ExxonMobil is being challenged on climate by an upstart investment group. The investor, Engine No. 1, has a US$50 million stake in Exxon Mobil, making it a niche player in the US$253 billion company. Yet, it has rallied enough institutional support to do what decades of environmental and shareholder activism never could: put ExxonMobil on the defence. The investor's push has been driven by…