Final answer:
The uniqueness of current climate change compared to earlier Earth history is primarily due to unprecedented levels of greenhouse gas emissions.
Step-by-step explanation:
The uniqueness of current climate change compared to earlier Earth history is primarily due to a) Unprecedented levels of greenhouse gas emissions. While natural cyclical patterns in Earth's climate and variations in solar radiation have played a role in past climate changes, they are not sufficient to explain the magnitude and rapidity of the current climate change.
Human activities, especially the combustion of fossil fuels, have significantly increased the levels of greenhouse gases in the atmosphere, leading to the enhanced greenhouse effect and global warming.