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Analysts disagree on whether the energy returned from solar is worth the energy invested. Oil producers are working harder than ever to replace their reserves and, increasingly, turning to nonconventional petroleum sources such as Canada's tarry bitumen and ultra-deepwater offshore wells. As they do so, they are investing more energy to deliver each new barrel of oil. Similar trends are playing out for natural gas and coal, and a growing number of energy analysts are worried. They see these accelerating trends what they call fossil fuels' declining energy return on investment, or EROI-as an ill portent for the global economy.