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Keeping in mind oil guzzlers and gas guzzlers and beer guzzlers, NPR science correspondent Robert Krulwich would like to extend the category to include those who guzzle trees.

ROBERT KRULWICH: When you think of NASA, says Professor Nalini Nadkarni, you think, of course, of space and of stars...

Dr. NALINI NADKARNI (Professor of Ecology, The Evergreen State College, Washington): And, you know, space rockets and the moon's surface. But about 20 years ago, NASA realized that they're also supposed to be studying planet Earth as well.

KRULWICH: So NASA satellites take pictures of our mountain ranges and oceans and forests. And it just happens that sunlight bouncing off trees has a particularly identifiable signature. So NASA was able to map where trees are on Earth, in forests, rural trees, trees in parks. I don't know if they can do city trees.

Dr. NADKARNI: No, no. We covered urban trees as well.

KRULWICH: In short, using NASA's data and tree density studies, Professor Nadkarni, who teaches at Evergreen State in Washington, was able to come up with a very rough count of how many trees we have on our planet.

Dr. NADKARNI: Now, obviously, it's not, you know, to a T. It's not completely accurate.

KRULWICH: But give or take, the Earth, she figures, has about 400,246,000,000 trees. And then she wondered, so how many trees is that per person? Which is kind of a cool question.

(Soundbite of laughter)

Dr. NADKARNI: Hey, I'm a scientist. I like being accurate.

KRULWICH: So how do you do this calculation then?

Dr. NADKARNI: Well, then you have to know how many people there are on Earth. And I found that there were...

KRULWICH: Six and a half billion...

Dr. NADKARNI: People as of midnight on December 31, 2005.

KRULWICH: OK. So now all she has to do is divide her trees by people. But before you did that, did you have any sense of the outcome of how this was...

Dr. NADKARNI: No, I had no idea whether there was, like, one tree per person or whether there would be a thousand trees per people.

KRULWICH: Have you had any hopes or some gut feeling?

Dr. NADKARNI: You know, I severely hoped that there was more than one tree per person, but I feared and thought that there would be fewer trees than people. You know, because...

KRULWICH: I'm trying to think what I - I thought there would be like - I always - I thought there'd be a hundred trees per person.

Dr. NADKARNI: Interesting.

KRULWICH: But when she did the calculation. So what's the...

Dr. NADKARNI: The magic number turns out to be 61 per person. Sixty-one trees per person.

KRULWICH: That's a pretty big number.

Dr. NADKARNI: Yes. I said, hooray! I get more than one tree. I'm in a sort of silly way. But I thought that's a lot of trees. You know, that means every person walking through Times Square, you know, has 61 trees. That's so fantastic. But then I talked to my husband about it, you know, at the breakfast table. And I said, Jack, we get 61 trees apiece. That's huge. And he looked at me in his quiet, sort of slow way and said, well, you know, I don't know. We use about that much in a couple of seasons of, you know, our wood stove and the lumber that made our home. So maybe it's not so many. And I've been struggling with that thought ever since. It's like, is it a lot or a little? I'm not sure.

KRULWICH: Because we all know there's lumber in our homes and furniture and toothpicks.

Dr. NADKARNI: But then you get into things like golf balls.

KRULWICH: Whose bounce comes from the gum of the gutta-percha tree. Or the number of disposable chopsticks in Japan every year.

Dr. NADKARNI: Over 20 billion pairs.

KRULWICH: That's 40 billion chopsticks thrown away every year, says Professor Nadkarni's former grad student, Jade Blackwater, who then said, in China the number is 900 billion.

Ms. JADE BLACKWATER (Former Forest Ecology Grad Student, The Evergreen State College, Washington): Which is about 25 million trees in equivalence.

KRULWICH: Nalini, Jade, and a team of students created a list of tree-based products. It's a long list.

Ms. BLACKWATER: Baby cribs and baseball bats. Barrels and landscaping bark.

Dr. NADKARNI: Nail polish.

Ms. BLACKWATER: Luggage, magazines.

Dr. NADKARNI: Party invitations, particle board.

Ms. BLACKWATER: Toilet paper, toothpaste, turpentine.

Dr. NADKARNI: Ping-pong balls.

Ms. BLACKWATER: Beds, billboards, birthday cards.

Dr. NADKARNI: Fireworks.

Ms. BLACKWATER and Dr. NADKARNI: Bulletin boards, tambourines, telephone books, umbrella handles, vacuum cornish, vitamins, xylophones and yo-yos.

KRULWICH: Yo-yos?

Ms. BLACKWATER: Wooden yo-yos.

KRULWICH: Oh yes. The point is there are so many tree-based products, there is no way to know how much tree any of us use every year. You could guess.

Dr. NADKARNI: My guess is that we are using more than our 61 trees because, actually, the cover of trees is decreasing over the surface of the Earth.

KRULWICH: And obviously, some of us consume more. Americans, for example, use 27 times more paper than Indians. But Nalini says this is not one of those, oh, I'm a bad, bad person stories.

Dr. NADKARNI: I don't want people to feel guilty about their relationships with trees and say, oh, my God, I can never touch another tree-created product again.

KRULWICH: Because, she says, remember, trees are not like oil. Trees are renewable. So if you think you're using more than your 61, the obvious solution is plant a few more. So, yeah, we may be consuming a lot...

Dr. NADKARNI: I would assume that we're the gobblers.

KRULWICH: But gobblers, she says, can be planters too. Robert Krulwich, NPR News.

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