Reuters: Burning of natural gas at oil fields surged since 2006 * Revenue needs from aging oil fields spur flaring * State oil firm Pemex vows to slash gas flaring by 2012 * Flaring curbs part of government’s voluntary carbon cut Activists and diplomats have hailed Mexico’s strong support for action against climate change, but a spike in natural gas flaring at offshore oil fields since 2006 has blemished its green reputation. President Felipe Calderon has vowed to …