Path to zero carbon emissions: Models show that to avert dangerous levels of climate change, global carbon dioxide emissions must fall to zero later this century. Most of these emissions arise from energy use. Daviset al.review what it would take to achieve decarbonization of the energy system. Some parts of the energy system are particularly difficult to decarbonize, including aviation, long-distance transport, steel and cement production, and provision of a reliable electricity supply. Current technologies and pathways show promise, but integration of now-discrete energy sectors and industrial processes is vital to achieve minimal emissions.Science, this issue p.eaas9793

CO ₂ emissions from the burning of fossil fuels are the primary cause of global warming. Much attention has been focused on the CO ₂ directly emitted by each country, but relatively little attention has been paid to the amount of emissions associated with the consumption of goods and services in each country. Consumption-based accounting of CO ₂ emissions differs from traditional, production-based inventories because of imports and exports of goods and services that, either directly or indirectly, involve CO ₂ emissions. Here, using the latest available data, we present a global consumption-based CO ₂ emissions inventory and calculations of associated consumption-based energy and carbon intensities. We find that, in 2004, 23% of global CO ₂ emissions, or 6.2 gigatonnes CO ₂ , were traded internationally, primarily as exports from China and other emerging markets to consumers in developed countries. In some wealthy countries, including Switzerland, Sweden, Austria, the United Kingdom, and France, >30% of consumption-based emissions were imported, with net imports to many Europeans of >4 tons CO ₂ per person in 2004. Net import of emissions to the United States in the same year was somewhat less: 10.8% of total consumption-based emissions and 2.4 tons CO ₂ per person. In contrast, 22.5% of the emissions produced in China in 2004 were exported, on net, to consumers elsewhere. Consumption-based accounting of CO ₂ emissions demonstrates the potential for international carbon leakage. Sharing responsibility for emissions among producers and consumers could facilitate international agreement on global climate policy that is now hindered by concerns over the regional and historical inequity of emissions.

What We're In For: What would our climate future look like if we stopped building any new infrastructure that used fossil fuels? Davis et al. (p. 1330 ; see the Perspective by Hoffert ) made the assumption that all existing infrastructure will be used until the end of its lifetime, after which it will be replaced by infrastructure that does not produce CO ₂ . They then calculated the amount of additional CO ₂ that will be added to the atmosphere. The outcome is that the concentration of atmospheric CO ₂ would remain below 430 parts per million (ppm) (present levels are 390 ppm) and global mean temperatures would increase to 1.3°C above preindustrial values (about 0.5°C above present), well below current targets of 450 ppm and 2°C, respectively.

Existing technologies, institutions, and behavioral norms together act to constrain the rate and magnitude of carbon emissions reductions in the coming decades. The inertia of carbon emissions due to such mutually reinforcing physical, economic, and social constraints is referred to as carbon lock-in. Carbon lock-in is a special case of path dependency, which is common in the evolution of complex systems. However, carbon lock-in is particularly prone to entrenchment given the large capital costs, long infrastructure lifetimes, and interrelationships between the socioeconomic and technical systems involved. Further, the urgency of efforts to avoid dangerous climate change exacerbates the liability of even small lock-in risks. Although carbon lock-in has been recognized for years, efforts to characterize the types and causes of carbon lock-in, or to quantitatively assess and evaluate its policy implications, have been limited and scattered across a number of different disciplines. This systematic review of the literature synthesizes what is known about the types and causes of carbon lock-in, including the scale, magnitude, and longevity of the effects, and policy implications. We identify three main types of carbon lock-in and describe how they coevolve: (a) infrastructural and technological, (b) institutional, and (c) behavioral. Although each type of lock-in has its own set of processes, all three are tightly intertwined and contribute to the inertia of carbon emissions. We outline the conditions, opportunities, and strategies for fostering transitions toward less-carbon-intensive emissions trajectories. We conclude by proposing a carbon lock-in research agenda that can help bridge the gaps between science, knowledge, and policy-making.

Seeking systems-based solutions: Without sustainable solutions, the world's most pressing environmental concerns will continue to persist or worsen. Achieving the goal of sustainability involves so many factors—from economics to ecology—that investigating one or even a handful of variables at a time often overlooks major parts of the problem. Liuet al.review systems-based approaches that are beginning to provide tenable ways to assess sustainability. Further integrating coupled human and natural components of a problem across multiple dimensions, including how one solution can create unintended consequences elsewhere, is essential for developing effective policies that seek global sustainability.Science, this issue10.1126/science.1258832