The world’s energy supply mix is currently dominated by fossil fuels. Now, coal, petroleum, and natural gas together supply roughly 80 percent of global primary energy demand. Traditional biomass, nuclear energy, and large-scale hydropower account largely for the remainder. Modern forms of renewable energy play only a relatively small role at present (on the order of a few percent of the world’s current supply mix). Energy security concerns—particularly related to the availability of relatively cheap, conventional supplies of petroleum and, to a lesser extent, of natural gas—continue to be important drivers of national energy policy in many countries and a potent source of ongoing geopolitical tensions and economic vulnerability. Nevertheless, environmental limits, rather than supply constraints, seem likely to emerge as the more fundamental challenge associated with continued reliance on fossil fuels. World coal reserves alone are adequate to fuel several centuries of continued consumption at current levels and could provide a source of petroleum alternatives in the future. Without some means of addressing carbon emissions, however, continued reliance on coal for a large share of the world’s future energy mix would pose unacceptable climate-change risks.

Achieving sustainability objectives will require significant shifts in the current mix of supply resources toward a much larger role for low-carbon technologies and renewable energy sources, including advanced biofuels. The planet’s untapped renewable energy potential, in particular, is enormous and widely distributed in industrialized and developing countries alike. In many settings, exploiting this potential offers unique opportunities to advance both environmental and economic development objectives.

Recent developments, including substantial policy commitments, dramatic cost declines, and strong growth in many renewable energy industries are promising. However, significant technological and market hurdles remain and must be overcome for renewable energy to play a significantly larger role in the world’s energy mix. Advances in energy storage and conversion technologies and in enhancing long-distance electric transmission capability could do much to expand the resource base and reduce the costs associated with renewable energy development. Meanwhile, it is important to note that recent substantial growth in installed renewable capacity worldwide has been largely driven by the introduction of aggressive policies and incentives in a handful of countries. The expansion of similar commitments to other countries would further accelerate current rates of deployment and spur additional investment in continued technology improvements.

In addition to renewable means of producing electricity, such as wind, solar, and hydropower, biomassbased fuels represent an important area of opportunity for displacing conventional petroleum-based transportation fuels. Ethanol from sugar cane is already an attractive option, provided reasonable environmental safeguards are applied. To further develop the world’s biofuels potential, intensive research and development efforts to advance a new generation of fuels based on the efficient conversion of lignocellulosic plant material are needed. At the same time, advances in molecular and systems biology show great promise for generating improved biomass feedstocks and much less energy-intensive methods of converting plant material into liquid fuel, such as through direct microbial production of fuels like butanol.

Integrated bio-refineries could, in the future, allow for the efficient co-production of electric power, liquid fuels, and other valuable co-products from sustainably managed biomass resources. Greatly expanded reliance on biofuels will, however, require further progress in reducing production costs; minimizing land, water, and fertilizer use; and addressing potential impacts on biodiversity. Biofuels options based on the conversion of lignocellulose rather than starches appear more promising in terms of minimizing competition between growing food and producing energy and in terms of maximizing the environmental benefits associated with biomass-based transportation fuels.

It will be equally important to hasten the development and deployment of a less carbon-intensive mix of fossil fuel-based technologies. Natural gas, in particular, has a critical role to play as a bridge fuel in the transition to more sustainable energy systems. Assuring access to adequate supplies of this relatively clean resource and promoting the diffusion of efficient gas technologies in a variety of applications is therefore an important public policy priority for the near to medium term.

Simultaneously, great urgency must be given to developing and commercializing technologies that would allow for the continued use of coal—the world’s most abundant fossil-fuel resource—in a manner that does not pose intolerable environmental risks. Despite increased scientific certainty and growing concern about climate change, the construction of long-lived, conventional, coal-fired power plants has continued and even accelerated in recent years. The substantial expansion of coal capacity that is now underway around the world may pose the single greatest challenge to future efforts aimed at stabilizing carbon dioxide levels in the atmosphere. Managing the greenhouse gas ‘footprint’ of this existing capital stock, while making the transition to advanced conversion technologies that incorporate carbon capture and storage, thus represents a critical technological and economic challenge.

Nuclear technology could continue to contribute to future low-carbon energy supplies, provided significant concerns in terms of weapons proliferation, waste disposal, cost, and public safety (including vulnerability to acts of terrorism) can be—and are—addressed.