For nearly half a decade, US natural gas and power traders had little to cheer about. Due to an influx of cheap shale gas, markets seemed to have settled in a rut, with abundant supplies and sluggish power demand growth placing a lid on volatility. At the gas and power desks of Wall Street banks, business slowed down as clients saw no reason to hedge, while many speculative traders, such as John Arnold's Houston-based hedge fund Centaurus Advisors, pulled out of the market.

Then came the polar