Answer: Coal and other fossil fuels.
China and India are on a immense growth trajectory. From an American perspective, it's easy to project our anemic growth rate on the rest of the world. But one glance at the combined energy consumption growth of China and India, in this chart from the EIA, shows where the action is on the globe.
Note that energy usage actually contracted in the OECD states in 2008 and is relatively flat out to 2015. That's not from energy efficiency -- it's from recession.
Strong economic growth will lead China and India to more than double their combined energy demand by 2035, accounting for half of the world's energy growth, according to the EIA International Energy Outlook.
That report predicts that India and China will consume 31 percent of the world's energy in 2035, up from 21 percent in 2008.
China surpassed the United States as the world's largest energy consumer in 2009. By 2035, China's energy consumption will be 68 percent higher than U.S. energy consumption, according to the report.
And although renewables are seen to be the fastest growing source of energy in the next 25 years, fossil fuels will remain earth's dominant energy source.
The EIA has been wrong before -- let's hope they're wrong on that last point.