Abrupt non-linear climate change, irreversibility and surprise
Introduction
Strictly speaking, a surprise is an unexpected outcome. In the context of climate change, a surprise would be said to have occurred if a climate change-induced phenomenon with a very low probability of happening became reality or if an event never before imagined took place. In IPCC (1996), “surprises” are defined as rapid, non-linear responses of the climatic system to anthropogenic forcing, such as a collapse of the “conveyor belt” circulation in the North Atlantic Ocean or rapid deglaciation of polar ice sheets. Potential climate change, and more broadly, global environmental change, is replete with such surprises because of the enormous complexities of the processes and inter-relationships involved (such as coupled ocean, atmosphere, and terrestrial systems) and our insufficient understanding of them.
Unfortunately, most climate change assessments rarely consider low-probability, but high-consequence extreme events. Instead, they primarily consider scenarios that supposedly “bracket the uncertainty” rather than explicitly integrate unlikely events from the “tails of the distribution.” Although researchers may recognize the wide range of uncertainty surrounding global climate change, their analyses are typically surprise free. Thus, decision-makers reading the “standard” literature will rarely appreciate the full range of possible climate change outcomes, and thus might be more willing to risk adapting to prospective changes rather than attempting to avoid them through abatement than they would be if they were aware that some potentially unpleasant surprises could be lurking. (Pleasant ones might occur as well, but many individuals and policy-makers, via insurance premiums, tend to insure against negative outcomes preferentially.) In fact, it is not even clear all such surprises are actually “low probability”, just very uncertain at this point given the state of knowledge is still evolving.
Events that are not truly unanticipated are better defined as imaginable abrupt events. For other events–true surprises–while the outcome may be unknown, it may be possible to identify imaginable conditions for surprise. For example, if the rate of change of CO2 concentrations is one imaginable condition for surprise (i.e., more rapid forcing increases the chances for surprises), we can deduce that the system would be less likely to undergo a “surprise” event if decision-makers chose as a matter of policy to slow down the rate at which human activities modify the atmosphere. To deal with such questions, the policy community needs to understand both the potential for surprises and how difficult it is for integrated assessment models (IAMs), and other models as well, to credibly evaluate the probabilities of currently imaginable “surprises”, let alone those not currently envisioned (Schneider et al., 1998).
Section snippets
“Imaginable surprises”: examples of abrupt non-linear response
Most global systems are inherently complex, consisting of multiple interacting sub-units. Scientists frequently attempt to model these complex systems in isolation, often along distinct disciplinary lines, producing internally stable and predictable behaviour. However, real-world coupling between sub-systems can cause sets of interacting systems to exhibit new collective behaviours—called “emergent properties”—that may not be clearly demonstrable by models that do not include such coupling.
Other limitations of the standard assessment paradigm
Most climate assessments also do an inadequate job of incorporating more near-term phenomena, including the transient effects of climate change as well as and impacts of changes in climate variability.
Incorporating non-linearities and surprises into damage estimates
A critical issue in climate change policy is costing climatic impacts, particularly when the possibility for non-linearities, surprises, and irreversible events is allowed. The assumptions made when carrying out such estimations largely explain why different authors obtain different policy conclusions. These issues are explored in the next several sections.
Discounting
Discounting plays a crucial role in the economics of climate change, yet it is a highly uncertain parameter. Discounting is a method of aggregating costs and benefits over a long time horizon by summing across future time periods net costs (or benefits) that have been multiplied by a discount rate, typically greater than zero. If the discount rate equals zero, then each time period is valued equally (case of infinite patience). If the discount rate is infinite, then only the current period is
Concluding remarks
The implications of such very long-term potential irreversibilities—melting ice caps, the shut-off of THC, and extinction of species, to name a few—are precisely the kinds of non-linear events that would likely qualify as “dangerous anthropogenic interference with the climate system” under the United Nations Framework Convention on Climate Change (e.g., see Mastrandrea and Schneider, 2004, for a review and analysis of a probabilistic framework for addressing the UNFCCC mandate). Whether a few
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