If we must take everything in order of most important to least important, then doing anything that doesn't affect this most important thing is a colossal waste of time.
If we are affecting the temperature of the planet in a way that will make it uninhabitable to us, it behooves us to do anything and everything we can to make our impact less. Whether or not it is the biggest contribution to the problem. Plus, solving the smaller problem may give us insights to the larger problem.
If we must take everything in order of most important to least important, then doing anything that doesn't affect this most important thing is a colossal waste of time.
If we are affecting the temperature of the planet in a way that will make it uninhabitable to us, it behooves us to do anything and everything we can to make our impact less. Whether or not it is the biggest contribution to the problem. Plus, solving the smaller problem may give us insights to the larger problem.