Here is a pop quiz: How many trees are on the planet?Most people have no idea.A new study says the answer is more than 3 trillion trees — that's trillion with a T, and that number is about eight times more than a previous estimate.Thomas Crowther was inspired to do this tree census a couple of years ago, when he was working at the Yale School of Forestry and Environmental Studies. He had a friend who was working with a group with an ambitious goal: trying to fight global warming by planting a billion trees. A billion trees sounded like a lot. But was it really?