The Importance of Ecosystem Thresholds in Assessing Safe Concentrations of Greenhouse Gases

There is a major strategic challenge in the public debate about global environmental change related to concentrations of greenhouse gases in the atmosphere that might lead to environmentally, socially, and economically unacceptable impacts. This project takes one approach to this problem: avoiding "dangerous anthropogenic interference" and "allowing ecosystems to adapt." But these phrases implicitly assume that the influences of climate change are likely to be gradual and that there will be substantial time for natural resources to adapt or for managers to cope with change. The current state of the science suggests that something quite different may be in the offing. The Intergovernmental Panel on Climate Change (IPCC) and other assessments of possible impacts now agree on two important points. One is that there is already well-documented evidence of the biological and ecological consequences of climate change - in the behavior of migratory birds, in corals bleached from the influence of warming ocean temperatures, in the loss of glaciers to warming air temperatures, and in the loss of sea grass beds to sea level rise. The second is that ecological systems may not in fact change gradually. Modeling studies and the historical record both suggest that changes in ecosystems can be rapid, large, and sometimes irreversible, i.e., there are thresholds that, once crossed, will present serious coping challenges to humans. Moreover, as suggested in a recent National Academy of Sciences (NAS) workshop on "Understanding and Responding to Multiple Environmental Stresses," dealing with threshold responses that may lead to sudden and dramatic change in societal or environmental structure and function will also require that we develop ways to proceed with decision-making despite the many uncertainties associated with thresholds. These observations present serious challenges to the modeling frameworks used in integrated assessment. Not only do the models have to characterize the dynamic behavior of ecosystems as they cross thresholds, but they also have to represent adaptation strategies that are promoted to cope with such sudden or irreversible changes. A major challenge in the discussion over the implications of tipping points and thresholds in natural resources and management systems is what lessons there are for debates over targets for concentrations of greenhouse gases in the atmosphere. Are there levels of greenhouse gases that would protect against ecosystems exhibiting tipping point behavior, for example? How does uncertainty in our knowledge of either the resources or the climate system influence margins of safety? What models and analytical tools are available for conducting the analyses that are needed to address these questions. The JGCRI's suite of integrated assessment models provide a systematic way of simulating different emissions and concentration scenarios that can then be used to investigate the climate triggers for ecological tipping points and thresholds.