یکپارچه سازی تجزیه و تحلیل اقتصادی و علم بی ثباتی آب و هوا

Scientific understanding of climate change and climate instability has undergone a revolution in the past decade with the discovery of numerous past climate transitions so rapid, and so unlike the expectation of smooth climate changes, that they would have previously been unbelievable to the scientific community. Models commonly used by economists to assess the wisdom of adapting to human-induced climate change, rather than averting it, lack the ability to incorporate this new scientific knowledge. Here, we identify and explain the nature of recent scientific advances, and describe the key ways in which failure to reflect new knowledge in economic analysis skews the results of that analysis. This includes the understanding that economic optimization models reliant on convexity are inherently unable to determine an “optimal” policy solution. It is incumbent on economists to understand and to incorporate the new science in their models, and on climatologists and other scientists to understand the basis of economic models so that they can assist in this essential effort.

Early analysis of climate change by economists evaluated the transition between two climate equilibriums, or at most, two climate paths that smoothly changed over time from today's climate to one characterized by a doubling of atmospheric concentrations of CO2 and other warming gases since the pre-Industrial Revolution (Mendelsohn et al., 1994, Manne and Richels, 1991 and Gaskins and Weyant, 1993, and references therein). Cline (1992) was the first to extend the analysis beyond a doubling, with CO2 emissions that were derived from simple models of economic growth (Manne and Richels, 1990, Nordhaus and Yohe, 1983 and Reilly et al., 1987). Cline input the CO2 emissions into a simple climate submodel that is used in the natural science literature for long-term analysis of the relationship between warming gases and temperature.2 Cline then presented a sensitivity analysis of the benefits and costs of avoiding climate change with respect to the discount rate that converts future damages and costs into present values. Nordhaus, 1992 and Nordhaus, 1994 developed an economic growth model (called “DICE”) that endogenously calculated the interest rate, coupled with a sophisticated climate submodel, where the social discount rate used for benefit cost analysis of policy alternatives was based upon the endogenous rate of return on capital. Nordhaus' (1994) climate submodel is based upon a model published in the early 1980s that has the feature of climate equilibrium, in which an increase in CO2 will eventually return to initial conditions (Schneider and Thompson, 1981).3 While Nordhaus (1994, p. 26, note 4) acknowledged that his equilibrium climate submodel is not applicable to a greater than doubling of CO2 equivalent gases, his model continues to be the basis for economic analysis beyond a doubling and is extended to analyze abrupt climate change. The science of climate change has advanced considerably in the last quarter of a century as these economic models were developed; yet the new understanding has not been incorporated in economic analysis of climate change. Paleoclimatic and paleoceanographic studies completed during the past decade demonstrate that Earth has experienced dramatic and abrupt environmental change, at scales and rates not experienced during recorded history (Dansgaard et al., 1993, Mayewski et al., 1997 and Alley et al., 2003). Scientific understanding of the nature of past climate change has been continuously advanced as climatic transitions are investigated at ever higher resolution and by application of new analytical techniques. These advances create a challenge for economic models in that specific climatic transitions that were only recently considered to be abrupt, but simple, steps of cooling or warming, have been discovered to actually consist of intervals of rapidly flickering climatic oscillation. Numerous rapid warming and cooling steps during the last 60,000 years have been 1/3 to 1/2 as large as the entire difference between the coldest glacial and warmest Holocene intervals, yet took only decades to years to occur (Alley et al., 1993, Alley and Clark, 1999 and Severinghaus and Brook, 1999). Past decadal-scale increases in local temperature were more than 10 °C at high latitudes, as great as 7 °C at mid-latitudes, and 1–2 °C in the tropics (Alley and Clark, 1999, Hendy and Kennett, 1999 and Lea et al., 2003). Both conventional and new hypotheses attempting to explain the forcing and amplifying mechanisms of such past climate instability have implications for future climate instability related to anthropogenic releases of greenhouse gases. One broadly accepted explanation of climate instability invokes switching of the North Atlantic deep ocean thermohaline circulation that keeps Europe warm and distributes heat from the tropics to higher latitudes. A shutdown or slowdown could cause a step into glacial cooling (Broecker, 1997), whereas resuscitation of circulation patterns similar to those of today could produce rapid warming steps (Ganopolski and Rahmstorf, 2001). A more recent explanation is the “Clathrate Gun Hypothesis” (Kennett et al., 2003). Briefly, they hypothesize that relatively minor changes in thermohaline circulation warmed intermediate-depth ocean waters, causing instability of methane hydrates at depths of 400 to 1000 m, triggering collapse of continental slopes and massive releases of methane (a powerful greenhouse gas in short time frames) that reached the atmosphere. This hypothesis implies that future global warming events can be greatly amplified by the release of vast quantities of methane stored in the sea floor and arctic permafrost. Past climate change was initiated by relatively small changes in the amount and distribution of solar insolation caused by cyclical changes in Earth's position relative to the sun (“orbital forcing”) or by other, still undetermined, forcing agents such as variations in solar output, interplanetary dust, volcanism, etc. (Hays et al., 1976 and Imbrie et al., 1992). The magnitude and rate of the resultant climatic changes, however, do not relate linearly to changes in the forcing function, because Earth's climate functions by stepping between a number of semi-stable operational modes of the connected atmosphere/hydrosphere/cryosphere system (Broecker and Denton, 1989, Denton et al., 1999 and Alley et al., 2003). Climatic flickering or abrupt transitions occur when thresholds in the combined ocean/atmosphere/climate system are reached. The critical values of these thresholds in terms of atmospheric composition, temperature, or oceanic circulation are yet unknown, yet abrupt transitions are features of many climate models dealing with ocean and atmospheric circulation (Manabe and Stouffer, 2000 and Ganopolski and Rahmstorf, 2001). Anthropogenic emissions of greenhouse gases could provoke similar instability by forcing large enough change to reach such a climatic threshold, followed by cascading feedbacks that cause further instability and climatic flickering. Economic analysis of climate policy is predicated upon the modeling assumption of a stable climate equilibrium at the present climate of the Holocene Epoch, an assumption that results in policy analyses concluding in favor of adaptation to climate change and against averting climate change. Environmental economists have not analyzed “climate instability”, although they have considered the possibility of a sudden climatic shift of known magnitude but unknown timing. Yet new scientific understanding is that climate change is frequently characterized by climate flickering and rapid, alternating changes of state that take place on the scale of decades or years (Alley et al., 2002). There is a broad consensus in the paleoclimate community on the timing, nature, rapidity, and geographic extent of these past abrupt climatic changes, in spite of substantial uncertainty and controversy about the triggers, thresholds, and processes involved. Thus far, economists have yet to examine the implications of true climate instability, including annual-scale climate flickering. Instead, economic models only admit a simple change in climate state in order to enable the calculation of an economic optimum. This limitation results in the policy conclusion to adapt to, rather than to avert, climate change.

The scientific understanding of the nature of climate change has gone through a revolution in the past decade with the discovery of numerous past climate transitions so rapid that they would have previously been unbelievable to reputable scientists. Reconstructed from a broad base of well-documented and consistent paleoclimatic records from around the world, large magnitude climatic transitions are now known to have been rapid (years to decades), large (3° to 10 °C over much of Earth's surface), and unpredictably irregular (climatic flickering oscillation, up to 1/2 as large as the entire transition, over periods as short as years). These transitions involved large stepwise shifts in surface and deep ocean circulation, atmospheric temperature, winds, aridity, and in the composition of powerful greenhouse gases that act as amplifying feedback agents. Abrupt jumps in the concentration of the greenhouse gases CH4 and CO2 provide a mechanism for globally synchronous warming events whereas shifts in the stable mode of conveyor-belt circulation directly and strongly influence the climate of the circum-Atlantic Northern Hemisphere. This new paradigm refutes assumptions that future climatic transitions are likely to be simple, gradual, or moderate, or consist of a single, abrupt switch to a new equilibrium state. Numerous positive and negative feedback processes that are beginning to be understood contribute to the irregular flickering that has characterized many past climatic transitions. In order to be accurate and pertinent to policy discussions, models of economic analysis of future global warming scenarios must include the best and most realistic understanding of the nature of global change. Climate instability has important implications for the applicability of some approaches to economic analysis. For economic optimization models with and without policy intervention, prices are endogenous, the most important of which is the interest rate. The endogeneity of the interest rate requires an equilibrium model to equate over time the rate of time preference for consumption to the rate of return on capital. The interest rate is intrinsically important as a basis by which we make economically optimal trade-offs between present and future consumption. If the equilibrium condition is not met, for the rate of time preference to equal the rate of return on capital, then policy conclusions based on economic efficiency arguments are inapplicable because the model is not endogenously selecting the interest rate. With irregular flickering between climate states, characteristic of past climatic transitions, we would expect the destruction of capital stock. If the flickering is forced by human activity, then policy or lack thereof results in the destruction of capital stock and a discontinuity of the rate of return on capital, violating the equilibrium condition. Moreover, with anthropogenically induced climate flickering that destroys capital, ex post of capital destruction, there is no guarantee that the rate of return on capital is positive. We conclude that reliable economic analysis of climate change will require two major alterations to economic analysis. First, economists need to understand and incorporate into their climate submodels the recent advances by paleoclimatologists and paleoceanographers. Second, climate scientists need to understand how economists are modeling damage from climate change, so that they can educate economists about the deficiencies in economic models.