Answer:
The phrase "at the mercy of" suggests that the Norse depended entirely on weather to succeed in colonizing Greenland.
Step-by-step explanation:
As you may already know, Greenland, like other Nordic countries, has a very cold winter. This was no different in ancient times, when the Nordic people depended a lot on the weather being favorable to their activities. In this case, we can say that the Nordic people were "at the mercy of" weather, that is, they needed the weather to be favorable for them to be successful in their activities, even in the activities related to region settlements, such as Greenland. In this case, unfavorable weather meant that the Nordics did not continue to colonize territories until the weather allowed it.