I love your question because it really gets us going and causes us to want to answer your question in all sorts of different ways.
Being an avid fan of the ancient alien theorists and such like, my imagination goes whirring with a question like this. I write science fiction based stories for young people and have had to think through this question many times before, in trying to decide how to present alien invasion type scenarios to the reader.
I have to presume that is where your question is going when you mention UFOs.
Do you mean to suggest we are being visited by alien beings in their spacecraft, frequently and that they have had some influence on the recent perceptible changes on our global climate?
If so, who knows what kind of technology has been dispelled on our planet from time to time? However the mind boggles and we would need some proof to actually accept this supposition.
From an ancient alien theorist's point of view, it would seem that we may have had 'visitors' before, influencing our genetics and who knows our global climate.
We live in a vast universe and it seems logical and probable therefore, that there has to be some kind of intelligent life out somewhere and life that is probably much more intelligent and superior to ours.
It is my guess we may be influenced at this moment as we 'innocently and naively' write our hubs for HubPages.
Which ever way we chose to look at this question for an answer, there may be aliens among who are already answering this question.
Thanks for the question.