2.2.2.1. Lines of Evidence

Attribution of observed changes in natural systems to the effects of climate change is analogous to attribution of anthropogenic greenhouse gases (GHGs) as causal factors of recent climate trends. Within the climate realm, the lines of evidence are as follows:

1. Knowledge of fundamental processes of atmospheric forcing by different gases and radiative features
2. Geological evidence that shows changes in particular atmospheric gases associated with changes in global climate
3. General circulation models (GCMs) that accurately "predict" climatic trends of the 20th century, based on fundamental principles of atmospheric forcing.
4. Analyses of global mean temperature and precipitation records that indicate large variances within and among station data as a result of genuine climate variance, as well as errors and biases resulting from instrument change, location changes, or local urbanization. There are large differences in the length of records because stations have been added over the century. Total record length may vary widely. This necessitates large-scale analyses that average the effects over many hundreds or thousands of stations so that the true climate signal can emerge.

Analogs in the biological realm are as follows:

1. Knowledge of fundamental responses of organisms to climate and extreme events. This knowledge is based on experimental work in the laboratory on physiological thresholds and metabolic costs of different thermal/ water regimes, as well as experimental work in the field on ecological thresholds and fitness costs of different temperature/water treatments. In addition to these controlled, manipulated experiments, there are onsite observations of individuals and populations before and after particular weather events (e.g., documentation of population evolution of body size in birds caused by a single winter storm or a single extreme drought, or population extinctions of butterflies caused by a single midseason freeze or a single extreme drought year). The biological community generally accepts the assertion that climate is a major influence on the abundances and distributions of species.
2. Geological evidence that shows changes in global mean temperature associated with changes in the distributions of species. Species' ranges typically shifted toward the poles by about 400-2,000 km between glacial and interglacial periods (change of 4°C).
3. Ecological and biogeographic theory and models that accurately "predict" current distributions of species, based on fundamental principles of climatic tolerances.
4. Analyses of biological records starting from the 1700s, when the first researchers began to systematically record the timing of biological events and the locations of species. There are some variances within and among the historical records for any given species or locality as a result of genuine variance of the biological trait as well as small errors resulting from changes in the recorder, methods of recording, local urbanization, and other landscape changes. There are large differences in the length of records because interest in taking such records gradually has increased over the centuries. Total record length may vary from 300 to <10 years. This necessitates large-scale syntheses that assess the effects over many hundreds of species or studies so that any true global climate signal can emerge.

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