| Interactions between subunits of the climate system (ocean, atmosphere, biosphere, and cryosphere) often lead to emergent properties—behaviors not evident when subunits are viewed in isolation. Abrupt climate change, such as could occur under thermohaline circulation (THC) weakening, constitutes one example that is increasingly evident in the paleoclimate record and model experiments. Ecosystem responses to abrupt climate change would likely feedback to the climate system and alter the flow of ecosystem services upon which humans depend.; Temperature sensitivity to THC is highest in the North Atlantic region, but this dissertation demonstrates that terrestrial ecosystem responses to THC weakening occur throughout the world. For example, precipitation changes in northern South America threaten local species richness. Currently, northern Amazonia has high levels of precipitation and relatively light land-use. For species richness this constitutes a good-good combination of climate and land use, respectively. In contrast, eastern Brazil has low levels of precipitation and heavy land-use (i.e., a bad-bad combination for species richness). THC weakening causes the precipitation patterns for these two locations to switch, but the loss of species richness associated with the good-good to bad-good transition in northern Amazonia far exceeds the gain in species richness associated with the bad-bad to good-bad transition in eastern Brazil. Thus, large losses in species richness occur. Similarly, plausible temperature changes in England threaten the remnant broadleaf deciduous habitat fragments upon which much remaining local biodiversity depends.; The broadly distributed ecosystem responses that occur, particularly due to changes in leaf area, also constitute significant feedbacks to local and regional climate. For example, large changes in the distribution of leaf area lead to local and regional changes in absorbed solar radiation. When globally aggregated, however, the changes in absorbed solar radiation and total terrestrial carbon storage change less than 1 percent. Therefore, accurate assessment of ecosystem responses and feedbacks requires spatially disaggregated analysis and careful consideration of scale.; Finally, biological responses to climate change depend not only on the final state of the climate system but also on the pathway of change. Thus, accurate projection of ecosystem responses to change requires consideration of transient climate responses. |
