Design Pics Inc/Getty With his executive order to open up the Arctic to drilling, Donald Trump is skating on thin ice. The president’s attempt to overthrow Barack Obama’s “permanent” block on exploitation of oil and gas there will attract a barrage of legal challenges, and rightly so. For starters, the law Obama used includes no provision to reverse a block. So, cautious good news for planet Earth, but it is complex and legal opinions differ. Are oil companies clamouring at the Oval Office door to grab leases?