Climate change is

Climate change is

a positive good as ice-capped lands are clearing
a negative good as sea water is rising
a global negative externality caused by Green House Gas emissions
a local positive externality caused by afforestation
This question was previously asked in
UPSC SO-Steno – 2017
Climate change is best described as a global negative externality caused by Green House Gas emissions.
Climate change is driven primarily by the emission of greenhouse gases (like CO2) from human activities. The costs associated with the impacts of climate change (such as sea-level rise, extreme weather events, etc.) are not borne by the emitters alone but by the entire global community, making it an externality. Since these impacts are largely harmful, it is a negative externality. Its effects are global in scope, affecting all regions.
Options A and B describe potential outcomes or consequences, not the fundamental nature or cause. Option D describes a positive externality (afforestation) which is a mitigation measure against climate change, not climate change itself.