CFE agenda 030915 - Laserfiche WebLink

Browse Search

RESEARCH ( RESEARCHARTICLE v v 0 cca v Process X Globally mixed, with continental /global threshold Planetary Control variable Safe operating space Global feedbacks W. W v 0 Local /regional a impacts Process Y Heterogeneous, with no continental /global threshold Planetary Zone of uncertainty: Increasing risk of impacts Control variable Dangerous level: High risk of serious impacts Local /regional thresholds Fig. 1. The conceptual framework for the planetary boundary approach, showing the safe operating space, the zone of uncertainty, the position of the threshold (where one is likely to exist), and the area of high risk. Modified from (1). but also allows society time to react to early warn- ing signs that it may be approaching a thresh- old and consequent abrupt or risky change. The developing science of early- warning signs can warn of an approaching threshold or a de- crease in the capability of a system to persist under changing conditions. Examples include "critical slowing down" in a process (22), in- creasing variance (23), and flickering between states of the system (24-26). However, for such science to be useful in a policy context, it must provide enough time for society to respond in order to steer away from an impending thresh- old before it is crossed (27 28). The problem of system inertia —for example, in the climate sys- tem (18) —needs to be taken into account in as- sessing the time needed for society to react to early- warning signs. Not all Earth - system processes included in the PB approach have singular thresholds at the global/ continental /ocean basin level (1). Nevertheless, it is important that boundaries be established for these processes. They affect the capacity of the Earth system to persist in a Holocene -like state under changing conditions (henceforth "resilience ") by regulating biogeochemical flows (e.g., the ter- restrial and marine biological carbon sinks) or by providing the capacity for ecosystems to tolerate perturbations and shocks and to continue func- tioning under changing abiotic conditions (29, 30). Examples of such processes are land- system change, freshwater use, change in biosphere in- tegrity [rate of biodiversity loss in (1, 2)], and changes in other biogeochemical flows in addi- tion to carbon (e.g., nitrogen and phosphorus). Placing boundaries for these processes is more difficult than for those with known large -scale thresholds (21) but is nevertheless important for maintaining the resilience of the Earth system as a whole. As indicated in Fig. 1, these processes, many of which show threshold behavior at local and regional scales, can generate feedbacks to the processes that do have large -scale thresholds. The classic example is the possible weakening of natural carbon sinks, which could further de- stabilize the climate system and push it closer to large thresholds [e.g, loss of the Greenland ice sheet (18)]. An interesting research question of relevance to the PB framework is how small - scale regime shifts can propagate across scales and possibly lead to global -level transitions (31, 32). A zone of uncertainty, sometimes large, is as- sociated with each of the boundaries (yellow zone in Fig. 1). This zone encapsulates both gaps and weaknesses in the scientific knowledge base and intrinsic uncertainties in the functioning of the Earth system. At the "safe" end of the zone of un- certainty, current scientific knowledge suggests that there is very low probability of crossing a critical threshold or substantially eroding the re- silience of the Earth system. Beyond the "danger" end of the zone of uncertainty, current knowl- edge suggests a much higher probability of a change to the functioning of the Earth system that could potentially be devastating for human societies. Application of the precautionary prin- ciple dictates that the planetary boundary is set at the "safe" end of the zone of uncertainty. This does not mean that transgressing a boundary will instantly lead to an unwanted outcome but that the farther the boundary is transgressed, the higher the risk of regime shifts, destabilized sys- tem processes, or erosion of resilience and the fewer the opportunities to prepare for such changes. Observations of the climate system show this principle in action by the influence of in- creasing atmospheric greenhouse gas concentra- tions on the frequency and intensity of many extreme weather events (17 18). Linking global and regional scales PB processes operate across scales, from ocean basins/biomes or sources /sinks to the level of the Earth system as a whole. Here, we address the subglobal aspects of the PB framework. Rock - str6m et al. (1) estimated global boundaries on- ly, acknowledging that the control variables for many processes are spatially heterogeneous. That is, changes in control variables at the subglobal level can influence functioning at the Earth - system level, which indicates the need to define subglobal boundaries that are compatible with the global -level boundary definition. Avoiding the transgression of subglobal boundaries would thus contribute to an aggregate outcome within a planetary -level safe operating space. We focus on the five PBs that have strong re- gional operating scales: biosphere integrity, biogeo- chemical flows [earlier termed "phosphorus (P) and nitrogen (N) cycles" (1, 2)], land- system change, freshwater use, and atmospheric aerosol loading. Table S1 describes how transgression of any of the proposed boundaries at the subglobal level affects the Earth system at the global level. For those processes where subglobal dynamics potentially play a critical role in global dynamics, the operational challenge is to capture the im- portance of subglobal change for the functioning 1259555 -2 13 FEBRUARY 2015 • VOL 347 ISSUE 6223 sciencemag.org SCIENCE