Table of Contents

• Section 1: Keeling's 50 year curve
• 1: How would YOU measure CO₂?
• 2: How Keeling measured CO₂
• 3: Parts per what?
• 4: Roadtrip anyone?
• 5: Let's go to Hawaii
• 6: The Keeling Curve at 3 (years old)
• 7: The Keeling Curve at 50
• Section 2: Reading Keeling's Curve
• 8: Within vs. between year fluctuations
• 9: Do the fluctuations change?
• 10: Past and future
• 11: 80% reduction in CO₂ emissions

Parts per what?

We're not going to worry about how Keeling's knobs, dials, and tubes worked, but... what units do you think Keeling used to measure CO₂? In other words, if you took a 10 cubic meter box of atmosphere, how would you report the CO₂ you measured? gallons? cubic centimeters? pounds? grams?

None of these seems quite right. You can't use gallons or cubic centimeters because the volume of the CO₂ will change, depending on how close together the molecules pack. And CO₂ (like all atmospheric molecules) is very light, so pounds or grams don't seem very useful.

Instead, think about what you know about the atmosphere — it is made of molecules of gas in constant motion, mostly nitrogen (N₂) and oxygen (O₂). Use the applet below to find the correct ratio of nitrogen to oxygen. Then click on the "Show me the CO₂" button to see how much CO₂ is present.

APPLET

Wow, that's not much CO₂. In fact, for every CO₂ molecule in the atmosphere, there are about 3000 other molecules (almost all of which are O₂ and N₂).

So, the way we measure CO₂ is in "parts per million" — i.e., how many CO₂ molecules are there for every million molecules in the atmosphere. By contrast, we measure O₂ and N₂ in parts per hundred (or percent) — about 20% O₂, and 80% N₂.