There is no integrated regime governing efforts to limit the extent of climate change. Instead, there is a regime complex: a loosely-coupled set of specific regimes. We describe the regime complex for climate change and seek to explain it, using interest-based, functional, and organizational arguments. This institutional form is likely to persist; efforts to build a comprehensive regime are unlikely to succeed, but experiments abound with narrower institutions focused on particular aspects of the climate change problem. Building on this analysis, we argue that a climate change regime complex, if it meets specified criteria, has advantages over any politically feasible comprehensive regime. Adaptability and flexibility are particularly important in a setting—such as climate change policy—in which the most demanding international commitments are interdependent yet governments vary widely in their interest and ability to implement them. Yet in view of the serious political constraints, both domestic and international, there is little reason for optimism that the climate regime complex that is emerging will lead to reductions in emissions rapid enough to meet widely discussed goals, such as stopping global warming at two degrees above pre-industrial levels.

Technological choices largely determine the long-term characteristics or industrial society, including impacts on the natural em·ironmcnt. However. the treatment o[ technology in existing models that are used to project economic and environmental [utures remains highly stylized. Based on work over two decades at llASA. we present a use[ul typology for technology analysis and discuss methods that can be used to analyze the impact or technological changes on the global environment, especially global warming. Our [ocus is energy technologies. the main sou rce or many atmospheric environmental problems. We show that much improved treatment o[technology is possible with a combination o[historical analysis and new modeling techniques. In the historical record , we identify characteristic "learning rates" that allow simple quantified characterization or the improvement in cost and performance due to cumulati,·e experience and investments. We also identi[y patterns. processes and timescales that typi[y the diffusion of new technologies in competitive markets. Technologies that are long-lived and are components of interlocking networks typically require the longest tim e to diffuse and co-evolve with other technologies in the network ; such network effects yield high barriers to entry even [o r superior competitors.These simple observations allow three improvements to modeling of technological change and its consequences for global environmental change. One is that the replacement of long-lived infrastructures over time has also replaced the fuels that power the economy to yield progressively mo re energy per unit of carbon pollution -from coal to oil to gas. Such replacement has "deca rbonizcd " the global primary energy supply 0.3'Yo per year. In contrast. most baseline projections for emissions of carbon, the chief cause of global warming. ignore this robust historical trend and show little or no decarbonization . A second improvement is that by incorporating learning cun·es and uncertainty into micro scale models it is possible to e11do!Je11011sly generate patterns of technological choice that mirror the real world. Those include S-shaped diffusion patterns and timescales of technological dynamics that arc consistent with histo rical experience: they also include endogenous generation o["surprises" such as the appearance of radically new technologies. Third. it is possible to include learning phenomena stylistically in macro-scale models; we show that doing so can yield projections with lessened cm·ironmcntal impacts without necessa rily incurring negative effect on the economy. Arriving on that path by the year c JOO depends on intenening actions. such as incentives to promote greater diversity in technology and lower barriers to entry for new infrastructures that could accelerate historical trends or dccarbonization. ['

This article examines the implications of the rising density of international institutions+ Despite the rapid proliferation of institutions, scholars continue to embrace the assumption that individual regimes are decomposable from others+ We contend that an increasingly common phenomenon is the "regime complex:" a collective of partially overlapping and nonhierarchical regimes+ The evolution of regime complexes reflects the influence of legalization on world politics+ Regime complexes are laden with legal inconsistencies because the rules in one regime are rarely coordinated closely with overlapping rules in related regimes+ Negotiators often attempt to avoid glaring inconsistencies by adopting broad rules that allow for multiple interpretations+ In turn, solutions refined through implementation of these rules focus later rounds of negotiation and legalization+ We explore these processes using the issue of plant genetic resources~PGR!+ Over the last century, states have created property rights in these resources in a Demsetzian process: as new technologies and ideas have made PGR far more valuable, actors have mobilized and clashed over the creation of property rights that allow the appropriation of that value+ International institutions have proliferated rapidly in the postwar period+ As new problems have risen on the international agenda, the demand for international regimes has followed+ 1 At the same time, international norms have become more demanding and intrusive-new rules on human rights, intellectual property, and food safety, for example, exert influence on national policies far "behind the bor-der+" 2 The growing density of international institutions, coupled to their newfound intrusiveness, has also been accompanied by a shift in political processes+ Governance systems dominated by elites have given way to more participatory We are grateful for comments on early drafts presented at Stanford Law School,

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