented advancements of the agricultural calendar for certain crops (notably grapes) that are mostly due to climate change (Figure 1). There are also indications of pests responding positively to warming requiring greater effort for pest control. In India, a reduction in expected rice production increases has been attributed to increasing night-time temperatures.
In forests, part of the recent increase in productivity in Northern Europe and North America can be attributed to warming and enhanced atmospheric CO2 concentrations. Semiarid regions in Southern Europe have seen declines in forest productivity, which can be linked to higher temperatures and moisture losses, a particular example being the extreme summer of 2003. Mediterranean woodlands experience strong increases in wildfire frequency and intensity, which is partly due to land abandonment but also to higher temperatures and drier soils. Also for trees, pests increase in some areas due to warming.
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