VIA planetark  
 WASHINGTON - Exxon Mobil CEO Rex  Tillerson said on Thursday it would be difficult to meet President-elect Barack  Obama's goal to significantly boost U.S. alternative energy  production.
 In a speech at George Mason University in Fairfax,  Virginia, Obama said he wanted the United States to double its output of  alternative energy sources during the next three years as part of his plan to  revive the American economy.
 "I think that's going to be  very challenging to do," Tillerson told reporters following a  speech at the Woodrow Wilson Center in Washington.
 He said the United States  will not be able to double biofuels output during that period and there was not  enough manufacturing capacity to build the wind turbines needed to meet Obama's  goal.
 "I'm not wanting to be  critical of his aspirational goal. Aspirational goals are good  because they challenge us all to go out and find the right answer," Tillerson  said of Obama's plan.
 But he added: "Let's be realistic  about timeframes. Let's not fool ourselves."
 Despite calls for more alternative  energy use, he said, "for the foreseeable future" the world will rely on oil and  natural gas for 60 percent of its energy needs.
 Renewable energy sources, which include solar, wind,  hydroelectric, geothermal, biofuels and other biomass, accounted for 7 percent  of U.S. energy supplies in 2007, according to the Energy  Department.
 Tillerson said the slowdown in the global economy would  be temporary and then strong energy demand from emerging nations will  return.
 He also said that even with the weak economy, Exxon  remained committed to investing in projects to meet future energy  demand.
 He said the company was sticking with its $125 billion  capital spending budget planned for the next five years, despite the steep drop  in oil prices and the global economic slowdown. Any changes to the capital  spending budget will be announced in March at a meeting with analysts, he  said.
 Separately, Tillerson  reiterated that it would be better for Congress to pass climate change  legislation that levies a carbon tax to curb greenhouse gas emissions, instead  of imposing a complicated cap-and-trade system that allows businesses to buy and  sell pollution permits to spew emissions.
 Tackling climate change is a top priority for the  incoming Obama administration and the Democratic-majority Congress.
 Tillerson said a carbon tax, which could be tacked on to  fuel supplies and electricity consumption, was the most efficient way to reflect  the cost of carbon.
 By comparison, a cap-and-trade  system is difficult to verify, requires new government regulators and "a Wall  Street of emissions brokers," he said.
 