Which event is most likely to cause a sudden change weather ?

We generally consider climate changes as taking place on the scale of hundreds or even thousands of years. However, since the early 1990s, a radical shift in the scientific understanding of Earth's climate history has occurred. We now know that that major regional and global climate shifts have occurred in just a few decades or even a single year. The most recent of these shifts occurred just 8200 years ago. If an abrupt climate change of similar magnitude happened today, it would have severe consequences for humans and natural ecosystems. Although scientists consider an abrupt climate change unlikely in the next 100 years, their understanding of the phenomena is still a work-in-progress, and such a change could be triggered instantly by natural processes or by human-caused global warming with little warning.

Natural processes or human-cause global warming

What are the choices?