Carbon is elemental. Its habits are fixed. It is what it is.

We, on the other hand, are adaptable, inventive, capable of change.

In this short summary, I root ... for us.

Happy Earth Day.

In this episode, No. 4 of our carbon series, we get to the heart of the matter.

If there's a growing concentration of CO₂ in our atmosphere, (and there is) why does that make the planet warmer?

In our cartoon and in the slideshow below, you see the CO₂ molecule getting hit by the heat energy from bouncing light. And instead of breaking up, it just gets excited (the chemical equivalent of getting warmer). That's essentially the chemical explanation for global warming.

As we've said all week, carbon's atomic structure, which gives it a tight grip on the oxygen atoms — fancifully called "Carbon In Love" in our cartoon — is the chemistry behind our climbing global temperatures.

Once again, the subject is carbon. So far we've been celebrating carbon's ability to bond with other atoms. Today we get violent — and break those bonds.

When you eat a carrot, set fire to a piece of paper, or put a match to a lump of coal, carbon atoms are being yanked, juggled and ripped out of each other's embrace. People have gotten very good at breaking carbon bonds: that's how we light our cities, drive our cars, power our tools. But let's look at this from carbon's point of view...

A couple of quick thoughts: when we break a carbon bond, there's not a sudden pop or release of energy. It's more subtle than that. Carbon, whenever free, immediately hooks up with a new, different set of atoms, and the new relationship is often more efficient, creating a net energy savings. Heat that used to be locked up is now released, and that's where we get our energy boost.

The Celery Exception?

Earth Day is coming up, so all this week we're talking about our planet, global warming and the element that makes global warming possible: carbon. Carbon, as we said in Episode 1, likes to hook up with other atoms using "bonds."

Science writer Natalie Angier, referring to the impeccable British spy ("The name is Bond, James Bond") of 20-odd films, says, "There is more than one way of being a Bond. You have your suave, supple, cat-like bond, your stiff-shanked bond, your uncommitted, barely there bond," and carbon has been bonding every which way for billions of years. Everywhere you look, you will find carbon snuggling with other atoms. Let's take a tour:

Tomorrow: Breaking Carbon Bonds. (Which you do when you eat a carrot.)

Cartoonist OddTodd's last appearance on Krulwich Wonders was our essay on "How Much Does A Hurricane Weigh?"

Well, it's that time of year. Friday is Earth Day, and this is the week that some of us pause to ponder the health of our planet (while others of us spend the week yelling at the people who are pausing to ponder the health of the planet). Being a pauser, not a yeller, I thought I'd spend this week sharing with you, especially the younger set of you, a series of cartoon essays about ... carbon. Why carbon?

"Water may be the solvent of the universe," writes Natalie Angier in her classic introduction to science, The Canon, "but carbon is the duct tape of life."