Brussels - The International Energy Agency, a group that advises industrialized countries, said Friday in a report that investments of at least $45 trillion might be needed over the next half-century to prevent energy shortages and greenhouse gas emissions from slowing economic growth.

    Nobuo Tanaka, the agency's executive director, called for "immediate policy action and technological transition on an unprecedented scale."

    Mr. Tanaka said the world would "essentially require a new global energy revolution which would completely transform the way we produce and use energy."

    The report sends a strong warning that the combination of growing demand for energy in countries like China and India, the dangers of climate change and the scarcity of resources are going to require huge shifts in the global economy. Countries will have to overcome objections to building nuclear power plants and to storing large amounts of carbon dioxide underground or beneath the ocean floor.

    The report also described emissions-cutting pathways that broadly match the advice of some leading scientists who have recommended cutting emissions in half by 2050 as a way of avoiding devastating climate change. Environment ministers from the Group of 8 industrialized countries have backed a 50 percent target. The ministers said governments should endorse that target at a G-8 summit in July.

    Among the International Energy Agency's chief messages is that current policies are unsustainable with carbon dioxide emissions expected to rise 130 percent and demand for oil expected to rise 70 percent by 2050. Tanaka warned that oil demand could be five times the current production of Saudi Arabia by that time.

    A crucial problem is that the rising cost of oil and gas is prompting a switch to coal, particularly in India and China. Coal is cheap and plentiful but its increasing use is contributing to the accelerating growth in emissions of carbon dioxide.

    The International Energy Agency offers advice on energy policy to its 27 member nations, including the United States, Canada, Japan, Australia, New Zealand, South Korea and most of Europe. It recommended taking measures now that would ensure that carbon emissions are down to at least present-day levels by mid-century by, among other strategies, using energy efficiency measures and reducing emissions from power generation.

    The agency also mapped out a second situation aimed at bringing emissions to half their current levels by mid-century by emphasizing technologies and strategies for "weaning the world off oil." The agency estimated the cost of that process at $45 trillion, or 1.1 percent of annual global output, over the period to 2050. Investments of $100 billion to $200 billion would be needed each year over the next 10 years, rising to $1 trillion to $2 trillion each year in the coming decades.

    To reach the goal of halving emissions, it said, among the most important measures would be equipping more than 50 gas and coal power plants each year with equipment to capture and sequester carbon dioxide. There would also be a need for 32 new nuclear plants each year, while the number of wind turbines would need to increase by 17,500 a year.

    Other strategies included accelerating the development of solar electricity and so-called second-generation biofuels that do not compete with food for farmland.

    The report acknowledged that numerous objections to these technologies would need to be overcome, in particular local opposition to building nuclear power plants and long-term nuclear waste repositories.

    But the most difficult and costly step, it said, would be reducing carbon emissions from transportation at a time when the use of cars, airplanes and ships would still be growing rapidly but few technologies would exist to limit emissions from those sources.

    Even so, Mr. Tanaka sounded an optimistic note, saying that although a global energy technology revolution "will be a tough challenge" it was "both necessary and achievable."

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