Yael: Have you ever been to the Arctic, Don?

Don: Nope.

Y: Well, if you're ever there, be sure to look up at the sky at night.

D: Okay. Why?

Y: You might see some really cool, luminescent clouds high up in the atmosphere.      Scientists call them night shining clouds.

D: That does sound cool. What makes them shine?

Y: That's been a mystery for a long time. But now a team of researchers is studying the      life cycle of the clouds in the Arctic and Antarctica. One thing they know is that over      the past few decades night shining clouds have become brighter and are appearing at      lower latitudes.

D: Do they know why?

Y: Well, the researchers suspect it may have to do with global warming. See, the clouds      form in the upper reaches of the atmosphere, where it's very cold. So the colder it      gets, the more clouds there are.

D: Wait a sec. I thought you said it had to do with global warming.

Y: I did. See, global warming happens when a thick layer of carbon dioxide builds up in      the atmosphere, trapping heat near the earth's surface. But all that heat trapped      below makes things colder in the very upper parts of the atmosphere, where the      layer of CO₂ is very thin.

D: Ah, I get it. So if there are more night shining clouds in the sky and they're getting      brighter, that could be one more sign of how global warming affects the atmosphere.

Y: That's right. And the more research they do, the more scientists will be able to tell us      about how the clouds form, and what makes them shine.  

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