Directions: Read the article “Mosses tell story of retreating ice” and then answer these questions.

1. In one sentence, what is the main idea of the article?

Possible student response: Analysis of frozen mosses in the Arctic shows that summers are now warmer than they have been in at least 40,000 years.

2. Where in the Arctic did paleoclimatologist Gifford Miller and his team find the mosses, and how were the mosses exposed?

Possible student response: Miller and his team studied mosses on Baffin Island in the northeastern Canadian Arctic. The mosses were found along the edges of diminishing ice sheets and were exposed as those ice sheets melted and retreated.

3. How were the mosses originally covered?

Possible student response: Earlier in geologic time, as conditions grew colder and ice sheets formed and advanced, live mosses were covered by the ice and remained frozen until now.

4. How many different samples were analyzed? How do you know this?

Possible student response: The article states that Miller’s team has identified 370 mosses with distinct ages, so at least 370 plant samples must have been collected and analyzed.  

5. What happens over time to a radioactive element? Define and give an example of a half-life mentioned in the article?

Possible student response: As unstable radioactive elements decay over time, they can give off radioactive particles, which transforms the element into a different one. The half-life of a radioactive element is the amount of time it takes for half of a radioactive sample to decay into another element. The half-life of carbon-14 is about 5,730 years.

6. How long does it take for radioactive carbon to completely “die” or decay? How is radioactive carbon generally used for radioactive dating?

Possible student response: Radioactive carbon almost completely decays in approximately 40,000 to 50,000 years. Because you know carbon-14’s rate of decay, by measuring how much carbon-14 an object contains and comparing that with the amount of decay by-products, you can estimate the time since that object was formed.

7. What is a limitation of radiocarbon dating?

After approximately 40,000–50,000 years, virtually all of the radioactive carbon has decayed away, so radioactive carbon dating cannot be used to estimate ages older than that.

8. Based on radioactive carbon dating, when were groups of moss buried on the island?

Possible student response: Some of the moss was frozen around 570 years ago, some around 900 years ago, some around 3,700 years ago and some at least 40,000 years ago (those moss samples did not have enough radioactive carbon left to get an age estimate).

9. What additional evidence can be used to estimate the age of the mosses that lack measurable radioactive carbon?

Possible student response: An ice core collected in nearby Greenland suggests that the planet experienced continuous cold from 40,000 to about 115,000 years ago, when the last warm interglacial period ended. Thus, some mosses might be as old as 115,000 years if they were buried and frozen as ice advanced at the end of the last warm interglacial period.

10. Why do you think this study is important for the modern world?

Possible student response: Summers are now the warmest they have been in 40,000 years, and summertime Arctic ice has retreated the furthest it ever has since that time. The world is warming rapidly due to human production of greenhouse gases, and will continue to warm unless we take dramatic action. Without action, the climate will be much warmer than it has ever been during human civilization, making it challenging to provide enough food, water and housing for everyone on the planet.