We are completely dependent on energy -- to fuel our cars, to heat our homes, to power the industries that produce our goods. We can't live without it. At the same time, we are providing for our energy needs primarily by burning fossil fuels -- and herein lies the problem.
The burning of fossil fuels (mostly coal and oil) releases dangerous gasses into the atmosphere, contributing both to global warming and destructive acid rain. In 1990, the Intergovernmental Panel on Climate Change estimated that global temperatures would on average increase by 1.5-4.5 degrees Celsius by 2050, resulting in increased precipitation and a rise in sea levels that could potentially flood thousands of square miles of coastal lowlands.
Environmental considerations are not the only issue -- current consumption of fossil fuels is not sustainable in the long run. Although estimates vary, proven oil reserves could easily be depleted by the end of this century, and proven reserves of coal by the next century. Coal and oil will fulfill our energy needs only temporarily.