By Suryadi PEKANBARU, Indonesia — Traffic stopped gradually, then suddenly, as the new year approached around an hour’s drive out of the city of Pekanbaru here on the Indonesian island of Sumatra. Within minutes, the arterial highway linking the island’s east and west coasts turned into a dark river. “The problem is complex,” said Sigit Sutikno, a peatland hydrologist at the University of Riau. “The floods will become more frequent, the impact will widen, and the duration will be longer.” Across much of Indonesia’s Riau province, motorcyclists coated in translucent ponchos scrambled under shop…