What is the Higgs boson of climate research?

That is essentially the question we ended the lunchtime meeting with today.

What is a big question that needs to be answered definitively by climate science? The idea is to provoke a community wide effort, funneling creativity, effort, and resources toward one big problem. The example in particle physics was the Higgs boson and the construction of the large hadron collider. Is there something equivalent in climate research?