There are 50 questions. Here are the answers. If you haven’t taken the quiz yet, take the quiz first.

If you get fewer than 20 right, you are a beginner. You really shouldn’t be talking about climate with others. You should learn first, and that’s what this website is for.

If you get 20-30 right, you’re on your way to understanding the earth’s complex climate system. Don’t be overconfident; keep learning!

If you get 30-40 right, congratulations, you have studied climate science and critical thinking. There are many resources on this site to continue your education. Help others learn by sending them here and volunteering to help me spread the word.

If you get more than 40 right, and you still think CO2 is causing climate change, let’s record a Zoom session so we can discuss and learn more from each other.

Warm-up question: the IPCC AR6 has 18 chapters and is roughly 6,000 pages long in total.

Now for the answers:

Part 1: easy questions

1. What percent of the earth’s atmosphere is CO2? 

A) 40 percent
B) 4 percent
C) 1.4 percent
D) 0.4 percent
E) 0.1 percent
F) 0.04 percent
G) 0.01 percent
H) 0.004 percent

F. 0.04 percent. Four percent of one percent. This is what 420 parts per million means. The last two digits are technically insignificant.

2. How thick is the CO2 layer in the atmosphere? 

A) About 1 cm
B) Less than 10 cm
C) Less than 30 cm
D) Less than 1 m
E) About 1,000 m
F) None of the above

F. CO2 is a well-mixed gas

3. Where is the CO2 in the atmosphere? 

A) It’s a thin layer at the top of the troposphere, about 10km up.
B) It’s more concentrated at the top of the troposphere — there’s more as you go up.
C) It’s mostly in the stratosphere, above 10km. 
D) CO2 is heavier than air. It’s mostly in the first kilometer above ground level.
E) None of the above. 

E. CO2 is a well-mixed gas. 

4. In the graph above, what was the key factor causing the global troposphere temperature to spike in 2023-2024?

A) Reduced albedo
B) More solar flares
C) More CO2 emitted post-Covid
D) Mostly the Hunga Tonga volcano

D. Explanation by Javier Vinos.

5. At what CO2 concentration do commercial greenhouses operate for optimal plant growth?

A) Below 250 PPM
B) 250 — 500 PPM 
C) 500 — 1,000 PPM
D) 1,000 — 1,200 PPM 
E) 1,200 — 5,000 PPM
F) Above 5,000 PPM

D. Between 1,000 and 1,200 PPM. 

6. What is the earth’s average surface temperature today? 

A) 30 degrees C
B) 23.6 degrees C
C) 18.2 degrees C
D) 16 degrees C
E) 15 degrees C
F) 12.5 degrees C

E. NASA

7. What’s the best estimate for the earth’s average surface temperature 90 million years ago? 

A) 30 degrees C
B) 28 degrees C
C) 23.6 degrees C
D) 18.2 degrees C
E) 16 degrees C
F) 14.5 degrees C

B. Global Mean Surface Temperatures for 100 Phanerozoic Time Intervals

8. How much of the earth’s surface is covered by water?

A) 49 percent
B) 51 percent
C) 67 percent
D) 71 percent
E) 76 percent

D. USGS.

9. If all the ice in the Arctic ocean melted, how much would sea level rise?

A) 0 cm
B) 1 cm
C) 10 cm
D) 25 cm (1 foot)
E) 50 cm
F) 84 cm

A. Arctic ice is floating.

10. The Intertropical Convergence Zone (ITCZ) is:

A) Where the Indian ocean meets the Pacific.
B) The climatic region around the equator.
C) The weather patterns that drive the Hadley cells
D) a belt of low pressure near the equator where the northeast and southeast trade winds converge.
E) None of the above

D. The ITCZ is the tropical weather zone that a) does not follow the equator because of land, altitude, currents, and weather patterns, and b) moves northward and southward to equilibrate the heat-transfer needs of the planet. As it moves, it shifts the distribution of warm water in the various large ocean basins. NASA.

11. This NASA diagram:

is fundamentally wrong, because:

A) It doesn’t take meridional heat transfer into account.
B) It omits albedo.
C) The atmospheric window is too small.
D) The insolation figure is only valid at the equator.
E) The numbers don’t add up.

A. The earth receives far more energy at the equator than at the poles. The tropics have a strong net gain, while the poles have a strong net loss of energy throughout the year. The meridional temperature gradient determines earth’s climate at a high level. See Vinos 2022.

12. Here’s a diagram of the earth’s adiabatic lapse rate:

Everyone who studies climate is familiar with this. Temperatures at 50km can be the same as those below on the ground. Explain what’s going on in the stratosphere?

A) The stratosphere warms as you go up because of incoming cosmic rays.
B) The stratosphere has no clouds, so there’s more sunshine hitting the O2 and N2 molecules.
C) Incoming UV radiation creates ozone, which absorbs shortwave radiation. Higher concentrations of ozone occur 20-30km up. As UV light is stronger higher up, the warming effect is stronger at higher elevations.
D) Most ozone is at the bottom of the stratosphere, which is denser, so the ozone heats up and radiates heat to space. The radiative heating of air molecules and convective upward air movement creates higher temperatures as you go up.
E) High clouds in the stratosphere have a greenhouse effect that warms the stratosphere. The heat coming off the clouds accumulates as you go up.

C. Higher energies higher up, as those ozone molecules receive shortwave radiation first. GPT explains this quite well - just ask it.

13. According to the IPCC, the effects of greenhouse gas forcing on changes in atmospheric dynamic and associated changes in drought occurrence is …

A) High confidence
B) Medium confidence
C) Low confidence

C. Low confidence. See section 11.6.1 of the AR6. 

14. According to the IPCC, general statements attributing changes in global flood events to anthropogenic climate change have: 

A) High confidence
B) Medium confidence
C) Low confidence

C. Low confidence. See section 11.5.4 of the AR6.

15. According to the International Union for the Conservation of Nature, whose job it is to count polar bears, the number of polar bears on earth in 1960 was under 10,000. Roughly how many are alive today?

A) 6,000–8,000
B) 8,000–12,000
C) 12,000–16,000
D) 16,000–22,000
E) 22,000–32,000
F) 32,000–40,000
G) 40,000–50,000 

E. Approximately 22,000–31,000 bears in the last count, though there could be more.

16. What was the major factor behind the high US temperatures in 1998 and 2016? 

A) Wet conditions in the South Pacific.
B) Bad farming practices.
C) Rising amount of CO2 in the atmosphere.
D) The AMO.
E) The PDO/ENSO.

E. Paper on weather extremes; also see Vinos.

17. Is East Antarctica warming? 

A) Yes, significantly, and that warming is accelerating.
B) Yes, significantly since 1980, though not accelerating
C) Yes, a bit, but not much
D) No, it has stayed the same
E) No, it has cooled a bit

D. “Beyond the Antarctic Peninsula there has been little significant change in temperature,” and Antarctica is losing ice from below, not from above.

18. Approximately how warm were temperatures in the Medieval Warm Period (900 - 1300 AD)?

A) Fake question. This is referred to as the Medieval Ice Age, everyone knows it was much colder then.
B) Less than today’s temperatures by about 2 degrees C
C) Less than today’s by about 1 degree C
D) About the same as temperatures today
E) Significantly warmer than today by at least 1 degree C
F) Significantly warmer by at least 2 degrees C

D. See Moberg et al.

19. What role do clouds play in the atmosphere?

A) They provide the tipping points that heat the earth to extreme levels. 
B) Troposphere (lower-atmosphere) clouds generally reflect heat during the day and retain heat at night, while stratospheric (upper-atmosphere) clouds are made of ice and reflect sunlight away. 
C) Water-rich low clouds over the tropical ocean have the greatest cooling effect and low-water ice clouds at high altitudes have the strongest warming effect. Overall, the cooling effect is greater.
D) Troposphere (lower-atmosphere) clouds greatly accelerate the greenhouse effect of CO2, while stratosphere (upper-atmosphere) clouds have no effect on temperature.

C. Interview with Bjorn Stevens, cloud expert.

20. What’s the general trend of precipitation in the continental US over the last 60 years?

A) Unusually wetter than the previous 60 years.
B) Generally wetter than the previous 60 years.
C) About the same.
D) Generally drier than the previous 60 years.
E) Remarkably drier than the previous 60 years.

B. See EPA graph.

21. What has been the trend of Arctic sea-ice extent for the last 15 years? 

A) Significant decrease
B) Gradual decrease
C) Flat
D) Increase
E) Significant increase

C. Flat. What does 45 years of daily arctic sea-ice data tell us?

22. What is the polar vortex? 

A) The polar vortex has little to do with climate but is responsible for the Northern Lights.
B) The polar vortex is a circular band of strong westerly winds in the stratosphere between about 10 and 30 miles above the northern polar region from about 65 degrees N to the pole, is strongest in winter, and can lose energy and change the jetstream. While the Northern polar vortex has more impact on humans, there is also a Southern polar vortex.
C) The polar vortex is a band of fast-moving cold air in the upper troposphere. It is where jets fly. 
D) The polar vortex is an irregular pattern of air above the north pole caused and influenced by greenhouse gases. It has grown tremendously in the last 50 years. 

B. Video explainer. 

23. The lowest 10km or so of the atmosphere is called the:

A) Atmosphere
B) Mezosphere
C) Lithosphere
D) Stratosphere
E) Troposphere

E. UCAR.

24. The earth’s albedo is roughly:

A) 1 percent
B) 8 percent
C) 18 percent
D) 30 percent
E) 44 percent
F) Nonexistent. Venus has an albedo, but earth doesn’t.

D. NASA.

25. The water in the earth’s atmosphere originally came from:

A) Deep underground.
B) Space. Water is relatively abundant in space.
C) It was created by heat and pressure on oxygen and hydrogen molecules.
D) It precipitated out of the original rocks.
E) None of the above.

B. Scientific American.

26. The CO2 in the earth’s atmosphere originally came from:

A) It was manufactured by animals breathing in oxygen supplied by plants.
B) It was created deep in the earth and came out through volcanoes.
C) Mostly from decaying plant and animal material buried underground.
D) Space. CO2 is fairly common in space.
E) None of the above.

D. UK Natural History Museum.

27. What phase is the Atlantic Multidecadal Oscillation in now? 

A. Neutral
B. Warm
C. Cold
D. None of the above, that’s not how the AMO works. 

B. Since the mid-1990s, it has been in a warm phase. 

28. What solar cycle are we in now? 

A) 18
B) 22
C) 23
D) 24
E) 25
F) 26
G) Solar cycles use names, not numbers.

E. We are in solar cycle 25. 

29. These maps use data from the US Historical Climate Network: 

One of these maps shows data from 1936, the other from 2023. Which is which?

A) The data is all faked and cherrypicked, it’s not from the USHCN. 
B) A is 1936. 
C) B is 1936. 

B. On an absolute basis, 1936 is possibly the US’s hottest summer on record, but it probably wasn’t the hottest year, because February 1936 was extremely cold. In describing earth’s climate, the concept of “hottest year on record” is meaningless.

30. According to NOAA’s tide-gauge network, the rate of sea-level rise since 1800 is:

A) Accelerating

B) The same

D) Slowing down

B. No acceleration in sea-level rise since 1880 (and since 1800 for tide gauges that have been keeping records that long). Paper.

31. Since 1968, temperatures have generally been rising. Has the rate of that rise generally:

A) Accelerated - the temperature curve is going up and bending upward even more
B) Been flat, no acceleration, linear increase
C) Declined - the temperature curve is going up but bending downward
D) We don’t know, there’s too much uncertainty in these measurements.

C. See Fig 12.11 in Javier Vinos’s book.

32. Recorded temperatures in Adelaide, Austrailia: 

One of these shows the temperature record for 1920, the other for 2020. Can you tell which is which? 

A) The gray one is obviously 1920. 
B) The orange dotted one is obviously 1920. 
C) I can’t tell. 
D) This is ridiculous. It’s totally fake data. 

C. One of these — it turns out to be the orange dotted line — is actually 2020, but there is no way to know. Is this what the IPCC says should happen? Isn’t CO2 a well-mixed gas? More such city graphs available at the Climate Discussion Nexus website. 

Part II: Intermediate questions

33. How does the greenhouse effect work?

A) Where the lapse rate is positive, it delays longwave photons as they escape, raising the emission height and shifting the curve toward higher surface temperatures; where the lapse rate is negative, it accelerates the escape of longwave radiation and shifts toward lower surface temperatures. 
B) It’s pretty much exactly like a greenhouse — it reflects heat back down to earth that would otherwise escape.
C) It concentrates more heat in the oceans. 
D) Same as A but without the negative pole nonsense. 
E) It’s not like a greenhouse, it’s like a blanket, trapping heat. CO2 is an insulator.
F) None of the above

A. More greenhouse gas molecules will shift the lapse rate toward higher ground temperatures, especially over the tropics, and the process works in reverse to cool the poles. See question 41 for more.

34. According to a 2022 blog post by Chris Landsea and Eric Blake at the National Hurricane Center in Miami, the number of hurricanes has risen from 1878 to today because …

A) CO2 added to the atmosphere by humans
B) A combination of CO2 and natural factors
C) Better determination and counting methods

C. See their blog post. 

35. According to Our World in Data, total cyclone energy, which is a measure of all cyclone strength per year combined, after adjusting for technology and measurement bias, has …

A) Risen dramatically in the last 20 years
B) Risen a bit
C) Been flat
D) Oscillated up and down over a 50-year cycle with no discernable difference between the end of the 19th century and early 21st. 
E) Trended down consistently

D. Our World in Data.

36. About 38,000 years ago, in the largest Dansgaard-Oeschger event #8, north Atlantic temperatures rose dramatically over a period of less than 120 years. According to a 2014 peer-reviewed investigation, how big was the temperature rise? 

A) 1–2 degrees C
B) 2–4 degrees C
C) 4–8 degrees C
D) 10–13 degrees C
E) 15–17 degrees C

D. Temperatures rose 11.8 degrees +/- 1.8 degrees C in under 140 years. The rise was natural and had nothing to do with CO2.

37. According to a peer-reviewed article in the journal Cryosphere, from 2009 to 2019, Antarctica’s ice shelves …

A) Lost 2,500 GT of ice
B) Lost 844 GT of ice
C) Have been stable
D) Gained 661 GT of ice
E) Gained 1744 GT of ice

C. “Over the last decade, a reduction in the area on the Antarctic Peninsula (6693 km2) and West Antarctica (5563 km2) has been outweighed by area growth in East Antarctica (3532 km2) and the large Ross and Ronne–Filchner ice shelves (14 028 km2).” See also NASA’s evaluation of Antarctic ice and this article.

38. According to a 2021 paper analyzing Greenland’s mass balance from 1992 - 2020, Greenland shed approximately 200 gigatons of ice per year, causing global sea level to rise by 0.4 millimeters per year. At this rate, how much will Greenland cause sea level to rise by 2100? 

A) 40mm = 4cm = less than 2 inches
B) 400mm = 40cm = 16.75 inches
C) 4,000mm = 400cm = 13 feet
D) 10,000mm = 10m = 33 feet

A. This is simple math. 

39. Has the Arctic Ocean been essentially ice-free in summer in any time during the past 10,000 years?

A) No; the last time the Arctic was ice-free was about 90 million years ago.
B) No; there was less ice 8,000 years ago, but no, not ice-free in summer.
C) Absolutely. In fact, the Arctic sea was ice-free in summer for a few decades in the 1600s.
D) Probably; the best studies we have point toward very little ice, and possibly no summer ice for hundreds of years in the range 6,000 — 10,000 years before present.

D. See Jakobsson, Jakobsson, and Stein.

40. This is a MODTRAN database graph showing the frequencies at which CO2 and methane are most effective as a greenhouse gas. What does it say?