Numerous recent changes in the biotic and abiotic environment can be attributed to recent global change processes. Among the most important drivers of change are climate (temperature, rainfall, extreme events), land-use change (deforestation, urbanization, land degradation), the invasion of alien species, and pollution. Many different data sources are used for statistical inference of impacts, which in many cases yields robust results. A bias might be suspected because there are many more studies of changing systems than there are of stable systems, but the majority of correlations are well supported by explanations of the underlying mechanisms.
Organisms react to changes in the environment in multiple ways: Their phenology (the timing of critical stages in their life cycle) is affected by the seasonal development of environmental conditions such as ambient temperature and moisture availability. Their spatial distribution is affected by the geographical range of suitable conditions. Their morphology and physiology are responsive to temperature, moisture, and the availability of nutrients (including CO2), while their risk of extinction is a result of both average and extreme event conditions in climate (e.g., frost or drought). Because these drivers show different changes in different regions, the actually observed attributable changes differ between major ecosystems and are presented by major biomes in the following subsections.
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