"Meeting this target of 50 percent cut in emissions represents a formidable challenge, and we would require immediate policy action and technological transition on an unprecedented scale," IEA Executive Director Nobuo Tanaka said.
A U.N.-network of scientists concluded last year that emissions have to be cut by at least half by 2050 to avoid an increase in world temperatures of between 3.6 and 4.2 degrees above pre-18th century levels.
Scientists say temperature increases beyond that could trigger devastating effects, such as widespread loss of species, famines and droughts, and swamping of heavily populated coastal areas by rising oceans.
Environment ministers from the Group of Eight industrialized countries and Russia backed the 50 percent target in a meeting in Japan last month and called for it to be officially endorsed at the G-8 summit in July.
The IEA report mapped out two main scenarios: one in which emissions are reduced to 2005 levels by 2050, and a second that would bring them to half of 2005 levels by mid-century.
The scenario for deeper cuts would require massive investment in energy technology development and deployment, a wide-ranging campaign to dramatically increase energy efficiency, and a wholesale shift to renewable sources of energy.
Assuming an average 3.3 percent global economic growth over the 2010-2050 period, governments and the private sector would have to make additional investments of $45 trillion in energy, or 1.1 percent of the world's gross domestic product, the report said.
That would be an investment more than three times the current size of the entire U.S. economy.
The second scenario also calls for an accelerated ramping up of development of so-called "carbon capture and storage" technology allowing coal-powered power plants to catch emissions and inject them underground.
The study said that an average of 35 coal-powered plants and 20 gas-powered power plants would have to be fitted with carbon capture and storage equipment each year between 2010 and 2050.
In addition, the world would have to construct 32 new nuclear power plants each year, and wind-power turbines would have to be increased by 17,000 units annually. Nations would have to achieve an eight-fold reduction in carbon intensity -- the amount of carbon needed to produce a unit of energy -- in the transport sector.
Such action would drastically reduce oil demand to 27 percent of 2005 demand. Failure to act would lead to a doubling of energy demand and a 130 percent increase in carbon dioxide emissions by 2050, IEA officials said.
"This development is clearly not sustainable," said Dolf Gielen, an IEA energy analyst and leader for the project.
Gielen said most of the $45 trillion forecast investment -- about $27 trillion -- would be borne by developing countries, which will be responsible for two-thirds of greenhouse gas emissions by 2050.
Most of the money would be in the commercialization of energy technologies developed by governments and the private sector.
"If industry is convinced there will be policy for serious, deep CO2 emission cuts, then these investments will be made by the private sector," Gielen said.
