Since the SAR, methods have been developed and applied to the detection of present impacts of 20th-century climate change on abiotic and biotic systems. Assessment of impacts on human and natural systems that already have occurred as a result of recent climate change is an important complement to model projections of future impacts. How can such effects be detected? Such detection is impeded by multiple, often intercorrelated, nonclimatic forces that concurrently affect those systems. Attempts to overcome this problem have involved the use of indicator species to detect responses to climate change and infer more general impacts of climate change on natural systems. An important component of this detection process is the search for systematic patterns of change across many studies that are consistent with expectations on the basis of observed or predicted changes in climate. Confidence in attribution of these observed changes to climate change increases as studies are replicated across diverse systems and geographic regions. Even though studies now number in the hundreds, some regions and systems are underrepresented. However, there is a substantial amount of existing data that could fill these gaps. Organized efforts are needed to identify, analyze, and synthesize those data sets.
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