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Paul A. T. Higgins

Reducing greenhouse gas emissions almost certainly requires adding a price to those activities that cause emissions. Policy makers have largely overlooked the most direct option, which is to set a price on emissions (an emission fee), and therefore may be missing an opportunity to reduce the risks of climate change. The advantages of emission fees are considerable, because they create a clear price signal to discourage emissions, help reveal who wins and loses from climate policy, are easy to administer, avoid nefarious market manipulation, and offer the potential for extremely strong emissions reductions in response to breakthrough opportunities. But emission fees also have notable disadvantages because they do not ensure limits on emissions, can be framed unfavorably in political debates, remain at an immature stage of policy development, and could be undermined by plausible political compromises. As a result, careful policy design is necessary to maximize the advantages of emission fees to society and to minimize their disadvantages. Critically, policy design can strive for favorable distributional effects, ensure a “safe” level of climate protection, and create the potential for even larger emission reductions if breakthroughs occur. Even then, additional climate policy needs will remain for both emission fee approaches, in particular, and climate change risk management more broadly. Most notably, a family of policies that includes mitigation, adaptation, and possibly geoengineering will be needed for comprehensive management strategies of climate change risks. Nevertheless, emission fees could provide one important component of this larger set of tools for dealing with the threat of climate change.

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Paul A. T. Higgins

Abstract

Biotic responses to climate change may constitute significant feedbacks to the climate system by altering biogeochemistry (e.g., carbon storage) or biophysics (i.e., albedo, evapotranspiration, and roughness length) at the land surface. Accurate projection of future climate change depends on proper accounting of these biological feedbacks. Similarly, projections of future climate change must include the potential for nonlinear responses such as thermohaline circulation (THC) weakening, which is increasingly evident in paleoclimate reconstructions and model experiments. This article uses offline simulations with the Integrated Biosphere Simulator (IBIS) to determine long-term biophysical and biogeochemical responses to climate patterns generated by the third Hadley Centre Coupled Ocean–Atmosphere General Circulation Model (HadCM3) under forced THC weakening. Total land surface carbon storage decreases by 0.5% in response to THC weakening, suggesting that the biogeochemical response would not constitute a significant climate feedback under this climate change scenario. In contrast, large regional and local changes in leaf area index (LAI) suggest that biophysical responses may constitute significant feedbacks to at least local and regional climate. Indeed, the LAI responses do lead to changes in midday direct and diffuse beam albedo over large regions of the land surface. As a result, there are large local and regional changes in the land surface's capacity to absorb solar radiation. Changes in energy partitioning between sensible and latent heat fluxes also occur. However, the change in latent heat flux from the land surface is primarily attributable to changes in precipitation that occur under forced THC weakening and not a result of the subsequent changes in vegetation.

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Paul A. T. Higgins and Jonah V. Steinbuck

Abstract

This study develops a new conceptual tool to explore the potential societal consequences of climate change. The conceptual tool delineates three quasi-independent factors that contribute to the societal consequences of climate change: how climate changes; the sensitivity of physical systems, biological resources, and social institutions to climate change; and the degree of human dependence on those systems, resources, and institutions. This conceptual tool, as currently developed, is not predictive, but it enables the exploration of the dependence of climate change risks on key contributing factors. In exploring a range of plausible behaviors for these factors and methods for their synthesis, the authors show that plausible assumptions lead to a wide range in potential societal consequences of climate change. This illustrates that the societal consequences of climate change are currently difficult to constrain and that high-consequence climate change outcomes are not necessarily low probability, as suggested by leading economic analyses. With careful implementation, this new conceptual tool has potential to increase public understanding of climate change risks, to support risk management decision making, or to facilitate communication of climate risks across disciplinary boundaries.

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Paul A. T. Higgins and John Harte

Abstract

Projections of greenhouse gas concentrations over the twenty-first century generally rely on two optimistic, but questionable, assumptions about the carbon cycle: 1) that elevated atmospheric CO2 concentrations will enhance terrestrial carbon storage and 2) that plant migration will be fast relative to climate changes. This paper demonstrates that carbon cycle uncertainty is considerably larger than currently recognized and that plausible carbon cycle responses could strongly amplify climate warming. This has important implications for societal decisions that relate to climate change risk management because it implies that a given level of human emissions could result in much larger climate changes than we now realize or that stabilizing atmospheric greenhouse gas concentrations at a “safe” level could require lower human emissions than currently understood. These results also suggest that terrestrial carbon cycle responses could be sufficiently strong to account for the changes in atmospheric carbon dioxide that occurred during transitions between ice age and interglacial periods.

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