More than of the world's population lives in cities, and of these people live in overcrowded slums, leaving them vulnerable to the effects of a changing climate.

I believe the answer is: half ; one-quarter

Economic factor played the most crucial role in creating this situation.
The reason why cities are overcrowded is because most of job opportunities that exist in every countries would centered in the city.
In the process of competing for job, some people would failed to obtain high paying jobs that forced them to sacrifice their well being to keep competing in the city.

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