What do the authors suggest are biggest problems with using continents?

* They are arbitrary. The boundaries of continents are often based on political or historical factors rather than on any natural features. This can lead to confusion and disagreement about which continent a particular area belongs to.

* They are too large. Continents are vast areas, and it can be difficult to generalize about their climate, geography, or culture. This can make it difficult to understand the world as a whole.

* They are outdated. The concept of continents was developed long ago, and it does not reflect the way that people live and travel today. For example, many people now live in transcontinental cities, which are located in two or more continents.