Greenspan called for a mixture of solutions, from plug-in hybrid cars to ethanol to nuclear power, to diminish the country's reliance on foreign oil.

Greenspan also sounded alarms about dwindling natural-gas supplies and the dangers posed by global warming.

He called for a range of solutions, including developing hybrid-fuel cars with rechargeable plug-in batteries and developing crops of switchgrass as a rich source of ethanol, a solution President Bush mentioned in his State of the Union address.

Greenspan said the nation may also need to develop more nuclear facilities and import more natural gas.