Interpretation of changes in marine organisms is difficult because of the strong influence of oceanic currents on dispersal and local temperatures. As the links between ocean currents and atmospheric conditions become better understood, linking changes in marine biota to climate change will become easier.
Tree rings provide long series of yearly data spanning centuries, and data are easily replicated across taxa and geographic regions. The width of an annual ring indicates growth for that year, but growth is affected by disease, herbivory, acidification, nitrification, and atmospheric conditions [carbon dioxide (CO2), ozone (O3), ultraviolet (UV) radiation], as well as by yearly climate (Bartholomay et al., 1997; Jacoby and D'Arrigo, 1997; Briffa et al., 1998). Correlative studies can be conducted to assess the relative impacts of different climatic variables on tree rings by focusing on the 20th century, for which independent climate data exist. If the correlations are strong, one can then attempt to reconstruct past climates (prior to the existence of climate stations) from the derived relationship. One cannot distinguish the primary cause of changes in ring width in any single case (Vogel et al., 1996; Brooks et al., 1998). However, because excellent geographic replication is possible, these complex causal factors can be statistically reduced to those with very large-scale effects; general global climatic conditions are one of the few factors that could simultaneously affect very distant organisms (Feng and Epstein, 1996; Tessier et al., 1997; Briffa et al.,1998).
Finally, an evolutionary response (a genetic change in a population/ species) is possible (Berthold and Helbig, 1992; Rodríguez-Trelles et al., 1996, 1998a). Modern molecular techniques make it possible to sequence DNA from small samples taken from museum specimens, which could then be compared to the DNA of current populations. Unfortunately, for most species, scientists do not yet know which genes are associated with climatic adaptations, so this method cannot provide useful data for more than a handful of species that have been intensively studied genetically (Rodríguez-Trelles et al., 1996; Rodríguez-Trelles and Rodriguez, 1998).
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