RS: And we start with the term "greenhouse gases." These are emissions that trap heat in the atmosphere but, Jackson says, not by definition a bad thing.

ROB JACKSON: "Without greenhouse gases, life wouldn't exist; the Earth would be a frozen ball of ice. When they trap extra heat, though, they warm the Earth more than has historically been usual and drive up the Earth's temperature."RS: "Well, we have three terms that we would like to know more about. One is carbon-neutral. What does it mean to be carbon-neutral?"ROB JACKSON: "Well, when something is carbon-neutral, it means that the activity has no net greenhouse gas emissions. So, for instance, you might offset the fuel that you use by using renewable energy or even by planting trees to offset your emissions. In practice, there are different levels of being carbon-neutral. When you hop in your car, do you count just the gasoline that you use? Or do you include the fossil fuels that went into making the car or building the roads you drive on? Most people just think about the fuel used."RS: "And what about your carbon footprint?"ROB JACKSON: "The term carbon footprint, it's come to mean just the amount of carbon dioxide and other greenhouse gases that come from a given activity, such as driving your car, again. To understand where it comes from, though, you have to know a little environmental history.

"Back in the early nineteen nineties an ecologist named Bill Reese coined the term 'ecological footprint,' the amount of land needed to support resource use and waste for a given population. So that's where the footprint half of carbon footprint comes from. In common use, through, it's lost the link to land area and just refers to net carbon emissions."AA: So we have carbon-neutral, carbon footprint and a third term -- carbon sequestration.

ROB JACKSON: "Well, carbon sequestration is the opposite of a carbon emission. So a sequestration activity is something that takes carbon dioxide back out the atmosphere and puts it into wood for trees, for example, or stores it below ground in an aquifer or in sediments in the ocean. So you're taking or removing carbon dioxide from the atmosphere."RS: "On Capitol Hill they're talking about 'cap-and-trade' and 'carbon trading.' What are these terms, what do they mean?"ROB JACKSON: "A carbon trade or a cap-and-trade works like this: a government or some regulatory body limits the total amount of a pollutant that enters the atmosphere. So this is what people call the cap.

"The government then issues credits, emissions credits, to the polluters who can sell or trade those credits. That's the term cap-and-trade. And the system works because it rewards companies that reduce pollution by letting them sell their credits for profit."RS: "And this is something that's popular now in Europe, that's actually in effect in Europe, correct?"ROB JACKSON: "It's popular in Europe, though, really because it worked here first, not for carbon dioxide but for sulfur. Back in the early nineteen nineties, through the Clean Air Act, we implemented a cap-and-trade system for sulfur emissions in the U.S. and it worked very effectively and very efficiently and very quickly to reduce sulfur emissions."AA: "Acid rain."ROB JACKSON: "That's correct. So the idea is to take that model and apply it to carbon dioxide and other greenhouse gases. Now it's more complicated with greenhouse gases but the principal is the same. And it really -- it rewards efficiency, it rewards companies financially for reducing their pollution."RS: "But it's become quite controversial."AA: "Right. That's right, there have been news reports recently that it's not quite as clear-cut as it might seem."ROB JACKSON: "Well, it's controversial for many reasons. It's controversial in how the emissions credits are given out, so who gets how many credits, how much a company pays for them. It's also controversial because you have to be careful that the carbon dioxide or the greenhouse gas that you're saving through your system isn't being re-emitted somewhere else.

"So, in other words, if one country sets up a cap-and-trade system to reduce its emissions, but all it does is shunt some activity to another country, and that carbon dioxide ends up in the atmosphere, you really haven't saved anything on a net global basis. It really points out the need for an international system."AA: "And are there terms that you use to describe a situation like that?"ROB JACKSON: "A cap-and-trade situation?"AA: "Or shunting off of -- passing the buck or something, or passing the carbon. I don't know what you would call it."ROB JACKSON: "Yeah, there is a technical term for it. It's called 'leakage,' and it sounds like your bathtub's leaking or something. But the idea is that use of some activity slides off to some other location or to some other time that's outside the bounds of your system, and therefore you're sort of losing track of it, you're not accounting for it, and that's why the term leaking is used."RS: We'll talk more about eco-language next week with Rob Jackson, a professor of biology and environmental sciences at Duke University in Durham, North Carolina.

AA: And that's Wordmaster for this week. You can learn more about English at our Web site, voanews.com/wordmaster. And our e-mail address is [email protected]. With Rosanne Skirble, I'm Avi Arditti.