Most analysts think that the world's demand for energy will keep growing in the near future. But they also believe that as time passes, renewable sources of energy — hydroelectric, biomass and perhaps nuclear energy, but above all wind and solar — will replace fossil fuels , reducing carbon emissions. The main disagreements are over how quickly that will happen. Most experts have concluded that change will come slowly. Analysts at Shell predict it will take a quarter of a century to reach a tipping point where the annual output of renewable energy matches the overall growth in demand and the amount of carbon being released into the atmosphere levels off. Their peers at BP think 30 years will pass before then; those at Exxon say 75. The International Energy Agency (IEA) largely agrees, though it recently cut its estimate from 60 years to 35.