What is the scope of God's will? Assuming that Protestant Christians believe that some outcomes and events in life are not necessarily God's will (correct me if this is an invalid assumption), what are some alternate explanations for an outcome if not God's will? Does the scripture give us instructions on how to determine (after the fact) if a particular outcome was God's will or not?

For example, in the discussion on this site regarding global warming, someone suggests that either God's will is that global warming should happen and therefore it must, or it is God's will that it must not happen, and therefore it won't - in either case, it is God's will and thus unavoidable regardless of what we do.

The above example seems to be a false dichotomy - what are the other alternatives that a Protestant Christian would allow in this situation?
These sorts of questions are always difficult to answer as they often require an understanding of God's sovereignty which in many ways is incomprehensible. What a wise being with infinite wisdom, knowledge of all and unlimited power can orchestrate, manipulate, foresee and advert etc. is impossible for us to ponder. In this sense virtually any external event may be used for some higher good as part of the divine 'emergency recovery operations' of a fallen world, even a global catastrophe like a global flood can have its purpose. Yet none of this is 'God's will' in the sense that we benefit from knowing if it is, or not.

However I think your specific question can side step the regular difficulties as you seem to be asking about God's direct will which can be determined to some degree. This is 'God's will' in the sense that he wants us to know it and that knowledge would be beneficial to us. Well from this standpoint his will is that we do not sin.  This really is God's will in the main sense.  We often get lost thinking philosophically about his hidden will, but we only need to concern ourselves with his clear will.  His will is that we love God with all our heart and our neighbor as ourselves, and these we can only do by faith in Christ as the atoning sacrifice for our sin.

So the question to ask is did an event come about by sin, or by love.  In whatever sense global warming has or will come about by greed it would not be God's will.  If it partially comes about because of attempts to secure shelter, safety and general prosperity for the poor, by faith in Christ it is God's will.

Unfortunately when we limit God's will to this clear scope most everything that happens in the world every day is not God's direct will.  It therefore makes the question less relevant as the answer is almost always no.

God's will will be followed in heaven.  Until then to whatever degree we rely on the death of Christ for the forgiveness of sins, and so in gratitude to his grace love and delight in God and become generous and kind to our neighbor, being also gracious to them by enjoying them in any way we can while publishing Christ's dying love, we are growing more and more in line with God's will.  But even then everything we do, wether eating or drinking has mixed motives in it, so part of our loyalty to our boss may be God's will and part fleshly and not his will.

You see it is all about the heart so we will only know in heaven what was God's will and what was not because our hearts are deceitful.  Most likely whatever seemed most like God's will on earth, as viewed by the world, we may find was not. It is by faith we see his will and by faith that we do it.