The world today depends on fossil fuels to meet over 80 percent of its energy needs, a simple fact of the way the industrial world has grown up. But dependence brings with it major challenges: rising demand because of economic growth and new consumers; the global distribution of resources; growing concerns about environmental impacts of energy production and use; and the timescales associated with transforming how we produce, deliver and consume energy.

All this places the United States and the world at an energy crossroads.

Meeting the world's hunger for energy without fundamentally altering the global climate, increasing geopolitical tensions or causing serious economic dislocation begs for, indeed requires, new technology solutions.

There is, however, no simple or single technology option: In the coming decades we will need a host of new technologies to diversify our fuel mix and control greenhouse gas emissions, and at the same time not hinder economic growth.

The challenge is large but there is also good reason for optimism-largely fueled by a range of new technologies. Some are ready for deployment. Others, though promising, may be a decade away. And some, while more uncertain and higher risk, could have far-reaching impact.

But this optimism must be tempered with realism. The scale of the energy industry is enormous. Therefore, so must be the scale at which these technologies operate if they are to have a major effect. Scale also translates into time.

Policies will have to be thought through and aligned. Also, since both markets and environmental challenges are global, international cooperation must be integral to effective solutions.

Of special urgency is the risk of climate change from global warming. Using atmospheric greenhouse gas concentrations before the industrial age as the baseline, a "business as usual" energy supply trajectory would nearly double those concentrations by mid-century, locking in average temperature increases of several degrees along with the expectation of severely disruptive impacts on human health and the environment. Such concentrations are thought by most engaged scientists to be at the upper limits of prudence.

Scenarios that address these challenges successfully, in response to policies that price carbon dioxide emissions, call for major advances in three key areas-energy efficiency, transportation fuels that are not petroleum-based and widespread electricity generation that yields little or no carbon dioxide into the atmosphere.

Greatly enhanced energy efficiency provides both the best short-term opportunity for addressing the major energy challenges and an essential component of a long-term strategy-perhaps a 40 to 50 percent reduction in primary energy use compared to mid-century "business as usual" needs, without a major impact on GDP.

But how to get there? The technology pathways for efficiency involve buildings, vehicles and industrial processes. Two-thirds of U.S. electricity is used for residential and commercial buildings.

Improved lighting, HVAC, appliances, active energy management, cogeneration and energy-efficient design could dramatically reduce our power requirements. Also, new approaches such as passive ventilation and daylighting can both reduce energy use and improve comfort.

In addition, new designs for the coming "gigacities" can minimize both energy use and pollution. We can also achieve dramatic improvements in vehicle efficiency. Options include advanced engine design integrated with new approaches to fuel utilization, hybrids and plug-in hybrids, "lightweighting," hydrogen and fuel cells, and others.

Hybrid technology appears ready in the next couple of decades, with further advances in battery technology, to deliver both very good overall efficiency and a considerable reduction in oil requirements. The second technology category includes technology options for alternative transportation fuels. This can include biofuels, conversion of coal or natural gas to liquid fuels, electricity and hydrogen.

Biofuels are currently receiving a great deal of attention, as they are renewable and strongly supported by the agricultural sector. Scientific and technological advances are needed to utilize agricultural and forest waste products and "designer" energy crops effectively and economically.

Such advances look quite promising over the next decade or two. Challenging issues also remain in the design of the appropriate infrastructure from field to fuel and of the regulatory structure for assuring fuel quality. And plug-in hybrids would lead to electricity

becoming a major transportation "fuel."

For the third technology category-electricity production without significant carbon dioxide emissions-we have to think across a wide range of options: nuclear power; renewables, including wind, solar, geothermal and waves; and fossil-fuel use with carbon capture and geological storage.

Nuclear power provides about a sixth of the world's electricity. Expansion will be based on evolutionary improvements of current technologies, such as passive safety systems and new construction techniques. More advanced technologies may include modular gas-cooled reactors for the midterm and possibly,for the long term, novel reactors and fuels that considerably mitigate waste management concerns.

Wind and solar renewables are expanding rapidly and demonstrating considerable cost reduction. Eventually, direct use of solar radiation appears the most promising energy option given the large amount of solar energy reaching the earth.

However, many scientific and technical advances are needed to realize massive deployment: new manufacturing techniques, new materials, new solar conversion processes and new storage technologies that enable use of a large-scale, intermittent energy supply.

Nevertheless,the competitiveness of solar technology in significant markets with high electricity prices is improving rapidly.

Coal can also be a "carbon-free" energy source if most of the produced carbon dioxide is captured and stored geologically. With current technology, this is expensive, but there is much promising research on new ways of converting coal to energy and less expensive carbon dioxide capture.

A major governmentled effort is needed to resolve remaining uncertainties, both technical and regulatory, around long-term geological carbon dioxide storage at large scale. This array of promising technologies-some ready today, others with an excellent prognosis in a decade or so, and still others as higher-risk candidates for "home runs"-offers an optimistic view of our capacity to deal with our energy needs.

However, as already observed, this optimism must take into account other realities. First is the issue of scale. For many of these technologies, overcoming key scientific and technical barriers is only part of the story. If biofuels were, for example, to replace half of current U.S. gasoline use, we would need about a hundred thousand square miles of land.

This raises issues not only of land use, but also of water resources, ecological stewardship, etc. As another illustration of scale: If all of the carbon dioxide emitted by U.S. coal plants today were compressed to a liquid for geological storage, its annual volume would be about 50 percent more than a year's worth of U.S. oil consumption.

These system challenges reflect the enormous scale of the energy enterprise. They will be met only through a complex interplay of multiple technologies, not some "silver bullet."

Second, policies that are synergistic with societal objectives are essential. U.S. energy policy does not currently incorporate societal imperatives such as oil security or climate change risks into energy prices, as it does for a variety of pollutants.

Instead, we face a complex and somewhat idiosyncratic set of incentives and subsidies that advance introduction of "winning" technologies. Also transforming the multi-trillion dollar energy business, with its vast, durable, and rather expensive infrastructure, takes time-about a half century for significant change.

Finally, these key energy challenges are global in nature and will need far more international cooperation than has been evidenced. Climate change risks clearly have global implications and require global solutions.

However, the global nature of the oil market similarly means that increased demand and security concerns of any region ripple through the world's economies.

Energy represents one of this century's grandest challenges:global in scale, powering economic growth, reducing poverty in developing countries, threatening to the environment and to human health, risking geopolitical conflict. Technology is a necessary but not sufficient enabler for resolving these problems.

The right mix of sustained research, technology investments and policies will, however, empower the nation's scientists, technologists and entrepreneurs to respond to these challenges. Getting that mix right will also present an opportunity for building a sustainable energy future for the 21st century and, considering the inherently long lead times, well beyond.