Ideas on open questions in Earth Science?

[PD: The result of this initiative is in this later post]

Searching for Unknowns consumes most of our research time and also makes the bulk of our research motivation. Wouldn't it be good to share the biggest open questions in our respective disciplines? 

I'm working on a list of challenging and sound geoscientific questions being researched today. Geoscience is here taken in the wide sense, including climate, hydrology, planetary science, ecology, geology, geophysics, etc.
I'm thinking of specific problems that are well established from a scientific point of view and that have an impact on at least a couple of subdisciplines. Examples would be: "What caused the Permian extinction?" or "What drives magnetic polarity reversals?" or "How much of the current climate change is anthropogenic?". But a bit more of elaboration and a key reference would be desirable.
I will try to summarize the compiled ideas in a later entry in this blog, but key references discussing each subject are welcome. I'm looking forward for suggestions or feedback, either as comments to this post (below) or at @danigeos on Twitter.

Alexandra Witze shares links (in a comment below) to the following relevant documents: 1, 2. Some of the questions summarized there will be useful as a general frame to what i aim at (they are big trans-scientific goals). But I would like to find the top key questions at a more detailed level, more specific, even if of interest only to a minority of subfields within Earth Sciences. Open problems that most assistants to the AGU, EGU, GSA or INQUA meetings could be curious about even if only a small percentage could really judge critically.

Update 2:
A reference to a key paper on the problem proposed will make every contribution much more valuable!

Update 3:
The result of this initiative is in this later post: