Sometimes in a market transaction there are benefits -- and costs -- that nobody pays for.

Like when you're walking down the street and you stop to enjoy the scent of your neighbour's prize roses. Your neighbour tilled the soil and bought the plants, but you get the floral blast of joy -- for free. That's what economists call a positive externality.

But there's a dark side to externalities too.

Some of the worst negative externalities involve the environment. The poisoned rivers and smoggy air that don't get tallied on the invoice

Over time, governments and businesses have come up with ways to deal with negative externalities. But many economists say that the bill has come due on one of the worst externalities of all -- and the planet is in peril if we don't pay up.

Dartmouth College economist Charles Wheelan will walk us through the world of externalities and market failures. Along the way, we'll hear how farmers, honeybees, and even the Chicago Cubs are dealing with externalities.

And finally, we'll hear from environmental economist Marc Jaccard about the bill we've run up on the planet's tab.