Final answer:
The aspect still involving considerable scientific uncertainty is the effects of deforestation on climate, as its complex interactions in the climate system are less understood than other well-established links such as the impact of greenhouse gases on global warming(option c).
Step-by-step explanation:
Among the options given, the one that still carries a relatively high degree of scientific uncertainty is c) The effects of deforestation on climate.
While the link between smoking and lung cancer, the cause of ozone depletion, and the impact of greenhouse gases on global warming are well-established with a strong consensus in the scientific community, the precise effects of deforestation on the climate involve complex interactions and feedback mechanisms that are less fully understood. Numerous factors, such as changes in albedo (the reflectivity of Earth's surface), the disruption of the water cycle, and the loss of biodiversity, all play a role in deforestation's impact on climate, and scientists are still working to fully quantify and model these effects.
Greenhouse gases like carbon dioxide and methane are well-documented contributors to global warming. Conversely, gases such as oxygen are not considered significant contributors to this process. Activities like the burning of coal and oil release large amounts of carbon dioxide, which is the primary greenhouse gas emitted through human activities and is the single most important factor to control to mitigate global warming.