A G8-backed goal to halve greenhouse-gas emissions by 2050 would take a global clean-technology investment of merely $45 trillion, the International Energy Agency said in a report Friday. That’s about 1.1 percent of the world’s average annual gross domestic product through 2050; more overwhelmingly, it’s also about three times the size of the current U.S. economy. To meet the 50-percent-by-2050 target, the world must build some 1,400 nuclear power plants by 2050; erect 17,500 wind turbines per year; and fit more than 50 coal- and gas-fired power plants per year with technology to capture and store carbon, says the IEA. Under current policies, warns the report, global CO2 emissions will rise 130 percent and oil demand 70 percent by 2050. It all “represents a formidable challenge,” says IEA Executive Director Nobuo Tanaka, “and we would require immediate policy action and technological transition on an unprecedented scale.” So hop to it.