(AP) — The world’s hopes for curbing climate change hinge on action by two giant nations whose relations are deteriorating. China and the United States both say they are intent on retooling their economies to burn less climate-wrecking coal, oil and gas. But tensions between them threaten their ultimate success. Climate scientists say countries need to move fast to avert catastrophic temperature rises. As political tensions rise, veteran climate negotiators John Kerry and Xie Zhenhua hope to find common ground on climate solutions. China and the United States are the world’s No. 1 and No. 2 carbon polluters, respectively.