Final answer:
Habitat destruction is the leading cause of current species extinctions, with global warming expected to significantly contribute to future extinctions.
Step-by-step explanation:
The leading cause of current species extinctions is habitat destruction. This results from various human activities such as deforestation, damming of rivers, and the expansion of urban areas. Moreover, with the growth in human population and unsustainable resource use, other significant causes include overharvesting, the introduction of exotic species, and more recently, climate change. While global warming is a rising concern and is projected to become a major cause of extinctions in the future, particularly affecting Arctic regions and causing sea-level rise, it currently adds to the extinctions caused by habitat destruction. Furthermore, the growing global trade and transportation have exacerbated the issue by introducing harmful exotic species to new environments.