If you took a quick glance at David de Rothschild and tried to guess his profession, you could be forgiven for thinking skateboarder, what with the beard, laid-back demeanor and current Northern California address. His height 6 foot 4 and lanky frame might suggest an underfed power forward. And with that British accent and seamless train of thought, you could always go with poet. But none of those is true for Mr. de Rothschild, who belongs instead to a multihyphened class of overachievers: eco-adventurer and green-evangelist. Just 30, Mr. de Rothschild has become the youngest Briton ever to reach both geographical poles; he helped break a world record for speeding across Greenlands shrinking ice cap and just recently has started popping up in places like The New Yorker magazine.